Volume 31, Issue 5, 01 May 1960
Index of content:
Preface: Proceedings of the Fifth Symposium on Magnetism and Magnetic Materials, November 1959, Detroit, Michigan31(1960); http://dx.doi.org/10.1063/1.1984587View Description Hide Description
31(1960); http://dx.doi.org/10.1063/1.1984588View Description Hide Description
This paper gives a brief critique, in elementary language, of the principal types of theoretical pictureswhich have been advanced concerning the electronic states of transition metals, especially those of the irongroup. It also calls attention to the possibility that some of the properties of these metals can be correlatedby the use of concepts which have an exact, not just approximate, meaning for a many-electron system.The Fermi surface is probably a concept of this type. Major conclusions are that in the iron group metalsthe 3delectrons ought not to differ radically from those in the free atoms either in number or in spatialdistribution, and that in most, though perhaps not all, of these metals the 3delectrons, magnetic or nonmagnetic,have an itinerant behavior.
31(1960); http://dx.doi.org/10.1063/1.1984590View Description Hide Description
The crystal distortion which arises from the Jahn-Teller effect is discussed in several examples. In thecase of compounds containing or at octahedral sites, the lowest orbital level of these ions isdoubly degenerate in the undistorted structure, and there is no spin-orbit coupling in this level. It is shownthat, introducing a fictitious spin to specify the degenerate orbital states, we can discuss the problem byanalogy with the magnetic problems. The “ferromagnetic” and “antiferromagnetic” distortions are discussedin detail. The transition from the distorted to the undistorted structure is of the first kind for theformer and of the second kind for the latter. Higher approximations are discussed briefly. In compounds likeFeO, CoO, and , the lowest orbital level is triply degenerate, and the spin-orbit coupling is presentin this level. In this case the distortion is dependent on the magnitude of the spin-orbit coupling relativeto the strength of the Jahn-Teller effect term. The distortion at absolute zero temperature and its temperaturedependence are discussed.
31(1960); http://dx.doi.org/10.1063/1.1984591View Description Hide Description
Ferromagnetic interactions of the rare earth elements in dilute solutions or in compounds with non-magneticelements have been discovered and will be described. The Curie points are essentially proportionalto the spin while the saturation moment follows the value for the effective moment. This ferromagneticinteraction, known to take place via the conduction electrons, follows criteria resembling closely those forthe occurrence of superconductivity.
It is shown that by suitable combination of similar superconductors and ferromagnets both phenomenacan happen simultaneously in the same crystal.
On the Laws of Magnetization of Ferromagnetic Single Crystals and Polycrystals. Application to Uniaxial Compounds31(1960); http://dx.doi.org/10.1063/1.1984592View Description Hide Description
One of the authors (L.N.) describes the mechanism of magnetization of a ferromagnetic single crystal:the elementary domains can be classified in several groups, called “phases” each having a certain direction ofthe spontaneous magnetization. The variation of the total magnetization takes place in various “modes,”corresponding to the number of coexisting phases in a given field. The calculation of the magnetizationcurve in these various modes explains the experimental results for single crystals of iron, pyrrhotine andmagnetoplumbite.
The results of measurements at room temperature on this latter compound, , are treatedin greater detail. The crystal is uniaxial; for different angles of the external field with the c axis, the magnetizationfirst increases proportionally to the field, followed by a gradual approach to saturation (exceptfor the angles 0 and 90°). The anisotropy constant is first determined ( is negligible). The two partsof the curve can be interpreted by two “modes,” with respectively, two and one “phases.”
The variation of the magnetization of polycrystalline samples can be explained by therotation mechanism only; the crystallites are too small in size to be divided into phases. The experimentalcurves can be explained by taking into account the interactions between crystallites in calculating the lawof approach to saturation.
31(1960); http://dx.doi.org/10.1063/1.1984593View Description Hide Description
Since the discovery of the magnetic yttrium and rare earth irongarnets, a systematic investigation hasbeen made of interactions of magnetic ions and the distribution of both magnetic and non-magnetic ionsin the garnets. The results of substitution of the tetravalent tin for ion in yttrium-iron garnet (balancedby substitution of for ions) have led to the development by Gilleo of a statistical interactiontheory which accounts well for the spontaneous magnetizations and Curie temperatures of the system.This theory has been further strengthened by results from zirconium substituted yttrium-iron garnets andby its successful extension to substituted rare earth irongarnets.
In the course of our investigations, many new garnets have been discovered. Several of these have enabledus to observe directly interactions between magnetic ions in dodecahedral and octahedral sites, dodecahedraland tetrahedral sites, octahedral sites only and tetrahedral sites only. The work on the garnets has alsoalready been adequately extensive to lead to the establishment of some rather simple rules pertaining to sitepreference of ions entering the garnets.
31(1960); http://dx.doi.org/10.1063/1.1984594View Description Hide Description
A crystal sphere of YIG was placed in either a doubly resonant transmission type coaxial cavity or intoa nonresonant transmission line device and biased with a dcmagnetic field. A low power microwave signalin the S band frequency range (cw or pulsed) was used to excite resonance. Current pulses of approximately1 μsec duration and low duty cycle were sent through a low impedance coil that was wound around thecavity or transmission line respectively. These pulses induced pulsed magnetic fields of the order of 1000gauss, which (vectorially) added to the existing dc field.
A microwave receiver, attached to the cavity output, detected weak oscillations at higher or lowerfrequencies. Shifts of 280 Mc have been obtained with the cavity, and shifts of 1280 Mc above and 530 Mcbelow the driving frequency have been obtained with the transmission type device.
The detected signal is associated with either the rise of the current pulse, or the decay, or two signals mayappear simultaneously corresponding respectively to the rise and decay. This depends upon the angularrelation of the dc field with respect to the cavity or transmission line.
31(1960); http://dx.doi.org/10.1063/1.1984595View Description Hide Description
Measurements of the absorption coefficient of (111) crystal sections of were made in the visibleand near infrared. Absorption peaks were observed centered at energies of about and .Reflectivity measurements on the crystals were carried out in the to range. Broad reflectivity peaks are found in the near ultraviolet at and , respectively.Very strong reflection peaks resulting from the lattice vibrations are present at energies smaller than .
Several quite sharp peaks are found in the absorption spectra of the rare-earth iron garnets of Dy, Ho,Er and Yb in the energy range. The circular dichroism of ytterbium iron garnet isdisplayed by means of absorption curves for the two senses of circularly polarized light.
31(1960); http://dx.doi.org/10.1063/1.1984596View Description Hide Description
Instabilities in YIG spheres that exist above certain threshold power values of cw microwave field arestudied using samples of about diam, that have low power line widths of 1 oe or less. Whereas thecharacteristic behavior such as asymmetrical line shape, “jump” effect etc. is somewhat similar to thatreported for disks, the phenomenon is generally different.
It has been determined that this instability, which can occur at cw power levels below the threshold forsignificant spin wavegrowth, is due entirely to the heating effect of resonance absorption upon the anisotropyenergy of the crystal lattice. As a result, the instability is characterized by a threshold curve thatfollows both the extrema and symmetry of the anisotropy curve for a given orientation. A straightforwardtheoretical explanation based on familiar relationships is outlined which fits the instability threshold vsorientation curve. The temperature instability provides a technique for measuring the “” factor that isbelieved to be more direct than previous methods.
31(1960); http://dx.doi.org/10.1063/1.1984597View Description Hide Description
A research program aimed at understanding our earlier line width and anisotropy measurements on YIGhas led to a study of ferrimagnetic resonance in doped crystals. YIG crystals have been grown containingappropriate concentrations (∼.01 to 5.0 atomic percent) of various impurities, including the 4f rare earthelements, members of the iron transition group, and several non-magnetic elements. This paper gives newresults for the variation of the field for resonance with crystal direction at several temperatures in the liquidhydrogen and liquid helium range. In the case of holmium, sharp peaks in the vs angle curve are seenwhich vary rapidly with temperature. The corresponding curves for the dysprosium containing sample donot show such sharp variations in (110), and at least in part do not vary with temperature below about 4°K.Though Kittel's recent theoretical results should apparently apply to the peaks in the holmium case, hisresults for the temperature dependence of the anomaly height and width do not correspond with our data.
31(1960); http://dx.doi.org/10.1063/1.1984598View Description Hide Description
Polycrystallinegarnets of the form have been prepared where was variedfrom 0 to 3 in six steps. Lattice constants were found to vary linearly from 12.374±0.005 A for yttriumirongarnet to 12.533±0.005 A for samariumirongarnet. The theoretical x-ray densities werecalculated and vary from 5.17 gms/cc for to 6.22 gms/cc for . Magnetic moments were measuredfrom 77° to 600°K. No magnetic compensation points were observed. The Curie temperature for thesegarnets is 570±10°K. Thermal magnetization curves for this series indicate that samarium substitution foryttrium produces very little change in the magnetization. The relative complex initial permeability wasmeasured from dc to 2 kMc for several temperatures. The results of these measurements are discussed briefly.
31(1960); http://dx.doi.org/10.1063/1.1984599View Description Hide Description
Complex initial permeability measurements have been made on polycrystallinegarnet toroids of theform where ranged from zero (yttriumirongarnet) to three (gadoliniumirongarnet). Both the real initial permeability and the imaginary initial permeability were measured at23°C, −78°C and −196°C over a frequency range of 1 kc to 2000 Mc. At room temperature, the low frequencyvalue of was found to decrease with increasing gadolinium content. At lower temperatures thelow frequency value of exhibits a minimum in caused by the temperature dependent behavior of thetwo different magnetic sublattices. The frequency at which the maximum value of occurs was found toincrease with the addition of gadolinium. No thermal relaxation was observed in these garnets, but ratherthe peaks in the curves shifted to slightly higher frequencies with decreasing temperature.
31(1960); http://dx.doi.org/10.1063/1.1984600View Description Hide Description
Heat capacitymeasurements of the irongarnets of Y, Gd, Er, Ho, and Yb between 1.4° and 20°K arepresented. Below 5°K, the specific heat of YIG can be represented by the sum of a lattice term proportionalto and the spin-wave contribution . This last term agrees satisfactorilywith that calculated from a spin-wave analysis, in which the exchange interaction coefficients were thosederived from Pauthenet's magnetization data. The results of the magnetic specific heat of the rare earthions could be interpreted in terms of a Weiss molecular field acting on these ions. For and , thisfield was found to be, respectively, about and below 20°K, in satisfactory agreementwith that derived from Pauthenet's data.
31(1960); http://dx.doi.org/10.1063/1.1984601View Description Hide Description
Large single crystals of yttrium and rare earth irongarnets, and yttrium and rare earth galliumgarnets,have been grown from molten solutions of lead oxide and lead fluoride. The crystals are grown by slowlycooling melts from about 1260°C to near 950°C. Sample compositions are: for YIG, 8 mole % , 22 mole% , 30 mole % PbO and 40 mole % ; for YGaG, 5.8 mole % , 14 mole % , 39.4 mole% PbO and 40.8 mole % .
This method provides crystals both large in size and of superior quality. Best results are obtained whenthe containing crucible is hotter near its top than at its base since nucleation is thereby reduced.
31(1960); http://dx.doi.org/10.1063/1.1984602View Description Hide Description
The paramagnetic resonance spectrum of in gallium and aluminumgarnets has been observed atliquid nitrogen and helium temperatures. The components of the principal tensor are 2.85, 3.60, and 3.74in galliumgarnet, and 2.47, 3.78, and 3.87 in aluminumgarnet. In aluminumgarnet, for a range of concentrations(relative to ) of from 2% to .1%, the line width varies linearly from 98 gauss to 20 gauss. Thelongitudinal relaxation time, for the same variation, increases from 0.001 sec to 0.02 sec.
31(1960); http://dx.doi.org/10.1063/1.1984603View Description Hide Description
The temperature dependence of magnetostriction in the gadoliniumferrite having structure of a garnethas been measured within the temperature range from liquid nitrogen up to the Curie point. At temperaturesabove the compensation point of sublattices, , the magnetostriction isotherms are of the same kind as forferromagnetics ( and have opposite signs and the vs curves exhibit saturation). In cooling below, there is an effect of superposition of large volume magnetostriction of paraprocess on the “ordinary”magnetostriction, which results in the distortion of the magnetostriction isotherms ( and are of thesame sign and without saturation). It is shown that the “ordinary” magnetostriction is caused by theinteraction of ions in sublattices and , while the volume magnetostriction of the paraprocess iscaused by the interaction of and ions.
31(1960); http://dx.doi.org/10.1063/1.1984604View Description Hide Description
The angular dependence of the maximum remanence parallel to the applied field, , and perpendicularto it, , when the specimen is rotated through an angle about an axis perpendicular to the field direction,is considered theoretically. For a rather wide range of materials the relation should hold. Foran assembly of noninteracting uniaxial single domains, it is shown that the distribution of the easy axescan be calculated from or and that , where is the saturationmagnetization.The above method yields a Legendre series for the distribution; it is deficient in that the high terms are quitesensitive to small changes in the remanences. Measurements of and with a vibrating-sample magnetometerwere carried out on commercially oriented Ferroxdure, on magnetically oriented Ferroxdurepowder, on oriented elongated single-domain iron particles and on anisotropic Alnico. For the first threematerials was a few percent lower, for the last up to 20% higher, than the value theoretically expectedfrom the relation between and . For Ferroxdure this small discrepancy is attributed to the reductionof by splitting into domains; for the elongated single-domain sample it is attributed to particleagglomeration. In order to furnish further magnetic evidence for the agglomeration of particles in magneticpowders, remanence curves are considered. Relations between remanences obtained by differentmethods are derived for the interacting-pair model. These relations differ from Wohlfarth's relations, whichhold for an assembly of noninteracting single-domain particles. Comparison with experiment favors the pairmodel.
31(1960); http://dx.doi.org/10.1063/1.1984605View Description Hide Description
The demagnetization curves of oriented elongated single-domain (ESD) iron particles have been measuredas a function of angle and packing fraction. The reduced coercive forces of dilute compacts show a maximumat an angle of about 50° associated with a bucklingmagnetization reversal. As the packing fraction is increased,particle magnetostatic interaction lowers the coercive force and suppresses the 50° max. The residualto saturation ratio appears as a distorted cosine function, which the simple single-domain particlemodel cannot account for without assuming particle orientations inconsistent with electron micrographs.A more realistic model is proposed which accounts for the experimental results by considering the effect onparticle domain structure of dendritic branches and cross-links to adjacent particles. The new model hasbeen analyzed by measuring the magnetic properties of arrays of pivoted magnets, which simulate the proposeddomain configurations at various angles and packing fractions.
31(1960); http://dx.doi.org/10.1063/1.1984606View Description Hide Description
This paper reports an experimental study of the size dependence of the coercive force and remanence ofessentially spherical cobalt,iron, and iron-cobalt alloy particles over the diameter range from 25 A to 2000 A.The particles were prepared by low-temperature electrodeposition into a mercury capillary jet, followed bythermal growth; size and shape distributions were determined by electron microscopy. The maximumcoercive force at 76°K was 1330 oe for cobalt, 890 oe for iron, and 1380 oe for iron-cobalt alloy; these maximaoccurred at a diameter of 30 A for cobalt, 130 A for iron, and 240 A for iron-cobalt alloy. At a sufficientlysmall diameter superparamagnetism reduced the remanence and coercive force to zero. This occurred at aparticle diameter below 25 A for cobalt, at 50 A for iron, and at 40 A for iron-cobalt alloy. The coerciveforce, remanence, and their temperature coefficients are discussed in terms of the approach to single-domainbehavior, the anisotropy constants which determine single-domain rotation, and the transition to superparamagneticbehavior. Crystal anisotropy predominates in the cobalt particles, a combination of shapeand crystal in the iron particles, and shape anisotropy in the iron-cobalt alloy particles.