Magnetic Susceptibility Instrumentation and Applications Including Lunar and Biotype 1. N. Mulay‘ and lndumati 1. Mulay, Materials Research Laboratory,l The Pennsylvania State University, University Park, Pa. 7 6802
I
on magnetic susceptibility, we survey briefly the trends in instrumentation and applications to chemistry including the solid state m d lunar studies. The first five reviews (72, 79-82) appeared during 1962 to 1970. This 1972 addition covers literature from about January 1970, to about December 1971. For the sake of completeness, we have included several introductory comments which appeared in the earlier surveys. The arbitrary element concerning the selection of the material continues in the present review. Hence, it should not be regarded as an extensive examination of a complete bibliographic nature on what may be commonly regarded as “magnetochemistry.” This statement partly reflects the authors’ agony in discovering the vast and ever-increasing growth in magnetics literature and their inability to cover all important aspects. The reviewer is then baffled by the decision of what to and what not to include. The titanic growth in magnetics literature refers to the overall growth in magnetics “technology” and, in part, to its “science.” We do not, therefore, review literature on magnetic materials and devices, magnetic measurements such as permeability, remanence, etc., which characterize magnetic materials; however, we list references to new books and review articles on magnetic materials research. We shall outline a few aspects of solid state chemistry or magnetic materials such as the non-stoichiometric oxides. Readers who need a thorough bibliographic list of references on all branches of magnetics or on magnetic susceptibility alone must be directed to the voluminous pages of chemical and similar abstracts. N THIS SIXTH REVIEW
N E W TRENDS
Having surmised that the “selectivity” of a review article in terms of the space available, may surprisingly continue to become inversely proportional 1 Inquiries should be addressed to this author, Also afliliated with the Solid State Science Program, an interdisciplinav graduate program in the physical sciences, leading to M.S. and Ph.D. degrees. 9 Laboratory for interdisciplinary research in Chemistry, Physics, Planetary Sciences, etc.
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to the growth of the literature (otherwise known as the “information explosion”), we have attempted to seek new trends in the realm of magnetics. Some of these are listed below and are succinctly reviewed or appropriately referred to in later sections. a. The new S.I. (SystAme International) units for magnetic quantities b. Coils to produce inhomogeneous field gradients for the Faraday method C. Vibrating Sample (or coil) Magnetometers with superconducting magnets d. Studies on geotype and lunar studies e. Magnetic and thermometric (miniature) instrumentation (especially for space exploration) f. Studies on magnetic oxides g* Ultra cryogenic measurements (below 3 OK) h. High pressure instrumentation i. Biotype investigations GENERAL LITERATURE
Abstract Services and New Addi-
tions. 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 but unfortunately ignores many interesting aspects of magnetochemistry. The general scope and use of these and other abstracting and indexing services were outlined in our previous reviews. We have also previously commented that 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, a t least in the general realm of chemistry, by the appearance of the “Current Index to Conference Papers in Chemistry” (1‘7). The first issue of Vol. I started in September 1969. These issues should be helpful in finding
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
the titles (and, in appropriate cases, the abstracts) of papers presented a t various chemistry-oriented meetings. Unfortunately, very little magnetochemistrytype work is reported a t such meetings; the general thrust has been on high resolution NMR, EPR and, to a modest extent, on Mossbauer Spectrometry. The same organization (17) also publishes 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 spectrometry are intricately related. Since resonance and Mossbauer spectrometry have developed as separate instrumental disciplines, literature in these areas (with the exception of studies on lunar samples) will not be reviewed here, in the belief that 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 a n integrated look a t both old and new problems of science and attempt to solve them. Indeed, “materials” covers a wider territory than “chemical compounds” of well-defined stoichiometry, with which the chemist is most familiar. I n the “materials science” approach, attempts are made to synthesize new materials, to “characterize” them (old and new), and to understand their structures a t the L‘macroscopic’’and L‘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. I n this respect, magnetic susceptibility and other related magnetic properties play a n important role. I n addition to the books listed below, there are a large number of treatises on “Materials Science,” “High Temperature Materials,” “Advances in Materials Science,” etc., which contain references to magnetic properties of several materials. Almost every major publisher nowadays has a special listing for materials science; hence, we refer our readers to such lists. An extensive chapter on magnetic susceptibility by one of us (L.N.M.) appeared recently in “The Physical Methods of Chemistry” (74). The chapter describes not only the wellknown, classical Faraday, Gouy, and the Quincke techniques and their important modifications,. but also various “change in flux” methods and the materials used in magnetic instrumentation. The author has included for the first time descriptions of new methods such as the “Meissner Effect” and “The Gordon Density Gradient” methods, along with a discussion of gadgetry for variable temperature and anisotropy. This chapter is essentially based on the material selected from his previous five review articles (79, 79-89) and covers literature generally up to the middle of 1969. The author has gratefully acknowledged therein the permission received from The American Chemical Society to use the material from these reviews. Volumes IV to VI of “Transition Metal Chemistry,” edited by Carlin (11) contain a number of chapters which have a bearing on the magnetochemical aspects of various coordination compounds. Typical examples are: “Paramagnetic Relaxation in Solutions” (W. B. Lewis and L. 0. Morgan); “Transition between Low-Spin and High-Spin Octahedral Complexes’’ (R.
L. Martin and A. H. White); “Electronic Structure of Ni(I1)” (L. Sacconi); “Copper Complexes” (W. E. Hatfield and R. Whyman); “Metal-Metal Exchange Interactions” (G. F. Kokoszka and G. Gordon); “Structure of Binuclear Coordination Compounds” (B. Jezowska-Trzebiatowska and W. Wojciechowski); “Amine Complexes of Cr(II1)” (C. S. Garner and D. A. House). Magnetic properties of transition metal clusters are surveyed by Johnston (64) in relation to compounds formed with *-acid ligands. Horrocks and Hall (49) have written a very useful review on paramagnetic anisotropy. Iizuka and Yonetani (62) discuss the spin changes in hemoproteins. One of the most comprehensive and masterly treatments depicting the correlation between the structure of metallic oxides and their physical properties (such as magnetism) is given by Goodenough ($4). He has summarized a vast amount of information (with about 550 references) and critical analysis of data on several oxides in a 250-page chapter in Volume 5 of “Solid State Chemistry.” He has reviewed the monoxides, the dioxides,the sesquioxides, the perovskjte and rutile type oxides of several transition metal ions. These include Ti, V, Fe, Co, Ni, Cu, Mn, Nb, Ru, Os, Rh, Ir, and Pt, to name a few. The author has written this and other articles in a style which both physicists and chemists can understand. He naturally discusses the behavior of individual oxides in terms of the band theory, the localized us. the itinerant electron behavior, polaron model, and so on; however, much of the discussion is presented in terms of the language of “molecular orbitals’’ (e.g., the role of bonding and antibonding orbitals, covalent and homopolar bonding), with which the chemist is quite familiar. I n another article, Goodenough (95) has focussed his attention on the vanadium oxides, which also exhibit anomalous properties, including magnetic properties. Several of the oxides of vanadium and titanium show anomalies in magnetic properties, which have been interpreted in terms of the semiconductor + metal transitions. Hebborn and March (46) present a theoretical treatment of orbital and spin magnetism and dielectric response of electrons in metals. Grant (38) similarly treats the magnetic excitations in nickel. Bosman and van Daal (6) discuss small-polaron us. band conduction in some transition metal oxides, with special reference to NiO, COO, a-FezOa and MnO. Geller et al. (30) have examined the crystal chemistry and magnetic structures of substituted Caz[Fe](Fe)Os. Fuller (86) has reviewed the physical basis of paleomagnetism with special reference to the remanent magnetism of ANALYTICAL CHEMISTRY,
rocks and the role of paleomagnetism in earth science. He reviews the magnetic mineral phases with emphasis on u-Fez0a (hematite) and various models proposed to explain its magnetism. Several other interesting aspects are discussed, which should prove helpful in understanding the magnetic properties of lunar samples. Aharoni (1) has surveyed applications of micromagnetics, with special reference to the recent advances in calculating various details of the hysteresis curves of ferromagnetic materials. The scope has been extended to cover almost all theories which use the concept of the magnetization vector as a continuous function of space, but an effort has been made to choose theories which are applicable to more than one special case and to avoid crude approximations or trivial extensions of previous calculations. Erber, Latal, and Harmon (22) discuss the origin of hysteresis in simple magnetic systems. Haas ( 4 4 , in his review entitled “Magnetic Semiconductors” discusses the influence of the magnetic properties on the band structure and the optical and electrical properties of magnetic semiconductors. After a discussion of the theory, the experimental data on two important classes of magnetic semiconductors, the europium chalcogenides and the chalcogenide spinels, are reviewed. Several informative reviews on thermometry-especially in the cryogenic range-are now appearing in instrumentation journals; references to these are given under the temperature measurement and control section. Senftle and Hambright (94) have surveyed the magnetic susceptibility of biological materials. Frei (25) has reviewed very interesting applications of magnetism. These do not involve the actual measurements of magnetic susceptibility or related parameters; however, the use of magnetic devices for removing foreign (ferromagnetic) objects from the eye and other parts of the body are quite interesting. Another aspect involves the electrical or electromagnetic activity of the body itself; the effect of external magnetic fields on biosystems is also examined. We had discussed some aspects of this in our 1964 and 1966 review articles (79, 80). Below are listed a few selected books in various areas of magnetics, which should be of interest to our readers: Strangway (104) has written a n introductory book in the field of paleomagnetism and rock-magnetism. Bradley ( 7 ) , who has authored several other books on magnetism, has added a description of materials for magnetic functions. A volume edited by Haidemenakis (4.9) and a book by Lee ( G I ) , both entitled “Magnetism,” have now appeared. White (111) discusses the
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quantum theory of magnetism. We were unable to obtain further information on these books by Haidemenakis, Lee, and White at the time of writing this review. Perhaps the most intriguing title ia the new magnetics literature is “U.F.O.’s and Diamagnetism,” by Burt (9). UFO means “unidentified flying objects” which presumably cone from outer space and have been “seen” by several individuals. The entire concept has been controversial; the author (9) has attempted to seek correlations of UFO’s and scientific observations. I n this context he has developed a new theory of diamagnetism, without affecting our concepts of para-, ferro-, ferriand antiferromagnetism. We hope to provide a resume of this book a t a later date. THE NEW S. 1. UNITS FOR MAGNETIC QUANTITIES
S.I. is now an internationally accepted abbreviation for “SystBme International d’Unit6s.” I t is an extension of the traditional metric system and incorporates many aspects of the M.K.S. and the rationalized Georgi system of units (cj. 73 and 7 4 ) . Pass and Sutcliffe (90) have given a brief account of the adoption of such units to the measurement of magnetic susceptibility and related parameters. In the S.1: system, the mass susceptibility xm is expressed in units of m8kg-1 and the molar susceptibility xm is expressed in units of mamol-l. Thus, the molar susceptibility for tris(acety1 acetonato)Mn(III) corrected for diamagnetism is said to be x,,,corr = 136,600 X 10-12m8mol-1. The magnetic moment, generally expressed in terms of the Bohr magneton, should now be expressed in the S.I. system as Am2. Thus 1 Bohr Am2, and magneton = 9.27 X a magnetic moment of 5.06 BM should be written on the S.I. system as 4.69 X Am2. It is easy to figure out the close correlation between the old claserg sical equation 1 BM = 9.27 x gauss-’ and the new 1 BM = 9.27 x Am2. The number (n) of unpaired electrons calculated from the well-known equation p = d n ( n 2) naturally remains the same in both systems because it is fundamentally 1) based on the formula p = d4S(S where S is the total spin quantum number. Reference should also be made to a valuable publication by McGlashan (6‘7) and to articles by Davies (18), as well as to a book by Drazil (20).
+
+
INSTRUMENTATION
Modifications, Error Analysis, Etc., for the Faraday Method. Lewis (63) has made a very significant contribu326R
tion toward the instrumentation of a Faraday balance, by designing coils to provide the dH/dx gradient, needed to produce a magnetic force on the sample. Traditionally the gradient is provided by pole tips of diderent shapes such as those designed by Faraday, Sucksmith, Heyding, Henry (cf. 73, 7 4 ) . These have the disadvantage that, in varying the Field H, one also automatically varies dH/dx so that the “constant force” region continuously varies with the field H, while maintaining a constant pole gap. [Generally the pole gap has to be held constant to a minimum dimension to accommodate low temperature cryostats (dewars) and high temperature furnaces.] Thus with all designs of pole tips, the region of constant force is large a t low values and small at high values of H . The Lewis coils, which are mounted on the flat poles of an electromagnet and energized by an independent dc power supply can provide different values of the gradient dH/dx, which can be soto-speak superimposed on the field H of the electromagnet. The authors describe the constructional details of the coils, which can handle currents up to 12 A. These provide a constant-force region (H.dH/dx) of 3 cm between the coils with a variation in the gradient which is less than *O.l%. This is more than adequate to handle large samples (up to -400 mg) in sample capsules, about 0.5-cm diameter and 1.3 cm tall. The authors describe various ways to avoid the effects of fields (generated by the electromagnet and the coils) on the electrobalance, and ways to avoid vibrations of the electrobalance, etc. Another approach to obtaining a field gradient (dH/dx) using a pair of coils is described by Cape and Young (10). They use Helmholtz coils (about 7.5-cm radius), canted et an angle of 29” and separated by a distance of 7.5-cm to obtain a “linear region” (that is, region of constant force) which is about 4 cm near the axial center of the coils. These coils are placed in between the poles of a standard electromagnet (with relatively large poles, a t least 15 cm in diameter). As in the Lewis design (63), the coils provide a means of varying the gradient dH/dx independently of the field H produced by the electromagnet. Cape and Young do not give any constructional details for the coils and the associated dc power supply. This situation is disenchanting for those wanting to adopt their design. They do report measurements of magnetisation (x.H) for a gallium-substituted yttrium iron garnet film. The authors designed the coils to especially obtain a good control of the force (= H .dH/dx) at very low values of the field, H. They contend that the use of special pole tips to obtain field gradients a t low
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
values distorts the gradients (H dH/dx) because of remanent magnetization in the pole tips. I n our opinion, the Lewis (63) design is superior to that of Cape and Young in many ways. It is more versatile, in the sense that it can be used from the very low to the very high fields produced by the electromagnet. I n this case, the poles can be kept close together to provide the necessary minimum gap for accommodating dewam, cryostats, etc. The Cape and Young configuration based on the principle of Helmholtz coils demands that the distance between the coils remain (at all times) equal to the radius of the coils. This would make it impossible to obtain high fields especially a t such large pole separation as 7.5 cm. Cape and Young suggest mounting the coils over the poles of the electromagnet and the use of superconducting coils to obtain high fields; this approach would possibly be very cumbersome. Another disadvanhge (pointed out by these authors) is that the sample must be located accurately at the same spot relative to the coils. This is not essential in the Lewis design which provides large (-3 cm) regions of uniform force (H.dH/dX). Wohlleben and Maple (11.2) provide a detailed account of a sensitive Faraday balance capable of measuring weak susceptibilities up to pressures of 20 Kilobars. A pressure clamp (that is, the sample vessel) about 1.5 cm in diameter and 3 cm tall is constructed to handle small samples under hydrostatic pressure up to these pressures, and the possibility of using Bridgman anvils up to 100 Kilobar pressures is pointed out, with perhaps a loss of sensitivity in the susceptibility measurements by about 50%. The hydrostatic pressure is obtained by using mixtures of 1:1 n-pentane and isoamyl alcohol. The basic principles used here are the same as described by Kawai and Sawoaka in 1967, and reviewed in our 1968 article (81). The unusual features of the paper include the following: (i) a very good discussion of materials for the pressure vessels; e.g. BeCu binary alloy, Berylco 25, GE 0-30 alloy, and WC; (ii) the susceptibility compensation technique in which a paramagnetic sample is attached to the diamagnetic BeCu vessel so that the susceptibility of the combination becomes nearly zero. This allows an increase in the ratio of the magnetizations M , / M . by two orders of magnitude; here the subscripts s and e refer to the sample and the pressurizing environment; (iii) a good description of various experimental procedures is given along with the procedures for eliminating errors due to the vessel magnetization; and (iv) relative accuracy (Ax/x) can be obtained in the to lo-‘ range. Another description of a high
pressure cell is given by Vaughn and coworkers (110). Ginsberg (32) reports magnetic susceptibilities of materials which may be used in magnetic instrumentation and cryogenic apparatus. This paper and the erratum (33) concerning the composition of epoxy and polymethane adhesives should prove useful in the fabrication of sample vessels, cementing of fiber suspensions, etc. Of particular interest are the names and properties of materials such as phosphor bronze, beryllium-copper (Berylco), germanium resistor, and the sources for obtaining them. The alloys are used in the fabrication of high pressure vessels in magnetic and other instrumentation. The susceptibilities were measured successfully at cryogenic temperatures with a “Cryotronics A.C. Inductance Bridge, Model ML155B.” In view of the comments we offered earlier on a n inductance bridge, manufactured by the Cryotronics, Inc., High Bridge, N.J. (see Ref 74 and Ref 79), it is gratifying to assume that i t has been modified to permit trouble-free measurements of susceptibility. Stewart (103) gives new information concerning the prediction of later instabilities in the Faraday magnetometer (balance). With reference to his earlier papers ( l o g ) , he points out that the expression he first derived for the field gradient ratio will be modified when the contribution to the horizontal force from the term @ d H / b x is taken into account. Thus, if the field component He is expanded about x = 0, the expression for the force in the x direction becomes
Stewart further points out that this equation had first been derived by Garber et aZ. (27) in an equivalent form and suggests that this should replace his (Stewart’s) equation which contained only the first term. The modified equation for the gradient G ratio is:
This equation shows that the lateral instability of the suspended sample will occur a t a lower value of the vertical force than that suggested by Stewart in his earlier papers (10.2). He further calculates that the second term in G-l contributes -3Q% of the total if the value of ( b H x / b z ) a t the ‘specimen position is taken to be -0.19 ern-’. As stated before ( 7 4 , all such factors should be taken into consideration in the construction of a Faraday balance for work of utmost precision and accuracy.
Derived Force Techniques. A torsion balance for magnetic susceptibility measurements on fluids is described by Splittberger and Gill (98). A “quadrant” type probe consisting of a Teflon (Du Pont) disk (1.8 cm diameter X 4 mm thick) with diametrically opposite holes is suspended from a quartz torsion fiber and placed directly above a (ferromagnetic) steel sphere (-1 .&inch diameter). This entire assembly is surrounded by a chamber, into which liquids are introduced, The probe and the sphere are placed between the poles of an electromagnet which, when energized, produces a torque on the quadrant, which is measured with a “torsion head” (see p 544, Ref. 74) similar to that used in an apparatus for measurement5 pG. The paper shows interesting and straight-forward applications of easily available electronic units for magnetic measurements for paleomagnetic research, which presumably can be carried out on a semiquantitative basis by looking a t a wide variety of samples from our planet and from outer space. For the measurement of magnetic properties (susceptibility, etc.), one has to depend on more sophisticated techniques such as the vibrating sample (or coil) magnetometers, magnetic balances, and so on. A low sensitivity apparatus for qualitative measurements on samples of rock is reported by Morris (69) for measurements in the -160 to 0 “C
region. The remanent magnetizations are measured with a “flux gate” magnetometer along different axes of rock samples, about 25 mm long and 25 mm in diameter. Zilstra (114) gives a n account of a very useful vibrating reed (sample) magnetometer for microscopic particles with very high magnetic moments such as those encountered in powdered magnetic materials (Ferroxdure, MnAl, SmCos). Unfortunately, the paper is written in an awkward fashion in that the theory, the experimental procedures, and the instrumentation are all mixed up. Reference to earlier microvibrational methods (for instance those developed by Yu and Morrish, Yousef, et ai. (and reviewed in Refs 73, ‘74) yould be most desirable. As best as we can judge, the following neasurements and procedures are needed for obtaining the “magnetic moment’’ (that is, “magnetization,” which in turn is equal to the “susceptibility times the field”) of a microscopic particle. A reed made of 38 p diameter gold wire and 2 cm long is vibrated between a pair of detection coils, which in turn are placed in an inhomogeneous field of an electromagnet. The axis of the coils is parallel to the direction of the field. A microscopic particle is cemented near the center of the strip so that it also lies a t the center of the coils. The coils are energized with an external audio oscillator which is tuned to the mechanical frequency of the reed. The amplitude of vibration naturally depends on the magnetic force acting on the sample. The deflection of the reed is measured using a microscope with a calibrated eyepiece and stroboscopic technique, which produces an apparently stable image of the reed. This deflection is said to be proportional to the magnetic moment of the sample. A lowest-detectable magnetic moment has been quoted to be about 2 X lo-* erg Oe-’, which is comparable to the magnetic moment of an iron particle of 10-lo gram or about 2 p size. The applicability of this technique to weakly paramagnetic materials is doubtful. The author reports a hysteresis curve for a 5-fi particle of SniCos, which is quite impressive. He discusses a t length the effects of the viscosity of the air on the damping of the vibrations of the reed, the quality factor for its mechanical vibrations, and so on. Although this information represents a critical analysis of the effectiveness of the apparatus, it tends to cloud the important features of this instrumentation. Creer and deSa (16)have developed a modification of the well-known Foex and Forrer (cf. Ref 74) horizontal magnetic balance for the measurement of variation in magr.etization with temperature. They employ a maximum field of -5000 Oe (400 kAm-l), a
furnace giving temperature variation up to -700 ‘C in about 30 minutes. The sample is vibrated horizontally with two small magnets and an inductance coil surrounding them. They describe a simple method of measuring the displacement of the specimen (proportional to its volume magnetic susceptibility) with the help of two parallel metallic disks, which form the capacitance part of a resonance network. The authors report nieasurements on (rock) samples as small as 2.5 mg; their instrumentation has several novel features which should be useful for thermomagnetic analysis. Other modifications of the Foex-Forrer technique have been successfully developed and applied to research on catalysis by Selwood (cf. Ref. 73, 7 4 ) . Variable Field and Other Magnetometers. Brankin, Eastham, and Rhodes (8) describe an “integrating magnetometer,” which in our opinion should mark a new era in instrumentation. The past decade has seen the development and commercial availability of the vibrating sample magnetometer (VSM). The authors point out that with the VSM the vibration or motion of a superconducting sample in a magnetic field may affect the penetration or expulsion of the magnetic flux. Also, in specific cases, the magnetization near the ends of the superconducting sample may be different from that of the central region. I n their integrating magnetometer, the sample (1 to 5 mm in diameter) is held steady inside a superconducting magnet and is surrounded by appropriate pick-up coils. The external field is then swept a t about 10 Oe/sec. (10’ A rn-lsec-’) and the voltage across the pick-up coils is measured and recorded on a x-y recorder. This voltage is given by
V
=
-(nl
- nz) A
diM dH dH dt
- -
here A is the cross-sectional area of the sample, and dm/dH is the slope of the magnetization curve; this slope defines the magnetic susceptibility x of the material. The inner pick-up coil has n1 = 2100 turns and the outer coil has n2 = 500 turns; both search coils are -5 mm long. Since the integrating magnetometer is apparently very easy to construct and less expensive than the VSM, it would be worthwhile to investigate its applicability to other materials of special interest to the chemist. It should be noted that the common VSM used in conjunction with electromagnets (giving fields to 20 kOe) is not capable of measurements on diamagcgs unit) or a weakly netic ( x -10-7 paramagnetic (x cgs unit). According to a recent report by Moss (70), it should be possible to measure
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
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such weak diamagnetic and paramagnetic susceptibilities with a specially designed VSM and a superconducting magnet (fields -100 KOe). A dynamic magnetometer for the measurement of anisotropy in ferromagnetic materials is described by Gessinger, Kronmuller, and Bundschuh (31). The apparatus has the facility to perform a Fourier analysis of the pick-up signal automatically by using a lock-in amplifier, sinusoidal reference signals which are synchronous to the pick-up signals. Temperature Measurement and Control. Over the past two-year period we did not come across any description of apparatus for the control and measurement of temperature specially designed for magnetic susceptibility type measurements employing the classical Faraday and Gouy type apparatus, nor for the newer techniques. However, there has been an upsurge in the measurement techniques, especially in the low cryoger?ic range (below -10 OK). Relatively few temperature controllers have been described in the recent literature, but the number of manufacmrers (too numerous to list here) offering such apparatus appears to have gone up steadily. Casual reference to various instrumentational journals reveals a number of tempting advertisements for low temperature cryostats, high temperature furnaces, and gadgets for thermometry. Space exploration efforts (for example, the continuous measurement and telemetering of temperature on different parts of the lunar surface) has given rise to a whole new area of instrumentation. This involves differential t,hermometry for measuring temperature gradients, etc.; and a number of exciting papers c o n t ~ u eto appear-especially in the Rwkw of Scientific Instruments, J a m 1 of I%ysics-Sectio?a E , and so on. Anyone interestec! in applying the new advanced technology to magnetic measurements should refer to a large number of articles which have appeared recently in such instrumentation journals. We outline briefly below only those aspects of thermometry, etc., which we believe can be applied directly and easily to magnetic measurements involving both the classical force-type and new “magnetometric” techniques. Swenson (107)has reviewed the area of Iow temperature thermometry (1 to 30 OK). The temperature scale (1 to 30 O K ) is defined in terms of primary thermometers (gas, acoustic, magnetic) and is maintained by secondary thermometers (vapor pressure, platinum resistance, germanium resistance), He considers recent work on low temperature thermometry which has involved all of these areas as well as the development of new temperature sensors and of 330R
new techniques for 4-terminal ac resistance measurements, Rosenbaum (98) has surveyed some secondary thermometers for possible applications a t very low temperatures (0.01-1.0 OK). These include the carbon resistors and germanium sensors, which are reported to be quite suitable for this interval. The feasibility of using 0.02 atom % Au-Fe thermocouples is also discussed. These are not particularly suitable for measurement of temperature in magnetic fields, because they have anomalously high thermopower a t increasingly low temperatures. The paper describes special techniques used in the measurement of temperature and the commercial sources for obtaining special products for such techniques, including spot-welders for thermocouples; reference should be made also to a paper by Ginsberg (32) concerning materials for thermistors. An ingeneous and an inexpensive device for the measurement of temperature in the liquid helium range is described by Lebeau and Pine1 (69). Their device essentially measures accurately the change in frequency of an ascillator circuit caused by changes in the resisGance of an Allen-Bradley carbon resistor, due to variations in temperature. Several workers have used such resistors for directly measuring the change in their resistance with the rather expensive Wheatstone bridge techniques. The present device gives a change ixi frequency from about 3 to 8 KKHz over a range of -2.2 to -4.2 OK. This variation is much greater (and can be measured more accurately) than the corresponding variation in the resistance of the carbon resistor. Johnson and Anderson (63) discuss the factors affecting the stability of carbon resistance thermometers and baking methods to improve their stability. These thermometers are generally used in the range 0.01 to 10 “If. The magnetic susceptibihty and specific heat of constantan in the cryogenic range 0.033 to 1 OK is reported by Yee and Zimmerman (113). Constantan used in conjunction with copper commonly serves as a thermocouple in magnetic measurements, in preference to other materials which are ferromagnetic. Constantan has been considered to be weakly paramagcetic; however, the authors report a phase transition near 0.7G OK and an antiferromagnetic type transition near G.08 O K , which was previously associated with a specific heat maximum. They measured the susceptibility with an ac inductance bridge described previously by Pillinger and coworkers (cf. 7 4 ); the cryogenic range of temperature was obtained by the adiabatic demagnetization of ferric ammonium sulfate. The paper provides information useful for work of extreme accuracy in the cryogenic range.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
Rubin and Lawless (93) report studies on a glass-ceramiccapacitance thermometer (SrTiOa glass ceramic crystallized a t 1100 “C)in intense magnetic fields (150 KG) and a t low temperatures between 1.5 and 4.2 OK. The authors discuss their results in detail and speculate on the possible applications of this thermometer. At present, for most magnetic susceptibility-type work of reasonable accuracy, selected thermocouples (Cu-Constantan or Pt-PtRhj and resistance thermometers (Pt) or thermistors (Ge) should prove more than adequate. Extensive data and analysis on germanium and platinum resistors is provided by Neuringer and coworkers (88) for work in the cryogenic range 3.5 to 78 OK and in higher magnetic fields up to 210 KG. Adequate description of the dc resistance measuring technique for the measurement of temperature is given along with a scheme for quick and semiautomatic nieasurements with reasonable accuracy. Fletcher (84) describes a precision solid state temperature controller for use over a wide range of temperature (4 to 300 OK) with a resolution of 5 mK. The apparatus has several at)traczive features, inchding the ease of incorporating commercial operational ainplifiers, etc. APPLICATIONS
Determination of Spin Concentration. Crippa, Urbinati, and Vecli (16) discuss the validity of various formulas generally used in the determination of spin concentration by the EPX techniques. They emphasize the necessity of considering the Weiss constant 0 i i i such determinations. The spin number ( N z ) is generally obtained by the following equation:
where the subscripts 2 and s refer to the unknown and the known sample, respectively, T is the temperature of measurement, and A is the area under the EPR absorption curve. The authors point out that the above formula can be used, provided the linewidths of the absorption signals are small in comparison with the external field, and that, the g-factors and spin quantum numbers for the unknown and the reference sample are the same. This is usually true in free radicals. Other instrumental conditions are expected to be the same including especially the temperature T . Generally 6 is assumed to be very small in comparison with T. This reduces the above equation t o A, A;, = kN,A, where k is the instrument constant.
The authors experiment on diphenyl picryl hydroxyl (DPPH), Varian Pitch, and Lignite BM-3 and suggest that the equation used by most workers is an oversimplification. For instance, they show that 8 for DPPH is --50 OK, for the Varian Pitch -0, and for Lignite BM-3 -+lo0 O K , These were obtained by plotting the reciprocal of the area (here 1/A is equivalent to l/x) us. the temperature. The authors point out that a “blind” use of Formula 2 gives, for the lignite BM sample, N L = 2.2 f 0.2 X lo1’ spins per g. a t T = 300 OK; whereas the use of Equation 1 with proper 8 values gives N L = 1.19 i 0.12 X 1017 spins/g. The authors give a critique of the equations and measurements reported by previous workers. Reference should be made to another very useful paper by Slangen (95) who states that an accuracy of 1257, in spin concentration can be obtained with the EPR method. We suggest, that, whenever possible, one should attempt to obtain t,he e values from magnetic susceptibility measurements over a range G€ temperature. Quite often the spin concentration can be determined frorli magnetic susceptibiiity measurements. Equilibria of Magnetic Species in Solution. Crawforcl and Swanson (14) discuss the measurement of magnetic susceptibility by Evans’ NhIR, method, which we have reviewed elsewhere (72-74). They describe the application of the teniperature dependent measurement of magnetic siaceptibility to structural equilibria in solution. T!ie experiments have been designed for undergraduate instruction, and one should not’ attempt to use the technique for purposes of research-uniess other well-established and sensitive techniques su.ch as spsctrophotometiy cannot be applied to a specific probleni. They describe solution equi!ibris such as ?,hoseinvolving acetyl acetonate type complexes of Cr, Fe, and Cu and of the Iron HEDA complex. The latter involves the monomer-dimer type equi!ibris, first, propoyed arid magnetically analyzed by one of 11s (LXMj. T h e applicability of this technique has been reviewed elsewhere (73, 74). We ’nave already rmde reference to a modification Gf the Quintke technique, adapted by Grnybill and cowmkers (39) for rnagnetic titrations. These authors also discuss a niimber oi equilibria irivoiving nagiletic species. We recorrimend tha.t such magnetic techniques he csed in musual zases where “tit,ration type” informmion oaniiot be obtain& by other metiiods, and also to eiucidats the electronic structures of species in solution. Intramolecular Antifenomagne*ism. There ~ R Sbeer. consideisb!e act’ivity ir: this area o’v‘er tRe past decade, and the mdecular systems displaykg exchange in:eractions bet w e n neighboring spins have prc-
liferated. Quite often such systems have been referred t,o as the “metal cluster” compounds. This terminology may turn out t o be confusing unless efforts are made to distinguish it from the “cluster” concept employed in super- (or collective) paramagnetism, An extensive chapter on this general subject by Hatfield (along with authoritative treatments in other areas) is expected to appear in a two-volume treatise on magnetic susceptibility now being edited by Boudreaux and Mulay (6) The thrust of wcrk in this area has been on the binuclear (or the dimeric type) complexes of transition metal ions. Recent!y Mulay and Ziegenfuss (85) have focussed their attention on the “Welo type” trinuclear complexes of iron. These include the following:
They have estimated the exchange interactions between Fe(1IX) ions and have proposed structures consistent with LfGssbauer data. Xdogaki and coworkers (61) have also investigated several trinuclear complexes. Typical studies on the linear antiferromagnetism in the Spin 1 system in (ionic) C,YiC13 are given by Smith and coworkers (86). Single crystal anisotropic measurements on C,CuC& are reported by Rioux and Gerstein (91). Gregson and Mitra (41) consider the anisotropy Gf copper formate. T h e Titanium-Oxygen System. A number of oxides of tit,anjuni have been proved to form pnases between and TiOl. These arc variously known as the rionstoichiornetric phases or, in certain cases, a% the M a g i d i pkiases. These and similar. oxides of rnost transition metal ions c!isp!ay interesting and unique phase trsiisitions, which a x readily v i s 4 i z e d as serniconductor-to-met,al transitions. Mulay and Dnnley (75) !lave applied the concepts of intramolecular (or constrr,iiied antiferromagnet,ism to explain the magnetic behavior of the titanium oxides in the semicondu.eriny region. More recently they (76) hsvg systematidly calculeted the efiectivc magnetic POnienw sf iil t,he metallic region to elucidate the “aie::troaic struotcres” (on a “band type’! model). hfuiay and. Houlihnn (77! 78) have successfa!!y ccrrelated their EPR mmsuremm’ls wit!] the rnngitetic susce$ibiliry type ;::;rameters t o dutidat.e t5e basic nature of the ilniqul: cooperative trsnsitions.
Greenwood and coworkers (40) have outlined the magnetic anisotropy studies on single crystals of VzOa. Synthetic and Natural Biosystems. We can excuse ourselves from summarizing a large part of the studies on naturally occurring biosystems (ennymes, proteins, etc.) without meaning t o belittle their importance, b y stating t h a t much of the work along t,hese lines is now based on a sound foundation and is proceeding routinely. Several qualified physical chemists (and physicists) who know the intricacies involved in magnetic measurements and their interpretation are now engaged in this area. Many are taking the trouble to extend their measurements to very low temperatures (down to 2 O K ) and to interpret their results on the basis of magnetic exchange interactions, ligand field theory, correlnticn with optical and EPR spectra and so on. Thus the days of finding excitement in qualitative discoveries of a paramagnetic component, etc., in well-isolated biosystems may be said to be over. In this section on biosystems, we have surveyed typical papers which deal directly or indirectly with the concept of intramoiecular antiferromagnetism (that is, systems involvirig exchange interactions between neighboring spins) and the concept of thermai equilibrium between the high- end low-spin species. At the end of the section, we have reviewed the very few examples of magnetic studies on living systems, mhicli appeared in the literature during the past two decades. The s;ibr,ormsl magnetic behavior of several complexes has been explained by many vorkers by ( i i assuming an intramolecular antiferromagnetic coupling between neighboring spiris or occasional!y by (ii) assuming a “therxial equilibrium” between the high- and lowspin states 3f a mo!ecular species. Kyoko a t d c.wwrkers (58) have postulated the seccicd rnechmiam for expiaicing the ;xagrietir behavior of a csta,iase--azide complex: The foilowing abstract of tiizir work ciarises this &w,tiGl?.
“’The noiar peraniagnetic EUSceptibilities (xrcole) per Fe3’ ion of free catalase and its fllioricie, cyanide, and azide compounds were 1fixis:irecl with a magnetic torsion balarm over the range from coni temperature to l i q d nitrogen temperature. Fioin the slopes of the jinea:. relatioilships for x m o i . - l / T , tlie vaiues of the effective Bohr magrieton ililmber (,n,ff)were found to be 3.96 for free catdasz, .j.?8 for the fluoride coifipounds, and 2.63 Toi’ the cyanidz C O ~ J ~ pound. Xowrver. i,h curve for xrnr,je US. Ii’T of the azide (:oinpou:id deviated markedly frcn; Ciirie’s lnw. ‘This can h expiakted by 3s-
ANALYTICAL CHEMISTRY, VOL. 44. NO. 5 : PF’KII 1972
E
331 R
sUming that there is a thermal equilibrium between the high-spin
(S l/*)
-states and the low-spin (6 = of the hematin irons of s/a)
this compound. From the data that the proportion of the low-spin form to the total concentration of the hematin irons of this compound increased on lowering the temperature, the energy of the low-spin state seems to be lower than that of the high-spin state. The epr spectrum of the azide compound was measured a t 20 OK and 77 OK. The spectrum a t 20 OK appeared to be more of low-spin type than 77 O K . This tendency agrees with results obtained from the magnetic susceptibility measurements.” Another study involving the highand low-spin complexes is reported by Torii and coworkers (108). The magnetic susceptibilities of horse erythrocyte catalase and its derivatives were measured in the range from room temperature to 4.2 O K . With high-spin compounds, such as free catalase and the fluoride compound, the temperature dependence of the magnetic susceptibility was calculated assuming that,the fine structure of the ground state of the iron ion can be expressed by a spin Hamiltonian, DS,*. I n this calculation, the paramagnetic, D, of the spin Hamiltonian were estimated to be 12 cm-1 for free catalase and 9 cm-l for the fluoride compound. The values of E were determined to be -0.3 cm-’ for free catalase and 0.23 cm-l for the fluoride compound, using the values of E/D previously determined from the EPR signals. From these values, the energy differences of the de orbitals, E, - E,, were calculated to be -700 cm-l for free catalase and -500 cm-l for the fluoride compound. With low-spin compounds, such as the azide and cyanide compounds, the values of the effective numbers of the Bohr magneton, peff, were estimated to be 2.63 for the azide compound and 2.39 for the cyanide compound from measurements in an extremely low temperature range. These large apparent values of perf may be due to the contribution of a small amount of high spin compound remaining in the test sample. Moss, Moleski, and York (71) report the magnetic susceptibilities for the oxy, deoxy, and met derivative of hemerythrin over a wide range of temperature. Their magnetic moments were found to be subnormal for a spin of 1 and ranged from about 0.08 to 0.2 Bohr magneton. The measurements were made with a superconducting coil vibrating sample magnetometer described previously in 1969, by Moss and coworkers (70). Below 205 OK all the methemerythrin derivatives have been reported to be diamagnetic. These findings are said to support the long-standing ideas that 332 R
two iron atoms per hemerythrin monomer are suf6ciently closely-linked to form an antiferromagnetically coupled pair. Garbett and coworkers (988) give evidence for an antiferromagnetically coupled dimeric complex of the type p-oxo-p-peroxo iron(II1) in hemerythrin. This evidence is obtained from Miissbauer spectrometry and is correlated with earlier magnetic susceptibility and related studies. By postulating structures for the oxy- and deoxy-hemerythrin they propose a mechanism and kinetic behavior for the oxygenation equilibrium between the two. Sullivan and coworkers (106) have measured the magnetic susceptibility of hemin from 3034.5 OK. The magnetic moments are found to be in agreement with the predictions of Harris a t 273 and 4.5 OK, whereas the value at 4 OK is roughly 9% below the theoretical estimate. The hemin data do not,fit the Griffith-Kotani equation, and the interpretation of susceptibility measurements using this equation has not been successful in determining the zero-field splitting parameter, D, for hemins. Moleski and coworkers (68) have studied the magnetic susceptibility of the oxidized and reduced iron-sulfur proteins adrenodoxin and putidaredoxin. The diamagnetism of the oxidized protein is said to suggest that the spins for the individual Fe(II1) are antiferromagnetically coupled. The authors suggest a need for the molecular orbital type theoretical studies on the bridged structures of iron. Kimura, Tasaki, and Watari (66) also investigate the adrendoxin and other adrenal steroid hydroxylases. They measured the magnetic susceptibility of adrenal iron-sulfur protein (adrenodoxin) in the oxidized and reduced states over the temperature range 4-260 OK. The oxidized protein exhibited a paramagnetism without any deviation from Curie’s law (S = l/2). None of the theoretical curves based on exchange interaction energy between 10 and 500 O K fit the experimental curve. Loew (64) has studied and carefully analyzed the magnetic properiies of some high-spin ferric heme compounds. These include hemin and its DEP derivatives (deuteroporphyrin IX-dimethyl esters with F-, C1-, Br-, and I-). We quote below a summary of her work. “Using a crystal field model and generalized expressions for magnetic properties, the magnetization, susceptibilities, and effective magnetic moments of a series of high spin heme compounds whose zerofield characteristics are known have been calculated as a function of both msgnetic field strength and temperature. None of the usual
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
assumptions limiting the applicability of the calculation to high tamperatures, small magnetic fields, and non-interacting magnetic states have been made. From this study we have determined the conditions under which the magnetic moments of high spin ferric heme compounds are significantly field dependent. We have also explained the observed magnetic saturation of these compounds a t high fields. Finally, we have used the results of these calculations to compare with experimental results recently obtained for the variation of magnetization and magnetic moments with field and temperature in five related ferric heme compounds.” Jori and coworkers (66) have proposed some novel concepts which suggest that paramagnetic ions such as Cs2+ and Cu2+ act as protectors to visible light and help to retain the enzymatic activity in various proteins. Diamagnetic ions such as ZnZ+are said to have the opposite effect. The binding of metal ions via the active lysozyme sites is discussed. I n this last category, we examine the living biosystems. Mulay, Mulay, and Worden (83) have presented a brief description of their microtechniques for the measurement of magnetic susceptibility of bulk biomaterials (tissues, fluids, etc.) and an apparatus for single cells at an International Instrumentation Conference in 1968. Murayama (86) showed that sickled erythrocytes, which sickle only when deoxygenated, definitely orient themselves perpendicular to an applied magnetic field. This behavior was expected on the basis that ferrohemoglobin is paramagnetic, whereas oxyhaemoglobin and carbon-monoxyhaemoglobin are diamagnetic. Arnold and coworkers demonstrate (4) that a sample of fresh rabbit psoas muscle is magnetically anisotropic and the difference in the susceptibility components R, and K2 (corresponding to the maximum susceptibility KLand another component Rza t right angles to it) is about 2 x 10-8 cgs units/cm8; the average susceptibility was suspected to be less than that of water (-0.7 X cgs unit cma). The authors (4) point out that “although the determination of magnetic properties of muscle before and after contraction would certainly be of great interest, our apparatus did not permit quantitative measurements.” They found that glycerated muscle preparations, as well as air-dried and freeze-dried muscle fibers are strongly anisotropic. Nerve and tendon from the rabbit were also found to he anisotropic, whereas a fresh beef liver sample appeared to be isotropic. Several others qualitatively investigated during the period 1942-1953 the magnetic sus-
ceptibilities of various materials. Nibkantan (mollusk and egg shells), 1935; Loeb and Welo (silk fibers), 1953; E. Cotton-Feytis and Faure Fremiet (silk, hair, horn and tendon), 1942. Reference has already been made to a survey on the magnetic susceptibility of biosystems by Senftle and Hambright (94). Chalazonitis and associates (la)have reported the rotation and alignment of a suspension of outer rod segments from the retina of frogs in an external homogeneous magnetic field. They attempt to interpret qualitatively their a n i s e tropic results in terms of various membrane currents, for example, “intrinsic” and “injury” biocurrents and photocurrents, created by the photoactivation of the isolated external segments. They have further endeavored to elucidate the phenomena of visual distortion produced by external alternating magnetic fields. Magnetic anisotropy and the orientation of retinal rods in a homogeneous magnetic field are reported by Hong, Manzerall, and Mauro (48). They have carried out a satisfying experimental and theoretical analysis of the problem in terms of the “Swan Theory” of liquid crystals. They propose that the orientation of retinal rods in a homogeneous magnetic field can be explained by the magnetic anisotropy of oriented molecules in the disk membranes of the rods. They calculate the energy of a single rod as a function of orientation in the field, the time required for the alignment of the rod in a viscous medium, and its fluctuations. Having ruled out the possibility that the orientation may arise from inhomogeneous fields, they propose that the anisotropy is primarily due to rhodopsin. Magnetic Studies on Lunar Samples. As we pointed out in our last review (sa),one of the most striking, epoch-making and satisfying applications of magnetic measurements is in the area of lunar exploration. A number of comprehensive papers on the static magnetic properties, magnetic resonance (inciuding Mossbauer), optical, and electrical properties on lunar samples have appeared in the “Proceedings of the Second Lunar Science Conference,” edited by Levinson (68). These and other papers on the physical properties of lunar samples provide intricately related information and constitute, in our opinion, one of the most exciting compilations of carefully executed research. Because of limitations of space, we will present only a few important features of these studies and abstracts of relevant papers. Sullivan and coworkers (106) have studied the magnetic properties of eight glass spherules (0.03-0.24 mg) from the Apollo 12 lunar fines, one fragment (44 mg) from glass spatter collected during the Apollo 12 mission, and eleven glass
spherules from the Apollo 11 fines. As in the case of the Apollo 11 specimens studied previously, the specimens show a strong paramagnetism and an easily and difEcultly magnetized ferromagnetic component. An intermediate ferromagnetic component was found which was small and contributed little to the total susceptibility. subsequent remeasurements of the spherules obtained from both the Apollo 11 and Apollo 12 miasions show gradual changes in the magnetic properties with time In many of the specimens. Selected specimens were heat-treated in controlled atmospheres to determine that the e!Tect of oxidation is primarily a surface effect. The metallic iron varies from about 0.01 to 1% and the total iron calculated from the magnetic measurements compares well with electron probe analysis. The data indicate that the titanium is essentially all in the Ti4+state. Several specimens also were studied at temperatures as low as 4.2 O K in order to determine if the Curie-We& law holds a t low temperatures. The data follow a CurieW e b law with a W e b temperature of about 3 O K . Studies on the remanent magnetization of samples from the Apollo 12 mission are reported by three groups of workers and have been compared with the findings of the Apollo 11 mission. The general conclusions are that the magnetic properties in both cases arise essentially from the ferromagnetism of the metallic iron, paramagnetism of the pyroxenes, and the antiferromagnetism of the ilmenite. Pearce and coworkers (89) examine in some detail the magnetic properties of two igneous samples from the Apollo 12 landing site. A weak natural remanent magnetization has been found that is believed to be of lunar origin. When added to previous evidence from the Apollo 11 samples, this suggests that the moon had a field 3.3 and 3.6 billion years ago. Evidence is presented to suggest that this ancient lunar field was leas than one tenth of the present field of earth. Data from the Apollo 12 magnetometer tend to confirm that the moon did have a magnetic field, since it is difficult to account for the local steady field except by the presence of rocks which have a remanent of magretism. The remanence is carried in part by iron but also in part by material with a lower blocking temperature near 500 “C. This may be troilite or very fine-grained iron. I n the lunar soil from the Apollo 12 site, the main magnetic constituent is nearly pure iron. Gromme and Doell (@) consider magnetization as a function of the field and temperature for two lunar samples (Apollo 12). Their data indicate that the ferromagnetic material in Apollo 12 samples 12052 and 12065 is spheroidal grains of iron with Curie temperatures of
782 and 788 OC, respectively. The samples carry antural remanent magnetization that is stable with respect to the earth’s field, of the order of 6 to 20 x 1 0 4 emu/gram in intensity. This magnetization consists of several components, all of which are destroyed by heating in vacuum (5 X lo-“ Torr) to temperatures below 600 OC. The ability of rocks to acquire thermoremanent magnetization below 200 “C is also destroyed by heating in vacuum to 600 “C. The NRM in these rocks is concluded to have resulted from several magnetizing events that occurred at moderate temperatures some time after the last cooling of the rocks. Nagata in Japan and his collaborators in the USA (87) demorlstrate that the natural remanent n.agnetization (NRM) of three Apollo 12 crystalline rocks ranges from 2 to 8 X lod emu/ gram in intensity, and in the case of rock 12038, is northward, eastward, and upward in direction. The magnetic p r o p erties of Apollo 12 crystalline rock 12053 and fines 12070 are not essentially different from those of Apollo 11 lunar materials, being mostly due to ferromagnetism of metallic iron, paramagnetism of pyroxenes, and antiferromagnetism of ilmenite. However, there is much less antiferromagnetic ilmenite present than in Apollo 11 material. The NRM and other magnetic properties of weakly, moderately, and strongly impacted microbreccias were specifically examined. More strongly impacted microbreccias have stronger and stabler remanent magnetization. The observed magnetic characteristics particularly the thermoremanent, piezoremanent, and shock remanent magnetization of Apollo 12 crystalline rocks suggest that the magnetic field on the lunar surface when the rock was formed between 500 and 3000 gamma. Possible sources of this ancient lunar field considered are (1) classical dynamoaction of an early liquid lunar core, (2) an acient solar wind field of the necessary intensity, and (3) enhancement of the field produced by a moderate pristine solar wind when hot conducting lava results in lunar unipolar generator action. The high electrical conductivity required by this latter model has been confirmed by measurements of lunar materials a t elevated temperatures so that it seems the most reasonable in the light of presently available information. Extensive information on the electrical conductivity has been obtained not only from direct measurements on lunar samples, but also from magnetometric measurements. Significant contributions in the latter area have been made by two different groups from the NASA Ames Research Center in California. Dyal and Parkin (21) report the elec-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
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trial conductivity of the lunar interior which was obtained from analyeis of magnetic field step-transient events measured simultaneously on the lunar surface and in circumlunar orbit. The data fit a spherically symmetric threelayer lunar model having a thin outer crust of very low electrical conductivity. The intermediate layer of radial thickness R1 - Rz, where 0.95 RmR1 < R m w n and Rz -0.6 R,,, has electrical conductivity uz -lo-‘ mho/meter; the inner core has radius R2 -0.6 R,,, and conductivity u2 ~ 1 0 mho/meter. For the example of an olivine moon, the temperatures of the layers are as follows: crust, -440 O K ; intermediate layer, -810 OK; core, -1240 O K . The bulk relative magnetic permeability of the moon has been calculated to be p / p o = 1.03 -I: 0.13. A steady field of 38 f 3 gammas has been measured a t the Apollo 12 site. Restrictions on size and locations of the source are calculated and discussed. Three-hour averages of the magnetic field during the first lunation indicate a compression of the steadyjisld by the solar Wkd. In another related study, Bonett and coworkers (97) obtain the lunar electrical conductivit.y profile from an iterative fit of theoretical electroma,gnetic t,ransfer functions to empirical ones based on joint power, spectral density analyses of data from the Apoilo 12 lunar Surface Magnetometer and the Ames Explorer 35 Magnetometer. Seven selected two-hour swaths and seven onehour swaths of data were used. The spectrum analyzed range in frequency from 0.00053 to 0.04 Xz. The amplification of the interplanetary magnetic field a t the lunar surface shows a 4 s tinct increase from -1 a t 0.001 I’lz to -3 a t 0.005 H a . At higher frequencies, the s!ope of the ampliEcat8ion 3s. frequency curve decreases unt.il the ampiification levels off a t about 4 a t 0.025 Hz. An amplification curve Yith such a distinct bend is best fit by 8. lunar conduc: tivity rnadel with a sharp maximum around 1500 kni radius, so that high frequency variations i ~ zthe magnetic field are compressed into the outer shell while iow frequency variations can penetrate to the interior. For it monotonic kinperatwe profile, the eiectrical conductkity model reqxires a change in composition around 1450 kni radius. A thermal and compositioml model which appears t o fit the data is a Sasult-like outer layer, an olioine-iikz core, and a temperatwe cf 469 “C at !he ~ i ~ ~ d ~ ~ peak, Increuring to XOG “C in fhe deep core. Genera! evidence for the abundance of iron in the luna,r sa,mples has been 05tained from both MGss’oauer and Electron Paramagnetic Resonance (EPR) investigations. I n order to emphasize the correlation of information obtained from the static (magretixation type;
studies and the dynamic (resonance type) studies, we give below the abstracts of work reported in this area with reapect to the Apollo 12 and Apollo 11missions. Herzenberg, Moler, and Riley’s (47) work on Mossbauer spectrometry shows that in Apollo 12 returned lunar samples iron is generally abundant, and it has been identified specifically only in the ferrous and, to a minor extent, in the metallic state. Substantial dserences among the spectra of the lunar crystal~line rocks indicate considerable diversity in the phase distribution of iron and in the moda! minemiogy. Two ciasses of crystalline rocks can be distinguished on the basis of their Mossbauer spectra. Analyses indicate that the Apollo 12 crystalline rocks contain less ilmenite and inore olivine than the Apollo 11 rocks. Spectra of lunar soils returned on Apollo 12 exhibit quite remarkable similarity, but are notably different from the characteristic Apo!lo 11 soil spect,ra, suggesting that the nuclear gamma resonance spectra of lunar soils provide regional soil signatures. In the Apollo I2 soils, ilmenite and metallic iron are less abunda.nt, and olivine is somewhat, more abundant, than in the Apollo 11 soils. Evidence for a resoEarlce associated with the “KREEP” component has been foiind (KREEP stands for Rare Earth Element Phosphorus). The unusual lunar mkrobreccia 12013 exhibits resonances indicating the presence of the major lunar minerals, with both olivine and ilmenite lese abundant than in any other Apollo 12 sample esamined. In addition, anomalous features are present which appear to be associated with the KREEP component in the cia.rk iitholngy and with a history of intense shock. Housley and his associates (60) m a !yze Mossbauer data on Apollo 12 fines swmples 12042,38 and 1%31!11?, co?e tube samples i2025,15; i2025,42; 12028,88; and 12C28,116; arid rooks 12038,47; 12052,16; and i2063,59. They find that the d u k gray h e grained material from widely separated surface !ocaticns a d different depths corit,ainsa nearly uciiorm dist.ributiori of inajor Fe containing phases suggesting either that extensive mixing has taken plme or iess likely that all t,hese samples were derived from simiisr rocks. The phase conp~sition of the separated minera! anti rock fragments indicabes t that i ~ irocks t y fairly ric,h in olivine 2nd poor ir, Ilirienik h.aw made t!ie major contributions to the fine% The fines COILtain about 0.5-1 w t $& rnetvilic 7 e which greatly exceeds the amount present in the igneous rocks. Most of this Ye metal is eilclosod in glassy materid and aboLlt half of it is in particles less than 40 h in diameter. There small Be par-
3 3 4 ~0 ANALYTICAL CHEMISTRY, WL. 44, Na. 5, APRIL ,972
ticles m y be partially responsible for the dark color of much of the glass. This fine Fe metal probably results from reduction of Fe*+ during impact events. Although the -0.1 w t 3’% metal content of the rocks is sufficient to contain all the Ni, thermodynamic analysis indicates that a large fraction may have been in olivine a t high temperatures. The Electron Spin (or Paramagnetic) Resonance (ESR or EPR) studies, while substantiating the previous findhgs have been useful in identifying the presence of “surface centers.” This was accomplished by (i) studying the adsorption of (paramagnetic) oxygen O A the surface (of freshly powdered) lunar samples, which decreased the original ESR absorption and (ii) by irradiation experiments. These produce valence changes of Fe3+ and also oxygen-vacancy type defects. Re!evmt abstracts are given below. Hanemsn and Miller (46) search for the preserice of dangling bonds, or unpaired electrons, on surfaces of lunar material. The adsorptive capacity for oxygen is also investigated. Since the surfaces of returned material were contamlnated by exposure to sir! new clean surfaces must be prepared in cltrs, high vacuum by crushing. Lunar fines (12070) and lunar rock (12021) surfaces are examined in vacuc) by electron paramagnetic resonance (X band). The string pre-existing ferromagnetic resofiance from the fines prevents detectior, of any new sinal! resonances after crushing. In the case of the rock, however, there are only relatively small resonances before crushing, and effects may be observed. After crushing, a new resonscce appears near g = 2.006. This resonance is due to surface centres, since i: may be altered (enhanced) by exposure to gases (oxygen). The densiby of siJ.rfz.ce spins is about 1 per lo4 surface atoms. The oxygen adsorption that takes ?!ace is reversible, since the eninancement in the resonitnce d i ~ s p pears when the oxygen is pumped away. Evidezce, cbtsiced by separabe techciques, demonstrates that adsorptiori takes place on non-paramagnetis sites. This adsorption is not reversible. The rate of uptakc decreazes markedly at abo:it one nionolayer coverhge. I t is suggested from Lhese resu!ts that the sxfxces of soil and rocks on the moon may well be contxninated with adsorbe3 gasps, because of the lo:ig period of exposure?. EPR studigs h a w been a!so carried out by Tsay and coworkers (iO9)z t both X-hand (9.5 GBn) an.d K-bard (34.8 GHz) frequeccies cn a selection of Apo!k, li and Apo11o 12 \ m a r samples
