Magnetic susceptibility. Trends in instrumentation and applications to

(165)

(186)

(167) (188)

L., Kline, D., Castle, J. G., Science, 167, 704 (1970) Weeks, R. A., Kolopus. J. L., Kline, D . , Chatelain, A., Proc. Apollo 17 Lunar Sci. Conf.. 3, 2467 (1970): Geochim. Cosmochim. Acta. Suppl. 1. Weeks, R. A., Proc. Third Lunar Sci. Conf., 3, 2503 (1972): Geochim. Cosmochim. Acta, Suppl. 3. William, D . J., Goeddle, A . D., Pearson, J. M., J, Amer. Chem. SOC., 94, 7580 (1972). Winstein, S., Moshuk, G . , Rieke, R . , Ogliaruso. M . , ;bid.. 95, 2624 (1973).

(189) Woischnik, E., Ohmes, E., Volkamer, K.. Addendum Tetrahedron, 28,4243 (1972) (197) Patton, E. V., West, R., J. Phys. Chem., 77, 2652 (1973). (190) Wong, S. K., Wan, J. K., J. Amer. Chem. SOC., 94, 7197 (1972) (198) Williams, L. F.. Yim, M. E . , Wood, D. E , (191) Yarnarnoto, D., Fukumoto, T . , Ikawa, N., J . Amer. Chem. SOC.,95, 6475 (1973) Bull. Chem. SOC.Jap., 45, 1403 (1972). (199) Nelsen, S. F., Gillespie, J. P., J. Org. Chem., 38, 3592 (1973). (192) Yarnamoto, D., Ikawa, I . , ibid., p 1405, (193) Yamamoto, D., Ozeki, F., ;bid., 1408, (200) Ahrens, W.. Berndt, A , , Angew. Chem.. 85 657 (1973). (194) Yoshida, 2.1 Sugimoto. T., Yoneda, s.9 J. (201) Cl;path, p,, zelewsky, A , v,, Chim, Chem. SOC.,Chem. Commun., 60 (1972). Acta. 55 52 (1972). v., lngold, K , y,, J. A ~ (195) Yoshimi, H., Kuwata, K., Mol. Phys., 23, (202) Malaiest;, 297 (1972). Chem. SOC..95, 6400 (1973). (196) Young, J. J., Stevenson, G. R., Bauld, N . (203) Danen, W. C., Richard, R. C . , ibid.. 94, L., J. Amer. Chem. Soc.. 94, 8790 (1972). 3234 (1972).

Magnetic Susceptibility: Trends In Instrumentationand Applications to Solid State Science L. N. Mulay’ and lndumati L. Mulay Materials Research Laboratory,2 The Pennsylvania State University, University Park, Pa. 16802

In this seventh review on magnetic susceptibility, we survey important trends in instrumentation and applications, especially in the realm of solid state science. The first six reviews (46, 49-53) appeared during 1962 to 1972. This 1974 addition covers literature from about January 1972 to December 1973. In response to an editorial plea, we have made this review more concise than the previous ones. In doing so, it seemed imperative that we should depict the exceptionally novel trends and eliminate those that we covered before, without, of course, implying in the least that these are no longer important. Hence, we shall not review the work on lunar samples, which was adequately covered in 1970 and 1972 (52, 53) and work on transition metal complexes including the biocomplexes, which we surveyed in all reviews (46, 49-53) up t o now. It should be noted that excellent surveys on the transition metal complexes are now available and are listed in a later section. Since this entire review issue is primarily concerned with instrumentation, we have focused relatively more attention on this aspect; yet, even here, we have attempted to point out the truly new trends in the hope that experimentalists will explore new avenues of instrumentation for specific problem-oriented research and that they will not remain chained to otherwise outmoded techniques. Since the classical methods, such as the Faraday and the Gouy techniques, quite surprisingly, continue to be very reliable for the measurement of weak susceptibilities and since these are relatively less expensive than some of the modern gadgetry, we shall continue to incorporate important modifications, tricks-of-the-trade reported by ingenious workers. Unfortunately, we shall not be able to survey various temperature controlling and measurement devices except to mention later another review and a few selected papers.

NEW TRENDS New developments in magnetic instrumentation are proceeding primarily along the following lines. (i) Use of relatively new phenomena in superconductivity ( e . g . ,the Josephson Tunneling Effect). (ii) Use of superconducting magnets in otherwise routine techniques such as the classical force methods, the vibrating sample magnetometer and so on. (iii) Use of the electron microprobe and magneto-optic I n q u i r i e s s h o u l d b e addressed t o t h i s a u t h o r . A l s o a f f i l i a t e d w i t h t h e S o l i d S t a t e Science P r o g r a m , a n i n t e r d i s c i p l i n a r y graduate p r o g r a m in t h e p h y s i c a l sciences, l e a d i n g t o M.S. a n d Ph.D. degrees. L a b o r a t o r y for i n t e r d i s c i p l i n a r y research in Chemistry, P h y s ics, P l a n e t a r y Sciences, etc.

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techniques in the measurement of magnetic properties of especially thin films. With regard to applications to solid state science (that is the chemistry and physics of solids), again we have to be very selective because this realm, as discussed below, is very vast. Solid state chemistry may appear to be a narrow and a highly specialized “discipline” to many chemists, who generally do all of their synthesis and analysis in solution media. However, it should be emphasized that under the rubric of solid state chemistry, as under materials science, one encounters a wide diversity of solid materials and an equally diverse number of physical methods for their characterization, elucidation of their structure and properties a t the macroscopic (bulk type) and at the microscopic (electronic type) levels. Speaking of classification by types of materials encountered under solid state chemistry, one can immediately think of (a) inorganic, ( b ) organic, (c) metalorganic ( e . g . , coordination complexes), and (d) organometallic ( e . g . , “sandwich” type or “metallocene”) compounds. Furthermore, one can classify materials according to the “ionic” or “molecular” nature of packing of individual species in crystal lattices. Generally, the types b, c, and d will fall in the category of molecular solids. All that we said so far can be generally grouped under the category of nometallic materials. This immediately suggests the existence of another category-namely, metallic materials consisting of pure metals, their alloys, and the so-called intermetallic compounds. True to say, the concept of inorganic chemistry, especially in highly departmentalized academia, has not generally gone beyond the domain of coordination complexes, organometallics, etc. and beyond the physical methods used for probing into their “structures,”-that is, their electronic bonding. Many chemists appear to focus their attention on molecular solids and attempt t o elucidate the “structure” of individual “molecular species,” again using solution techniques such as the optical (UV, Visible, IR) spectroscopy and high resolution nuclear magnetic (NMR), and electron paramagnetic (EPR) resonance spectroscopy. Somehow, very few attempts are made to understand the bulk properties of the solid (except melting point and color) and even fewer in elucidating their structure-property correlations. Thus, in the realm of nonmetallic materials alone, the solid state chemistry aspects of molecular solids a r e often ignored, notwithstanding that it is these aspects that have a direct societal relevance t o modern device-oriented technology. This technology encompasses the synthesis, characterization, and often tailor-made properties of materials useful in various ways, only a few of which are noted below. (a) Materials for the production and conservation of en-

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L. N. Mulay is a professor in the Solid State Science Program and a senior member of the Graduate School at Pennsylvania State University He received his BS (1944) degree in chemistry and physics and his MS (1946) and PhD (1950) degrees in physical chemistry from the University of Bombay He held various research and teaching positions in chemistry at Northwestern and Harvard Universities and the University of Cincinnati before joining the faculty at Penn State in 1963 Dr Mulay is the author of about 110 research publications and a monograph on “Magnetic Susceptibility ” HIS research interests have centered on magnetic probes, such as susceptibility. broad-line NMR, EPR, and Mossbauer spectroscopy for the characterization and structural elucidation of solids at the macro and microscopic levels Dr Mulay has traveled widely and contributed to international meetings and research conferences and to the interdisciplinary program at Penn State He is a member of several professional organizations and was chairman of the Central Pennsylvania Section of the ACS (1965-67) He has been a regular contributor to Analytical Chemistry s biennial Fundamental Review issue since 1962

ergy; for superconductivity; for lasers for the detection and measurement of electromagnetic radiation. (b) Materials for memory storage in high speed computers; for radio and high frequency telecommunications including television. (c) Materials for high-pressure, high-temperature as also cryogenic applications. Fortunately, new thrusts are being made in solid state science, in the U.S.A. These thrusts are exemplified by new symposia on solid state chemistry, solid state organic chemistry, and materials science, which were sponsored by the American Chemical Society during the past five years. Having offered the above prologue, we shall consider briefly two new trends in the applications of magnetic susceptibility techniques to solid state science. These are: (i) The organic charge-transfer complexes which were recently found to show a very large increase in electrical conductivity a t cryogenic temperatures. These observations have raised the hope of producing organic (polymer) superconductors. Another system consisting of coordination complexes of platinum shows one-dimensional electrical conductivity. (ii) AmoTphours materials such as the chalcogenide glasses, which display very interesting electrical fast switching properties. These properties are likely to be used for memory storage applications.

GENERAL LITERATURE Abstract Services and New Additions. The Cumulative Solid State Abstracts and the STAR compilation of the National Aeronautics and Space Administration (NASA) of the U.S. Government continue to provide excellent abstracts in magnetics at a somewhat slow pace. The “Index to Literature in Magnetism,” published by the Bell Telephone Laboratories is highly specialized and unfortunately ignores many interesting aspects of magnetochemistry such as the diamagnetism of organic and organometallic compounds. The general scope and use of these and other abstracting and indexing services were outlined in our previous reviews (50-53). We have also previously commented t h a t there was no way for any reader to locate even the title of a paper that had been or was to be presented a t any of the meetings, conferences, or symposia sponsored by organizations of professional interest to him. This situation appears to have been partly corrected, at least in the general realm of chemistry, by the appearance of the “Current Index to Conference Papers in Chemistry” (cf. 5 3 ) . The first issue of Volume I started in September 1969. These issues should be helpful in finding the titles (and in appropriate cases, the abstracts) of papers presented at various chemistry-oriented meetings. Unfortunately, very little magnetochemistry-type work is reported a t such meetings; the general thrust has been on high resolution NMR, EPR, and, to a modest extent, on the Mossbauer Spectroscopy.

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lndumati L. Mulav has been a research associate and coilaborator in the Materials Research Laboratory at Pennsylvania State University since 1963. She received a BS in chemistry and MS in biochemistry (1953) from the University of Bombay. She also earned an MS (Radcliffe College) in 1957 and a PhD in biology (Inst. Divi Thomae) and did postdoctoral research at the University of Cincinnati before joining Penn State. Her main research interests include radiation genetics, trace metal analysis, EPR studies on cancer tissues. and the effect of magnetic fields on biological matter She has published several papers and reviews in these areas and contributed a chapter to a book on biomagnetism &&&

The same organization (cf. 53) also yrblishes the “Current Index to Science and Technology, which very likely covers all those areas of magnetism in which our readers may be interested. The annual “Magnetism and Magnetic Materials Conference” previously published all presented papers in the March issue of the Journal of Applied Physics. Since 1971, the organizers have decided to publish the papers in a special “Proceedings” issue to be published by the American Institute of Physics. This is, in our opinion, a backward step in that not all libraries (even the affluent ones) will consider buying such “Proceedings” and the information, which otherwise would have reached a larger number of readers, will be restricted to a very limited audience (probably, to the contributors of the papers and their closest associates). Contributed Chapters, Reviews, and Books. Several contributed chapters, reviews, and books with a direct or indirect bearing on magnetic susceptibility, magnetic materials, and, so to speak, on “magnetochemistry” have recently appeared. Static magnetic susceptibility, magnetic relaxation, magnetic resonance, and Mossbauer spectroscopy are intricately related. Since resonance and Mossbauer spectroscopy have developed as separate instrumental disciplines, literature in these areas will not be reviewed here, in the belief t h a t these will be covered elsewhere. With reference to “materials,” we should like to point out that the concept of “materials science” is not new; the name was introduced about a decade ago, and, as outlined below, several excellent contributions to magnetism have resulted from this approach. It is unfortunate that some chemists (and physicists) find it difficult to comprehend the “materials science” concept. Briefly stated, it is an emerging “interdisciplinary” approach in which the chemist, the ceramist, the metallurgist, the physicist, the engineer, etc., take an integrated look a t both old and new problems of science and attempt to solve them. Indeed, “materials” covers a wider territory when “chemical compounds” of well-defined stoichiometry, with which the chemist is most familiar. In the “materials science” approach, attempts are made to synthesize new materials with tailor-made properties, to “characterize” them (old and new), and to understand their structures a t the “macroscopic” and “microscopic” (or “electronic”) levels. Characterization a t the macroscopic level includes special efforts to understand the widely variable “defect state” of the material by various techniques including those related to magnetic, electrical, optical, and mechanical properties. A study of certain properties on the one hand helps in the “characterization” of materials and, on the other, in the “elucidation” of their electronic structure. In this respect, magnetic susceptibility and other related magnetic properties play an import a n t role. In addition to the books listed below, there are a large number of treatises on “Materials Science,” “High Tem-

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perature Materials, ” “Advances in Materials Science,” etc., which contain references to magnetic properties of severaf‘materials. Recently a number of books under the title Solid State Chemistry” have appeared. Almost every major publisher nowadays has a special listing for materials science; hence, we refer our readers to such lists. Two excellent series of reviews have appeared during the past couple of years. The first is known as the M T P International Review of Science, named after the Medical and Technological Publishing company, University Park Press, Md. (U.S.A.). This serial publication is organized under many diverse headings including the more general topics such as “Inorganic,” “Physical,” and “Analytical” chemistry. The following typical reviews of magnetochemical interest should prove useful to our readers: Sharp and Mays (69) have reviewed work on (the compounds of) transition metals; Bagnall (5), lanthanides and actinides; Roberts (67) Solid State Chemistry. Ditchfield (15) surveys magnetic susceptibility of diamagnetic molecules; Abrahams (1) surveys inorganic magnetic materials. There are many more topics, which are too numerous to list here. The second review series is known as the Specialist Periodical Reports published by the Chemical Society, London. Typical topics include: Electronic structure and magnetism of inorganic compounds by Day (14) and Inorganic chemistry of transition elements by Johnson (33). Zeiger and Pratt (77) discuss in detail the “Magnetic Interactions In Solids.” The book is written with a theoretical rigor particularly to focus on the effect of magnetic interactions on the behavior of electrons in crystalline solids. First, the basic magnetic Hamiltonian is developed, including all electromagnetic interactions. These results are then applied to hydrogenic atoms and generalized to many-electron atoms and ions. Following this, the manyelectron ion is considered in a crystalline environment, as described by crystal field theory, including the effects of covalence. Consideration of itinerant electron states in crystals leads naturally to the discussion of band electrons and their interactions with electromagnetic fields, where topics such as Landau levels, cyclotron resonance, the de Haas-van Alphen effect, and coupling to both magnetic fields and lattice vibrations are encountered. Next, effective mass theory is discussed for the full magnetic oneelectron Hamiltonian, leading to the treatment of the magnetic properties of impurity states and excitons. Finally, indirect magnetic interactions in metals are considered. The purpose of this book according to the authors is not to cover the more traditional topics in magnetism such as ferro-, ferri-, and antiferromagnetism, spin waves, or domain structure. McCoy and Wu (43) have written an authoritative and comprehensive account of the two-dimensional Ising model. This model is not only the most thoroughly investigated but it is also the richest and most profound among all systems in statistical mechanics on which exact calculations have been made. A number of spin correlation functions are discussed in detail. Topics include specific heat and magnetization and a discussion of the one-dimensional Ising model which he first proposed in 1925. McCoy and Wu have integrated much of the widely dispersed information on the subject so as to provide an understanding of phase transitions on the basis of the Ising models. Myers (55) has written “Molecular Magnetism and Magnetic Resonance Spectroscopy” with a view to correlate our understanding of the molecular basis of magnetism and the nuclear and electron resonance spectroscopy. He has succeeded in presenting a unified account of Van Vleck’s well known methods and the Spin Hamiltonian developed for magnetic resonance spectroscopy. He has shown how the vector model, as adapted from the commutation rules of quantum mechanics, can serve as a bridge between the classical and quantum theories of magnetism. He points out that although it has become fashionable in recent years to reject the vector model as a vestige of an old and abandoned quantum mechanics, the vector model has some usefulness, if properly used. Most teachers recognize the pedagogical importance of the vector model as an instrutional aid in advanced undergrad492R

uate and beginning graduate level courses. The author has admirably succeeded in his efforts to clearly and concisely present the two interrelated (static and dynamic) aspects of magnetism. The book contains several original illustrations, for instance, those that clearly depict the parallel and perpendicular components of the magnetic moment and other magnetic parameters. Nesbitt and Wernick (56) have written an up-to-date account of rare earth permanent magnets. Both authors have been engaged in research on the magnetic properties of materials a t Bell Laboratories for many years. Several portions of the book summarize their original work. The development of rare earth-cobalt intermetallic compounds such as SmCoa have revolutionalized the design and manufacture of permanent magnetic materials. The new materials provide significantly larger energy products ( B x H) than the conventional Alnico (Al-Ni-Co composition) magnets and the new materials also provide simplifications in their preparation. The book is written in the style of R. M. Bozorth’s “Ferromagnetism,” giving detailed information on the metallurgy of the rare earth compounds and extensive data of their physical and magnetic properties. This is accompanied by simple explanations-elementary aspects of magnetism are developed first to provide an understanding of the subject of permanent magnetism. Another book in the general area of rare earth permanent magnets is written by Wallace (72), based partly on his extensive research in the chemistry department a t the University of Pittsburgh. Magnetic properties of rare earth metals are described by Elliott (19). The topic of magnetically ordered crystals containing impurities is covered by Izyumov and Medvedev (32). The book deals essentially with the theory of ferromagnetism of crystals containing impurities. Hooper and de Graaf (28) have edited a volume dealing with magnetic properties of various amorphous materials. These include properties ranging all the way from the diamagnetism of chalcogenide glasses, “paramagnetism” of dilute alloys (these are often called the “spin glasses”), to the superparamagnetism of widely varying systems. Several authors of contributed chapters, based on the papers presented a t the First international Symposium on “Amorphour Magnetism” (Sept. 1972), discuss the theory, synthesis, characterization, and properties of amorphous materials. In addition to a discussion of the magnetic and the thermal properties, some of the electrical properties (including the recently discovered “switching phenomena”) are presented. It would be fair to say that “amorphous magnetism” is now becoming a fashionable topic for study in many laboratories and is attracting considerable attention because of its technological significance in catalysis, “switching,” and related device applications. The question of magnetic ordering in amorphous systems is indeed very intriguing and is expected to be a focal point of magnetics research in years to come. “Chemically Induced Magnetic Polarization” edited by Lepley and Closs (38) should be of great interest to biochemists, molecular biologists, biomedical and pharmaceutical researchers in the detection of molecular species in plant and animal studies. The following succinct description of the contents, which appears in the book, should be of special interest to chemical and physical researchers. “The discovery of spin polarization induced by chemical reactions in 1967 has added an entirely new and potent dimension to the field of magnetic resonance. Indicative of this potential is its widespread applicability to many sciences, especially biology and medicine. Until now, this versatile technique had remained in the hands of a small group of specialists. This book offers the first comprehensive review of the origin and subsequent theories of this new scientific development, as seen by eleven of the major contributors to the field. But most important, a broad range of applications is discussed so that a scientist, unfamiliar with the technique, will know when and where chemically induced spin polarization occurs, how to detect it, and how to interpret observed results.” The first two chapters provide background material on the general theory of nuclear and electron spin polarizations. Applications are then covered in the remaining six

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chapters. Topics range from CIDNP (Chemically Induced Dynamic Nuclear Polarization) in reactions of carbenes, azo compounds, and photo-excited carbonyl compounds to CIDNP in reactions of radical ions; and from thermal and photochemical decompositions of aliphatic acyl peroxides to aroyl peroxide decompositions. Throughout the book, special emphasis is given to particular aspects of theory which are the direct outgrowth of experimental results and to detailed examples of applications a t a readily understandable level. In addition, many figures and diagrams supplement the text which are not available elsewhere. An excellent and up-to-date tabulation of magnetic materials is given by Connolly et al. (11). The third volume of electronic properties of materials is edited by Grigsby (25). The following announcement by the publisher aptly describes the value of the book. Magneticists will be particularly interested in information on materials which display ferro- and ferrimagnetic and superconducting properties. “This series, continually revised and updated, presents the results of the Electronic Properties Information Center’s indexing effort. It contains a modified coordinate index using an uncomplicated descriptor vocabulary. Descriptors consist of material names and properties; both materials and properties are cross-referenced and a glossary is included. As the series covers semiconductors, insulators, ferroelectric dielectrics, metals, ferrites, ferromagnetics, electroluminescent materials, thermionic emitters, and superconductors, anyone working in the field is sure t o find the information he needs in a time-saving, easyto-use form.”

BizSes as a Hall effect “magnetometer” for reliable low temperature use (1.1 to 78 K). The term “magnetometer” here simply means a “probe” and its most practical application is to measure the magnetic field inside various cryostats placed between the poles of a magnet. The authors describe how single crystals of the n type semiconductor material (BizSes) are grown, its advantages over other materials such as InAs and the techniques for mounting a single crystal with appropriate leads for measuring the Hall effect voltage. Technical data for the Hall resistivity measured as a function of fields up to 7 Tesla (70,000 Oe) and temperature (up to 300 K ) are given. Magnetic Units and Magnetic Calculations. In our last review ( 5 3 ) ,we surveyed important aspects of the SI (Systemme International) units for magnetic quantities. In this section we list a n important observation by Hoppe (29) on the significance (or the lack of it) attached to the so-called unit “pB” or ‘‘p” used for reporting magnetic moments. We also offer comments on the calculation of magnetic moments from experimental data, and the use of computer programs. Hoppe (29) gives an interesting comparison of the effective magnetic moment ( p e r f )calculated on the basis of the CGS and the SI units, which we reviewed before ( 5 3 ) . He emphasizes t h a t perf is a number and is dimensionless in both systems of units and as such it does not have the “unit pB.” He suggests that although the present symbol can be retained, the current name given to peff is unsatisfactory and that the “Bohr Magneton number” would be a preferable name. With regard to the use of the relatively new SI units, and any newer still, which the international commissions may adopt, a significant number of magneticists, because of habit, will continue to follow the old CGS system unINSTRUMENTATION less every journal strictly enforces the use of the SI units in all branches of physical science. Perhaps it will be deCalibration of Susceptibility A p p a r a t u s a n d Magnetsirable for all authors of scientific publications for the ic Fields. Mulay (47) has listed a number of reliable comnext five years or so to report all data in SI units and give pounds of known magnetic susceptibility for the calibratypical values also in the CGS system. This mental “contion of various magnetic balances and magnetometers. To version or adjustment” period will perhaps help the budthis list, we add cerium magnesium nitrate and gadoliding and senior scientists to “communicate” with (or “renium oxide. These compounds are very useful in measurlate” to) each other before strictly enforcing a changeover ing the susceptibility of various samples by the relative to the new system. method ( 4 7 ) . Quite often some of these can be used as A number of laboratories including the writer’s magnetparamagnetic thermometers-that is, for measuring the ics laboratory a t The Pennsylvania State University have temperature in a cryostat or oven via their known magdeveloped computer programs (13) for calculating the netic susceptibility. Some of these aspects and a Hall efmagnetic moments of ions from their Russell-Saunders fect sensor for measuring the magnetic field are outlined ground state terms by the “spin only,” the “spin and orbelow. bital,” and the “total angular momentum methods.” An apparatus for the growth of large single crystals of cerous magnesium nitrate [Ce(NO3)6]~[Mg(H~0)6]3. These programs are useful in obtaining a quick compari6H20 is described by Fisher ( 2 3 ) . Flawless high puson of such theoretical values with the experimental mority, oriented single crystals in cyclindrical form (5cm ments. Brown (9) reports the development of a Fortran IV diam x 5.5-cm long) were grown with the C axis parallel c o m w t e r Droeram. which is available from him on reto the cylinder axis. It should be recalled t h a t cerous magque& (J. P . Brown, Morgan State College, Baltimore, nesium nitrate makes an excellent calibrating agent for Md. 21239). magnetic susceptibility measurements and the availability It should be noted that in obtaining the pelf, most workers assume a straightforward Curie law (xm = C / T ) of single crystals is quite essential in calibrating magnetic and use the simplified equation 1: torquemeters such as the Krishman’s flip angle method [ c f . Mulay ( 4 7 ) ] .The Fisher apparatus can be also adaptperi = 2.832&7 (1) ed for the growth of other crystals. Reference should be made to the original article for constructional details. However, it is important t o obtain a large number of data A technique for the high temperature calibration of a for xrn as a function of temperature and to see if the exvibrating sample magnetometer (made by the Princeton tended Curie-Weiss law is more applicable, namely, xm = Applied Research Corp, Princeton, N.J.) is suggested by [ C / ( T - 8 ) ] + a , where C and 8 are the Curie and the Hines and Moeller ( 2 7 ) .In our opinion, this technique can Weiss constants, respectively, and a is the temperature be applied to other magnetometers and magnetic balindependent term representing the Van Vleck “high freances. For calibrations up to 1050 K, the authors have quency” paramagnetism. Quite often 8 and/or a can be used a cubic GdzO3 as a standard (with a Curie constant C significantly large quantities. If this is so, then p e f f is = 4.254 x 10-2 emu K/gram and a Weiss constant 8 = more correctly represented by -17.5 K ) to obtain an offset correction for the difference between the sample and the oven temperature. The known susceptibility of Gdz03 expressed in terms of the A “blind” use of Equation 1 for calculating the peff constants given above [xm = C / ( T - e)] enables one to implies that one is assuming both a and 0 to be negligible. “measure” the actual temperature a t the sample relative While many workers appear to obtain the Weiss constant to the one measured by a thermocouple placed near the (e) from the plots of l / x m us. T they assume the a term sample. The offset correction was found to be linear to be zero, and thus they use Equation 3: through the entire temperature range of the oven. The authors describe several advantages of this technique over other suggestions such as establishing a thermal contact Yet some groups of workers assume an arbitrary value between the sample and the thermocouple. Woollam, Beale, and Spain (75) describe the use of for a and others seldom feel the obligation to give any raA N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 5, A P R I L 1974

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tionale even for a reasonably selected value. Sometimes the so-called rationale is in poor judgment. Thus, needless to say, there has been a considerable confusion in the magnetics literature, reported especially in chemistry oriented journals. It is very important to statistically analyze the xm us. T data described by the general equation

and to obtain a curve giving the best fit. The above equation is intrinsically nonlinear [cf. Draper (ZS)] and a nonlinear least squares analysis is required for the best “values” for the magnetic parameters C, 8, and a. The “best value” solution has been feasible only with the advent of large digital computers and the recent development of computer programs to solve such nonlinear curve fitting problems [cf. Marquart (4Z)I. We have extensively used such programs ( 5 4 ) for our magnetochemical studies on the trinuclear coordination complexes of Fe(II1) and on the mixed valence oxides of titanium (22). This statistical analysis has also been useful in obtaining the best xm us. T curves with a Calcomp plotter (California Computer Products, Palo Alto, Calif.) coupled t o an IBM-1410 computer. Modifications, Error -Analysis, Etc. for the Faraday and Gouy Balances. Zatko and Davis (76) recommend the use of sapphire filaments as hang-down supports for the Faraday and even the Gouy methods. Sapphire (A1203) filaments are now commercially available (Tyco Labs, Waltham, Mass. 02154, U.S.A.) and have a high tensile strength. Filaments 0.254 mm (10 mil) in diameter can be bent into circles less than 10 cm in diameter. Thus, the sapphire filaments do not break as easily as the quartz filaments which otherwise pose exasperating experimental problems. Quinn and Knauer (64) describe a few mechanical modifications to a Faraday balance using a Cahn R.G. balance. They also describe the use of manometric techniques for regulating temperatures of liquid helium baths in the very low temperature regions (2 to 4 K ) . A brief outline of a fully automated Faraday magnetic balance is given by Donini and coworkers (16). Magnetic susceptibilities can be measured routinely and automatically in the temperature range - 196” to +200 “C. Lateral instabilities in the Gouy balance are discussed by Stewart (70). He points out that these are of the same nature as those encountered in the Faraday balance, which he described previously. He derives an equation for the stability condition and points out that the magnetic image effects produced by the sample are quite negligible, being less than 0.5%. New Superconducting Devices. As pointed out before, a number of new techniques based on the discovery of relatively new phenomena in superconductivity have become available. One particular device outlined below is based on the Josephson tunneling effect. I t should be noted in passing that new techniques based on the rather well known and well established Meissner effect in superconductors also emerged during the past decade. One such technique was reviewed by us before ( 5 1 ) . Reference should be made to Mulay (47) and to the references listed therein. A number of SQUID techniques have emerged over the past decade and some have now become commercially available. SQUID is an acronym for Superconducting Quantum Interference Device. A miniaturized SQUID probe, as outlined below, can be used for measuring magnetic fields ( t h a t is, as a magnetometer) or for measuring gradients in a magnetic field, or as a gradiometer. Since a distortion of a uniform magnetic field is caused by introducing a sample in the field and since such change in the field is dependent on the susceptibility of the sample, the principles of the so-called gradiometer can be applied for measuring magnetic susceptibilities of various samples. In this mode, the apparatus has been designated as a susceptometer by Goree (24). A system consisting of two identical pick-up coils (P) and a field coil (F) all connected in series are placed inside a superconducting solenoid such that the axis of the pick-up coils (P) is parallel to the field axis. The field coil 494R

(F) is coupled to a drive-and-sense coil (S) which is wound around a se%sor. The placing of a sample in one of the pick-up coils (with a standard material in the other coil) produces a signal in the sense coil (S) which is electronically detected, amplified, and measured in a routine way. The signal intensity is proportional to the susceptibility of the sample. The operation of this susceptometer or magnetometer depends primarily on the sensor which is a quantum interference device and is based on the “tunneling effect” first predicted by Brian Josephson in 1962. (He shared the 1973 Nobel Prize in physics for this work.) He showed that supercurrents could tunnel through a thin dielectric barrier without developing a voltage drop. He further predicted that a critical current exists above which a voltage would be developed. The sensor consists of a thin dielectric film ( - 10 A) or a point contact held between two superconductors (such as strips of Nb-Sn alloy). This sensor is known as the Josephson junction. For finite voltages (V)the supercurrent in the junction oscillates a t a frequency ( Y ) directly proportional to the voltage given by the Josephson equation hv = 2 eV where the symbols have their usual meaning. It has now become customary to use the “flux quantum” unit 40 in the literGauss ature on the SQUID techniques [& = 2.07 x cmz]. It is beyond the scope of the present survey to furnish even briefly the many intricate principles of superconductivity in general and of the Josephson quantum tunneling effect in particular. The readers are, therefore, urged to familiarize themselves with excellent accounts of the phenomena and theory given by Kittel (35),Morrish (45),and Williams (74).Literature supplied by the Develco Company is also very useful (24). It should be noted in passing that over 20 books and reviews on the science and technology of superconductivity have appeared during the past five years. Indeed this topic, while continuing t o be an integral part of magnetism, is growing into a major subdiscipline of physics because of the extensive research being carried out in several prominent laboratories all over the world. Microprobe and Magneto-optic Apparatus for Magnetic Materials. In this section we present three papers on novel instrumentation developed for measuring the magnetic parameters of magnetic materials, such as coercivity, anisotropic constants, etc. Wichner (73) has constructed a versatile microprobe analyzer for magnetic thin films. He points out that the versatility of a cross-wire RF magnetic microprobe tester is increased with the use of orthogonal 60 Hz and dc magnetic fields. Thus, a single instrument provides a large number of magnetic parameters for thin films such as coercivity, anisotropy field, magnitude of angular dispersion, skew, magnetostriction coefficient, and the general shape of the Stoner-Wohlfarth astroid. The new techniques are quite sensitive and simple for obtaining reliable results. Schematics of the microprobe system are given along with typical results for a 500-A thick film of an alloy (Ni (81%)-Fe (19%)] with uniaxial anisotropy. The microprobe technique can be used with thin films only; nevertheless, it shows elegantly the interaction of electromagnetic radiation with electronic spins in ferro- or ferrimagnetic domains. A signal processing technique to obtain the Kerr effect M-H loops is outlined by Mapps and McQuillin (40).The M-H surface relationship in a manganese-zinc ferrite crystal is obtained using a laser beam, a photomultiplier tube, and signal-averaging techniques. Unwanted magnetostrictive components are eliminated by computer-aided Fourier analysis. The method is used to obtain surface M-H loops using the longitudinal Kerr effect to detect the fluctuation in the surface magnetization of ferrites. Modifications to a magneto-optic hysteresigraph are outlined by Horne and Sawatzky (30). These modifications to a previously reported instrument for the direct recording of Faraday rotation and magnetic hysteresis have increased the sensitivity by a factor of ten. Kerr effect and in-plane Faraday rotation measurement capability have been added to the original instrument. The maximum available field has been doubled to about 9000 Oe. Details of the normalizing and detector circuitry are provided along with hysteresis loop results for a Fe& sample.

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These results consist of the in-plane Faraday effect and the longitudinal Kerr effect. Hysteresis Loop Tracers a n d Vibration Type Magnetometers for Magnetic Materials. A large number of papers are reported on the hysterigraphs, or hysteresis loop tracers, and special types of magnetometers involving a vibration of the sample or the coil are being developed. This type of upsurge in magnetic instrumentation merely reflects the deep and ever-growing interest in the synthesis and characterization of magnetic materials for new applications. Magnetic hysteresis loop tracer using operational amplifiers is outlined by Fay (21), who discusses in some detail the theoretical basis for his technique and the electronic circuitry for tracing hysteresis loops of cylindrical nonconductive ferromagnets. Fields up to 10,000 Oe peak amplitude are obtained from a water-cooled solenoid driven a t 60 Hz, and the change in field is detected with a coil coupled to the sample. Two operational amplifiers convert the input variables to analogs of the local field H and the induction B or magnetic intensity I. The sample area and are used as parameters and the demagnetizing factor (N) a correction for the excess coil area (A,) is introduced. Correct loops are obtained except when N and A, are very large. Typical results for the rapid determination of the magnetic properties of a hard ferrite are described. Suggestions for the measurement of permeabilities via rapid magnetization and demagnetization are offered. Lenaerts and Vanwormhoudt (37) describe a low frequency hysterigraph capable of continuously recording a series of well defined B-H loops both symmetric and antisymmetric. The instrument can be programmed before starting various sequences of measurements with a simple read-only memory. Typical recordings of a hysterigraph with major and minor loops is given along with a brief discussion of the results and the advantages of this apparatus. The reproducibility of the B-H loops is stated to be very good and free of drift. Nielson and Zaitev (57) have constructed a vibrating sample magnetometer especially suitable for magnetic anisotropy measurements. The description of their apparatus seems to imply t h a t it can be used only with magnetic materials, that is with ferro-, or ferrimagnetic samples. In view of its novel features and sensitivity, it would be interesting to see if its sensitivity can be further enhanced to measure very weak dia- and paramagnetic anisotropies. The vibration of the sample is obtained through an electrostatic excitation of a resonance mode in a high Q mechanical vibration unit. Results on the anisotropy of a single crystal of nickel are reported. The parallel and perpendicular components of the magnetization as a function of the field ranging up to 50 KOe are given and a good fit is reported with values calculated for a known anisotropy constant K1 = -12.3 x lo5 erg-cm-3. The two components of magnetization are detected by two separate sets of detector coils mounted inside a superconducting magnet. Constructional details for the electrostatic driving unit are given and its advantages are discussed. A vibrating coil magnetometer for the simple yet accurate determination of the magnetization and coercive force of soft magnetic materials is designed by Drake and Hartland 117). A magnetized sample is placed in a uniform demagnetizing field ( H D ) produced by a long solenoid. The current in the solenoid is steadily increased from zero to the point where the HD reduces the sample magnetization to zero. The sample magnetization is detected by a vibrating coil connected to a frequency selective amplifier. The detector system is said to be simpler than a rotating search coil or the Forster probe employed by earlier workers. The vibrating coil moves longitudinally over the stationary sample and, with the electronics described, gives excellent performance. Measurements on samples of “relay” iron and of low carbon steel gave results in excellent agreement with those obtained by using a compensated permeameter a t the National Physical Laboratory, England. The coercivities measured were in the range 55 to 550 A m - l . The authors give a good mathematical rationale for the designing of the solenoid, etc. The technique is indeed very simple and has been shown to be very reliable for work on soft magnetic materials. Rotating a n d Vibrating Sample Magnetometers. It is

gratifying to note t h a t a rotating sample (RS) magnetometer has been designed for diamagnetic susceptibility measurements, which can be naturally expected to measure weakly paramagnetic susceptibilities. Chemists and biochemists are generally very interested in these types of measurements. Hudgens (31) describes a relatively simple RS magnetometer in which two sample tubes (one containing a standard of known susceptibility) mounted vertically alongside a shaft are rotated in a magnetic field applied a t right angles to the shaft. A pickup coil mounted in the vicinity of the rotating tubes picks up the emf generated by the periodic “breaking” up of the magnetic flux. The signal voltage is proportional to the susceptibility of the sample. T h e shaft is rotated a t a frequency of 55 f 0.7 Hz and the pickup signals are detected via a “boxcar” integrator. The magnetometer is said to measure 3 x 10-9 cgs change in the volume susceptibility of AX unit or approximately within 1% of the volume susceptibility of a typical diamagnetic solid. The authors discuss the advantages of their RS method (which also allows anisotropic measurements) over the vibrating sample (VS) techniques. Hudgens (31) points out certain difficulties with regard to the observed fluctuations in the sample and the standard from one measurement to the next. The magnetometer is designed for room temperature measurements; however, the possibility of variable temperature measurements is pointed out. The paper gives sufficient constructional details, block diagrams for electronics enough to tempt instrumentation lovers to build their own (relatively inexpensive) apparatus. It appears t h a t further work needs to be done to improve the overall performance and the versatility of the apparatus. All in all this is an excellent paper and has contributed substantially to the techniques of susceptibility measurements in which chemists are most interested. A simple vibrating sample magnetometer (VSM) for work with weakly paramagnetic frozen solutions is described by Redfield and Moleski (65). We earlier reviewed (52, 53) important aspects of work in protein research carried out by Moss, Moleski, and coworkers. The simplicity of their apparatus arises from the vibration and detection system in t h a t the small superconducting coil which produces the static field up to 30 K Oe also detects the periodic (vibrating) magnetizations generated by the sample. The detection is accomplished with a high impedance ratio transformer ( l : l O 5 ) , the low impedance side of which is connected to the split-superconducting coil, and the high impedance is connected to a lock-in amplifier. The sample is vibrated a t an odd frequency of ( 2 0 / 7 ) Hz to avoid interference from the regular 6 0 - H ~line frequency. The amplitude of vibration is rather large ( - 2 cm peak to peak), but is found to be necessary to obtain the desired sensitivity. Although measurements at high fields (-30 K Oe) were planned to elucidate the paramagnetic saturation behavior of various species, no useful information was obtained and as such the authors confined their measurements to about 3 K Oe. A short term resolution of about AX = 10-9 cgs volume unit was attained with excellent reproducibility on a day to day basis corresponding to A x = 4 X cgs volume units for runs over 1.4 to 77 K. Thus, in very small samples (-0.7 ml), a small fraction ( - 2 x 10-7 mole) of paramagnetic species with the lowest moment of 1.83 Bohr magnetons (that is, for one unpaired electron, S = Yz and g = 2) could be detected and distinguished from the signal arising from the overwhelming diamagnetism of water encountered in protein research. The authors give adequate information on instrumentation, which has several advantages over the very expensive commercial magnetometers employing more sophisticated detection systems. The authors point out other advantages over the SQUID techniques (described before) and force techniques such as the Faraday type. They also describe their techniques for preparing (frozen) samples for protein research. Bowden (8) has discussed the design of detector coil systems for use in VSM with special reference to those magnetometers which are mounted on conventional iron core magnets. He presents experimental results for two distinct types of coils which can be fitted in a 2-in. (5.1 cm) pole gap. One particular design provides a broad saddle point. With this design, the equilibrium position of

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the vibrating sample can be displaced by *2.6 mm, *6.4 mm, or 4.8 mm in the x , y , or z directions without causing a change in the output voltage from the detection coils. Constructional details for this coil (called the j t h sensing coil) and a mathematical rationale for its design and comparison with other coils is elegantly given. Operation of a conventional vibrating sample (VS) magnetometer a t liquid 3He temperatures (-1.15 K) and in high magnetic fields is described by Oliviera and Foner (58). They describe simple modifications of the tail section of a commercial VS magnetometer, made by Princeton Applied Research Corp., Princeton, N.J. The sample is immersed in liquid 3He for good thermal contact. The tail section of the VSM fits inside a superconducting solenoid giving fields of 60 K Oe. The modifications are carried out without losing the sensitivity, field range, and other advantages of the commercial VSM. A sample of cerous magnesium nitrate was used as a paramagnetic thermometer over the range 0.5 to 1.5 K. A comparison of the sample temperature as deduced from its susceptibility us. the temperature of the liquid 3He bath (expected from the pressure of the 3He vapor) showed excellent agreement within about 0.005 K. No numerical data are provided by the authors. However, sufficient constructional details are given. Reeves (66) describes a novel instrument (an alternating force magnetometer) for measuring the magnetic moment of a small specimen. AC techniques are applied t o the traditional force magnetometer of the Faraday type. The new instrument is closely related to the vibrating sample magnetometer (VSM), although the principles of the two appear to be quite different a t first sight. A field gradient is provided by a set of coils and the steady field is provided by a n electromagnet. The gradient coils activated by AC voltage set the sample in motion. This motion is modified depending on the susceptibility of the sample and is detected by a piezo-electric device. A detailed discussion of the forces acting on the sample (as in the Faraday balance) and the vibrations in a VSM is provided. Various sources of error and practical difficulties are analyzed along with a candid evaluation of the advantages of the new instrument. A sensitivity of 0.2 nA2 is attained. The entire apparatus can be placed in a vacuum bottle. Undoubtedly the instrument is novel and the paper is very well written. Reference should be made to papers by Hines et al. (27) who suggest procedures for the calibration of VSM a t high temperatures. Drake et d.(17) and Nielsen et al. (57) describe VSM for measurements of large magnetizations of magnetic materials. A VSM in combination with high frequency oscillators is described by Boschi (7) and coworkers. These aspects are covered elsewhere under appropriate headings. High Frequency Oscillator Techniques Including Magnetic Resonance Spectroscopy. A high frequency ( - 125 KHz) oscillator circuit essentially similar to that described by Baghosian et al. (4) has been adopted for the measurement of susceptibilities ( x ) over the range 4.3 to 300 K by Rothwharf and coworkers (68). The authors suggest that the frequency shifts ( c y = vblank/vsample) measured for the blank coil and coil with sample provides a good measure of the susceptibility when the shift ( a ) is compared with another factor a0 obtained from the inductance ( L ) and the capacitance ( C ) of the coil under similar conditions. They claim that the product xF (where F is the filling factor) and, hence, x can be determined to about 0.005% over a wide range of temperature. Unfortunately no susceptibility data for standard materials are provided. The main thrust of the paper is on the construction of a variable temperature probe which can be operated in conjunction with the R F coil. Susceptibility measurements a t high frequencies using a versatile and sensitive vibrating sample technique are outlined by Boschi et al. (7). The measurements in principle yield the static part of the susceptibility x = x ’ real and it is shown that the imaginary part X” is negligible since the measurements are done in frequencies of lo6 Hz in small magnetic fields (few Oersteds) which eliminate the possibility of paramagnetic resonances. The solid sample (with volumes as small as 0.1 cm3) is glued to a nylon wire placed within a small magnet gap and is vibrated 496R

with an electromechanical device. A square wave motion at 6 Hz with an amplitude of 1.5 cm is obtained within the gap of the permanent magnet of the ferrite horseshoe type magnet. A detector coil wound around the magnet detects the change in frequency ( 6 v ) of an R F Oscillator caused by the sample. The ratio ( 6 v / v ) is shown to be proportional to the static susceptibility. This shift (6v/v) is caused by the change in the permeability of the space in the pole gap. The apparatus in a sense uses the principle of the heterodyne method ( i e . , the R F Oscillator method) and yet by using the ferrite toroid magnet, the method overcomes the disadvantages of the regular R F Oscillator method [cf. Mulay (47)] with regard to the dielectric losses caused by the insertion of the sample in a R F coil. Frequency shifts as small as 1.6 x 10-8 can be easily detected. This corresponds to a change in volume susceptibility emu for samples as small as 0.1 cm3. The of -3 x authors point out that a further increase in sensitivity is possible. They report very good results for the susceptibilities of several dia-, para-, and ferrimagnetic samples and give a very convincing rationale for the development of their technique. The electromechanical and electronic instrumentation is very straightforward. The apparatus can be modified for variable temperature operation. A radio frequency oscillator technique is used in the construction of a magnetometer which should prove useful for geochemical and geophysical work. The magnetometer measures the iron contamination in various powders. Moore (44) has shown that a frequency change, A f , occurs when a sample is inserted and withdrawn from a coil which forms a part of a resonant circuit. Several other workers have also used similar techniques [cf. Mulay (47)]. However, the novel feature of Moore’s apparatus is the incorporation of a Faraday screen, which ensures t h a t only magnetic effects are monitored. By compressing the samples to different bulk densities but to the same geometric shape, it is found that Af is proportional to the mass of the sample, and the constant of proportionality gives quantitative data on iron contaminant. The Moore technique is empirical in that it is found to depend on the concentrations of the magnetic components, [Fe] and [Fe304]. Although the magnetometer is not capable of detecting the socalled nonmagnetic Fez03, it is sensitive down to the level of about 0.1% Fe and 0.01% by weight of metallic Fe. The technique is admittedly suitable for the quick determination of the two contaminants in various powders. It would be desirable to see if the Faraday shield can be adapted to more sensitive but less stable RF oscillators described elsewhere [cf. (47)] in order to facilitate measurement of weak (dia- and paramagnetic) susceptibilities and for temperature measurement. Ostfeld and Cohen (59) caution against the use of the Evans high resolution NMR method [cf. Mulay (47)] for measuring the magnetic susceptibilities of paramagnetic solutions as a function of temperature. They point out that the application of the Evans method will lead to erroneous results unless the variation of the solvent density with temperature is taken into account. For a solvent with a volume change of about 10% for a 100 “C interval, the error in the slope of the reciprocal susceptibility us. temperature will be about 20 to 25%. This will result in incorrect values for 0, k e f f , and any thermodynamic parameters that may be sought in equilibrium studies. Nuclear magnetic susceptibilities which are consideraemu) than the electronic susceptibilbly smaller ( ities ( - 10-5) have been of interest not only for theoretical purposes but also for the measurement of very low temperatures in the milli-degree K region. The nuclear susceptibilities have been determined by various techniques, including nuclear magnetic resonance spectroscopy. Andres and Wernick (3) describes a sensitive magnetometer consisting of a superconductor transformer and a flux-gate magnetometer for the measurement of the moments (susceptibilities) of nuclei in various metals. They show that AuInz can be used as a standard for nuclear thermometry in the region 1.7 Mk to 0.5 K. The authors point out that their technique is not as sensitive as the SQUID (Superconducting Quantum Interference Device) technique described before. However, the simplicity of their apparatus is impressive and is particularly suited for

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nuclear thermometry using AuInz which has a large nuclear susceptibility per unit volume due to the large nuclear moment of indium. Zuckerman (78) describes a method for the absolute determination of change in magnetic susceptibility due to electron spin resonance. This change is represented by (dx"/dH)n [where X ' ' is the imaginary part of the (para)magnetic susceptibility, H is the field, and n is the filling factor] and x'' is obtained by modifying a standard E S R spectrometer. The paper gives a very good analysis of the needed electronic circuitry in terms of the fundamental E S R theory. However, it is important to establish how this information in the changes in susceptibility during resonance can be used for chemical or other types of applications. Various Coil Systems. Parker and coworkers (60) have designed coil systems to produce orthogonal linear magnetic field gradients. A careful mathematical analysis for designing coils is given. Although the coils were designed for application in t h e pulsed NMR experiments involving a study of diffusion constants, the information should prove helpful in designing or modifying coil systems such as those described by Lewis [cf. ( 4 7 ) ]for producing field gradients required for the Faraday method. Pulsed magnetic fields in the range 100 to 200 tesla (1 tesla = lo4 Oe) are generated using lightweight aluminum coils. Herlach and McBroom (26) describe various coil configurations, and capacitor bands for obtaining field pulses of 2-microsecond duration. Knoepfel and Luppi (36) have extensively reviewed the generation of very high magnetic fields in single turn solenoids. The single-coil structure is one of the most simple coil structures for generating pulsed magnetic fields. Temperature Control. An excellent review on the measurement and control of very low temperatures was recently published by Marson (42). The author points out t h a t techniques and sensors available for the measurement of low cryogenic temperatures come in a more confusing variety than those for higher temperatures. He reviews the various types of sensors (thermocouples, paramagnetic salt and resistance thermometers, helium vapor pressure measurements, etc.) and discusses the electronic techniques needed to give accurate display and control of cryogenic temperatures. Most magnetic measurements are carried out at low temperatures for the simple reason t h a t the magnetic susceptibilities (or magnetizations) increase with decreasing temperature for para-, ferro-, and ferrimagnetic materials. Hence, the review by Marson should prove informative to the magneticists. A large number of papers continue to appear in the Review of Scientific Instruments, Journal of Physics-E (Scientific Instruments), etc. on the subject of thermometry in general. Again, for want of space, we shall not review such papers. Only occasionally one comes across a paper describing a cryostat suitable especially for the Faraday and Gouy type magnetic balances and other magnetometers. One such paper is reviewed below. Lewis (39) describes a truly simple method for varying the temperature of a sample from 4.2 t o 800 K . This method was primarily designed for use in a Faraday type magnetic bzlance. The probe, t h a t is the tube or chamber surrounding the sample, is made of stainless steel. A small electrical furnace is placed inside the probe a t the sample posirion which is used to heat the sample and a heater is placed a t the bottom of the probe which is used to generate cold helium gas. Cooling of the sample is carried out in two ways. First, the cold helium gas can be used to cool the sample t o temperatures higher than that of the bath. Second, the sample can be cooled to the temperature of the bath itself by using a control valve to establish a pressure differential between the helium Dewar and the probe. This causes liquid helium to rise in the probe and surround the sample. Other operational procedures and important constructional details are given in a short note. The method is simple indeed and shows promise of extension to other magnetic measuring systems.

APPLICATIONS Introduction. In the following section on applications, we review a few selected papers on the organic solid state

chemistry, which have a bearing on their magnetic and high electrical conducting properties. In a later section, we summarize aspects of work on amorphous materials. For many years, researchers have been enamored by the prospect of producing organic superconductors. In recent years, some organic charge transfer complexes were found to show very large electrical conductivity at cryogenic temperatures. Readers not familiar with the principles of electronic transport properties should refer to books by Kittel (35), Morrish (45), and Williams ( 7 4 ) and also special topic books on transport phenomena in organic solids. Reference should be also made to an interesting paper on the possibility of a n organic magnet by Pohl (63). Organic Charge Transfer Complexes with High Electrical Conductivity. Perlstein and coworkers (22) investigated the electrical conductivity and magnetic susceptibility of a group of organic donor-acceptor complexes. They synthesized a ccmplex between the electron donor tetrathiofulvalene (TTF) and the electron acceptor tetracyano-p-quinodimethane (TCNQ) shown below.

TTF This new compound not only behaves like a metal over a large temperature range, but has by far the largest maximum electrical conductivity ( u ) of any known organic compound. They found u = 1.47 x lo4 ohm-' c m - l a t 66°K. Prior to this the Heeger group (20) a t the University of Pennsylvania had observed a conductivity of u = 170 to 400 o h m - l cm-' a t 200°K for a complex known as the NMP-TCNQ, where N M P stands for Nmethylphenazinium. Their work will be reviewed in a later section. TTF is known to form good electron-conducting cation radical complexes (TTFf-C1- 1. Similarly T C K Q forms highly conducting radical anion complexes containing one-dimensional conducting chains of face-to-face stacked TCNQ groups. Perlstein and coworkers (22) have briefly reviewed the mechanisms for the electrical conductivity of these complexes. For instance, the electrical conductivity of T C N Q complexes has been interpreted in terms of a mobility-activated electron transfer between a high density of localized states. the localization being induced by either the random orientation potential of the asymmetric cation or possibly by defects along the one-dimensional chain. An alternate mechanism proposes that electrons are localized by their Coulomb correlation and that transport occurs with carrier activation proportional to the Coulomb correlation energy according to the Mott-Hubbard model. Perlstein et al have given a n excellent rationale for their choice of the (TTF)(TCNQ) complexes as a candidate for electrical conductivity studies. They point out that (a) T T F is highly symmetric (point group Dzh) and thus random potentials due to asymmetric orientation are less likely than for other cations previously investigated. ( b ) T T F is highly polarizable because of the presence of sulfur, and thus Coulomb repulsion between electrons on neighboring TCNQ sites should be considerably less than suggested for the T C N Q complexes prepared with nitrogen containing heterocyclic cations of similar size to TTF. The authors observed a semiconductor-tometal type transition a t 66°K in that the conductivity plotted as In u u s . the reciprocal of temperature showed a transition peak around 66°K. They also observed a positive temperature coefficient of the magnetic susceptibility from 2 to 359°K and a conductivity of 1 o h m - l c m - l perpendicular to the long axis of a crystal. The activation energy for conduction AE below 8°K was found to be about 0.0062 eV (or 72°K) close to the semiconductor-to-metal transition. In subsequent work, the Perlstein group (61) a t the Johns Hopkins University discussed their observations on the (TTN)(TCIVQ) complexes also. Here T T N stands for tetrathianaphthacene. For the TTF-TCNQ, the magnetic susceptibility (x)was found to be diamagnetic below 20 K with a minimum showing the presence of 0.07% impurities. Above 20 K , x showed a paramagnetic behavior increasing monotonically up to 359 K . The authors point out

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that the magnetic properties are more typical of the lower conductivity TCNQ salts, although their attempts to fit the data to a singlet-triplet model were unsuccessful. They show that the data fit an equation of the form

‘

4N0fie-A 3KT -I- x p where the 4/3 factor accounts for the Curie paramagnetism and diamagnetism of the thermally excited electrons and holes, A is the band gap, xP contains the Pauli-Peirls term for the TTF+ metallic excitations as well as any high frequency terms, representing the VanVleck paramagnetism. NO represents the Avogadro’s number when the bandwidth is assumed to be zero. They estimated A = 0.065 eV and x P = +lo0 x 10-6 emu/mole from their magnetic data, and they examined the results in terms of various models for electrical conduction. The above brief survey is representative of solid state science research, carefully planned and well executed. It shows the potential of magnetic susceptibility not only as a mere characterization tool, but also for examining transport properties of solids in a quantitative fashion. Reference should be made to work by Mulay et al. (&), who used susceptibility technique for the elucidation of the semiconductor-to-metal transitions in the oxides of titanium. The mechanism for such transitions is quite different from those discussed above. Perlstein (now with the Kodak Research Laboratories, Rochester, N.Y.) and coworkers (62) have also investigated the electrical conductivity of a cyano complex of platinum [ K ~ p t ( c N ) ~ B3r ox H20] and the quinolinium (TCNQ)2- complex. An abstract of their paper in the Materials Research Bulletin is given: “Experimental evidence is presented to show that the one-dimensional chains of Pt ions in K ~ P ~ ( C N ) ~ B ~ O .and ~ . Xthe H ~one-dimensional O columns of TCNQ molecules in quinolinium (TCNQ)2- contain localized electronic states created by a random potential along the chains which originates a t the cationic sites. T h e random potential in KzPt(CN)rBro,aanHz0 is most likely created by partial site occupancy of K + and Brsites whereas in quinolinium (TCNQ)2- t h e random potential is caused by orientational disorder of the quinolium cation. The electrical conductivity for both materials follows a n exp -(To/T)l law a t low temperatures with n = 2 for a one-dimensional system. To = 5.9 x lo4 K for the Pt chain and 5.0 x 103 K for the TCNQ columns. At intermediate temperatures u a exp - S E / k t with AE = 0.085 ev for the Pt chain and 0.023 ev for the TCNQ system. At still higher temperatures, a semiconductor to “metal” transition occurs in the T C N Q system a t T = A E / k . For the Pt chains, decomposition occurs before the transition. It is suggested that the “metallic” region is not due to extended states but to correlated electron hopping. It is suggested that the transition temperature of the TCNQ system may be lowered by using more polarizable cations.” The Heeger group first published a very comprehensive account of their work on the metal-insulator transition and antiferromagnetism in a one-dimensional organic solid-namely, the (NMP)(TCNQ) complex-to which a reference was made earlier. We quote below an abstract of their work ( 2 0 ) , in order to maintain the conceptual and factual accuracy of their research and in order to illustrate the thoroughness of the theoretical rigor with which these physicists worked on (hopefully well characterized) organic solids: “An experimental study of the organic charge-transfer salt N-methylphenazinium tetracyanoquinodimethan (NMP-TCNQ) is presented. Magnetic-susceptibility, specific-heat, spin-resonance, and conductivity measurements indicate a metallic state above 200°K with a continuous transition t o a small-band-gap magnetic Mott insulator below 200°K. The groundstate and low-lying excitations indicate t h a t this system can be quantitatively described in terms of the one-dimensional Hubbard model with a transfer integral of 2.1 x 10-2 eV and an effective Coulomb interaction of 0.17 eV. These values are discussed in terms of the fundamental molecular physics of the TCNQ- anion in the NMP-TCNQ crystal. It is con-

x=

498R

cluded that in addition to the Heitler-London correlation which reduces the interaction between two excess electrons on a TCNQ molecule, the NMP cation polarizability plays a significant role in reducing the effective interaction. The transition to the metallic state is attributed to electron-hole correlations which become important when the number of excitations is large. These correlations persist into the metallic state where the electronic system behaves as a quasi-free-electron Fermi liquid as indicated by the unenhanced Pauli susceptibility and simple transport properties. The low-temperature one-dimensional antiferromagnetic state is studied using spin-resonance and specific-heat techniques. The linear temperature dependence of the specific heat predicted for the one-dimensional antiferromagnet has been observed. Electron-spin-resonance linewidth studies indicate motionally narrowed dipolar widths with the correlation time determined by the Fermi velocity in the metallic state and by exchange in the insulating state. The large fluctuations expected for a onedimensional “phase transition” show up as a maximum in the correlation time, which, however, never exceeds sec. The spin-lattice relaxation of the conduction electrons via phonons and molecular vibrations in the metallic state has been observed. The results are consistent with Elliot’s theory in which T1-l (Ag/g)2TR-1, where 7R is the scattering time as determined from the resistivity.” Subsequent to this work the Heeger group ( I O ) carried out extensive research on the complexes of TCNQ with TTF and its derivatives. This work was fully reviewed in Physics Today (May 1973). The July issue of the same publication contains excellent articles on superconductivity. Reference should be made to other papers including a recent contribution by Bardeen ( 6 ) , who discusses the mechanisms for superconductivity fluctuations in one-dimensional organic solids. Amorphous Materials with Exciting Electrical and Magnetic Properties. During the past decade, there has been a great upsurge of activity on amorphous materials, often referred to as glasses because these lack a long range order as in crystalline solids. Several amorphous materials have been found to show semiconducting and magnetic properties. We listed references to selected papers on this subject in our last review (53).A group of materials, notably the chalcogenides, have been found to exhibit the electrical switching properties. It is beyond the scope of this survey to state the different types of switching phenomena in various materials. However, it will suffice to point out that the memory switch behaves initially like a “threshold” switch. Thus, conductivity is very low until a given voltage is exceeded. After a delay time, there is a very rapid switching to a highly conductive state. If the current is quickly reduced below a critical value, the device will switch back to the resistive state? exactly like a “threshold” switch. However, if the device is held in the conductive state for the order of second, it will remain highly conducting, even after the external field is removed. Once locked in the ON state, the material can be returned to the original OFF state by applying a short intense pulse of either polarity. The above description is taken from a n elegant review by Adler (2). Wachter (71) has covered the vast subject of the optical, electrical, and magnetic properties of the europium chalcogenides and the rare earth pnictides. He discusses in detail the effect of the s-f interaction on the transport properties in magnetic semiconductors. Another paper of interest is by Kasuya ( 3 4 ) , who discusses the s-f interactions and magnetic semiconductors. Several papers dealing with the magnetic properties of various amorphous systems have appeared in a book edited by Hooper and deGraaf (28) and as such will not be referenced individually. This collection of papers includes another contribution by Wachter concerning the long and short range magnetic interactions and electrical switching in highly doped “amorphous” ferromagnetic semiconductors. Tauc and coworkers (cf 28) have made an extensive study of the magnetic susceptibility of chalcogenide glasses. The authors report the differences in the magnetic susceptibility x of pure semiconductors in the amorphous and crystalline states and on the effect of doping impurities. They show that in the crystalline state x can be a complicated

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function of T but in the amorphous state XT can usually be divided into an almost temperature independent diamagnetic part xC. The component (xT) is observable a t low temperatures and can be described by the Curie law ( X = C / T ) except a t very low temperatures. Tauc et al. have examined the chalcogenide glasses As& and AszSes (pure and doped). The glasses were found to be more diamagnetic than the corresponding crystals, but the difference in good glass forming chalcogenides is small. No evidence for disorder induced singly occupied states with concentrations above amroximatelv 3 x ~ m was - ~ found in these glasses: 'Their preGiously reported paramagnetic contributions to t,he magnetic susceptibility were traced to iron impurities. In general, this paper rep-

LITERATURE CITED (1) A b r a h a m . S. C., "Cooperative Phenomena in inorganic Magnetic Materials," Vol. 11 "Physical Chemistry" Series, MTP. International Review o f Science, University Park Press, University Park, Md., and Butterworth, London, 1972. (2) Adler, D.. CRC Crit. Rev. Solid State Sci., 2 ( 3 ) , 1 (1971). (3) Andres, A,, Wernick. J. H.. Rev. Sci. Instrum., 44, 1186 (1973). (4) Baghosian, C.. Myer, H., Rives, J., Rev. Sci. Instrum., 42, 528 (1971) (5) Bagnall, K. W., "Lanthanides and Actinides," Vol. 11, "Inorganic Chemistry" Series MTP International Review of Science, University Park Press, University Park, Md., and Butterworth, London, 1972. (6) Bardeen, J., SoIid-State Commun., 13, 357 (1973). (7) Boschi, A . , Bucci. C., Vigrati, C., Rev. Sci. Instrum., 44,899 (1973) (8) Bowden, C. J., J. Phys. E. Sci. Instrum., 5, 1115 (1972). (9) Brown, J. P., J. Chem. Educ., 49, 408 (1972). (10) Coleman, L. B., Cohen, M. J.. Sandman, D. J.. Yamagishi. F G., Garito. A. F., Heeger, A . J., Solid State Commun., 12, 1125 (1973). (11) Connolly. T. F.. Copenhaver, E. D . , "Bibliography of Magnetic Materials, etc." "Solid State Physics Literature Guide," Vol. 5, Plenum Press, NewYork. N.Y.. 1972. (12) . , Danlev. W. J.. Mulav. L. N . . Mater. Res. Bull., 7, 739 (1972). ' (13) Danley, W. J.. Mulay, L. N., Ziegenfuss, H . G.. "Computer Programs for Magnetic Parameters from Experimental Data," t o be published. (14) Day, P., "Electronic Structure and Magnetism of Inorganic Compounds." Vol. I . Specialist Periodical Reports, Chemical Society, London (1972) (15) Ditchfield. R . , "Magnetic Susceptibility of Diamagnetic Molecuies." Vol. 2 "Physical Chemistry,'' Series. MTP International Review of Science. University Park Press, University Park, Md.. and Butterworth, London (1972) (16) Donini. J. C., Hollebone. 8 . R., Koehler. R. A . . Lever. A. B. P.. J. Phys. E, Sci. Instrum. 5 , 385 (1972). (17) Drake, A. E.. Hartland, A.. ibid., 6, 901 11973). (18) Draper, N . R . . Smith, H., "Applied Regression Analysis," Wiley, New York. N.Y. (1967). (19) Eiliott, R . J.. "Magnetic Properties of Rare Earth Metals," Plenum Press, New York. N.Y., 1972. (20) Epstein. A. J., Etermad. S., Garito, A. F , Heeger. A J., Phys. Rev., 5,952 (1972). (21) Fay, H.. Rev. Sci. Instrum., 3, 1274 (1972). (22) Ferraris. J., Cowan. D . 0.. Walatka, V., Perlstein, J. H . . J. Amer. Chem. SOC., 95, 948 (1973). (23) Fisher, R . A., Rev. Sci. Instrum., 43, 386 (1972). (24) Goree, W. S.. Rev. Phys. (France), 5 , 3

resents a successful application of the magnetic susceptibility technique (in combination with ESR) to the elucidation of the electronic and optical properties of the chalcogenide glasses. These materials continue to be interesting because of their technological significance in memory storage devices for computer applications. Mulay and coworkers [cf. (28)] have published two papers on the superparamagnetism of iron dispersions in ( a ) the zeolite and (b) the glassy carbon matrices.

ACKNOWLEDGMENT We are grateful to Mildred Proffitt and Shirley LeFrancois for their expert and unfailing assistance in the preparation of this review.

(1970). See also brochures available from the Develco Inc.. Mountainview. Calif. 94040. (25) Grigsby. D. L.. "Electronic Properties of Materials-A Guide to the Literature," Vol. 3 in two parts, Plenum Press, New York, N.Y., 1973. (26) Herlach. F.. McBroom. R., J . Phys. E. Sci. Instrum., 6, 652 (1973). (27) Hires. W. A., Moeller, C. W., Rev. Sci. Instr. 44, 1544 (1973). (28) Hooper. H. O., de G,r,aaf, A . M . , Ed., "Amorphours Magnetism, Plenum Press, New York (1973) (29) Hoppe, J. I . , J . Chem. Educ., 49, 505 (1972). (30) Horne, D . E.. Sawatzky. E., Rev. Sci. Instrum., 43, 1842 (1972). (31) Hudgens. S. J., ibid., 44, 579 (1973). (32) lzyumov, Yu. A.. Medvedev. M. V.. "Magnetically Ordered Crystals Containing Impurities," Plenum Press, New York, N.Y., 1972. (33) Johnson, B. F. G.. "Inorganic Chemistry of Transition Elements," Vol. I , "Specialist Periodical Reports." Chemical Society, London, 1971. (34) Kasuya, T., CRC Crit Rev. Solid State Sci.. 3 ( 2) , -131 ( 1972). (35) Kittel, C., "Introduction to Solid State Physics." Fourth ed., Wiley, New York, N.Y., 1971. (36) Knoepfel, H., Luppi, R . , J. Phys. E, Sci. Instrum.. 5 . 1133 119721 - -, (37) Lenaert;: JT,. Vanwormhoudt, M., ibid., p560. (38) Lepley, A . H., Closs, G. L., "Chemically Induced Magnetic Polarization." Wiley, New York, N . Y . , 1973. (39) Lewis, R. T.. Rev. Sci. Instrum., 44, 518 (1973). (40) Mapps. D.J.. McQuiliin. J. D. R . . J. Phys.ESci. Instrum., 5 , 461 (1972) (41) Marquart. D . W., "Least Squares Estimation of Non-linear Parameters," IBM Share Library, Distribution No. 3094, IBM, New York, N . Y . , 1964. (42) Marson, G. B., Amer. Lab., October 1973. p 27. (43) McCoy, B M., Wu. T. T.. "The Two Dimersional lsing Model." Harvard University Press, Cambridge, Mass., 1973. (44) Moore, G. S. M., J. Phys. E, Sci. Instrum., 6. 1170 119731. ~, (45) Morrish. A.,,H.. "The Physical Principles of Magnetism, Wiley, New York, N . Y . , 1965. (46) Mulay, L. N . , Anal. Chem.. 34, 343R (1962). (47) Mulay, L. N.. "Magnetic Susceptibility" ( A reprint monograph), Interscience, New York, N . Y . , 1966; "Techniques for Measuring Magnetic Susceptibility," in "Physical Methods of Chemistry,'' Weissberger and Rossiter. Ed., Wiley, New York, N.Y. 1972. (48) Mulay, L. N.. Danley. W. J., Bull. Amer. Phys. SOC.,Ser. 2, No. 14, 350 (1969): and J. A w l . Phys., 41, 877 (1970); Mat. Res. Bull., 7, 739 (1972). (49) Mulay, L. N.. Mulay. I , L.. Anal. Chem., 36, 404R (1964). \

~I

(50) (51) (52) (53) (54)

(55) (56) (57) (58) (59) (60) (61)

(62) (63) (64) (65) (661 .

I

(67)

(68) (69)

(70)

(71)

~

(72) (73) (74) (75) (76) (77) (78)

lbid., 38, 501R (1966). Ibid., 40, 440R (1968) Ibid., 42, 325R (1970). lbid., 44, 324R (1972). Mulay, L. N., Ziegenfuss. H . G., Bull. Amer. Phys. SOC., 16, No. 1, 50 (1971). A series of papers on the magnetic properties of trinuclear complexes will be published during 1974. Myers, R . J., "Molecular Magnetism and Magnetic Resonance Spectroscopy," Addison Wesley, Englewood Cliffs, N.J., 1973. Nesbitt, E. A.. Wernick, J. H . , "Rare Earth Permanent Magnets." Academic Press, NewYork, N.Y., 1973. Nielsen, 0. V., Zaitev, V . I . , J. Phys. E , Sci. Instrum., 6, 1022 (1973) Oliviera, N. F., Jr., Foner, S . . Rev. Sci. Instrum., 43, 37 (1972). Ostfeld, D.. Cohen, I . A , , J. Chem. Educ., 49,829 (1972). Parker, R . S . , Zupanctc, I , Pirs, J , J . Phys. E , Sci. Instrum., 6, 899 (1973) Perlstein, J. H., Ferraris. J P., Walatka, V . V . . Jr., Cowan, D. D . . Magnetism and Magnetic Materials Conference. Denver, Colo.. Nov. 1973, AIP Proc., 6, 1494 (1972). Peristein. J. H.. Minot, M. J., Walatka. V . , Mat. Res. Bull., 7, 309 (1972). Pohl, ti. A . , Phil. Mag.. 26, 593 (1972). Quinn. R . K., Knauer, R . C., Rev. Sci. I n strum., 43, 1543 (1972). Redfield. A. G., Moleski. C., ibid.. 760. Reeves. R.. J. Phvs. E. Sci. Instrum. 5 . 547 (1972). Roberts, L. E. J., "Soiid State Chemistry," Vol. 8, "Inorganic Chemistry'' Series; MTP International Review of Science, University Park Press, University Park, Md., and Butterworth, London (1972) Rothwharf. F., Ford, D., Dubeck. L. W , Rev. Sci. Instrum., 43, 317 (1972) Sharp, D . W. A., Mays, M J.. "Transition Metals" (Parts I and 1 1 ) . Volumes 5 and 6. "Inorganic Chemistry'' Series. MTP international Review of Science, University Park Press, University Park, Md., and Butterworth. London, 1972. Stewart, A. M . , J. Phys-E, Sci. l n s t r u m . 5 , 978 (1972). See also. 2, 851 (1969); and 3, 251 (1970). Ph.D. Thesis, University of London, 1971 Wachter, P., CRC Crit. Rev. Solid State Sci.. 3. 189 119721. Walla-;e. W. E . . ' "Rare Earth Intermetailics." Academic Press. New York, N.Y.. 1973. Wichner, R., Rev. Sci. Instrum., 43, 1307 (1972). Williams, J. E. C., "Superconductivity and its Applications," Pion Ltd., London, 1970. Wooilam. J. A.. Beale. H . A., Spain, I . L . Rev. Sci. Instrum,, 44, 434 (1973) Zatko, D A,, Davis. G. T , ibid.. 43, 818 (1972). Zeiger, H. J., Pratt. G. W., "Magnetic Interactions in Solids," Oxford University Press, London, 1973 Zuckerman, B Rev. S o . I n s t r u m , 44, 1118 (1973)

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