bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical Magnetic Scattering of Neutrons in Chromium
Authors: Hamaguchi Y. , Tsunoda Y. , Kunitomi N.
Published: February 2004

The magnetic properties of the metallic chromium can be interpreted in terms of itinerant electron model. The angular distribution of the neutron diffuse scattering around its (1δ0) satellite reflection has been measured with the temperature of TN−20°C≤T≤TN+33°C. Improvement of the instrumental resolution has made it possible to observe the correlation range more accurately and to determine its anisotropic property.

Correlation range of the spin system above the Néel temperature, which was analyzed from the experimental data on the basis of the Ornstein-Zernike-type correlation function and the static approximation, has been obtained as κ12(Q∥) ∝ (T − TN)0.70±0.2 and κ12(Q⊥) ∝ (T − TN)0.54±0.2. The temperature dependence is remarkably smaller than those obtained in various substances and the theoretical conclusions of Ising or Heisenberg model.
Temperature Dependence of Isomer Shift and Hyperfine Field near the Curie Point in Iron
Authors: Preston Richard S.
Published: February 2004

Current theories of critical-point phenomena predict that in the region below the Curie temperature TC, the spontaneous magnetization M of a ferromagnet varies with temperature T according to M/M0 = h(1 − T/TC)β for T→TC. Values of h and β have been calculated for theoretical models, and β is of special interest because of the possibility that it is a simple number such as ⅓ or ½. Measured values of β for various ferromagnets seem to cluster about these two values.

Pure iron is a favorable case for determining h and β by the Mössbauer effect since the hyperfine field should be closely proportional to M. However, previous results for iron1 showed a rather sharp discontinuity in the isomer shift at TC, and this is difficult to reconcile with the accepted opinion that the magnetic transition in iron is of higher than first order. If this were a first-order transition, then β would not be a meaningful parameter.

A more careful study of the Mössbauer effect in the vicinity of this transition is now being made. The temperature control has been further improved since the most recent results of this work were published2 and the uncertainty in temperature during any run is now of the order of 0.03°C. It now appears that the jump in the isomer shift, although sharper than it seemed to be originally, is not really a discontinuity, but is in fact spread out over a temperature range of about 0.3°C. When the values h, TC, and β are simultaneously adjusted for least-squares fits of the theoretical relation to the new data taken in two temperature ranges, the results are: β=0.342±0.004, h=1.49±0.03 for 0.99TC≤T≤0.999TC, and β=0.37±0.02, h=1.7±0.2 for 0.999TC≤T≤0.9999TC.
Inelastic Magnetic Scattering from RbMnF3 in the Neighborhood of its Néel Point
Authors: Nathans R. , Menzinger F. , Pickart S. J.
Published: February 2004

A study of the inelastic neutron scattering from antiferromagnetic RbMnF3 in the vicinity of the Néel point reveals the presence of a diffusive-type peak, accompanied by spin-wave modes that persist above the transition temperature. The width of the diffusive peak for small-momentum transfer is found to be strongly temperature-dependent above TN, indicating a slowing down of the spin fluctuations near the transition.
Magnetic Hyperfine Fields and Spin-Lattice Relaxations Measured by Nuclear Orientation NMR
Authors: Barclay J. A. , Brewer W. D. , Holliday R. J. , Matthias E. , Shirley D. A.
Published: February 2004

Nuclear magnetic resonance detected by resonance destruction of γ-ray angular distributions from polarized nuclei has recently been shown1,2 to be a powerful method for the study of nuclear spin systems in ferromagnetic lattices. From measurements of this type it is possible to obtain accurate values of nuclear g-factors, gN internal fields, Hint, and spin-lattice relaxation times, T1, in ferromagnets. This technique is discussed for the cases of 54Mn in Ni and 125Sb in Fe. The corresponding resonance frequencies at 0.02°K, including an external polarizing field of 2 kG, are v(54Mn in Ni) = 271.3±0.5 MHz and v(125Sb in Fe) = 132.5±0.5 MHz. With the g-factor of |g| = 1.101±0.002 for 54Mn2 the internal field of Mn in Ni is Hint = 321.3±1.0 kG. We have observed the resonance of Sb in Fe as a function of external field. After a least-squares fit we obtain from the slope: g = 0.749±0.018 and from the intercept: Hint = 231±6 kG. The sign of the field is shown to be positive from the sign of the slope. Measurements of the relaxation of the resonance indicate a value of T1T = 2.2±0.3 sec°K if we assume an exponential decay.
Resistance Anomaly in Palladium-Silver Alloys Containing Rare Earth Impurities
Authors: Chen C. W. , Edwards L. Roger , Legvold Sam
Published: February 2004

Electrical resistivities of Pd and three Pd–Ag alloys (Pd0.5Ag0.5, Pd0.35Ag0.65, and Pd0.25Ag0.75) containing small amounts of Gd and Nd have been determined at temperatures between 1.2° and 40°K. All lanthanidedoped alloys were observed to have resistance anomalies which were similar in character for the heavy and light rare earth solutes. However, the anomalies were found to vary drastically with the host lattice. With decreasing temperature, the resistivity-temperature curves show (1) a sudden increase in slope in the matrix of Pd, (2) a minimum in Pd0.5Ag0.5, and (3) a minimum and a maximum in Pd0.35Ag0.65 and Pd0.25Ag0.75. The present alloys, which constitute a continuous series of solid solutions having a common magnetic impurity, have thus exhibited a gradual shift from one to another of the three possible types of anomalous resistance behavior predicted for dilute magnetic alloys by Kondo and other workers. The shift signifies that a dominant role is played by the host lattice in the mechanism leading to a resistance anomaly. The difficulties in reconciling the present results with predictions on the Kondo theory as related to the sign of the effective s-f exchange integral and earlier EPR measurements are discussed.
Mössbauer Study of Metamagnetic Eu3O4 and Ferromagnetic EuO
Authors: Wickman H. H. , Catalano E.
Published: February 2004

The Mössbauer technique has been employed to study the metamagnetic oxide Eu3O4 and the ferromagnet EuO. In Eu3O4 both di- and trivalent europium ions are present (in the ratio 1:2) with isomer shifts at 77°K of −1.25±0.02 and +0.06±0.02 cm/sec, respectively. In the helium temperature range the divalent ions in Eu3O4 showed magnetic hfs, with a hyperfine field of 305±8 kOe at 1.2°K. The temperature dependence of the magnetic hfs of the Eui+ nuclei was followed from 1.2°K to the zero magnetic field Néel point of 6.2±0.2°K determined during these measurements. The data are only modestly well approximated by a Brillouin function with J=72. No hfs was found at the trivalent side. In EuO, the temperature dependence of the hyperfine field was followed from 77° to 4.2°K. The data are compared with predictions of two models for a cubic ferromagnet.
Magnetically Induced Electric-Field Gradients in Octahedral Fe2+:RbFeF3
Authors: Ganiel U. , Shtrikman S. , Kestigian M.
Published: February 2004

A theory for magnetically induced electric-field gradients in ferrous compounds, where the Fe2+ ion occupies a site of octahedral symmetry, is outlined. From the sign of the quadrupole splitting, as determined from a Mössbauer measurement on a powder sample, one can, in this model, determine the direction of the magnetic axis in the crystal. Comparison of the theory with the experimental results on RbFeF3 in the antiferromagnetic region leads to the prediction that the magnetic axis should be parallel to a (111) direction. Recently, however, a small tetragonal distortion has been observed in RbFeF3 below TN, indicating that the proposed model cannot account for the appearance of the quadrupole splitting in this case. Below 85°K, RbFeF3 undergoes a second transition to a state with spontaneous magnetization, which is a crystallographic transition.
Internal Fields at the Two Sites of the Spinel System Ni1+xGexFe2−2xO4 by Mössbauer Effect
Authors: Fatseas G. A. , Krishnan R.
Published: February 2004

In the study of ferrites, some authors have found two Mössbauer resonances of Fe3+ in tetrahedral (A) and octahedral (B) sites. The lower value of the internal field at the nucleus of Fe3+ in A sites has been attributed to the dipolar field, resulting from deviations from cubic symmetry, and to the more covalent nature of iron-oxygen bonds.

When the A site behaves as a perfect tetrahedron, the quadrupole interaction being absent, the dipolar field is excluded and the decrease in the field HA can be attributed to the effect of covalence alone. Those requirements are fulfilled in the spinel system Ni1+xGexFe2−2xO4. No quadrupole interaction is evident in any of these spectra.

The lower value of HA in all cases is attributed to the covalent effects in Fe3+–O2− bonds in A sites. The variation of HA and HB with x at 300°K is shown to be the effect of Curie temperature. At 80°K HB is independent of x, whereas HA decreases with an increase in x. This diminution in HA is attributed to the modification brought about by the presence of Ge4+ in A sites making the Fe3+–O2− bonds more covalent. This hypothesis is supported by the lower isomer shift of A sites.
Influence of Crystallite Size on the Magnetic Properties of Acicular γ-Fe2O3 Particles
Authors: Berkowitz A. E. , Schuele W. J. , Flanders P. J.
Published: February 2004

Magnetic properties have been measured for a number of samples of acicular γ-Fe2O3 particles of the type used in magnetic recording tapes. The average particle size and shape were approximately the same for all samples. However, the average crystallite sizes of the samples, as determined from x-ray line broadening, ranged from 50 to 700 Å. All magnetic properties measured showed a strong dependence on average crystallite size. Saturation magnetization at room temperature, σ, decreased sharply with decreasing crystallite size. An excellent fit to the σ vs crystallite size data was made by assuming that the crystallites were separated by a nonmagnetic grain boundary on the order of 6 Å wide. Room-temperature coercive force decreased with decreasing crystallite size, and the ratio Hc(83°K)/Hc(293°K) increased sharply with decreasing crystallite size.

The data support a particle model in which the constituent crystallites interact magnetostatically across nonmagnetic grain boundaries. A critical crystallite diameter of order 400 Å may be inferred. Below this crystallite size, superparamagnetic behavior is observed; above this size, noncoherent magnetization reversals in the crystallites are suggested. This model is quite consistent with the ``fanning'' mode of reversal based on a ``chain-of-spheres'' model which has been considered by several authors to be most compatible with their data on γ-Fe2O3 particle assemblies.
Coercivity of Mischmetal-Cobalt Alloy Powders
Authors: Strnat K. , Olson J. C. , Hoffer G.
Published: February 2004

A single-phase alloy of the type MMCo5 prepared by fusing cobalt with cerium-rich mischmetal (MM: 54.5 w% Ce, 26 La, 13 Nd, 5 Pr) is the cheapest of several new candidate materials for fine-particle permanent magnets. Results of experiments with powders produced by mechanical grinding and tempering are reported. The coercive force is limited by crystal imperfections introduced during milling. Maximum values of MHc are between 2500 and 4600 Oe, depending upon the milling conditions and the magnetizing field. Vibration grinding produces higher Hc than ballmilling, but ballmilled powders align better in a field (loop squareness Mr/Mp≈0.9 vs 0.7). Vacuum annealing at 300°–400°C improves the coercivity somewhat (3770 to 4600 Oe); higher temperatures reduce Hc drastically. Mortar grinding under liquid nitrogen yields considerably higher coercive forces than grinding at room temperature.
Band Structure of Nickel and Low-Energy Optical Transitions
Authors: Hanus J. , Feinleib J. , Scouler W. J.
Published: February 2004

Low-energy structure observed at 0.25, 0.40, and 1.30 eV in the temperature-modulated reflectance of nickel films has been assigned to interband transitions in a band scheme in which the conduction band is lowered with respect to the d bands such that L2′ is below L32 in both the majority and minority spin bands. A recent self-consistent band calculation agrees with the assignment of the 0.25-eV peak in the majority and the 0.40-eV peak in the minority spin bands at the L point, while the 1.30-eV transition is at the W point in the minority spin band.
Magneto-optic Properties of Nickel, Iron, and Cobalt
Authors: Krinchik G. S. , Artemjev V. A.
Published: February 2004

The frequency dependence of the real and imaginary parts of the nondiagonal components of the dielectric permeability tensor of iron, cobalt, and nickel is calculated in the region 0.2–6.0 eV from experimental curves of the equatorial Kerr effect. The curves obtained can be reconciled with results from photoemission by assuming that the curve of positive values of ε2′ corresponding to transitions in the 3d ↓ subband is pushed down by the curve ε2′ with the opposite sign corresponding to transitions in the 3d ↑ subband.
Magnetic Impurities in Metals
Authors: Blandin A.
Published: February 2004

This paper discusses some features of the electronic structure of magnetic impurities in metals.

For impurities in normal metals, applications of the model of resonant scattering states are made in the following directions: (1) discussion of the orbital degeneracy within the Hartree-Fock approximation and applications, particularly to cerium; (2) discussion of the magnetic solutions: effective values of J, occurrence of magnetism, and the Kondo effect; (3) influence of correlation.

The properties of impurities in transition metals are also discussed: magnetic moments, specific heat, resistivity. Evidence for two-current conduction in ferromagnetic metals is given. The paper ends with a discussion of dynamical properties of impurities.
The Problem of Scattering by Magnetic Impurities in Metals
Authors: Suhl H.
Published: February 2004

The ``s-d'' model of interaction of a free electron gas with a magnetic impurity is treated by the methods of scattering theory. Older transport-theory results are reviewed, and two additional measurements are proposed: frequency dependence of the resistivity (at low frequencies), and the Hall coefficient. The latter should show a broad maximum. It is suggested that because the ``s-d'' model leads to strong coupling effects, it is necessary to return to fundamentals. A preliminary indication of how this might be done is given at the end of the paper.
Magnetoelastic Resonance in Ferrite Cores
Authors: Boyd E. L.
Published: February 2004

The magnetostrictively coupled elastic resonance (MER) of a ferrite core is a useful tool for studying the magnetic response of a core and for magnetic-materials studies. The magnetic response of a linear ferrite and two square-loop ferrites are described in detail. The MER technique is applied to the study of an interesting composition in which the magnetostriction is a function of heat treatment.
Hyperfine Coupling Constants from Specific Heats Down to 0.02°K (Au–Fe and Pd–Fe Dilute Alloys)
Authors: Dreyfus B. , Michel J. C. , Thoulouze D.
Published: February 2004

We present nuclear specific-heat measurements of dilute Au–Fe and Pd–Fe alloys. The results give integrated information about the hyperfine field induced by the magnetic impurity on the matrix nuclei. In the case of Au, the results are in agreement with the theory. In the case of Pd, an upper bound is given for the number of Pd nuclei affected.
Determination of Nuclear Indirect Exchange and Pseudodipolar Interactions in Alloys by Spin Echoes
Authors: Alloul H. , Froidevaux C.
Published: February 2004

In alloys the spatial variations of the electronic density induce a distribution of the Knight shifts as well as a spread of the quadrupolar couplings.1 These effects create an inhomogeneous broadening of the NMR spectrum and have a strong influence on the shape of the spin-echo envelope of a given nuclear species.2 A general calculation involving 90°–180° pulse sequences has been performed assuming a broadening much larger than the contribution of the spin-spin interactions. It shows that the spin-echo envelope is a product of two factors, the first involving the quadrupolar effects and the second the spin-spin couplings. Each of these factors may have an oscillatory behavior the frequency of which is characterized either by the strength of the electric-field gradients or by the indirect spin-spin interactions.3

Experimental data illustrate this theoretical model in the case of a spin I=½. It yields values of the Ruderman-Kittel and the pseudodipolar coupling constants in Pb and Pt. For Sn and Cd only an order of magnitude of the exchange coupling constant has been deduced actually. The difficulty in the interpretation of the data in these last cases is due to the complicated crystal structure of these metals. These experimental data are summarized in the following table: bJMetal(kc/sec)(kc/sec)Pt0.39±0.084.0±0.2Pb2.2±0.34.8±0.5Sn≲5Cd≲0.25
Hyperfine Interactions in Pd3Fe and Pt3Co Alloys
Authors: Stetsenko P. , Avksentjev Y.
Published: February 2004

The specific heat of Pd3Fe and Pt3Co Alloys was measured in the temperature interval 0.15°–-1°K at the different degrees of atomic order in the alloys. The constants of magnetic hyperfine interactions were determined by the values of nuclear terms estimated from the total measured specific heat. For the Pd3Fe alloy the values of the effective magnetic field on Pd nuclei were calculated. The increasing of the atomic order degree in the Pd3Fe alloy leads to increase of the absolute value of effective field on Pd. This corresponds to the increase of local magnetic moments of Pd atoms, determined previously by neutron diffraction. In the case of Pt3Co alloy it is very difficult to determine the values of effective magnetic field on Pt and Co separately. The total hyperfine constant due both to Pt and Co vs order degree shows a flat maximum.
Mössbauer Studies of Iron-Rare Earth Intermetallics
Authors: Bowden G. J. , Bunbury D. St. P. , Guimaraes A. P.
Published: February 2004

The Mössbauer effect has been used to study the hyperfine field at the Fe sites of DyFe2, HoFe2, ErFe2, TbFe2, and SmFe2. The last three of these each show two superimposed hyperfine spectra with an intensity ratio of about 3:1. A similar result was found for ZrFe2 and TmFe2 by Wertheim et al.,1 who showed that it was consistent with the magnetization lying along a (111) direction. In DyFe2 and HoFe2 however we find only one Fe hyperfine spectrum at room temperature; it is shown that this would be expected if the magnetization is along a (100) direction. An explanation of these results is given on the basis of a simple ferromagnetic (antiferromagnetic) model for the light (heavy) rare earth intermetallics in which the direction of magnetization is determined by the easy direction for the rare earth ion.

The hyperfine fields at the Fe and Dy sites in DyFe2 have also been measured as a function of temperature. This parallels the work of Cohen2 on TmFe2 and allows a comparison of the rare earth-iron exchange in these intermetallics.
Magnetic Properties of SrRuO3 and CaRuO3
Authors: Longo J. M. , Raccah P. M. , Goodenough J. B.
Published: February 2004

Magnetization measurements of SrRuO3 and CaRuO3 confirm that SrRuO3 is ferromagnetic with TC = (160±5) °K and that CaRuO3 has antiferromagnetic exchange interactions dominant. They further identify in CaRuO3 a Néel temperature TN = (110±10) °K and parasitic ferromagnetism below TN having a σ0 = (3.2±0.4) × 10−2 emu/g at 4.2°K. High-field (to 125 kOe) magnetization and neutron-diffraction data for SrRuO3 are consistent with a reduced spontaneous ferromagnetic moment μ0 = (1.4±0.4) μB and no ordered antiferromagnetic component. Resistivity measurements confirm a low resistivity (ρ<10−3Ω·cm) at room temperature having a metallic temperature coefficient. It is concluded that both compounds represent spontaneous collective-electron magnetism and that the reduced moment of SrRuO3 is due to holes in both the up-spin and down-spin bands. Antiferromagnetic CaRuO3 is predicted to have narrower ``4d'' bands.
Magnetic Structure of RbNiF3
Authors: Pickart S. J. , Alperin H. A.
Published: February 2004

Magnetic measurements1 on hexagonal RbNiF3 indicate that this compound exhibits a spontaneous moment of approximately ⅓ the total Ni2+ moment, in contrast to the isomorphous compound CsMnF3, which is antiferromagnetic.2 In this structure (space group C63/mmc) there are two inequivalent cation sites (2a and 4f), and it has been suggested1 that the moment arises from antiparallelism of the cation spins on these two sites. Alternatively, canting of successive antiparallel c planes (such as occur in CsMnF3) toward each other within the plane could account for the observations. Powder and single-crystal neutron data on RbNiF3 have been obtained which indicate that neither of these two simple models can be correct. A more general model with different canting angles for the 2a and the 4f sublattices, however, is consistent with both neutron and magnetic data.
Study of the Acentric Compound FexGa2−xO3 with Polarized Neutrons
Authors: Delapalme A.
Published: February 2004

The cross terms between nuclear and magnetic scattering amplitudes in FexGa2−xO3 belonging to space group Pc21n are measured with polarized neutrons. The magnetic amplitudes are linear functions of the moments and the form factor of the atoms. By minimizing the difference between the form factor deduced from our measurements and the form factors experimentally (Nathans) known for Fe3+, we obtain simultaneously: the ferrimagnetic structure (+−+−) on crystallographic sites noted Fe1, Fe2, Ga1, Ga2, the proportions of iron atoms on each site p(1)=0.75, p(2)=0.83, p(3)=0.65, p(4)=0.07 and the magnetic moment of the Fe3+ iron (4.8 μB±0.07).
Temperature Dependence of the Anisotropy Constants of Gd, Tb, and Dy
Authors: Brooks M. S. S. , Goodings D. A.
Published: February 2004

The sources of magnetocrystalline anisotropy have been studied for the ferromagnetic rare earth metals Gd, Tb, and Dy. In the weak anisotropy case of Gd the contributions to the anisotropy constants from the crystal field have been calculated by spin-wave theory treating the crystal-field terms as a perturbation. Estimates have also been made of the contributions to K1(T) from real and pseudo-dipole-dipole interactions and from the magnetoelastic coupling making use of magnetostriction data. The resulting agreement with the measurements of Graham and others of K2(T), K3(T) and K4(T) is good, but the situation is less satisfactory for K1(T). From the available experimental data the various crystal-field parameters have been estimated.

In Tb and Dy, in which there is a strong axial crystal field, the anisotropy constants are no longer well defined, except for small-angle deviations from the easy direction. The data of Rhyne and Clark for the effective anisotropy coefficients K20(T) and K66(T) have been analyzed in this light taking account of the torque balance conditions. The measured value of K20(0) for Tb leads to a value of the effective crystal-field parameter about 100 times larger than the corresponding value for Gd.
Resonance Properties of Ni1+xGexFe2−2xO4 Single Crystals
Authors: Krishnan R.
Published: February 2004

Single crystals of the Ni1+xGexFe2−2xO4 system with 0≤x≤0.41 have been investigated in order to throw some light on the ionic distribution and properties of Ni2+ ions. The properties studied were K1, K2, g factor, and ΔH by means of ferrimagnetic resonance techniques in the temperature range of 4.2° to 393°K.

An increase in x results not only in a general decrease in K1 but also in a modification in its temperature dependence. For x≥0.36, K1/M and K1 actually begin decreasing below 80°K. From single-ion model of anisotropy a linear decrease of K1 with increase in x is predicted, whereas experimentally K1 falls off more rapidly for x≥0.36. To interpret this result an ionic distribution having Ni2+ in A sites is necessary, since from crystal-field considerations A site Ni2+ ions could contribute a positive K1. Since this contribution is larger at lower temperatures it counteracts the usual increase in -K1, thus diminishing the value at 4.2°K. The K1 found for x=0.32 could also be attributed to such A site Ni2+.

The above hypothesis is supported by the fact that for a sample with x=0.08 quenched from 1000°C which is known to increase A site Ni2+ concentration, K1 was found to vanish at 170°K.

The anomalous variation of g and the sharp increase of ΔH at 4.2°K with x are characteristic of A site Ni2+.

Our results indicate the presence of Ni2+ ions in A site for crystals of x≥0.3.
Origin of Magnetic Anisotropy Energy of Fe7S8 and Fe7Se8
Authors: Adachi K. , Sato K.
Published: February 2004

Based on the electronic energy state of Fe2+ ion in the crystalline fields in Fe7S8 and Fe7Se8 (4c) with a pseudo NiAs structure, the expression of magnetic anisotropy energy were derived by perturbation method under an appropriate approximation. Then the experimental results of magnetization curves were analyzed from this theory and the physical constants for the anisotropy energy, such as orbital angular momentum in the lowest state and the anisotropy energies of the second- and the fourth-order, were estimated. Using these constants, the experimental results of the temperature dependence on the spin angle (the second-order transformation) without magnetic field were well explained. On the other hand, a possibility of the occurrence of the first-order phase transformation for spin angle appearing in Fe7Se8 (3c) and FeS (α-point) was discussed from the same basis of the theory, and was given an explanation for the nuclear quadrupole shift at this transformation point in the Mössbauer experiment.
Magnetic Anisotropy and Its Field Dependence as Measured by an Induction Method
Authors: Flanders P. J.
Published: February 2004

A magnetic-induction technique which employs a rotating-sample magnetometer and lock-in amplifier has been used to study anisotropic magnetic behavior at 20°C. When measuring anisotropy constants (K) in ferromagnets, anisotropic spontaneous magnetization (σ) or anisotropic susceptibility (χ⊥−χ∥), outputs vary as K/H, σ and (χ⊥−χ∥) H, respectively; absolute values of K can be determined by measuring the ratio of the second-harmonic component to the fundamental. In Ni, a value of (1/K1)dK1/dH<0.04%/kOe was obtained between 2 and 20 kOe.
Conditions for Ferromagnetism in Real Gases
Authors: Braeter H. , Gruner R. , Heber G.
Published: February 2004

We consider a gas consisting of particles (molecules) with spin ½ and vanishing electrical charge. We assume the existence of two-body potentials of interaction between these particles, which are spin dependent. Our aim is to establish conditions for the appearance of ferromagnetism in such a system. Calculations are done for a two-body potential of modified Lennard-Jones type. The sum over states is expanded with respect to cluster integrals, which are calculated approximately up to the fourth order. Clusters of higher order seem to give only unimportant contributions to the mentioned conditions. The conditions for ferromagnetism are fullfilled if the density of the gas is higher than some critical value and below some critical temperature. The dependence of magnetization on temperature is given. It is supposed that a liquid of the same composition with the same interaction will also show ferromagnetism. The results seem to hold also for potentials, which are not of Lennard-Jones type, but have the appropriate spin dependence.
On the Exact Frequency-Dependent Susceptibility of the Two-Dimensional Ising Model
Authors: Allan G. A. T. , Betts D. D.
Published: February 2004

Fisher has found exactly for the square and honeycomb lattice the static initial perpendicular susceptibility of the quantum-mechanical version of the Ising model defined by the Hamiltonian H=−J ∑ nnσizσjz−mHx(t) ∑ iσix,where all symbols have their usual meanings. As the magnetization, Mx = mΣiσix, does not commute with the Hamiltonian, the system can relax and Kubo's linear response theory can be applied. We have been able to evaluate the response function exactly and hence obtain simply the exact frequency-dependent initial perpendicular susceptibility for the honeycomb lattice Jχ⊥(ω,T)/Nm2=¾[〈σ0zσ1z〉−〈σ0zσ1zσ2zσ3z〉]ω02/(ω02−ω2)+3[¾〈σ0zσ1z〉+¼〈σ0zσ1zσ2zσ3z〉]ω02/(9ω02−ω2),where ω0 = 2J/ℏ and 〈σ0zσ1z〉 is the nearest-neighbor correlation evaluated by Onsager. A similar expression is obtained for the square lattice. For the four-particle correlation 〈σ0zσ1zσ2zσ3z〉 the other three spins are nearest neighbors to spin zero, and we have obtained its value from Fisher's transformation theory. The behavior of χ⊥ as a function of T and ω is described and its physical significance is discussed.
Local Magnetization of a Ferromagnet with Point Defects
Authors: van de Braak H. P. , Caspers W. J. , Dekker A. J.
Published: February 2004

If a ferromagnetic lattice, consisting of localized magnetic moments, contains nonmagnetic impurities, the local magnetization of the ions in the neighborhood of such an impurity will be affected by the absence of the magnetic moment. This effect is studied in the temperature region 0≤T<TC for the cases that the host lattice spin equals 72 and ½, respectively. For the upper part of the temperature region numerical solutions of molecular-field equations are obtained. For the lower part a first-order perturbation calculation in the spin-wave model is made, and compared with results of a perturbation treatment in the local picture, where the method of the double-time temperature-dependent Green function is used. The latter seems to be more suitable and yields a magnetization curve that fits closely to the molecular-field curve.
Dynamical Spin Correlations in Many-Spin Systems
Authors: Tahir-Kheli Raza A.
Published: February 2004

Time-dependent correlations in many spin systems with anisotropic Heisenberg exchange interactions of arbitrary range are analyzed, using quantum-statistical Green's functions. The discussion is restricted to the ferromagnetic case and to two sublattice antiferromagnetic and ferrimagnetic orderings.

A scheme of higher-order random phase approximations, i.e., RPA II and modified RPA, is developed, thus extending the usual treatments involving the unembellished RPA.

The RPA II leads to a set of coupled nonlinear integral equations which cannot be solved at general temperatures. However, due to the simplicity of the ferromagnetic ground state, at low temperatures a rapidly convergent iterative procedure leads to a satisfactory solution which reproduces the spin-wave results.

The intra- and inter-sublattice longitudinal correlation functions are calculated in the modified RPA scheme, which consists in approximating the RPA II by the substitution of the usual RPA solutions for the transverse Green's functions, and the results are found to be satisfactory over the entire range of temperatures.
Electron-Magnon Interaction in Ferromagnetic Transition Metals
Authors: Davis L. C. , Liu S. H.
Published: February 2004

The purpose of this work was to determine some of the consequences of the electron-magnon interaction. We have calculated the self-energy of an electron in a ferromagnetic transiton metal due to the virtual emission and absorption of a magnon. The calculation was carried out in second-order perturbation theory, but since the lowest-order vertex correction vanishes on account of spin conservation, the result is thought to be rather accurate. The enhancement of the electron mass at the Fermi surface was found to be as much as 50%, which is consistent with experimental estimates. It appears reasonable to conclude that the electron-magnon enhancement can be as important in the ferromagnetic transition metals as is the electron-phonon enhancement. We have also shown that a kink, similar to a Kohn anomaly, exists in the electron-energy spectrum in the ↑ band at εF−ℏω0 and at εF+ℏω0 in the ↓ band, where ω0 is the frequency of a magnon emitted or absorbed when an electron or a hole scatters in the forward direction from one spin band to the other near the Fermi surface.
Specific Heat of Impure Heisenberg Ferromagnets
Authors: Hone Daniel , Vogelsang Karl
Published: February 2004

A Heisenberg ferromagnet containing a single substitutional impurity, differing from the host atoms in spin magnitude and/or exchange coupling to its neighbors, has been studied recently by several authors. The problem has many similarities to that of the lattice vibrations of an impure crystal, but the simplifications introduced by the harmonic approximation for the phonons are applicable only to unrenormalized spin waves. In particular, even in the RPA, the single spin-flip excitations are renormalized by the presence of other such excitations. Because of these interactions the free energy cannot be expressed simply in terms of a density of states of these RPA spin-wave modes; interaction energies are multiply counted. We have expressed the specific heat in terms of transverse and longitudinal spin-correlation functions, or the corresponding Green's functions. We have calculated the specific heat for a finite (but low) concentration of randomly distributed impurities, neglecting coherent scattering of spin waves from different impurity atoms. The results are discussed for weakly coupled impurities with spin magnitude equal to that of the host atoms. In this case there exist low-lying spin-wave resonances, leading to enhanced low-energy densities of states and specific-heat anomalies.
Spin-Wave Theory of Principal Susceptibilities of the Heisenberg Antiferromagnet
Authors: Kanamori Junjiro , Itoh Yoshimasa
Published: February 2004

The perpendicular susceptibility of MnF2 below TN(67.3°K) measured by Trapp and Stout decreases with increasing temperature at low temperatures and passes a minimum at 45°K. Calculation based on existing spin-wave theories of the susceptibility turns out to decrease too fast above 15°K compared with the data. It is pointed out through a new approach that the theories neglect an important contribution made by excitations of three and more magnons of finite wavenumber; they arise from the nondiagonal part of the spin-wave interaction with the perturbation of external field. The new contribution starts with a 1/S2 term in the 1/S expansion and increases with temperature. A calculation which takes into account whole terms of 1/S2 is in good agreement with the data up to 40°K. A calculation of the parallel susceptibility is also presented, which takes into account a shift of the spin-wave frequency caused by external field through the spin-wave interaction. It agrees with experiment even at 50°K.
The Hydrothermal Growth of Rare Earth Orthoferrites
Authors: Kolb E. D. , Wood D. L. , Laudise R. A.
Published: February 2004

Rare earth and yttrium orthoferrites, RFeO3, where R is Y, Yb, Ho, and Tb, have grown under hydrothermal conditions from 20 ml KOH and NaOH solutions at 375°C, ∼8000 psi, and Δt=30° in the 〈110〉 and 〈001〉 directions at rates as great as 6 mil/day. Ease of growth appears to be inversely related to the ionic radius of R3+. TbFeO3 has been deposited on TbAlO3 seeds under similar conditions. The grown orthoferrite has excellent optical properties (much less OH than Y3Fe5O12 grown under similar conditions) and is of good perfection as indicated by Schulz pictures and optical scattering measurements. The proton concentration correlates with the alkali and alkaline earth concentration in the grown crystal and is lower than in garnet because the distribution constants for alkalies and alkaline earths are lower in the orthoferrite lattice. Hydrothermal growth appears to be an advantageous technique for the growth of large orthoferrite crystals for magnetic studies and applications.
Metamagnetism in TbAlO3
Authors: Holmes L. , Sherwood R. , Van Uitert L. G.
Published: February 2004

Magnetization measurements have been carried out on single crystals of orthorhombic TbAlO3. The compound is antiferromagnetic below 3.8°K, where a sharp maximum in dχ/dT is observed. Single or double metamagnetic transitions, depending on the orientation of the applied field, are seen in the a-b plane for fields above 7.5 kOe at 1.5°K. The data are interpreted with four sublattice model in which magnetic moments of 9 μB per ion are fixed by the crystal field at angles ±36° from the a axis in the a-b plane.
Band Model of Ferromagnetic Thin Films
Authors: Szczeniowski S. , Wojtczak L.
Published: February 2004

Spontaneous magnetization of ferromagnetic thin films in the band model is calculated in a manner analogous to that used for bulk bodies. The thin film is considered as a superposition of monoatomic layers. To calculate the mean values of the numbers of electrons with oriented spins as traces of the electronic density matrix in Bloch representation the electron-energy spectrum is obtained. The assumptions made are that the periodicity conditions are satisfied in the plane of a layer and that the lack of a part of nearest neighbors for the boundary layers is taken into account in the direction perpendicular to the surface. The Fermi level is found in the same way. The behavior of the Curie temperature with the decrease of the film thickness is in a better accord with the experimental data than that obtained in the band theory but by means of another procedure. The important role of the surface states in the results obtained is shown. The spin-wave energy spectrum is discussed.
Coupled Ferromagnetic Films at Microwave Frequencies
Authors: Kooi C. F. , Waksmann B. , Buder R.
Published: February 2004

We have studied spin-wave resonances at 9.3 GHz in polycrystalline double layers of gadolinium on Permalloy and of triple layers of Permalloy on gadolinium on 50% cobalt-50% Permalloy, deposited at 5×10−9 Torr. The spin-wave resonances observed as a function of temperature between 100° and 400°K are those corresponding to coupled oscillators, whose frequencies, being directly proportional to their magnetizations, vary with temperature and cross at temperatures below 300°K. A strong interaction between the atomic layers of Permalloy, or of cobalt Permalloy and gadolinium, which are in contact, is indicated.
Spin-Wave Resonance in Epitaxial Fe–Ni Films and in Coupled Double Layers of Epitaxial Fe–Ni (Ferromagnetic)—Fe–Ni–Mn (Antiferromagnetic)
Authors: Waksmann B. , Massenet O. , Escudier P. , Kooi C. F.
Published: February 2004

We have studied spin-wave resonance at 9.3 GHz in epitaxial [111] permalloy films both alone and in contact with an antiferromagnetic Fe–Ni–Mn film. In the first case we find very weak higher-order spin-wave modes such that the intensity ratio I (higher order)/I (principal) <10−3. From this we conclude that the surface anisotropy constant is Ks<0.1 erg/cm2. In the second case, we observe ferro-antiferro-magnetic coupling across the interface. This is exhibited by the excitation of strong higher-order modes of odd symmetry, which disappear upon heating the Fe–Ni–Mn to its paramagnetic state. This is correlated with a shift of the hysteresis loop of the Permalloy, caused by the same interaction, which also disappears upon heating.
Magnetization of Some Single-Crystal Alloys of Rare Earth Metals
Authors: Bozorth R. M. , Gambino R. J. , Clark A. E.
Published: February 2004

Single crystals have been prepared of rare earth hexagonal solid solutions having 4f electron concentrations (10 to 11) near which the Stevens' factors change sign. Compositions are Ho3Er, HoEr, HoEr3, DyEr, and DyEr3. Moment measurements have been made on spheres in the a, b, and c directions (a⊥b⊥c) at 4.2° in fields to 140 kOe. From the data can be derived the cone angles in the ferromagnetic structures, the break-down of the cone structure (when H ⊥ c), the approach to saturation against anisotropy forces, and the field strengths at which fanning occurs when H ∥ a or ∥ b. The cone angles so determined are a linear function of the 4f electron concentration, in agreement with neutron-diffraction data. In Ho3Er the b direction is the easiest in the ab plane, as in Ho, whereas in HoEr and HoEr3 a is easier than b. When H ⊥ c in Ho3Er, the moment approaches saturation smoothly against anisotropy forces, whereas in others fanning occurs in a more complicated manner.
Exchange Interactions and the Spin-Wave Spectrum of Terbium
Authors: Goodings D. A.
Published: February 2004

The spin-wave spectrum of terbium obtained by Mo/ller and Houmann by inelastic neutron scattering at 90°K has been analyzed on the basis of a model consisting of Heisenberg exchange and axial and hexagonal crystal-field terms. By examining the conditions required to fit the calculated spin-wave energies to the experimental results at certain points of high symmetry in the Brillouin zone, values were found for the interatomic exchange constants Jij extending out to sixth-nearest neighbors. These provide a convenient interpolation scheme for obtaining the spin-wave energies throughout the Brillouin zone. The effect of interactions among spin waves arising from the strong crystal-field terms has also been studied. With the aid of an approximation often employed in spin-wave studies of magnetic anisotropy, it was found that the crystal-field parameters of the noninteracting case become multiplied by appropriate powers of the reduced magnetization. This accounts for about half of the temperature dependence of the spin-wave energies measured by Mo/ller and Houmann.
A Molecular-Field Solution for the Magnetic Properties of the Tb–Y Alloy System
Authors: Wollan E. O.
Published: February 2004

The magnetic properties of the heavy rare earth metals and their alloys are considered in terms of a molecular-field approximation. Because of the complications of the problem particular attention is given to the Tb–Y and the Gd–Y (in the ferromagnetic range) alloy systems which embody most of the variety of magnetic properties exhibited collectively by all the heavy rare earth systems. These include the initial transitions to a spiral or ferromagnetic type of order and the change of TN and the turn angles ω with the spin variable, and the change of ω with temperature and the approach to ferromagnetism when it occurs. The molecular-field solution obtained for the Tb–Y system gives a nearly quantitative fit to all the initial transition temperature data, and it accounts very well for the low-temperature properties of the Tb–Y alloys over the whole composition range. The molecular field is consistent with an interaction of the RKKY type but it includes effects arising from axial ratio and volume changes as previously reported and also an assumed anisotropic exchange term to account for the temperature-dependent behavior.
Thermal Conductivity of Dysprosium, Erbium, and Lutetium Single Crystals
Authors: Boys Donald W. , Legvold Sam
Published: February 2004

The thermal conductivity of dysprosium, erbium, and lutetium single crystals has been measured as a function of temperature from 5° to 300°K. Measurements were made along the 〈0001〉 (c axis) direction and along a basal plane direction. Electrical resistivity measurements were made on the same crystals so that the Lorenz functions could be calculated. The magnetic transitions in dysprosium and erbium produced a pronounced effect on the c-axis thermal conductivity. With increasing temperature a precipitous drop occurred at the ferromagnetic-antiferromagnetic transition. A sharp rise occurred as the antiferromagnetic-paramagnetic transition was approached. Only slight effects were seen in the basal plane measurements. The results were interpreted on the basis of magnetic superzone energy gaps. The thermal conductivity of nonmagnetic lutetium showed a temperature dependence similar to non-rare earth metals. The Lorenz-function values were considerably higher than expected for pure electronic heat transport in all cases except the c-axis lutetium crystal. This indicated that magnons and/or phonons contribute significantly to the heat transport. In the paramagnetic temperature region considerable anisotropy exists between the c-axis and basal-plane thermal conductivities. This is explained in terms of the anisotropic Fermi surfaces of the rare earths.
Effect of Hydrostatic Pressure on Magnetic Properties of Tb and Dy
Authors: Tatsumoto E. , Fujiwara H. , Fujii H. , Iwata N. , Okamoto T.
Published: February 2004

Measurements of weak-field ac susceptibilities have been made on Tb and Dy under hydrostatic pressures up to about 6 kbar, to investigate the pressure effects on the magnetic transition temperatures, and on the effective number of Bohr magnetons neff in the paramagnetic state. In addition, the compressibility necessary for the correction was measured. In the paramagnetic temperature range, the measured susceptibilities followed the Curie—Weiss law well, and neff decreased with pressure; (1/neff) (dneff/dp) is −9.6×10−7 bar−1 for Tb and −6.1×10−7 bar−1 for Dy. These negative values simply suggest the transition of electrons from the 4f shell to the conduction band with decreasing atomic distance. An anomaly in the compressibility was observed at the magnetic transition temperatures in agreement with the thermodynamical Ehrenfest relation for a second-order transition.
Magnetic and Crystallographic Properties of the System MnCr2S4–MnInCrS4
Authors: Darcy L. , Baltzer P. K. , Lopatin E.
Published: February 2004

The system MnInxCr2−xS4 was prepared and its crystallographic and magnetic properties studied in order to better understand the anomalous antiferromagnetism observed for MnInCrS4. X-ray studies showed that the compounds throughout the series formed spinel structures having unit cell parameters which increased as a linear function of composition from x=0 to x=1. The A sites are always occupied solely by the Mn2+ ions. Therefore, all the In3+ ions are substituted for Cr3+ ions on the B sites. The materials in the compositional range 0≤x<0.4 were found to be ferrimagnetic. For 0.4≤x≤1.0, the samples were antiferromagnetic. The magnetization, Curie and Néel temperatures, and the high-temperature susceptibilities were measured. The Curie constant and the Curie-Weiss θ were determined for each composition. As observed previously for MnCr2S4 and MnCr2O4, the Curie constants of this system were found to be lower than those which are theoretically expected. A discussion of the possible exchange interactions responsible for the antiferromagnetism of MnInCrS4 is presented.
Resistivity Anomaly in Conducting Europium Chalcogenides
Authors: von Molnar S. , Methfessel S.
Published: February 2004

Ferromagnetic and semiconducting Eu chalcogenides exhibit a negative magnetoresistance effect near the magnetic transition temperature (TC) which is much too large to be explained in terms of free electron spin disorder scattering. In order to define experimentally the parameters involved in this phenomenon we have measured the temperature dependence of resistance and magnetoresistance in the chalcogenide series using diamagnetic (e.g., La) or magnetic (e.g., Gd) trivalent RE ions. The trivalent ions donate an excess electron and change the character of the material from insulating to conducting. The resistivity increases for all dopants exponentially with decreasing temperature. This corresponds to an activation energy for carrier mobility which depends more on the carrier concentration than on the character of the anions and dopants. The resistivity rise is followed by a decrease below TC. The resulting peak is dependent on the magnetic order and decreases markedly with applied field strength and increasing electron concentration. Some possibilities for describing strong interactions between conduction electrons and localized spins will be discussed.