Magnetic Suscept ibiIity Aspects of Instrumentation and Applications Including Lunar Studies I . N. Mulay,‘ Solid State Science ProgramtZMaterials Research laboratoryt3 and lndumati I . Mulay, Center for Air Environment Studies, and Materials Research laboratory,3 Pennsylvania State University, University Park, Pa. 7 6802
I
review on magnetic susceptibility, we survey briefly some selected aspects of instrumentation and applications to chemistry including the solid state. The first four reviews (54, 63-66) appeared during 1962 to 1968. This review covers literature from about January 1968 to December 1969. As stated in earlier reviews, the elements of arbitrariness which may appear to some readers concerning the selection of the material continue to exist in the present review. Hence, it should not be regarded as an extensive review of a complete bibliographic nature on what may be commonly regarded as “magnetochemistry.” This statement partly reflects the agony of discovering the vast and ever-increasing growth in magnetics literature and one’s inability to cover all important aspects. The reviewer is then baffled by deciding what to select and what not to select. 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, references to new books and review articles on magnetic materials research are included. We will outline a few aspects of solid state chemistry or magnetic materials such as nonstoichiometric oxides and superparamagnetic materials. Readers who want 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 other similar abstracts. N THIS FIFTH
W H A T IS N E W ?
Having surmised that the selectivity of a review article, in terms of the space available, may surprisingly appear to become inversely proportional to the growth of the literature (otherwise Enquiries should be addressed to this author. Interdisciplinary graduate program in the physical sciences leading to N.S. and Ph.L). degrees in Solid State Science. a Laboratory for interdisciplinary research in Chemistry] Physics, Planetary Sciences] etc.
known as the information explosion), we have attempted to seek what is exciting and new in the realm of magnetics. Some of these exciting aspects are listed below and are succinctly reviewed or appropriately referred to in later sections. Magnetic properties of lunar samples Studies on synthetic and natural biosystems Intramolecular antiferromagnetism Nonstoichiometric oxides of transition metals Superparamagnetic systems Error analysis in classical techniques. (This analysis has been carried out almost a century after the discovery of these techniques.) GENERAL LITERATURE
Abstract Services.
T h e Cumulative
Solid State Abstracts and the S T A R compilation of the National Aeronautics and Space Administration (KASA) of the U.S. Government continue to provide excellent abstracts in magnetics a t a somewhat slow pace. T h e “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 abstracting and indexing services were outlined in previous reviews by us (64, 65). Monographs, Books, Contributed Chapters, and Reviews. Several books, reviews, etc. 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 disciplines, literature in these areas will not be reviewed here in the belief that these will be covered by other reviewers. 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 materials science approach. I t is unfortunate that some chemists (and physicists)
find it difficult to comprehend the “materials science” concept. Briefly stated, it is a newly emerging “interdisciplinary” approach in which the chemist, the ceramicist, 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 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 materials (old and new), and to understand their structure at the macroscopic and microscopic (or electronic) level. Characterization a t the macroscopic level includes efforts especially 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 p r o p erties play an important role. I n addition to the books listed below, there are a large number of treati,qes 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; our readers are advised not to ignore such listings. Earnshaw (24) has written a concise book on magnetochemistry (about 120 pages). It is outstanding with regard to a clear discussion on the application of the crystal field theory to transition metal complexes, and topics in intramolecular antiferromagnetism, etc. It gives important derivations and “working equations,” which are generally difficult to extract from the vast amount of literature on the subject. This book is indeed valuable as a classroom text for specific topics and for research as well. Cotton (19) and Kotani (41) have also covered magnetochemical aspects. An earlier monograph on “Magnetic Susceptibility” by Mulay (66) which encompassed some of the experimental
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
325R
and application aspects, is expected to appear as an enlarged and revised edition in the near future. “Elements of Theoretical Magnetism” by Krupicka, Sternberk, and Bardo (49),and “Spin Waves” by -1khiezer (2) should provide physicists’ approach to magnetic phenomena. Theoretical and experimental aspects of rotational magnetic moments of molecules have been outlined by Ramsey (71). This will be useful to workers studying molecular structures by molecular beams. An extensive review (113 pages) on the theoretical and experimental aspects of the exciting electrical and magnetic properties of transition metal oxides has been presented by hdler (1). This reviews literature up to mid-1967, with about 350 references. It is very well written and should satisfy the reviewing needs of chemists, physicists, and device-oriented scientists, who continue to be intrigued by the unusual properties of these oxides. Seitz, Turnbull, and Ehrenreich (76) continue to come out with up-to-date reviews of modern topics in Solid State Physics, including old and new magnetic phenomena in metallic and nonmetallic systems. Berkowitz and Kneller (9) have written two volumes on magnetism and metallurgy. Bradley (14) has briefly reviewed techniques of magnetic measurements under high pressure. However, a book on “Advances in High Pressure Research, Vol. 3” edited by him ( I S ) contains a very extensive chapter on magnetically ordered materials a t high pressures written by Block and Pavlovic. Brout (16) discusses magnetic aspects of phase transitions. Barfield, Busch, and Nelson (6) have given an extensive review on iron, cobalt, and nickel complexes having anomalous magnetic moments. This covers the interesting aspects of “intramolecular antiferromagnetism,” which we outlined in earlier reviews. Reference should be made also to Cotton (18, 19). The theory of infrared and optical spectra of antiferromagnets has been reviewed in detail by Loudon (45). The discovery and identification of a two-magnon adsorption band in FeFz in 1965, and of magnon side bands on sharp absorption lines in MnFz in the same year has given rise to a n exciting new area of research on the optical properties of antiferromagnets. Loudon’s forty-page review discusses especially the contributions of magnons to the optical properties of antiferromagnets having the rutile structure. Applications of the theory are made to Dxperimental results on excitons and magnons in hlnFa, FeF2, and CoF2. RbMnF, is also reviewed. The Magnet Laboratory of McGill University has now an extensive program in this area 326R
under the direction of Professor R. Stevenson. Fowles (29) presents an interesting review of some coordination complexes with coordination number five of transition metal ions, especially of titanium, vanadium, and chromium. Thioether complexes of TiCh appear to be antiferromagnetic. The author concludes that five coordination is now commonplace. Most of the complexes reviewed relate to metals with relatively few d electrons and the bulky ligands seem to force the complex into a transtrigonal bipyramidal configuration. An introductory review on “Magnetic Materials” has been written by Lee (44). It presents a few basic principles, and applications of ferro- and ferrimagnetic materials including those of thin films. -4nderson (4) has written a book on “Magnetism and Magnetic Materials.” Equally interesting are books on the “Magnetic Properties of Materials” by Thompson (85), and on “Magnetic Domains” by Tebble (83). Another book by Tebble and Craik (84),amongst other things, focuses attention on orthoferrites and ilmenites. The Institute of Electrical and Electronics Engineers, (38) in addition to providing chapters on new magnetic materials, e.g., bubble domains in orthoferrites, has come out with a special issue on “Advances in hIagnetics.” These have been written by well known workers in the field. Galasso (SO) gives useful information on EPR studies, magnetic, optical, and electrical properties of perovskite-type compounds. “Soft Ferrites” which describes properties and applications, has been written by Snelling (78). The “1968 hlagnetism and hIagnetic Materials Digest” has been edited by Chang and McGuire (17). We could not obtain any information on the status of the 1969 digest a t the time of writing this review. Goldsmid (32) has edited a book on “Problems in Solid State Physics.” It should be very helpful to students at the undergraduate and beginning graduate levels. This contains particularly a chapter on dia-, para-, ferro-, antiferro-, and ferrimagnetism by Morrish, who earlier published an extensive book on physical principles and applications of magnetism. The book by Goldsmid contains useful chapters on “Superconductivity” and on “Magnetic Resonance.” Izyumov and Ozerov (39), in addition to giving basic information on magnetic neutron diffraction, provide an account of modern views on atomic magnetic ordering etc. “Physics of Solids in Intense Magnetic Fields” by Haidemenakis (35),books on “Introduction to Superconductivity” by Rose-Innes and Rhudevick (73),“Structure and Application of Galvanomagnetic Devices” by Weiss (88),and
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
solenoid magnet design by Llontgomery (51)and by Koran (40) should be of general interest to magneticists. “CRC Critical Reviews on Solid State Science, Etc.” edited by RIeites, Zweig, and Sunshine (50) contains four chapters on interesting topics: “Medical ADplications of AIagnetism” (by Frei), “Geophysical Aspects of Paleomagnetism” (by Fuller), “Xagnetic Semiconductors” (by Hass), and “Low Temperature Thermometry” (by Swenson). INSTRUMENTATION
Error Analysis Etc. for Classical Methods. Sloot, Massen, and Poulis (77) discuss elegantly the reliability of magnetic susceptibility data obtained with the Gouy balance and focus their attention on large errors caused by the misadjustment of the sample holder. These errors were not suspected by any of the previous experimenters and it now seems difficult to believe that the accuracy in most of the published data has been better than 1 part in lo3, despite claims to the contrary. The authors derive an equation for a correction factor f in terms of angle a representing the deviation of the sample from the vertical axis, and 2, the distance between the lower end of the tube and the center of the magnet. The authors show that even for a small deviation of a = l o , and small values of R, which is used for defining the polar coordinates of positioning, an error of 0 to 2% is introduced in the measurement. For high values of field obtained by minimizing the pole-piece diameter, the positioning of the sample becomes even more important. Experimentalists, having regard for ultimate high accuracy and precision, must read the original paper, which gives references to earlier analysis of Brownian motion of the balance and several other critical factors. Stewart (80) gives a careful analysis of errors sometimes caused by the specimen holder being attracted toward one of the poles and swinging out of the vertical in a Faraday magnetic balance. Stewart considers various factors such as the gradient forces, image forces arising from the magnetic image of the sample in the pole, and the overall behavior of the Faraday system. He gives equations for predicting the instabilities which occur when the downward magnetic force exceeds a critical value, which is (surprisingly) the same for para- and ferromagnetic materials, and is independent of the electromagnet current. A11 conscientious experimentalists should do well to follow the valuable directives contained in this paper. Calibration procedures for determining regions of constant force a t various
fields and for different pole gaps for 4” “Heyding”-type pole tips are given by Martin and Hill (48). Using these tips and the method of Senftle and Thorpe, reviewed earlier by us (54, 63) and in the monograph b y hlulay (55), absolute susceptibility measurements on standard platinum samples have been made and used to obtain the field profiles. Martin and Hill discuss valuable methods for measuring the region of “coiistant force” ( H . d H / dZ) and experimental procedures for obtaining the field-profiles. Analysis of the data b y the Honda-Owen method for obtaining absolute (corrected) susceptibilities is also presented. Another contribution by Hill was recently reviemed by us (65). Griest and Ostertag (33) point out the difficulties encountered in the calibration of a Cahn balance, which has now been well adapted for the Faraday type magnetic susceptibility measurements and suggest a simple method to surmount these difficulties. The calibration is generally carried out on the sample loop A , which necessitates quite often breaking the vacuum, separating the hangdown tube, etc. to insert the calibration weight. The authors describe in detail procedures in which only the readily accessible counterweight (loop C) is used. The authors’ instructions have certainly facilitated magnetic susceptibility measurements in many laboratories which use the Cahn balance. Magnetic susceptibility measurements on borosilicate glass, quartz and suprasil, which are quite often used in the fabrication of containers for samples in the Faraday (and Gouy) balance are reported by Marshall and coworkers (47’). The authors find that borosilicate glass and suprasil show distinct paramagnetic susceptibilities arising from impurities below 50 and 4.2 O K , rebpectively. High purity quartz produced by Worden Laboratories, Houston, Texas, is diamagnetic all the way down to 2 O K . Even here the diamagnetic specific susceptibility decreases cgs from -0.399 to -0.072 ( X units) over a range 300 to 2 O K . This indicates again the presence of minute amounts of some paramagnetic impurities. The values reported here and those reported in our earlier reviews (63-65) will be useful for applying corrections for the sample holder in high precision work. Modifications of Faraday Balance. hlulay, Mulay, and Rorden (66) have outlined the applicability of a very sensitive all quartz manual and automatic microbalance for magnetic susceptibility measurements. The Worden balance is particularly useful for studies on the adsorption (of even corrosive) gases, vapors, etc. This balance was briefly described before (65). A complete description has now
appeared in the literature (66). Reprints of this paper and other information is available from Worden Quartz Products, Inc., 6121 Hillcroft Avenue, Houston, Texas, 77036, U.S.X. Morris and Wold (52) give rather well known and well established constructional details for a Faraday balance using the Cahn balance and a commercially made dewar for cryogenic measurements. A discussion of the Honda-Owen method for making corrections for ferromagnetic impurities and discussion of problems arising from thermomolecular flow, electrostatic effects, etc. contained in this paper will be useful to all experimentalists. The authors point out that when the vertical force exceeds about 1 mg, the sample bucket tends to swing toward one of the magnet poles and causes additional problems. The authors make simple suggestions for limiting this force to about 500 pg for a wide variety of samples, It is fortunate t h a t now a complete analysis of errors arising from this effect has become available, as shown in the previous section. An automatic vacuum microbalance of the pivot type has been built and tested by van Liehr (87). It can be easily adapted for measurements of susceptibility of especially diamagnetic materials, adsorption experiments involving low surface areas, thermogravimetry, etc. I t s unique features include the handling of high loads up to 2 g, stability over long periods of time, and infrequent calibration. It has a sensitivity of 88 p V / p g in the most sensitive region. Zero shifts have been reduced by a novel frame construction. The main limitation is said to be the precision of the recorder. The balance is automated by an optical beam, a solid-state null indicator, and associated circuitry which is described in detail. It uses magnetic damping; the oscillation period of the balance is about 11 seconds. Torque Magnetometers for Anisotropy Measurements. -4 rather unique application of the Cahn-R.G. electronic microbalance for measuring rather small torques in anisotropy measurements is presented by Bransky et al. (16). The balance, with a horizontal beam attached to a sensitive microammeter type coil movement, is rotated through 90’ so that the axis of the coil and the beam arm face down. The arm is now modified to hold a light weight tube (soda straw) which is then attached to the specimen (ferromagnetic thin films). The electromagnet is mounted on a rotating stand, which permits measurements, of the torque (via the associated electronics and recorder) for various angular settings. I n our opinion, the apparatus should have ample sensitivity for measurements even on feebly magnetic speci-
mens. A sensitivity of about 2 x dyn-cm. can be obtained and improved upon. Several torquemeters using sensitive moving coil ammeters have been previously reviewed by us (54, 63-65). One may surmise that the Cahn balance was a direct result of this torque principle and was developed as a microbalance for weighing, that is for measuring vertical forces; it is now amusing to see that the Cahn balance has been been put back to use to measure torques. A digital vacuum torque magnetometer for the 300-1000°K temperature range is described in detail by Fletcher and coworkers (27). I t s innovation lies in using a modified ballistic galavanometer as a torque head, which allows torque-free electrical contacts and a means for easy replacement of the suspension fiber. The associated electronics for digital readout and/or 2-y plotting of magnetic data are described. The measurable range of torques, 10-l to 103 dynes, allows measurements on both natural rock samples and synthetic ferrimagnetic crystals. Fields u p to 12 KOe are used and a compact vacuum furnace enables measurements over a wide range of temperature. X torque balance (magnetometer) for measuring small torques superimposed on large torques as encountered in the measurement of magneto-induced anisotropy (-lo3 erg c111-~) in the presence of magnetocrystalline anisotropy (-lo5 erg cm+) has been constructed by Gerber and Vilim ( 3 1 ) . It was used for the doctoral thesis research of the first author. The paper gives details of construction of the magnetometer, cryostat, etc. The temperature dependent K was measured on a copper manganese ferrite and the energy splitting E t ‘v 1540 cm-l of the trigonal component of the spinel crystal field was determined. The paper (and presumably the thesis also) contains a good detailed discussion of experimental and theoretical problems and their solutions. Maxim (49) reports the construction of a very sensitive automatic torquemeter for the measurement of very small anisotropies in the presence of large magnetic moments. This high sensitivity is attained by using a new frictionless method which eliminates unwanted lateral displacements of the specimen, suspended in an inhomogeneous field. Specially designed pole faces for the magnet are described and the recording apparatus consisting of a light beam, mirror, and two photocells is briefly outlined. Constructional details for a high load magnetic balance and torquemeter for magnetic and thermogravimetric studies are furnished by Beaton ( 7 ) . The apparatus is especially useful for studies on oxidation, phase transitions,
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
327 R
etc. (e.g., in steel). The system can handle masses up to 500 g and the transistorized electronic circuitry is capable of continuously monitoring weight changes between 0 to 50 mg and up to the maximum range of 0 to 10 g. Vanderkooy (86) describes a torque magnetometer for use with superconducting magnets, which usually have limited accessibility. Vibrating Samples (or Coil) Magnetometers. Noakes and coworkers (70) present relatively simple principles for improving the performance of vibrating sample magnetometers, especially with regard to eliminating the microphonics and the usual sensitivity of the apparatus to field noise. This is accomplished by the addition of trimming coils which are adjusted to produce no signal when the applied field is deliberately driven at a frequency which is to be used for vibrating the sample. The design of a mechanical chopper which discriminates against even and third harmonics is discussed. The use of eddy currents to precisely align a sample in a null measurement of magnetization is also described. A simple vibrating coil magnetometer has been developed by Farrell (26) for use a t temperatures above 1.5 O K with a sensitivity of emu. The apparatus was specially designed for measurements on Type I1 superconductors and to test recent aspects of their theory. A brief discussion of problems in this area is given and measurements on a single crystal of pure niobium are reported. A vibrating sample magnetometer is described by Foner and McKiff (28) which is specifically adapted for magnetic moment measurements in high D C fields as obtained from water-cooled magnets giving fields u p to 220 KOe. This uses a motion of the sample at very low frequencies between detection coils, integration of the induced voltage, and the recording of each cycle, which allows a further time averaging of the output. These features were not available in the earlier magnetometers designed by Foner. The authors give detailed description of their apparatus, a general description of flux integration techniques, and other essential features which include operation with superimposed field modulation, variable temperature, additional electronic filtering, and so on. A11 components for the magnetometer, except the mechanical system for vibrating the sample are available commercially. It has a moderately high sensitivity (10-l to emu) and can be used for differential magnetic susceptibility measurements on a wide variety of samples such as metals, alloys, insulators, superconducting materials, and highly anisotropic materials such as the rare earth metals. The paper, about ten pages 328 R
long gives all the necessary constructional details and a theoretical discussion, enough to tempt experimentalists into building one. Reference should also be made to a n earlier review on flux integration techniques by Dwight (25) to gain an insight into the theory and operation of other types of magnetometers. Resonance and Inductance TechA dynamic ( E P R type) niques. method for measuring the temperature dependence of saturation magnetization has been developed by Bhagat and Lucas ( I O ) . Although the utility of the technique is doubtful for extensive ferromagnetic studies, the technique is reviewed here because it displays beautifully the correlation between the demagnetizing field (Hdem)and the EPR line shape of a free radical such as D P P H (a,a - diphenyl 0 picryl hydrazyl). The authors describe an experiment in which a small paramagnetic crystal of DPPH is placed in the center of a rectangular microwave cavity. Another similar crystal is kept about 2 cm above the first one and the EPR experiment is performed a t 10 KHz and a n external field of 3000 gauss. Since this field was directed along the (l,l,l) easy axis, the nickel was saturated, thus making the Hdem proportional to the saturation magnetization Me. The authors claim that the field separation observed between the two resonances for the D P P H crystals should give directly the temperature dependence of M,. They obtain an accuracy of a few parts in lo4 by this method, which was applied over a wide range of temperature (-4 to 300 OK). This approach will be extremely useful in demonstrating both the principles of EPR and the effects of demagnetizing, etc. fields, as a n advanced laboratory experiment. The technique is reminiscent of the broad line N M R measurement of the magnetic susceptibility of MnO in which two orthogonal proton samples are surrounded by MnO powder and the separation between the two N M R signals is measured. This has been described by us in earlier reviews (54) and in a monograph (55). Seehra (75) describes another method for measuring the static magnetic susceptibility by paramagnetic resonance. I n our opinion, although it is a ne^ method, it is not a t all useful for such measurements because of the complexities involved, the poor accuracy (in some cases up to 10%) and its natural restriction to paramagnetic compounds only. However, the merit of the paper lies in a clear analysis of the correlation between large magnetic moment in a sample and the line width, and other parameters in EPR. The paper is indeed of great value to workers in EPR and the magnetic susceptibility area. The author has shown that the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5. APRIL 1970
EPR line broadens with increasing magnetic losses without changing its “g” value. The extra line width A u ~ is shown to be a simple monotonic function of the static susceptibility x0, the Larmor frequency u t , the filling factor, N , and the loaded Q t of the cavity: A w ~=
~T~QLX~WL
The theory has been compared with experiments on copper formate Cu ( H C 0 0 ) ~ . 4 H z 0with reasonably good agreement. An antiferromagnetic type maximum in the susceptibility of this salt around 70 O K , observed previously by routine methods, is confirmed by this EPR method along two directions in its crystals. -4 differential ballistic technique for sensitive magnetization measurements on ferromagnets a t low temperatures is described by Stevens (79). I n this method orginally developed by Barnett in 1952, the sample is quickly moved from one inductance coil to another. Several sophisticated modifications of this technique have appeared in the literature and were reviewed by us in earlier articles (54). The attractive features of Stevens’ paper relate to an analysis of measurement changes in fractional magnetization, a detailed description of the apparatus, and the associated cryostat. A sensitivity of 1 part in lo5is stated for measurements on gadolinium metal. An apparatus for the continuous recording of coercive force, maximum magnetization of ferromagnetic materials is described by English (25). It furnishes 60 Ha B-H parameters as a function of temperature on time. The associated circuitry appears to use standard electronic equipment (Philbrick amplifiers, etc.). Data are given for 1% Cr-l% C steel. Temperature Control for Magnetic Techniques. Birss, Gibbs and Wallis (11) have presented a very careful analysis of four different methods to obtain temperatures between 4.2 and 77 OK. They have used a method involving heat exchange between the coolant vapor and the specimen, which is not in common use. I n their cryostat (which can be used over the range 4.2 and room temperature) the rate of boil-off of liquid helium by means of an electrical heater is controlled by an electronic circuit. The required temperature is obtained by altering the heat exchange between the vapor and the specimen. Constructional details for this cryostat, especially designed to accommodate a very sensitive torque magnetometer [designed by Birss and Wallis and reviewed by us (63, 6 4 ) ] , and the performance of the cryostat in relation to rate of boil-off of helium and the specimen temperature are given. This apparatus should prove very useful for other magnetic techniques also.
n a i l and Knapp (20) describe a novel temperature device for accurate measurements in conjunction with a Faraday balance. I t s unique features include establishing a close physical contact between the sample and the temperature sensor (carbon resistor) and the ability to connect the device to a temperature controller. This allows a simultaneous control and measurement of temperature of the sample, especially between 4.2 and 30 O K . The sample, the sensor and light weight input coils forming a part of the inductive coupling circuit (air core transformer) are all suspended from the Cahn microbalance. The connection between the sensor and the input coils is made with fine wires, attached to the regular (quartz-fiber type) suspension. The change in resistance of the sensor with temperature is measured externally through a n AC type bridge, a lock-in amplifier, a 2-KHz signal source, etc. The temperature control is provided by a helium dewar, heating coils surrounding the sample, and an exchange gas. The authors describe relevant circuitry for their gadgetry. This should prove most useful for other temperature ranges (above 30 OK) with the use of other sensors. I n our opinion it is always desirable to measure the temperature when the magnetic field is turned off, since certain types of sensors have magneto-resistive properties, which are likely to vitiate the temperature measurement. I n this connection, reference to a paper by Belanger ( 8 ) describing the behavior of carbon resistors in high magnetic fields rvill be most valuable. Keuringer et al. (69) describe low temperature thermometry in high magnetic fields using carbon resistors. Equally useful information on the fabrication of small carbon resistors is given by Booth and Ewald (12) and the use of modified Speer resistors is outlined by Robichaux and coworkers (72). A cryostat for the measurement of magnetic susceptibility between 0.3 and 20 OK using the AC mutual induction bridge is described by D e Oliviera and Quadros (2f). The sample with variable positioning facility is enclosed in a dewar immersed in a 4He bath. The measuring coils surround the small dewar and are submerged in the 4He bath. Inside the dewar aHe, 4He, and Hz can be separately liquefied by means of the external bath. By using an electrical heater and a carbon thermometer, temperatures between 4 and 14 O K are obtained and measured with a precision of 2.67& The paper gives sufficient details for the liquefaction of various components. Window (89) describes a very simple electronic circuit for controlling temperature. It makes use of inexpensive amplifiers and avoids the use of com-
plicated and expensive gadgetry such as oscillators, phase sensitive detectors, etc. This should prove very useful for all types of magnetic work. APPLICATIONS
Magnetic Studies on Lunar Samples. One of t h e striking and epoch making applications of magnetic measurements is in the area of lunar exploration including a search for the magnetic monopole, which gave negative results. Researchers in t h e past have attempted to study the intriguing geomagnetic and paleomagnetic phenomena such as the reversal of the earth’s magnetic field in the hope of learning something about the history of their own planet, the Earth. Somewhat similar and more sophisticated techniques have been used and will continue to be used in the future for lunar exploration in the hope of learning more about the perhaps potentially intertwining history of both the Earth and the Moon. [We had earlier reviewed some of the books in the area of geomagnetism etc. (65).] Seven papers on the static magnetic properties, two on magnetic resonance, one on optical-electrical properties, and four on the Mossbauer spectroscopic studies on lunar samples have appeared in a special issue of Science. These and other papers on various 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 review a few features of the “magnetic susceptibility” type studies and quote from other abstracts. -4lvarez and cotvorkers (3) have beautifully outlined the concept of the magnetic monopole, as predicted by Dirac and their attempts to find it in the lunar sample. They describe a detection technique which relies on the electromotive force induced in a coil by a moving monopole (of the north or south type). A search for such monopoles in 28 samples gave negative results. We give below an excerpt from their paper which summarizes the concept of the monopole and reasons for their search in lunar samples. “For several years now, the hunt has been on for particles that would interact with the magnetic field just as electric charges interact with the electric field, acting as a source for the field and being accelerated by it. These particles, called monopoles, would be stable. They would have a magnetic charge measured by an integer Y, the Dirac charge 3 X 10-8 emu being used as a unit. Their existence would give credence to the only known explanation for the extraordinarily accurate phenomenon of charge quantization. According to a recent theory, they would be the
most fundamental particles, the building blocks of the universe. However, no such particle or combination with a net nonzero magnetic charge has ever been found. “In view of the negative results of these experiments, the lunar surface is considered to be the most likely hiding place for monopoles, whether they belonged to the primary cosmic rays or were produced in the collision of a highenergy cosmic ray particle with a nucleon of the lunar surface. I n either case, the lunar material would slow the monopole down and trap it. The reasoning that favors the lunar sample involves its great age, 3 to 4 x 100 years, and the small depth to which the surface has been churned during the long period of time.” Strangway and coworkers (81) point out that the moon does not have a significant magnetic field. I n an orbiting vehicle 300 km above the surface, the magnetic moment of the moon was found to be less than 4 X lozocgs; the present value of the earth is approximately 8 x loz5. The regional surface field must be less than 16 X 10-5 oersted. The natural remanent magnetization found suggests that the moon once had an intrinsic magnetic field of its own, much larger than the present one, or that it wa5 in the vicinity of another body such as the earth, which had a significant field. A breccia sample (10023) was found to have a strong natural remanent magnetization. If this was not magnetized by the local fields in the spacecraft or in the lunar receiving laboratory, it must have been magnetized on the moon. The authors suggest various mechanisms for this magnetization, e.g., cooling through the Curie temperature, by continuous thermal cycling, etc. Thermomagnetic studies show the presence of iron with about 1% nickel (igneous), iron with about 5 to 10% nickel (meteoritic), iron with about 33y0 or more nickel (meteoritic), and ilmenite Two abstracts, ( a ) by S a g a t a and coworkers (68) and ( b ) by Runcorn and coworkers (74) summarize information relating to the magnetic composition and natural remanent magnetization of lunar samples: (a) “Magnetic measurements have shown that nondiamagnetic minerals in a lunar crystalline rock of type B are (free F e z + in paramagnetic pyroxenes) : (antiferromagnetic FeSiOl) : (antiferromagnetic FeTiOl) : (ferromagnetic iron) = 4.3:7.20 :0.08 in weight percentage. The abundance of ferromagnetic F e in the lunar fines is about 7.5 times its abundance in the crystalline rock. The natural remanent magnetization of the emu/g i n crystalline rock of 7.5 x intensity may not be attributable to its thermoremanent magnetization.” ( b ) “The magnetic properties of samples of rock, fines, and magnetic separate from the fines from hpollo 11
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
329R
4001 figure
1. MAGNETIC SUSCEPTIBILITY AFTER KEYS AND MULAY
X
I
t h e compositions T i 3 0 5 (containing paramagnetic Tia+ with one unpaired spin) and TiOz (with the diamagnetic Ti4+ ion) several distinct nonstoichiometric phases are known to exist. These are described in general by Ti,02n--1 (n is an integer such that 4 n < 10). The unusual magnetic and electrical properties of these compounds have intrigued a number of investigators. Exploratory magnetic studies were previously reported by Mulay et al. and have been since reviewed by Adler (1) and by us (65). A detailed magnetic susceptibility investigation of the pure single phases Ti30j, TiaOT, Tis09, and Tisol, was recently reported by Mulay and Danley (58). The results of previous exploratory work are summarized in Figure 1. I n this study relatively few measurements were made around the transition, which was then assumed to be strictly of the Nee1 type. More recent detailed studies on pure single phases of Ti&, Ti407, Tis09, and Ti6011are reported in Figure 2. These clearly show sharp cooperative magnetic transitions, which are seen to be different from those in Figure 1. TiaOs displayed an abrupt transition from a low temperature crystal form to a high temperature form with a reproducible thermal hysteresis (transition temperatures during heating 462 OK, cooling 432 OK). Though such thermal hysteresis was not studied in detail earlier by Mulay et al., they briefly report this phenomenon. TirOT, TisO9, and Ti6011were found to show sharp transitions a t 150, 130, and 122 OK, respectively. Above these transitions the magnetic susceptibility of these phases was shown to follow a Curie-Weiss type behavior. From these and related studies the following model is proposed by Mulay and Danley (58) to explain the observed phenomena. Below the transition, groups of cations are distributed periodically throughout the lattice. Within these groups the d-electrons are delocalized; however, “coiistraiiied type” antiferromagnetism (which is a “short-range” intramolecular type) sets in between specific neighboring d-electrons through homopolar bonding of cations. Xeighboring groups interact via thermal excitation of electrons, Above the transition this type of magnetic ordering is modified by changes in crystal structure; here 3d overlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice. This behavior is described by the free electron gas model. I n addition, in all but Ti305a Curie-Weiss law phenomenon is seen to be superimposed on this transition, which may result from relatively small (2%) interactions between the upaired spins. In proposing this model, calculations of the effective electronic m*, and Goodenough’s equation for
