Volume 38, Issue 3, 01 March 1967
- general session
- compounds and oxides. i
- critical point phenomena
- hard magnetic materials; induced anisotropy
- metallurgy of soft magnetic materials. i
- compounds and oxides. ii
- kondo effect; hyperfine fields in metals
- metallurgy of soft magnetic materials. ii
- memory applications
- magnetoelastic devices and interactions
- metals and alloys. i
- insulators: resonance and relaxation
- domains; instrumentation
- metals and alloys. ii
- thin‐film physics
- rare‐earth materials
- rare-earth materials
- microwave materials and applications
- thin‐film materials and instrumentation
- compounds and oxides. iii
- magnetic measurements
Index of content:
- GENERAL SESSION
38(1967); http://dx.doi.org/10.1063/1.1709688View Description Hide Description
For some time NMR and ESR have been applied to the study of biological materials. Two somewhat distinct applications have developed: as a means of detecting a given complex and following it through a reaction, and as a source of information on the electronic state and structure of a material. Mössbauer spectrometry has recently been applied to biological studies, principally in the latter mode. Unfortunately, the method may be used only on atoms with suitable nuclear properties. Work has been reported on hemoglobin, myoglobin, various cytochromes, ferritin, ferridoxin, and xanthine oxidase, all of which contain iron. Hemoglobin has been the most intensively studied. The magnetic compounds (e.g., the azide, fluoride, nitric oxide) are sufficiently dilute that reasonably well‐resolved hyperfine spectra are seen at low temperatures. Many are consistent with ESR data and one (the azide) can be predicted from the g values. Recent measurements in applied magnetic fields confirm the original interpretations. The nitric oxide spectrum provides information on the spread of the unpaired NO spin onto the iron ion. Some spectra are not yet understood, with Mössbauer and ESR spectra indicating different spin states. A number of interesting problems relating to these, to spin‐spin interaction, and to other cooperative iron‐iron effects remain to be investigated.
38(1967); http://dx.doi.org/10.1063/1.1709689View Description Hide Description
Experimental investigations on the characteristic length over which two magnetic atoms interact in a nonmagnetic metal have been carried out. Two methods are used: (a) measurement of the transition temperature as a function of the magnetic atom concentration in zero field, and (b) study of the magnetization distribution in alloys in the region just below the transition temperature as determined from the magnetic hyperfine field distribution.
These experiments have been done using palladium as the nonmagnetic and iron as the magnetic atoms. In the FePd alloys it is found that the transition temperature and the nature of the magnetic transition depend strongly on the method of sample preparation and in particular on the annealing of the samples. Samples quenched from the melt to room temperature show a diffuse transition and a relatively slow rise in magnetization with decreasing temperature. Those which have been annealed show a relatively sharp transition and a narrower magnetization distribution below the transition temperature, indicating that the process of annealing produces a more homogeneous distribution of the iron atoms. Interpretation of this phenomenon and its relation to the range parameter are discussed.
Assuming that the magnetic interaction between two iron atoms is due to the overlap of the matrix polarization, the range obtained in this investigation from a simple statistical model is in excellent accord with neutron diffraction measurements.
38(1967); http://dx.doi.org/10.1063/1.1709690View Description Hide Description
Ferroelectricity is a very general phenomenon. Yet it is possible to divide it into three main groups with essentially three different mechanisms: the order‐disorder of hydrogen bonds, the trigger effect of nonspherical radicals, and the 4π/3 catastrophe due to high electronic polarizabilities. Recently it has been realized that some of the metallic bronzes crystallizing in the perovskitestructure may also be ferroelectric. At present, it is very difficult to determine ferroelectricity in these metals.
38(1967); http://dx.doi.org/10.1063/1.1709691View Description Hide Description
The relevance of domain studies to both intrinsic and technical magnetic properties and the reasons for the continued interest and activity in the subject are discussed. The particular advantages of certain of the many methods of observing domains are noted, without attempting any comprehensive survey. Reference is made to some recent developments using x‐rays, to the interpretation of powder patterns, and to dynamic studies. There is no general domain theory, i.e., no theory which predicts the domain structure which will exist in a specimen with specified shape and intrinsic properties. However, it is possible to evaluate, with varying degrees of approximation, the energies corresponding to some observed or postulated structures. Domain observations can then be used to verify magnetostatic calculations, and can give measurements of certain intrinsic properties. Also, precise calculations of magnetization curves can be made for postulated structures in the hope that these will elucidate the technical properties of real materials.
38(1967); http://dx.doi.org/10.1063/1.1709692View Description Hide Description
In general, the symmetry of the spin‐wave spectra is determined by the magnetic space group characteristic of the spin order in the crystal. For many magnetic materials, however, interactions such as dipole‐dipole, antisymmetric exchange, and higher‐order anisotropic exchange are quite small, so that most of their properties are essentially determined by isotropic Heisenberg exchange plus a few crystal field parameters. It was pointed out previously that this fact is reflected in the spin‐wave spectra in terms of additional symmetry not predicted by the magnetic‐space‐group theory and that this symmetry is describable in terms of spin‐space groups which include independent spin and space rotations. In this paper the theory of magnetic space groups and spin‐space groups and the characterization of the spin‐wave spectra in terms of their representations is briefly reviewed. The effect on the spin‐wave spectra of the various types of interactions that have been proposed for the rare‐earth metals is then analyzed in detail and compared with the recent experimental data for Tb. The calculation of selection rules for neutron scattering is also discussed and applied to Tb.
- COMPOUNDS AND OXIDES. I
38(1967); http://dx.doi.org/10.1063/1.1709693View Description Hide Description
The compounds (RE)2 3+Cr3+Sb5+O7 (RE=Y or a rare earth except Ce, Pm, Lu) with pyrochlore structure have asymptotic Curie temperatures between 4° and 16°K; Ho2CrSbO7 and Dy2CrSbO7 are ferromagnetic below T c=10°K and T c=16°K, respectively. The moments of the chromium and the rare‐earth ions are oriented parallel in these latter two compounds. The positive Cr3+–Cr3+ interaction in the pyrochlore structure is compared to the interaction between chromiumions in other compounds.
38(1967); http://dx.doi.org/10.1063/1.1709694View Description Hide Description
The structure of CaFe2O4 has orthorhombic symmetry (space group Pnam‐D 2h 16) and is characterized by Fe–O–Fe–O chains within which the Fe–O–Fe bond angle is ∼130°. These chains are cross linked by means of Fe–O–Fe bonds in which the angle is ∼99°. The compounds (Ca (Cr x Fe1−x )2O4, with 0≤x≤0.5, have been found, by means of neutron measurements, to exhibit two magnetic structures in each of which the chains are internally antiferromagnetic, with spins alternating +−+− along the chain. In one structure the chains are linked ferromagnetically through the 99° bonds, with magnetic symmetry Pn′a′m, and in the second they are antiferromagnetically coupled, corresponding to the space group Pna′m. A phase diagram has been constructed showing both single‐phase regions and a region in which the two phases exist in equilibrium. Transition temperatures as well as the temperature dependence of magnetic moments and phase concentration have been determined as a function of chromium content. The problem posed by the coexistence, at equilibrium, of two magnetic phases belonging to a single‐crystal structure will be discussed.
38(1967); http://dx.doi.org/10.1063/1.1709695View Description Hide Description
Electrical conductivity, Hall effect,magnetoresistance and thermoelectricproperties have been studied on a number of undoped and doped polycrystalline CdCr2S4 and CdCr2Se4 samples. Undoped CdCr2S4 and CdCr2Se4 are n‐type and p‐type, respectively. With suitable doping, the conductivity of CdCr2Se4 can be changed by about six orders of magnitude [10−5 to 10 (Ω·cm)−1] without changing the magnetic properties. In Ag‐doped p‐type CdCr2Se4 the mobility at room temperature is 32 cm2/V·sec and rises with decreasing temperature. By doping with In, CdCr2Se4 can be made n‐type. The mobility of electrons is found to be two orders of magnitude lower than the mobility of holes. The conductivity of p‐type CdCr2Se4 decreases monotonically with decreasing temperature without any anomaly around the Curie temperature (T c=130°K1). However, the conductivity of n‐type CdCr2Se4 has a minimum at 146°K and rises sharply with further decreasing temperature. This n‐type material also exhibits a large negative transverse magnetoresistanceeffect which has a maximum at the Curie temperature (Δρ/ρ0=−0.8 for H=6 kG). The magnetoresistance of the p‐type material is positive in the paramagnetic range and negative in the ferromagnetic range. Optical transmission experiments on thin single crystals show that the absorption edges of CdCr2S4 and CdCr2Se4 are at 1.6 and 1.3 eV, respectively, at room temperature.2 For the ferromagnetic range an anomalously strong shift of the absorption edge takes place to higher energies in CdCr2S4 and to lower energies in CdCr2Se4. The edge is shifted further in both materials by an externally applied magnetic field. This additional field shift exhibits a linear magnetic dichroism and has its maximum at the Curie temperature. The observed anomalous optical properties and also the strong increase in conductivity of n‐type CdCr2Se4 in the ferromagnetic range are closely related to magnetic ordering. Apparently the strong shift of the absorption edge decreases the energy difference between the donor level and conduction band and results therefore in an increase of the carrier density. A complete account of this work will be published elsewhere.
38(1967); http://dx.doi.org/10.1063/1.1709696View Description Hide Description
Mössbauer spectra of 57Fe in Fe2SiO4, which has iron atoms in two distinct crystallographic sites, have been obtained from 9°K to 300°K, a temperature range in which two magnetic transitions occur as indicated by magnetic susceptibility measurements. Above the first transition temperature, 66°K, the spectra consist of a pure quadrupole split line. Below this temperature Zeeman splitting is observed, resulting in composite spectra attributed to the two different iron sites. Magnetization curves indicate the presence of weak ferromagnetism between 66°K and the second transition temperature, 20°K. Below the second transition temperature Fe2SiO4 is an ideal antiferromagnet. The hyperfine spectra were analyzed by numerical solution of the complete interaction Hamiltonian. From this analysis values of the magnetic fields, the electric field gradients, and their relative orientations are obtained. These results are compared with the spin assignments that have been made on the basis of neutron diffraction studies.
38(1967); http://dx.doi.org/10.1063/1.1709697View Description Hide Description
The ferromagnetic resonance of single crystals of CdCr2S4 and CdCrSe4 has been investigated as a function of temperature at 9.49 and 24.24 GHz. The CdCr2S4 crystals were grown by vapor transport, and the CdCr2Se4 crystals were grown from CdCl2 flux. The magnetocrystalline anisotropy and linewidth were found to be dependent on sample. The K 1 at 4.4°K varied between 3.8×103 and 4.4×104 erg/cm3 for CdCr2S4 and 2.2×103 and 1.8×104 erg/cm3 for CdCr2Se4. These values are in good agreement with those based on a calculation using the single‐ion model of anisotropy. The linewidth of the more anisotropic samples was found to be orientation‐dependent and broad compared to the less anisotropic samples. The narrowest linewidth observed at 4.4°K was 37 Oe. This value is higher than expected for these materials and may be caused by crystalline imperfections. It is suggested that the observed variation of properties with sample may be due to small concentrations of divalent chromium.
38(1967); http://dx.doi.org/10.1063/1.1709698View Description Hide Description
PbCrO3 has been synthesized in the high‐pressure ``belt'' apparatus from PbO and CrO2 at 1150°C and a pressure in excess of 50 kbar. Single‐crystal and powder x‐ray diffraction at room temperature show the compound has the cubic perovskitestructure with a 0=4.00 Å. No deformation of the cubic structure was discerned in neutron diffraction patterns at 77° and 4.2°K. At low temperature the magnetic moments of the chromium atoms are ordered in the antiferromagneticG‐type structure in which the spins on each Cr are antiparallel to those of the six nearest neighbors. Assuming the magnetic form factor of chromium can be approximated by that of Cr3+, the magnetic intensities give a moment of 1.9 μB per chromium atom, approximately the spin value for Cr4+ (3d 2). The temperature dependence of the (111) magnetic peak was measured and the results fitted to a Brillouin function with T N≈240°K. The magnetic susceptibility does not show a maximum at the Néel point but does obey a Curie‐Weiss law above T N, consistent with the neutron‐diffraction results.
38(1967); http://dx.doi.org/10.1063/1.1709699View Description Hide Description
The distribution of magnetic ions and the crystallographic parameters remain essentially constant throughout the system Hg1−x Cd x Cr2S4. A study of the magnetic properties of this system was therefore undertaken in order to further elucidate the origin of the large difference in the magnetic properties already reported on the endpoints. The Curie temperature was found to increase nonlinearly from 36.0° to 84.5°K as x varies from 0 to 1. The low‐temperature F‐AF (ferromagnetic‐antiferromagnetic) transition and metamagnetism of HgCr2S4 penetrate well into the system, up to x≃0.37 at which the F‐AF transition temperature approaches 0°K.
The distribution of magnetic ions remains constant, but the crystallographic parameters vary in the system Zn1−x Cd x Cr2Se4. Nevertheless, since ZnCr2Se4 is an antiferromagnetic with large positive θ, while CdCr2Se4 is a ferromagnet, a magnetic study of this system provides further evaluation of the theory of the ferromagnetic spinels. In addition, the evaluation of the nonlinear variation of ferromagneticCurie temperature from 22° to 129.5°K, observed for x=0.35 to 1, permits qualitative evaluation of the exchange parameters of ZnCr2Se4. The Néel temperatures were found to be ≃20°K for the whole antiferromagnetic region of the system x=0 to 0.35. F‐AF transitions in zero field and/or metamagnetism are also observed over a large compositional region.
38(1967); http://dx.doi.org/10.1063/1.1709700View Description Hide Description
Single crystals of CdCr2Se4 were grown for the first time. Individual pills of CdSe and CrCl3, in close contact, were heated around 700°C for three days. Perfect octahedra up to 2 mm in size grew in the CdSe pill by liquid transport. Formation of CdCr2Se4 occurred only in the presence of platinum. The technique and its reactions are described in detail.
38(1967); http://dx.doi.org/10.1063/1.1709701View Description Hide Description
Previous EPR measurements on Gd3+ pairs in LaCl3 have shown that the interactions between nearest and next‐nearest neighbors are accurately described by isotropic bilinear exchange and magnetic dipole coupling, and they indicated that the ferromagnetic ordering of GdCl3 at 2.2°K is due to dominant next‐nearest neighbor exchange. Similar measurements have now been made using EuCl3 as the host lattice. The results are similar to those in LaCl3, and since the lattice spacing in EuCl3 is very close to that of GdCl3, it is now possible to use the interaction constants measured for the Gd3+ pairs to make quantitative predictions of magnetic bulk properties of GdCl3. Excellent agreement is found for the high‐temperature susceptibility; for the specific heat, there is a small discrepancy. The measured interaction constants are close to one of a set of values recently used by Marquard to predict some highly anomalous spin‐wave spectra for GdCl3.
38(1967); http://dx.doi.org/10.1063/1.1709702View Description Hide Description
The NaCl‐type compounds Eu1−x Gd x Se form a continuous series of solid solutions between the ferromagneticinsulator EuSe and the antiferromagnetic metal GdSe. We have studied the temperature dependence of electrical resistivity ρ, in Eu1−x Gd x Se single crystals and polycrystals between 2.5° and 300°K; we find, in qualitative agreement with earlier observations, for small x, an exponential increase in ρ similar to normal semiconductors but a very sharp drop in ρ below the magnetic transition temperature. The resulting peak in ρ decreases sharply with applied magnetic field strength and increasing Gd concentration. The temperature dependence of the Hall effect in single‐crystal samples of Eu0.95Gd0.05Se indicates as origin of the resistivity anomaly an unusually large effect of the magnetic order on the carrier mobility. Preliminary thermoelectric power measurements support this conclusion. The possibilities of describing strong interactions between conduction electrons and localized spins by scattering theory and by magnetic self‐trapping of electrons in the spin lattice are discussed.
38(1967); http://dx.doi.org/10.1063/1.1709703View Description Hide Description
Magnetic and electrical measurements are reported on single crystals of CdCr2Se4. The measurements include ferromagnetic resonancelinewidth and magnetic moment vs temperature, magnetic anisotropy constant K 1 and g‐factor at 4.2°K, and the sign of the Seebeck coefficient at 300°K. A theoretical calculation of K 1 at 4.2°K is also presented.
- CRITICAL POINT PHENOMENA
38(1967); http://dx.doi.org/10.1063/1.1709704View Description Hide Description
Measurements of the specific heat have been made on three samples of dysprosiumaluminumgarnet in the critical point region of 2.5°K, using large optical‐quality single crystals and temperature steps down to 50 μdeg. The results for all three samples were very similar but they could not be fitted unambiguously to any of the presently proposed singular functions. In all cases, rounded maxima 2 to 3 mdeg wide were found, with peak values between 4.2 and 5.1R. Below these maxima, the results could be fitted to simple laws of the form C=A_ln | T−T N− | +B_, but above the maxima a logarithmic form could only be fitted by choosing TN+ more than 5×10−3 °K higher than T N−. Even then the agreement was not very good, and an apparently better fit could be obtained using the law of the form C=A′(T−T N)−α+B′ with T N+=T N− and α=0.31±0.02. This value of α would be considerably larger than predicted by any of the present theories. These experiments suggest that small broadening effects present even in apparently homogeneous samples can lead to considerable uncertainties in the analysis of critical‐point behavior.
38(1967); http://dx.doi.org/10.1063/1.1709705View Description Hide Description
A neutron‐diffraction study of antiferromagnetism and the wave‐vector‐dependent susceptibility of Tb near its Néel temperature shows a susceptibility which can be represented over a range of temperatures by χq −1=χ Q −1 + (qz−Q)2/(eq 0)2 + (qx 2+qy 2)q 0 2 with e=2.6 and q 0=0.017 (4πb 3) and with a χ Q which roughly follows the expression [(T−T N)/T 1]−4/3, with T 1=6°K. There is a more rapid decrease in χ q than predicted by the Lorentzian form for large values of (qz−Q) and a less rapid decrease than predicted for large values of qx . The wave vector for which χ q is a maximum shifts further away from ferromagnetism the higher the temperature.
38(1967); http://dx.doi.org/10.1063/1.1709706View Description Hide Description
The temperature dependence of the 57Fe hfs in FeF2 has been obtained with the Mössbauer effect in the region below the Néel temperature. In the low‐temperature region (0°−50°K) the present measurements are in good agreement with the more precise determination of the temperature dependence of the 19F NMR by Jaccarino. In the region just below the Néel temperature the data were analyzed in terms of the equation,yielding β=0.325±0.005 and D=1.36±0.03. This value of β differs slightly but not significantly from that found in MnF2 by Heller. D is larger than those previously reported in other materials because of the larger magnetic anisotropy in FeF2.