bluuebunny/crossref_metadata_2025 huggingface dataset:
∂M/∂T Measurements in Ferromagnetic Pd(Fe)
Authors: Oder R. R.
Published: February 2004

An ac temperature technique is employed in direct measurements of ∂M/∂T for a 1%(at.) Fe in Pd sample whose Curie temperature is 40–50°K. Temperatures between 1.5° and 4.2°K and fields up to 30 kG are employed. The temperature and magnetic field dependences of ∂M/∂T are analyzed to obtain the spin-wave exchange stiffness coefficient D. The temperature dependence of ∂M/∂T extrapolated to H = 0 gives D≃550±34(°K − Å2) in good agreement with independent measurements on alloys of similar iron concentration. A point by point analysis of ∂M/∂T in H≳5 kG, however, yields a strong (T, H) dependence for D and suggests that a simple spin-wave description is not applicable. If we separate a spin-wave component with D=550(°K − Å2) and g = 2 from the measurement, the remaining component of ∂M/∂T is not neglibigle and has a strong (T, H) dependence even at 30 kG.
Magnetic Susceptibility of Dilute Pd(Rh) and Pd(Ag) Alloys
Authors: Doclo R. , Foner S. , Narath A.
Published: February 2004

High-resolution measurements of magnetic susceptibility χ of dilute alloys of Pd1−aRha (a = 0.001, 0.075, 0.015, 0.03, and 0.07) and Pd1−aAga (a = 0.001, 0.005, 0.012, 0.023, and 0.025) are presented for temperatures from 4.2° to 300°K and for fields up to 55 kG. The general features show a broad maximum of χ(4.2°K) between 0.03 Rh and 0.07 Rh for Pd(Rh), then a rapid decrease in χ(4.2°K) to the value of χ for pure Pd, and a further decrease of χ with increasing Ag content. Small effects of Fe impurities are noted and briefly discussed. The very rapid variation of χ versus Rh content is consistent with earlier measurements. The maximum of χ(T), observed at about 80°K for pure Pd, moves toward 0°K as Rh content increases to 0.03, but the maximum in χ(T) is almost independent of increasing Ag content. The measured susceptibility and electronic specific heat are used to estimate the Stoner exchange enhancement in Pd and Pd0.95Rh0.05 and the results are in reasonable agreement with independent high-field results discussed elsewhere. The relevance of recent high-resolution band calculations is briefly discussed. Finally, for simplicity, a rigid band model and the χ(4.2°K) data for the Pd(Rh) and Pd(Ag) alloys are employed to construct the density of states versus energy for Pd.
Observation of Field-Dependent Susceptibility in Pd(Rh) Alloys at High Magnetic Fields
Authors: Foner S. , McNiff E. J.
Published: February 2004

Earlier high-field magnetic moment measurements in Pd(Rh) alloys have been extended to 200 kG at 4.2°K. A B3 field dependence is clearly resolved at fields above 150 kG and is consistent with previous deviations barely detected at 150 kG for alloys near the maximum susceptibility (χmax≃1.8 χPd for 3 to 7% Rh in Pd). Only data to 150 kG were presented here. These results combined with recent high-resolution band calculations permit evaluation of the exchange enhancement D = 1/[1 − N(0)V] in the Pd(Rh) alloys with a minimum number of parameters, and place further limits on D in Pd.1 A field dependence has also been detected at 77°K for some of these alloys. These results were compared with the temperature and concentration dependence of the magnetic susceptibility of Pd(Rh) alloys and related electronic specific-heat data. Present estimates of band calculations were compared with estimates of exchange enhancement. The detailed results of these experiments will be published elsewhere.
Susceptibility of Palladium Metal
Authors: Misetich A. A. , Watson R. E.
Published: February 2004

The unenhanced paramagnetic spin susceptibility, χ0(q, ω), has been obtained for Pd metal. Band-structure effects have been accounted for utilizing the pseudopotential scheme of Hodges, Ehrenreich, and Lang.1 The contribution of the fifth band (which is responsible for the ``jungle-gym'' Fermi surface) is inspected and compared with the total susceptibility. Spin-orbit quenching of band spin character and interband terms significantly affect the magnetic response.
Electron Tunneling into Films of Ni, Pd, and NiPd Alloys
Authors: Rowell J. M.
Published: February 2004

Film tunnel junctions Al–I–M (M is an evaporated film of Ni, Pd, a series of NiPd alloys, Fe, and Gd) have been investigated by measuring the I-V characteristic and G(dI/dV) vs V over the range 0–2 V. The aim of the investigation was to see if excitation of spin waves by the tunneling electron is observable in the tunneling characteristic. The result to data is that the rather complex results cannot be interpreted using such simple arguments. At voltages >500 mV the G vs V plot is noticeably asymmetrical about V=0 for Ni and the alloys, the conductance being smaller for the bias corresponding to electrons tunneling into nickel. This may be a weak reflection of the low density of d-band states well above the Fermi level compared to the high density near and below the Fermi level. At low voltages in Ni a peak in conductance is observed which is asymmetrical about V=0; its width scales approximately with the Curie temperature in the NiPd alloys. Drastic changes at both high and low voltages were obtained when Ni was evaporated in an applied magnetic field. Interesting but very small features, which are observed in G vs V for Al–I–Pd junctions, are presented.
Domain Wall Width Measurement in Cobalt Films by Lorentz Microscopy
Authors: Suzuki T. , Wilts C. H.
Published: February 2004

A recent report by Suzuki et al. indicates that the domain wall width of 76% Ni–Fe alloy films with thickness 200 to 1800 Å is much wider than those calculated on the basis of the simple wall models. It was suggested that theoretical calculations might be in serious error due to gross overestimation of the magnetostatic stray field energy. In the present work, similar measurements of domain wall width have been made by the defocused mode of Lorentz microscopy for evaporated polycrystalline Co films with thickness from 200 to 1500 Å. Only the intensity profiles of the divergent wall images were taken into account, and a small out-of-focus distance (1.1 mm) was used, so that classical interpretation of the image would be allowable. The present study indicates (1) the wall width in Co is much greater than predicted by present theories with minimum value of 3000 Å at 500-Å film thickness and (2) the change in wall width with film thickness is much less than in the case of Ni–Fe films.
Bloch Wall Motion in Ferromagnetic Films Excited by Fast-Rising Hard-Axis Pulses
Authors: Bourne H. C. , Kusuda T. , Lin C. H.
Published: February 2004

Bloch wall motion along the hard direction of Permalloy films due to fast-rising hard-axis pulses is studied. Experimentally, it is found that for a given value of ḣ⊥, the time rate of change of the applied field, wall motion occurs only for fields in excess of a certain minimum or threshold value hth. Two distinct regions exist in this characteristic threshold curve. When ḣ⊥ is large hth is constant and when ḣ⊥ is small hth is a linear function of ḣ⊥. Above hth the wall displacement or movement per pulse increases linearly with driving field until this field approaches the value where the effect of Néel lines becomes important. A theoretical model taking account of both Bloch-Néel wall transitions and the gyromagnetic effect of the spins in the walls and in the surrounding domains is proposed to account for the experimental results. The calculation assumes a reasonable wall structure and that Bloch walls are allowed to move freely except for a coercive force, i.e., a possibly more complicated interaction between the Bloch walls and Néel lines is neglected. With appropriate values of the constants and film parameters, the computation gives very good agreement with experimental results.
Investigation of Free Pole and Magnetization Distributions in Micron-Sized Ferromagnetic Whiskers
Authors: Lipschutz M. R. , Rodgers J. M.
Published: February 2004

Using Lorentz microscopy, measurements of magnetic field integrals (of the form ∫Bxdz) have been made in the neighborhood of small ferromagnetic whiskers. The whiskers were single crystals, rectangular in cross section, of Ni, Ni–Co, and Fe, and had cross-sections of from 3–0.5 μ. Simple models of the magnetization distribution and resulting free poles at the whisker tips were postulated and the stray fields due to each model were calculated. Comparison of measured and calculated results has enabled us to make some observations about the nature of departures from uniform magnetization in these small crystals. At the tips of the whiskers, spreading of the free poles from the terminating faces to the prism faces was always observed. There is evidence for closure domains in whiskers as small as 1 μ across. The component of magnetization normal to the whisker axis was always found to be substantially reduced below Ms close to the whisker tip. Quantitative comparisons are restricted in accuracy by the limit of validity of classical analysis of Lorentz microscopy. Errors in measurement range from 10% in best cases to cases where order-of-magnitude estimates only can be made.
Neutron Scattering Studies of Thin-Film Magnetization
Authors: Spooner S. , Wrege D. E. , Livesay B. R.
Published: February 2004

The total external reflection of monochromatic neutrons has been measured on cobalt and iron thin films evaporated on glass substrates. Neutron scattering from the thin-film neutron mirrors was studied with a magnetic field applied parallel to the plane of the film. The changes in the cobalt mirror reflectivity curves with field strength can be correlated with local spin direction variations brought about by perpendicular anisotropy. By contrast, iron films show no field dependence which is consistent with measured planar coercive fields and the absence of a strong perpendicular anisotropy.
Nature of the Anomalous Resonance in the Ferromagnet K2CuCl4·2H2O
Authors: King A. R. , Dweck J.
Published: February 2004

An anomalous magnetic resonance observed in ferromagnetic K2CuCl4·2H2O with the microwave and dc magnetic fields parallel, was tentatively interpreted as a forbidden hyperfine resonance involving simultaneous electron and nuclear spin flips. A theory explaining many of the observed features has been proposed. The theory depended, however, on the somewhat implausible assumption that the FMR relaxation time was dominanted by a term proportional to the third power of the magnon wavevector. In order to check this assumption, and to re-examine the unusual frequency dependence reported for the ratio of the forbidden to allowed intensities (ν4±1), extensive measurements were made at several microwave frequencies on samples of varying shapes and sizes. We found, surprisingly, that at a given frequency the intensity ratio was a function of the sample size, with the intensity of the forbidden line changing much more rapidly than the sample volume. In addition, the spacing between the forbidden and allowed lines, predicted to be a measure of the hyperfine interaction, was found to be a function of the sample shape. From these findings and additional evidence presented, it is concluded that the data can be more fully explained on the assumption that the anomalous forbidden resonance is in fact the (2, 1, 0) magnetostatic mode.
Infrared Faraday Rotation in Ferrites
Authors: Zanmarchi G. , Bongers P. F.
Published: February 2004

The absorption and Faraday rotation of crystals of the spinel ferrites Li0.5Fe2.5O4, NiFe2O4, and MgFe2O4 and of the hexagonal ferrites BaFe12O19 and Ba2Zn2Fe12O22 have been measured at 300°K between 1 and 8 μ. The rotation consists of a nearly wavelength-independent term θM arising from the magnetic resonance and of a strongly dispersive term θd originating mainly from electronic transitions of Fe3+. As in YIG, the Fe3+ ions in tetrahedral and octahedral coordination contribute differently to θd. The experimental curves of the Faraday rotation of YIG, Li0.5Fe2.5O4 and MgFe2O4 can be described rather accurately with the same constants for the rotary powers of Fe3+ in tetrahedra and of Fe3+ in octahedra. Also the contribution of Ni2+ to θd in NiFe2O4 at 80°, 300°, and 373°K has been determined.
Photo-Induced Magnetic-Susceptibility Changes in Mn-Doped ZnS Crystals
Authors: Andreadakis N. C. , Williams Ferd
Published: February 2004

The magnetic susceptibilities χ of several Zn1−fMnfS cyrstals (0.004<f<0.4) have been measured (77°−400°K). From the temperature dependence of χ, a large negative Weiss constant is found, consistent with indirect exchange between neighboring Mn atoms. Measurements have also been made with the mixed crystals subjected simultaneously to optical radiation from a 450 W Xenon source. A decrease Δχe not ascribable to heating and attributed to electronic excitation is observed and becomes greater with increasing concentration of the Mn. This effect occurs only for wavelengths below 5400 Å. Multiplicity changes of the Mn, which accompany the optical excitation, are believed to be the main factor to which this photo-induced effect can be attributed. The lifetime of the excited states, as obtained from the decay of the susceptibility change and separately from the magnitude of the change combined with the incident photon flux, is longer than that predicted from the luminescence. We conclude that the Mn atoms responsible for Δχe are not the isolated Mn which contribute to luminescence but are more probably interacting Mn atoms. The analysis of the data is confirmed by a similar study of ZnS:Cr crystals, where an enhancement of paramagnetism is observed, in agreement with spin resonance and optical studies.
Transverse Kerr Magneto-Optic Effect and Optical Properties of Transition-Rare-Earth Alloys
Authors: Ferguson P. E. , Romagnoli R. J.
Published: February 2004

The transverse Kerr magneto-optic effect and optical properties have been measured in Fe and GdFe5 for the optical wavelength range of 0.35 to 1.0 μ. The metals were evaporated onto the base of glass prisms at 1×10−9 mm Hg vacuum. The properties were measured at two angles of incidence. The maximum transverse Kerr effect at a 60° angle of incidence is 5.55% for Fe and occurs at 0.7 μ. The maximum transverse Kerr effect at a 60° angle of incidence for GdFe5 is 1.9% and occurs at 0.65 μ. From the experimental measurements, the spectral distribution of the diagonal and off-diagonal components of the dielectric tensor were determined. Structure indicative of optical transitions was found in the absorptive parts of both components for GdFe5 but was absent for Fe. A discussion is given relating this structure to the conduction electrons in Gd.
Spin Reorientation in Mixed Samarium-Dysprosium Orthoferrites
Authors: Pierce R. D. , Wolfe R. , Van Uitert L. G.
Published: February 2004

Flux-grown single crystals of solid solutions of the rare-earth orthoferrites SmFeO3 and DyFeO3 have been studied from 2° to 500°K. At elevated temperatures all compositions have iron moments antiferromagnetically aligned along [100] and slightly canted toward [001]. In SmFeO3 and all but the Dy-rich mixtures a transition (a-c reorientation) to antiferromagnetic alignment along [001] with canting toward [100] occurs at temperatures decreasing from about 450°K for SmFeO3 to below 30°K at 80 at.% Dy. DyFeO3 and the Dy-rich mixtures undergo an a-b reorientation to antiferromagnetic alignment of the iron spins along [010] with no overt canting. The temperature at which the ferromagnetic moment vanishes decreases from about 30°K for DyFeO3 to about 15°K at 15 at.% Sm. Both types of transition occur gradually over well defined temperature intervals, and are believed to be associated with temperature variation of the second order anisotropy coefficients of the iron spins. The phenomenological theories proposed to describe the a-c reorientation are extended to include the a-b reorientation also, and stability conditions for the three spin arrangements are deduced.
Chemical and Magnetic Study of Layered Strontium Lanthanum Manganate Structures
Authors: MacChesney J. B. , Potter J. F. , Sherwood R. C.
Published: February 2004

Previously published work describes the magnetic properties of the calcium manganate (IV) phases having tetragonal structures consisting of perovskite layers separated along the 001 direction by non-magnetic layers. Anomalous magnetic behavior exhibited by these was attributed to a persistence of short-range, two-dimensional order above the Néel temperature. In the present study, phases having similar structures were prepared having the general composition SrO[(Sr1−xLaxMnx3+ Mn1−x4+)O3]y, where y = 1, 2, 3 corresponding to single, double, or triple perovskite layers. Magnetization measurements over the range 1.4°–300°K showed that ferromagnetism expected of the Mn3+-O-Mn4+ interaction was achieved. At low temperatures, remanent moments as high as 3.2 μB/MN ion were observed. With increasing temperatures, these decreased in an anomalous manner. At intermediate temperatures there was a sharp decrease-like that expected at a Curie temperature. However, at its termination a significant moment remained. This behavior is thought to be related to the two-dimensional nature of these layered structures and may depend upon a long spin-correlation length which is expected of such structures.
Effect of Y and Bi Substitution on Spin Reorientation and Optical Absorption in ErFeO3
Authors: Wood D. L. , Remeika J. P. , Holmes L. M. , Gyorgy E. M.
Published: February 2004

The effects of Y and Bi substitutions on spin reorientation in ErFeO3 have been studied through optical absorption and magnetization measurements. Below the reorientation temperature Tr, optical absorption lines from low-lying Er3+ levels show splittings of several cm−1. The splittings decrease above Tr. With additions of Y or Bi, Tr decreases linearly from 102°K for ErFeO3. For crystals of Er1−xYxFeO3, 0≤x≤0.6, and Er1−xBixFeO3, 0≤x≤0.03, dTr/dx≃ −120° and −840°K, respectively. The internal energy for the Er3+ ions above and below Tr may be calculated from the optical data. The change in Er3+ energy at Tr in ErFeO3 is comparable with the energy required to move the Fe3+ spins from the a axis to the c axis in YFeO3. The behavior for substitutions of Y3+ may be understood as a simple dilution effect, whereas for Bi, having additional electrons outside the closed shell, the behavior is anomalous.
Spin Axis Rotation in Hematite
Authors: Flanders P. J.
Published: February 2004

Detailed measurements of m⊥, magnetization normal to the C axis, of pure synthetic hematite crystals as a function of H⊥ (0–20 kOe) and T (210°–273°K) indicate a 3 or more step process for rotation of the spin axis from the [111] AF or C axis, θ=0°, into the (111) WF plane, θ=90°, as H⊥ and/or T is increased. The rotation starts from θ=0° with a smooth change in θ, increasing in rate for higher T; this is followed by an abrupt jump, decreasing in amplitude for lower T, and finally as θ approaches 90° there is a smaller more gradual jump.
Biquadratic Exchange and Quadrupolar Ordering
Authors: Blume M. , Hsieh Y. Y.
Published: February 2004

We have studied in the molecular field approximation the statistical mechanical properties of the Hamiltonian H= ∑ mnJ(m−n)Sm·Sn− ∑ mnj(m−n)(Sm·Sn)2,for spin one ions. Other authors have already shown that the biquadratic term can cause the magnetic transition to become first order.1 Closer examination shows that one must consider, in addition to the usual long-range order parameter 〈Sz〉=M, the possibility of ``quadrupolar'' ordering in the independent parameter Q=〈Sz2〉−23. If the biquadratic exchange is sufficiently large the quadrupolar ordering will appear at a temperature TQ which is different from that at which dipolar ordering M appears, so that two separate phase transitions are found. The phase diagram is discussed as a function of the ratio of bilinear to biquadratic exchange, and the interplay between quadrupolar and dipolar ordering is considered.
Crystallographic and Magnetic Properties of Solid Solutions of the Phosphides M2P, M = Cr, Mn, Fe, Co, and Ni
Authors: Fruchart R. , Roger A. , Senateur J. P.
Published: February 2004

Several phosphides with the chemical formula M2P crystallize in a hexagonal structure: Mn2P, Fe2P, and Ni2P. The structure of Co2P is orthorhombic. These two structures are related via an identical elementary subcell consisting of a tetrahedral-site and a pyramidal-site M-atom pair. This investigation shows that solid solutions between two hexagonal end members, such as Fe2P or Ni2P with Mn2P, may exhibit orthorhombic structures at intermediate compositions. Fe2P shows complete solid solubility with Ni2P. Curiously, hexagonal and orthorhombic symmetries alternate with decreasing number of 3d electrons, Ni2P(hex) -Co2P(ortho) -Fe2P(hex) -FeMnP(ortho) -Mn2P(hex). Lattice parameter variations with composition and Mössbauer studies reveal atomic ordering in the mixed systems, Mn and Cr substituting for pyramidal-site iron in FeMnP and FeCrP while Ni and Co substitute preferentially for tetrahedralsite iron in FeCoP and FeNiP. While neither Co2P nor Mn2P are ferromagnetic, intermediate phases are, the Curie temperature and magnetization reaching the maximum values 310°C and 3.03 μB/molecule for MnCoP. Metamagnetism appears for a range of compositions about (Mn0.7Co0.3)2P. The Co2P-Fe2P system is also ferromagnetic with a maximum Curie temperature near FeCoP. This system shows a sharp discontinuity in magnetization, but only a small discontinuity in Curie temperature, across the phase transition. Sensitivity to stoichiometry in the magnetization of Fe2-εP is attributed to an electron/atom ratio near that for the appearance of metamagnetism.
Experimental System for Studying Itinerant d-Electron Ferromagnetism
Authors: Cloud W. H. , Jarrett H. S. , Bouchard R. J. , Butler S. R. , Frederick C. G. , Gillson J. L.
Published: February 2004

Typical theoretical models of an itinerate electron ferromagnet consist of a single narrow band, but experimental data have been obtained primarily from the ferrous metals whose behavior is more complex because of overlapping broad s bands and narrow d bands. Transition metal sulfides of the cubic pyrite structure appear to have a doubly degenerate conduction band arising from atomic d orbitals of eg symmetry without overlapping s bands.1 The full band would contain four electrons per metal atom. Metallic conductivity is observed, except for the empty FeS2, exactly half-filled NiS2, and filled ZnS2 bands. The electron concentration, n, can be varied from zero to one electron per metal ion in the Fe1−xCoxS2 systems (n=x) and can be extended to higher values by use of the Ni1−yCoyS2 system (n=2−y).2 In these systems the effective moment obtained from paramagnetic susceptibility measurements is that of the isolated cobalt and nickel ions. It appears, therefore, that a model based on strong electron-electron correlation at the metal-ion sites would be applicable to these pyrite compounds. Such a model for a nondegenerate band has been given extensive theoretical treatment by Hubbard.3,4 Solutions for the degenerate case have also been indicated,5 and calculations of the Hubbard model for the two-fold degenerate band are currently being carried out.6 Significant features of the pyrite system which should be qualitatively explained by a theoretical model are:

(1) Ferromagnetism exists down to very low electron concentrations. A plot of Curie temperature vs n goes to zero at n≈0.05. For n<0.05 the electrical resistivity vs temperature curves exhibit shallow minima at 20°K indicative of localized states. The loss of ferromagnetism below n≈0.05 may thus be due to loss of itinerancy and not to a critical electron concentration.

(2) The ferromagnetic moment, μs, is equal to one Bohr magneton per electron over a wide range of electron concentration 0.15 ≤n≤0.95. Ferromagnetic behavior is observed for 0.05<n<0.15, but the curve of magnetization vs applied field does not saturate, and μs appears to be less than n.

(3) In Ni1−yCoyS2 (n>1) the saturation magnetization, Curie temperature, and Weiss constant decrease rapidly with Ni concentration, indicating strong antiferromagnetic Ni–Co interactions. Ferromagnetism disappears for n>1.1. This would be expected if strong correlations at the Ni sites dominate.

(4) In Fe1−xCoxS2, μs decreases from 0.95 to 0.89 as n is increased from 0.95 to 1.0, but the effective moment in the paramagnetic state continues to increase linearly with cobalt concentration. Neutron-diffraction studies indicate a collinear spin state.7 A localized moment model which seems to explain much of the magnetic behavior cannot account for this decrease in μs with increasing Co concentration.
New Permanent Magnet Materials Containing Rare-Earth Metals
Authors: Nesbitt E. A.
Published: February 2004

Previously, most basic magnetic properties of the hexagonal intermetallic compounds involving the rare-earth (RE) and 3-d transition elements were reported in the literature. In addition, some of these compounds were investigated by various workers for permanent magnet properties by grinding ingots of the compounds into very fine powder. Recently, we discovered new permanent magnet materials which were based mostly on the hexagonal intermetallic compounds Co5RE. Replacement of cobalt with copper in some of these rare-earth compounds resulted in solid materials having substantial permanent magnet properties. An outstanding feature of these alloys is their high value of intrinsic coercive force. High-energy product alloys were found in the Co5−xCuxSm and Co5−xCoxCe systems with iron additions. Small pieces of Co3.5Fe0.4Cu1.35Sm exhibited BR=6450 G, BHC=4000 Oe, and (BH)m=9×105 G Oe. Also, small pieces of Co3.5Fe0.5CuCe exhibited BR=5100 G, BHC=3500 Oe, and (BH)m=5.2×106 G Oe. In general, these alloys are prepared by inert-electrode arc melting, resulting in a relatively rapid freezing from the molten state. The samples are then given a low-temperature anneal. Cross sections of the specimens show microstructural directionality which is a result of freezing in a direction normal to the cold surface of the hearth. These directional effects appear to be associated with the directional magnetic properties that are important because of the high crystal anisotropy of the alloys. When a specimen of the composition Co3.45Fe0.25Cu1.35Sm was annealed at 1100°C and examined by optical-microscope and electron-probe techniques, particles rich in Co5Sm were observed to be embedded in a Cu5Sm-rich matrix. The iron appeared largely in the cobalt-rich phase. The cobalt-rich particles were approximately 2.5×10 μ. Further work is in progress to improve directional properties and thus further improve the magnetic properties.
Preparation of α-Manganese Single Crystals and Their Physical Properties
Authors: Kunitomi N. , Yamada T. , Nakai Y. , Fujii Y.
Published: February 2004

Single crystals of α-manganese were grown by the distillation method. These weigh about 300 mg. Crystallographic parameters which define the positions of manganese atoms have been determined by x-ray diffraction. The magnetic structure was determined with better accuracy than that determined by the powder method. It is concluded that the collinear model cannot explain the experimental data very well and therefore a noncollinear model is proposed, in which the magnetic moments in sites I and II are much larger than those in sites III and IV.
Divergences of the Magnetic Properties of CrBr3 near the Critical Point
Authors: Ho John T. , Litster J. D.
Published: February 2004

We have used the Faraday effect to study the magnetic properties of chromium tribromide along several isotherms near the critical point. Using external fields up to 1000 G, we have measured the reduced magnetization σ from 0<σ<0.25 along 21 isotherms from 400 mdeg K below Tc to 2.2 deg above Tc. For T>Tc the susceptibility along the critical isochore (σ=0) diverges like (T/Tc-1)−γ with γ=1.215±0.02. Along the critical isotherm σ∼H1/δ with δ=4.28±0.1, and along the coexistence curve, σ goes to zero as (1-T/Tc)β with β=0.368±0.005. The value of δ agrees with the scaling law prediction δ=γ/β+1. Our data are consistent with an equation of state of the form proposed by Widom: viz. H=σΦ[(T/Tc-1), σ1/β], where Φ is a homogeneous function of degree γ of its arguments. This experiment is the first confirmation of the scaling laws for an insulating ferromagnet.
Critical Indices for a System of Spins of Arbitrary Dimensionality Situated on a Lattice of Arbitrary Dimensionality
Authors: Stanley H. E.
Published: February 2004

A model Hamiltonian H=−JΣ(ij)Si·Sj is considered, where Si are isotropically interacting D-dimensional classical spins. Thus the S=½ Ising, classical planar, and classical Heisenberg models are obtained if D=1, 2, and 3, respectively; moreover, as D→∞, the free energy corresponding to H approaches the free energy of the exactly-soluble spherical model of Berlin and Kac. H can also be solved exactly providing the lattice dimensionality d=1. For lattices of higher dimensionality, we must resort to approximation techniques; hence we calculate high-temperature expansions for arbitrary D and d of the susceptibility, internal energy (specific heat), and the second moment. (With a few exceptions, such expansions have been available in the past only for D=1, 3 and d=1–3.) Somewhat smoother series to facilitate more reliable extrapolation are then obtained by re-expanding all of the series in terms of the nearest-neighbor spin correlation function for a one-dimensional lattice. We then study the dependence on spin and lattice dimensionality of the critical temperatures Tc(D, d) and the critical exponents γ(D, d) [(χ∼(T-Tc)−γ], α(D, d) [C∼(T-Tc)−α], ν(D, d) [inverse correlation range κ∼(T-Tc)−v]. These ``critical properties'' are all found to vary monotonically (and in most cases, smoothly) with D and d. The assumptions behind making critical phenomena predictions from a dozen or so terms (of an infinite series!) are given increased plausibility by the fact that we have extended the various expansions for the spherical model to 80–100 terms and find that none of the predictions based upon ∼10 terms is changed [e.g., Tc(∞, 2)=0, γ(∞, 3)=2, etc.].
Nuclear Resonance Studies of Critical Fluctuations in Ferrous Fluoride
Authors: Gottlieb A. , Dupré F. , Heller P.
Published: February 2004

Near and above the Néel point in FeF2, we observe an anomaly in the F19 NMR line width about six times stronger than the corresponding anomaly1 in MnF2. In a field H0=14 kG applied along the c (antiferromagnetic) axis, the line width δνc increases by a factor of 20 upon cooling from Tc+5°K to Tc+0.04°K. An empirical formula is δνc = Acε−nc, with nc = 0.69±0.05, Ac = 35 kHz, and ε = (T-Tc)/Tc. With the applied field along the a direction we find δνa = Aaε−na, with na = 0.50±0.05, and Aa = 45 kHz. These relations hold for 10−3<ε<10−1. At Tc+0.1°K, δνc/δνa = 2.5±0.4. From the theory of Kubo and Tomita,2 the line width may be expressed3 as δν(T) ∝ Σk[Szz(k, 0) + ½Sxx(k, ωL) + ½ Syy(k, ωL)], where S(k, ω) is the diffuse part of the scattering function, z is along H0, and ωL = γ19 H0=3.8×108 sec−1. Assume that the line width is due mostly to modes for which |k-k0|⪝ν1, where k0 is the antiferromagnetic mode and κ1 ∝ εν is the inverse correlation range. Then the temperature dependence of the relaxation rate Γc(k0) of the staggered mode can be deduced from our data. We find Γc(k0) ∝ κ1s where s = 2.1±0.25, when we assume ν=0.67. If we take ν=0.5 as was reported4 for MnF2, we find s=2.5±0.3. It is then possible that Scc(k0, 0) is significantly larger than Scc(k0, ωL) near Tc. In fact, this must be the case in order to explain the observed value of δνc/δνa within the framework of the Kubo-Tomita expression. Alternatively some additional source of broadening may be present. One possibility is the indirect nuclear-nuclear interaction5 as enhanced by the large spin susceptibilities in the critical region. A detailed account of our experiments will be published elsewhere.
Fermi Surfaces and Generalized Susceptibilities for Neodymium and Praseodymium
Authors: Fleming G. S. , Liu S. H. , Loucks T. L.
Published: February 2004

Using the energy bands which we have previously calculated for double hexagonal-close-packed neodymium and praseodymium, we have calculated the generalized susceptibilities, χ(q), along several directions in the ΓMLA plane of the Brillouin zone. The bands were calculated using the relativistic APW method and the procedures used in calculating the susceptibility were similar to those employed by Evenson and Liu for their work on the heavy rare-earth metals. There are several relatively flat pieces of Fermi surface nearly parallel to the KMLH plane which nest into each other. This nesting produces a peak in the susceptibility curve along ΓM at approximately 0.13b1. This wavevector corresponds to the b-axis periodicity of the magnetic ordering for these two elements determined by neutron diffraction measurements.
Magnetic Properties of CoSexS2−x
Authors: Johnson Vancliff , Wold Aaron
Published: February 2004

Single crystals of the solid solutions CoSexS2−x, 0≤x≤0.25 and a well homogenized powder sample with x=0.30 have been studied by magnetization and susceptibility measurements between 4.2° and 300°K. Over the range 0≤x≤0.20 the Curie temperature decreases linearly, while the moment remains constant at 0.88±0.02 μB/Co atom at 4.2°K. For compositions with x>0.20 both TC and the moment, σ4.2°,10kOe, decrease rapidly, and at x=0.30 no well-defined ferromagnetic transition temperature was observed down to 4.2°K. The limit of existence of ferromagnetism in this system at 0°K appears to be close to x=0.20. The reduced moment observed for apparently homogeneous preparations with x=0.22 and 0.25 is suggestive of a noncollinear spin arrangement at low temperatures for these compositions. For 0.20≤x≤0.30 a minimum is observed in the χM−1, T curve, in some cases above a well defined lower ferromagnetic transition temperature.
Magnetism of a Strongly Correlated s-Band
Authors: Tahir-Kheli Raza A. , Jarrett H. S.
Published: February 2004

To study the translationally uniform magnetic solutions of a strongly correlated, narrow s-band, improved random-phase approximations and moment-conserving decoupling approximations are developed.
Effects of Imperfect Fermi-Surface Nesting on Properties of Itinerant Electron Antiferromagnet
Authors: Liu S. H.
Published: February 2004

In the itinerant electron theory of antiferromagnetism one postulates that the Fermi surface of the metal has two sheets which can be brought into point-by-point fit by a translation in the crystal momentum space. The wave vector of the spin-density wave corresponds to the amount of translation. In real metals the pieces of Fermi surface do not nest exactly. We have explored some consequences of imperfect Fermi-surface nesting on the basis of a one-band model. We find that there exists a minimum value of the interaction strength below which antiferromagnetism does not occur; and when antiferromagnetism does occur the ratio 2Δ0/kTN is always greater than 3.5. These results were also obtained in more elaborate model of Kimball and Falicov. However, in contrast to Kimball and Falicov, we find a second-order phase transition at TN. This is perhaps due to the oversimplification of our model. If the exchange interaction is not strong enough to stabilize antiferromagnetism, the result is a nearly antiferromagnetic material in which a new type of paramagnon may be found. Some physical properties of these virtual modes are discussed.
Experimental Determination of the Fermi Radius, Velocity, and g Factor in Pd and Pt
Authors: Windmiller L. R. , Ketterson J. B. , Hornfeldt S.
Published: February 2004

Modern techniques for the measurement of the de Haas-van Alphen effect now allow many properties of the Fermi surface to be determined with great precision. In addition to the traditional measurements of extremal cross-sectional areas, measurements of effective cyclotron mass, orbital g factor, and Dingle temperature have been made in palladium and platinum. From these measurements the Fermi radii, Fermi velocities, and density of states are calculated. Comparison of these data with recent band structure calculations gives estimates of the magnitude of the many-body corrections. Such corrections include not only the effects of electron-electron and electron-phonon interactions but also the effects of the exchange enhancement due to short-lived spin fluctuations.
Magnetic Properties of FeTi2S4
Authors: Morris Bernard L. , Johnson Vancliff , Plovnick Ross H. , Wold Aaron
Published: February 2004

The low-temperature magnetic properties of FeTi2S4, a metallic compound with the Cr3S4 structure, are strongly dependent on the value of the magnetic field Hc in which the compound is cooled to 4.2°K. For Hc=8000 Oe the hysteresis loop at 4.2°K is strongly displaced along the σ axis, with a coercive field greater than 10 000 Oe. For Hc=30 Oe the loop is only slightly displaced along the σ axis, with a coercive field of about 100 Oe. For Hc=30 Oe the temperature dependence of σ is as follows: at 4.2°K it is fairly small, but increases with increasing temperature, reaching a maximum at a temperature Tm. Above Tm, σ slowly decreases with increasing temperature. If Hc is approximately 1000 Oe or more, the magnetization is fairly large at 4.2°K, but decreases monotonically as the temperature is raised, meeting the Hc=30 Oe value at a temperature slightly above Tm. The exact value of σ at any temperature depends on Hc. It is suggested that the observed properties are caused by localized ferromagnetic domains of iron ions present in an antiferromagnetic host lattice. These interactions are consistent with the theory of exchange anisotropy.
Theoretical Coercive Field for an Interacting Anisotropic Dipole Pair of Arbitrary Bond Angle
Authors: Bertram H. N. , Mallinson J. C.
Published: February 2004

The coercive force for a pair of identical interacting uniaxially anisotropic dipoles of arbitrary bond angle is calculated for the case of the applied field coincident with the common anisotropy axis. The technique used is to examine the shape of the total energy surface about the equilibrium state as the applied field is lowered from an arbitrarily large initial value and subsequently reversed. At the nucleating field, the energy surface curvature disappears in some eigendirection which can be related to the initial mode of switching. An examination of the stability of available states to which the system may switch shows that for this simple model the nucleating field is identical to the coercive field. The results, which coincide with a planar solution, are expressed as coercive field versus pair bond angle for various pair separations. They differ significantly from the usual ``local field'' approach in which the magnetization is presumed to remain parallel to the applied field until nucleation. This exact collective treatment leads to a nonlinear reduction of coercive force with packing fraction for an ensemble of independent randomly oriented pairs, whereas the ``local field'' approach yields a coercivity independent of packing.
Study of α1 and α2 Phases in Ticonal 600 Alloy
Authors: Drapier J. M. , Coutsouradis D. , Habraken L. , Bronner C. , Haberer J. P. , Sauze J. , Planchard E.
Published: February 2004

The changes in the magnetic properties of Ticonal 600 (Fe-24% Co-14% Ni-8% Al-3% Cu) and the evolution of the microstructures, as observed on thin foils by transmission electron microscopy, both resulting from the interruption of the magnetic cooling by a water quench from temperatures in the range 850°–600°C, were studied at room temperature before and after double tempering. These data were used in connection with a system of approximated relations to estimate the saturation magnetization, σs1 and σs2, and the compositions of α1 and α2 phases, respectively. During the magnetic cooling, σs1 first decreased slightly, between 850°–800°C, and then increased, while σs2 decreased continuously over the entire temperature range. Tempering induced a decrease in σs1; and an increase in σs1; however, the latter effect become less important as the prior magnetic treatment progressed to lower temperatures. Cooling down to 600°C followed by tempering resulted in a σs2 value of the order of 93–100 emu/g, while σs1 was found between 184–195 emu/g. After the same cycle of heat treatments, the chemical compositions of the phases were calculated as being (in wt.%): Fe (50–57)-Co (37–45)-Ni≤5-Al≤0.3-Cu≤5 (α1 phase) and Fe (42–52)-Co≤7-Ni (26–36)-Al (18–19)-Cu≤7 (α2 phase).
High-Coercivity Powders of Partially Reduced Metal Oxides
Authors: Morrison J. R. , Speliotis D. E. , Judge J. S.
Published: February 2004

Recent magnetic recording studies have indicated the following modifications in particulate media to improve their recording performance: increase particle coercivity and magnetic moment density, and decrease coating thickness. In this study we attempted to modify the traditional iron oxide particles used in magnetic recording media by substitution of other ions such as Co2+ and Ni2+, and by partial or full reduction to the metallic state. The alloy powders were prepared by hydroxide precipitation of the mixed oxides with subsequent partial reduction at elevated temperature under hydrogen, and by oxalate precipitation followed by thermal decomposition to the oxides and hydrogen reduction to the metal. The particles thus produced consist of a partially reduced metal oxide enclosed by an outer oxide shell, with attractive magnetic properties, particularly in the Fe–Co–O system.
Magnetization and Hyperfine Structure of 57Fe in CsFeF3
Authors: Eibschütz M. , Holmes L. , Guggenheim H. J. , Levinstein H. J.
Published: February 2004

Magnetic measurements and Mössbauer effect (ME) studies of 57Fe in CsFeF3 are reported. The compound is hexagonal at room temperature, but undergoes a crystallographic transformation at 83°K to a structure with lower symmetry. Magnetization data show a spontaneous magnetization appearing at TC=62°K, with a saturation magnetization at low temperatures of 1.72 μB per formula unit. In the magnetic state ME yields well resolved hfs corresponding to the two magnetically inequivalent iron sites. At 4.2°K the following hyperfine structure parameters have been obtained: Heff(A)=318 kOe, Heff(B)=274 kOe; Q.S. (A)=−2.85 mm/sec, and Q.S. (B)=−1.9 mm/sec, with the axes of the EFG tensors at 40° to the direction of the magnetic moments.
Magnetic Behavior of the FeSiO3–MgSiO3 Orthopyroxene System From NGR in 57Fe
Authors: Shenoy G. K. , Kalvius G. M. , Hafner S. S.
Published: February 2004

The magnetic behavior of (FexMg1−x)SiO3 with 0.269≤x≤1.0 was investigated between 300° and 1.7°K using NGR of 57Fe. For x≳0.76 magnetic ordering was observed at the following temperatures: x=1.0, TC=38°K; x=0.866, TC=18°K; x=0.758, TC=11°K. Ferrous ions occupy two different lattice sites (M1 and M2) of low symmetry. The resonance spectra consist of two superimposed hyperfine patterns, which reflect different electronic ground states for Fe2+ at the two sites. The M2 site is characterized by a smaller magnetic hyperfine field and a more negative isomer shift compared to the M1 site. Typical values for FeSiO3 at 4.2°K are 113 kOe, 1.29 mm/sec, 45 MHz at M2 and 290 kOe, 1.34 mm/sec, 69 MHz at M1. Above TC two quadrupolar doublets are observed. The quadrupole coupling of the M1 site shows a strong temperature dependence, while that of the M2 site remains fairly constant. The magnetically more dilute samples (x≲0.4) did not show ordering in the temperature region above 1.7°K, but produced paramagnetic relaxation spectra with only the smaller of the two hyperfine fields present, since only the M2 site is occupied sufficiently with Fe2+ for the resonance to be observed.
Mössbauer Investigation of Metamagnetic FeCO3
Authors: Forester D. W. , Koon N. C.
Published: February 2004

Mössbauer measurements have been made on single crystals of metamagnetic FeCO3 (siderite) at temperatures from 2.5°K to above the Néel temperature (TN=38.4°±0.2°K) and in static magnetic fields of 0–120 kG applied parallel to the spin-ordering axis. Computer analysis of the spectra assuming a stochastic model for the atomic-spin relaxation is used to determine the order parameter η(T) and the atomic-spin relaxation frequency ΩS(T). The temperature dependence of the order parameter is consistent with that expected of a highly anisotropic, Ising-type magnetic system. The relaxation frequency is small compared to the nuclear precession frequency below T=0.7 TN and increases rapidly with temperature. Other parameters obtained are: the saturated hyperfine magnetic field Hsat≡Heff(0)=184±2 kG, quadrupole splitting e2qQ/2=2.06±0.03 mm/sec, and isomer shift I.S.=1.36±0.03 mm/sec relative to iron metal. There was no change in the latter parameters over the range of temperatures and applied fields used in this study. In addition, the sign of the effective magnetic field at the 57Fe nucleus was determined to be positive.
Magnetization of Impurity Fe2+ Ions in MnF2 and CoF2
Authors: Wertheim G. K. , Guggenheim H. J. , Buchanan D. N. E.
Published: February 2004

Measurements in FeF2 show that the effective magnetic field obtained from the Mössbauer effect nuclear hfs of 57Fe provides a good measure of the magnetization of divalent iron in rutile structure fluorides. Similar measurements in MnF2 containing 0.7 mole % FeF2 show that the spin deviation of the iron impurity increases much more slowly with temperature than that of the host lattice. A fit of the MnF2 data to a molecular field model, considering only the next-nearest neighbor exchange, gives JFeMn=1.87 cm−1. However, systematic deviations show that the in-band contribution to the spin deviation which dominates at low temperature cannot be ignored. Results of measurements on Fe2+ in CoF2 are also given.
Cation Distribution in Gallium-Substituted Yttrium Iron Garnets by Mössbauer-Effect Spectroscopy
Authors: Czerlinsky E. R.
Published: February 2004

Recoilless nuclear-resonance absorption was measured on Y3GatFe5-tO12 powders with 1.75≤t≤4.0. A constant acceleration spectrometer was used, and the investigations were performed at room temperature, that is above the Curie temperature of these compounds. For each material we observed three resonant absorption maxima with different heights. The position of the maxima on the velocity scale is independent of composition. For each spectrum two doublets which arise from the quadrupole splitting of the 57Fe nuclei are assigned; one doublet arises from the 57Fe nuclei in the tetrahedral (d) sites, whereas the other arises from the 57Fe nuclei in the octahedral (a). From the relative values for the intensities D and A for the d and a sites together with the known amount of t, the fraction x/t of the Ga3+ ions in the d-sites was determined for each compound x/t=[D(t−2)+3A]/(A+D)t. The results were compared with work on cation distribution in substituted garnets, obtained from (a) magnetization measurements,1,2 (b) x-ray and neutron diffraction,3 and (c) thermodynamics of solid solutions.4 At Ga concentrations t>1.75, our results deviate from the ones obtained from magnetization data. The distribution function proposed in Ref. 4 is consistent with our experimental data over the total range of t. A more complete account of this work is intended for publication elsewhere.