bluuebunny/crossref_metadata_2025 huggingface dataset:
Singlet Ground State of the s-d Hamiltonian
Authors: Klein Michael W. , Tsay Yeun C.
Published: February 2004

The ground state of the s-d Hamiltonian with antiferromagnetic exchange interaction J and spin 1/2 was shown by Mattis1 to be a singlet. An exact solution of the decoupled Nagaoka2 equations by Zittartz3 does not give a singlet and results in a magnetic susceptibility which becomes negative at very low temperatures. This indicates that the Nagaoka equations do not give the appropriate description of the Kondo effect at very low temperatures. Zittartz3 has examined the quantity P defined by P=∫〈sc(r)·S〉d3r, where sc(r) is the conduction electron polarization as a function of the distance r from the impurity, and S is the impurity spin assumed to be 1/2, and the brackets 〈 〉 indicate thermal averages. According to Mattis proof P = −S(S+1) = −3/4. The quantity P is related to the two particle Green's function Γkk′ defined by Nagaoka,2 where Γkk′=〈(σαβ·S)ck′β|ckα*〉. We let mkk′=−4 ∫ −∞∞ImΓkk′(ω)f(ω)dω, where Im stands for the imaginary part, and f(ω) = (eβω+1)−1, where β = (kBT)−1, σαβ is the quantum mechanical expectation value of the Pauli spin operator between the electron spin states α and β. Then P= ∑ klim lim k′→kmkk′. In this paper we give a short summary of an investigation in which we examine the ground state of the s-d Hamiltonian with antiferromagnetic J and spin 1/2. The complete results will be published elsewhere. Nagaoka derived the equation of motion of the one- and two-particle Green's function and decoupled the resulting three-particle Green's functions and obtained an integral equation for Γkk′. We felt that the Nagaoka equations do not give the correct ground state because they neglect important contributions to Γkk′ and thus to P given in Eq. (4). In order to obtain a correction to Γkk′ we derived the equation of motion of the three-particle Green's function and decoupled the resulting four-particle Green's function to lowest nonvanishing order. We found that the higher-order Green's functions give important corrections to Γkk′ which are neglected on the Nagaoka equations. In particular we find that when we expand our terms on a power series in the exchange constant J and retain terms to order J2 the ground state is a singlet. The magnetic susceptibility χ is positive for all temperatures, and χ ∝ {T[1+(Jρ/N)2 log (D/T)]}−1 where D is an energy of order of the Fermi energy, N the number of sites in the solid, and ρ the density of states at the Fermi surface.
Superconductivity and Magnetic Order in La–Ce Alloys
Authors: Wollan J. J. , Finnemore D. K.
Published: February 2004

Superconductivity and magnetic order have been studied both above and below the Kondo temperature for the La–Ce system. Electrical resistivity measurements on La 0.2, 1.0, 2.0, 3.2, and 4.0 wt% Ce have been made from 0.060 to 20.0 K. All samples showed a Kondo-like minimum Tmin in the resistivity with a subsequent maximum Tmax. Tmin was found to be proportional to n1/7, where n is the concentration, and Tmax(H=0) was proportional to n2. The resistivity due to the magnetic impurities went as αln (T/T0) on both sides of Tmax. Above Tmax, α was almost independent of n, and below Tmax it was proportional to n2. Applied magnetic fields had little effect on α even through Tmax increased linearly with H. The magnitude of the resistivity at the peak was essentially independent of n, but decreased for increased H. The superconducting transition temperature was depressed as n increased, but more slowly than predicted by Abrikosov and Gor'kov, and the critical field curves were considerably depressed below the multiple pair breaking theory of Fulde and Maki.
Transport and Magnetic Properties of Th–Np Alloys
Authors: Nellis W. J. , Brodsky M. B.
Published: February 2004

The magnetic susceptibilities and electrical resistivities of dilute Th(Np) alloys containing 0.1, 0.5, and 1.0 at.% Np have been measured from below 2°, 59°, 300°K. At the lowest temperatures of the measurements, the resistivities decrease monotonically with temperature, and are proportional to T2. The susceptibilities follow a Curie-Weiss law of the form χ=χ0+C/(T−θ), where θ=−11° and −19° for 0.5% and 1.0% Np, respectively. The Curie-Weiss constant C yields effective moments of 0.8 and 1.1 μβ per Np atom for 0.5% and 1.0% Np. These values are considerably lower than would be expected from the free ionic configuration of 5f46d17s2, and are probably due to hybridization of the 5f electrons, as is the case in pure Np and Pu. The data indicate the formation of a broad 5f virtual bound state at the Fermi level, which then causes paramagnon-conduction electron scattering, as has recently been found for Th(U).1
Magnetic Fields in Astrophysics
Authors: Parker E. N.
Published: February 2004

Magnetic fields appear in association with most astrophysical objects and spaces. Their general effect is to promote fast-particle and enhanced-temperature activity. It appears that the magnetic fields are generated by the combined nonuniform rotation and cyclonic turbulence of the objects. The nonuniform rotation shears the fields, producing strong fields in the azimuthal direction. The cyclonic turbulence interacts with the azimuthal field Bφ generating azimuthal vector potential Aφ at a rate described by the dynamo equation (∂/∂t−η▿2) Aφ = ΓBφ, where Γ is essentially the cyclonic velocity of the turbulence. The fields of Earth, the Sun, and the galaxy appear to be generated in this way.
Dynamic Earth
Authors: Maxwell John C.
Published: February 2004

The earth's surface, in the context of geologic time, may be likened to a boiling vat of maple syrup. The crust, with its high-standing continents, is analogous to the scum which rises from boiling syrup, coalesces, drifts apart, and rejoins in different patterns on the surface of the convecting liquid. The earth's crust is a similarly thin scum of relatively light rocks ``floating'' on the mantle, a zone of heavier materials extending halfway to the earth's center and overlying the inner metallic core. By some cosmic accident the earth has been endowed with a magnetic field, apparently for much of its 4.5-billion-year history. Changes in the field with respect to a point on the surface are recorded by successively formed sequences of rock. Analysis of these ancient magnetic fields gives convincing evidence of extensive differential movements in the earth's crust. The composition of the crust and the forces which cause its deformation are apparently determined by gravitational and thermal instabilities within the outer few hundred kilometers of the mantle. Temperature within the earth increases downward at a rate exceeding the adiabatic gradient for the upper few hundred kilometers, hence this outer zone is intrinsically unstable. Vertical movements, once initiated, tend to be self-propagating. These instabilities may give rise to lateral and vertical movements approximating convection currents in liquids. Rising currents have apparently occurred largely in oceanic areas, bringing new mantle material to the surface in the oceans and sweeping older oceanic rocks towards and perhaps beneath the high-standing continents. The great mountain ranges which border many continents are believed to be related in some way to the convective overturn of rocks in the earth's crust and upper mantle.
Evidence of the Intrinsic Metallic Character of Some Rare-Earth Pnictides
Authors: Gambino R. J. , Eastman D. E. , McGuire T. R. , Moruzzi V. L. , Grobman W. D.
Published: February 2004

From measurements of the low-temperature specific heat of bulk crystals of YSb and GdSb and photoemission spectra from a single crystal of GdSb we conclude that these compounds are intrinsic metals. The electronic contribution to the specific heat of YSb was found to be 0.9 mJ/mole deg2 which may be compared with 0.65 for Ag, a good metal. Photoemission spectra for GdSb show structure at the Fermi level EF which is consistent with an s-like band at EF which overlaps a ``valence band'' with a peak at 2.2 eV below EF. This band picture offers an alternative explanation for the absorption spectra which have been interpreted as evidence for a large bandgap in the rare-earth pnictides.
Effect of Substitutional Anions on the Magnetic Properties of the Mononitrides of Uranium and Gadolinium
Authors: Kuznietz Moshe
Published: February 2004

The mononitrides of uranium and gadolinium have the NaCl-type structure and are good electrical conductors. UN is antiferromagnetic of type I (TN∼50 K); GdN is ferromagnetic (TC∼70 K). In UN limited oxygen substitution raises TN, while carbon substitution lowers TN, and UN0.9C0.1 does not order. Oxygen substitution in GdN lowers the paramagnetic Curie temperature θ and GdN0.95O0.05 and GdN0.90O0.10 do not order or at most have short-range antiferromagnetic order; carbon substitution in GdN raises TC or θ. The different orderings in UN and GdN and the variations in TN or TC due to anion substitution are accounted for by a simple model, originally proposed for UX compounds, in terms of variations in the number Z of conduction electrons per cation. Assuming U4+ and Gd3+ cations and N∼3− anions, and RKKY-type interactions between the localized f electrons, the observed ordering in UN is most stable for Z∼1 and in GdN for Z∼0.1. Substituting O−2 anions for nitrogen increases Z in both UN and GdN; carbon substitution decreases Z. The variations of magnetic ordering and θ with Z are discussed for anion-substituted GdN and UN as well as other monopnictides of Gd and U.
Spectral Dependence of the Photoconductance in CdCr2Se4
Authors: Amith A. , Berger S. B.
Published: February 2004

The spectral response of the photoconductance in In-doped single crystals of the ferromagnetic semiconductor CdCr2Se4 was measured in the range of photon energies from 1 to 3 eV, between 80° and 340°K. The photoresponse is attributable to three distinct transitions: I varies between 1.2 and 1.35 eV, II and III remain constant near 1.45 and 1.8 eV, respectively. Transition I is identified with that which had been previously observed in absorption measurements, and shows the same characteristic ``red shift'' associated with magnetic ordering as the temperature is lowered. Transitions I and II are superimposed near room temperature, and become separated only below 200°K, after I had shifted to lower energies. Transitions II and III are relatively temperature insensitive. The relative photoresponses associated with transitions I and II remain nearly unchanged over a temperature range in which the total photoresponse changes by orders-of-magnitude. This points to the likelihood that the carriers excited by these two transitions move in the same final state. Transition III corresponds to a high-lying one which had been previously inferred from magneto-optical measurements.
Anomalous Exchangestriction in Ferromagnetic Pyrite and Chromium Chalcogenide Spinel Compounds
Authors: Bindloss W.
Published: February 2004

Results are presented of x-ray measurements of lattice constant versus temperature in the range 4°–500°K of the ferromagnetic compounds CoS2 and Co0.75Fe0.25S2. Earlier measurements in the range 4°–300° on CdCr2S4 and CdCr2Se4 are extended to the range 4°–700°K. For purposes of comparison, x-ray data are also presented on the nonmagnetic compounds FeS2 and CdIn2S4. The results of measurements of dTc/dP of CoS2 and Co0.75Fe0.25S2 are also reported. The magnitude of the volume exchangestriction of the ferromagnetic compounds is determined by using Grüneisen-Debye theory to fit the high-temperature lattice constant data, and extrapolating this fit into the temperature range below Tc. The volume exchangestriction is found to be anomalously small when compared to thermodynamic predictions based on values of dTc/dP and the magnetic energy determined from the high-temperature susceptibility. The proposed explanation involves atomic displacements internal to the unit cell which modify the magnetic exchange integral J. This phenomenon is related to the coupling between optical phonons and magnons.
Raman Determination of 2F7/2 Crystal Field Levels in YbAlG and YbGaG
Authors: Argyle B. F. , Wadsack R. L. , Chang R. K.
Published: February 2004

Raman scattering from YbAlG and YbGaG at 80 K was investigated in order to obtain reliable crystal field levels of the Yb3+(2F7/2) ground manifold. Our 2F7/2 assignments for YbGaG (0, 546, 610, and 624 cm−1) are complete (four levels predicted for D2 site symmetry) and are consistent with the levels determined from absorption by Buchanan et al. [Phys. Rev. 159, 245 (1967)]. For YbAlG our assignments (0, 587, 621 cm−1, and a doublet at 758 cm−1) differ significantly from the work of Buchanan et al. who may have adopted the wrong assignments due to numerous vibronic side bands present in their original absorption spectra. Further confirmation of our results for YbAlG was provided by a companion study of LuAlG because of its closed 4f shell (no electronic scattering) and the nearly identical masses of Lu and Yb (i.e., equivalent phonon spectrum).
Interaction of Light with Coherent Spin Waves in YIG
Authors: Le Gall H. , Jamet J. P. , Desormiere B.
Published: February 2004

The susceptibility of the one-magnon Raman scattering induced by the magnetic dipole coupling of light with a ferromagnetic crystal is derived and compared with the scattering due to the electric dipole coupling. For a given magnon state, it is shown that the ratio of the magnetic and electric Raman susceptibilities is defined by the ratio of the quadratic power of the nondispersive and the dispersive specific Faraday rotations. We deduce from the present calculation, that the light-spin-wave interaction increases linearly with the magnons and incident photons populations and is proportional to the reverse of the spin-waves relaxation rate. These theoretical results agree well with our experimental data obtained with coherent spin-waves excited by a microwave parallel pumping at X band and subjected to infrared light peaks produced by a Nd laser at 1.06 μ. By decreasing the dc field at constant rf field, we detect an increase of the spin-wave-light interaction with the propagation vector of the parametric magnons except between 1400 and 1600 Oe because of the rise of the three-magnon relaxation between the parametric states in this range.
Raman Scattering in FeBO3
Authors: Shepherd I. W.
Published: February 2004

The phonon Raman spectrum has been measured in single crystals of the transparent ferromagnet FeBO3. A pure sample (Tc = 350°K) and a sample containing 10% Ga (Tc = 272°K) were investigated, and in each case three strong lines of Eg symmetry were seen at 279, 401, and 1210 cm−1. Three weaker lines were observed at 535, 641 (Eg), and 926 cm−1 (A1g); the 535 cm−1 line was only visible at liquid-helium temperature. The temperature dependence of the spectrum was measured from 20° to 370°K, and it was found that for both samples in the region of Tc the magnitudes of the lines decreased relative to the 926-cm−1 line by factors of 2.45 (1210 cm−1), 2.1 (644 cm−1), 2.5 (401 cm−1), and 1.3 (279 cm−1). This behavior is similar to that of the 168-cm−1 Raman line of CdCr2Se4.
Exchange-Enhanced Optical Absorption of Fe3+ in Al2O3
Authors: Krebs J. J. , Maisch W. G.
Published: February 2004

The polarized optical absorption of isolated and exchange-coupled Fe3+ ions in Al2O3 has been measured at room, liquid-nitrogen, and liquid-helium temperatures. Single crystals were grown from a PbO–PbF2 flux and contained Fe3+ concentrations c ranging from 0.1 to 2 at.%. The single ion optical spectrum of Fe3+ in the distorted octahedral surroundings of Al2O3 is characterized by the following parameters: B=660 cm−1, C/B=4.75, and 10Dq=1510 cm−1, in reasonable agreement with the recent work of Lehmann and Harder on natural iron-bearing corundum crystals. At sufficiently low temperatures, the spectrum is due only to single ion transitions for all concentrations. However, as the temperature is raised, there is a striking increase in the intensities of the 4A1, 4Ea, and the 4Eb bands in the higher concentration samples. At room temperature, the intensity of the 4A1, 4Ea band increases more rapidly than c. Furthermore, the 4Eb band contains a partially resolved low-frequency component which represents the first observation of an optical transition due to exchange-coupled Fe3+ pairs. For c≥0.5 at.%, the room temperature spectrum in the visible region is dominated by exchange-enhanced transitions for which ΔS=0. Similar spectra have been reported previously for the isoelectronic Mn2+ ion.
Infrared Absorption and Raman Scattering by Two-Magnon Processes in NiO
Authors: Dietz R. E. , Parisot G. I. , Meixner A. E.
Published: February 2004

Temperature-dependent Raman scattering by two magnons has been observed in both green and black (from excess oxygen) NiO single crystals, peaking at 1555 cm−1 at 1.4°K. The shape of the band is reasonably well described by a dominant intersublattice exchange of J2=148 cm−1, in excellent agreement with the value J2=153 cm−1, obtained from neutron scattering.1 The high-frequency cutoff of the Raman scattering lies near 1900 cm−1 in agreement with the J1=0 two-magnon joint density-of-states cutoff at 176 cm−1. The absorption line2 peaking at 1940 cm−1 has been reexamined in light of this data, and is found to exhibit a cutoff on the high-frequency side at 2460 cm−1, and a threshold near the peak of the Raman scattering. These data support a model for the absorption band in which two magnons are created simultaneously with a photon, with the cutoff corresponding to the creation of two zone boundary magnons and an L0 phonon of 580 cm−1 at k=0. Such a phonon sideband of a two-magnon transition probably derives its intensity from the phonon modulation of exchange as suggested by Koide et al.3
Absence of Antiferromagnetism in NbSe2 and TaSe2
Authors: Ehrenfreund E. , Gossard A. C. , Gamble F. R. , Geballe T. H.
Published: February 2004

NbSe2 and TaSe2 are layered metallic compounds in which antiferromagnetic ordering has previously been deduced from anomalies in magnetic and electrical behavior. The anomalies consist of magnetic susceptibility maxima, Hall coefficient sign reversals and for TaSe2, an electrical resistivity kink, at T∼30°K and T∼110°K for the two compounds respectively. We have now studied the nuclear magnetic resonances of 93Nb in NbSe2 and of 77Se in TaSe2 above and below these temperatures. Observation of resonances from T=4.2°K to T=300°K shows that the materials are definitely not magnetically ordered. In NbSe2, an axially symmetric 93Nb quadrupole powder pattern is observed at 77° and 300°K but at 4.2°K is found to be further split. This implies that the transition is instead a low-temperature structural distortion. This distortion may be responsible for the previously observed anomalies and for the absence of magnetic order. On the basis of the observed change of the electric field gradient, the corresponding change of the niobium d-orbitals occupancy is calculated.
Magnetic Transition in Rhodium-Doped Hematite Single Crystals
Authors: Morrish A. H. , Eaton J. A.
Published: February 2004

Single crystals of hematite doped with up to 1.6-mole% rhodium oxide have been grown by the flux method. The amount of dopant, which electron microprobe measurements show to be nonuniform, is linearly correlated with the Morin transition temperature TM determined by a colloidal technique. The increase in TM by about 50°C/mole% Rh2O3 is suggested to have its origin in the effects of the lattice distortion produced by the larger rhodium ions and characterized by a small average lattice expansion of +0.04%/mole% Rh2O3.
Effect of Pressure on the Magnetic Properties of Ca1−xSrxMnO3
Authors: Kafalas J. A. , Menyuk N. , Dwight K. , Longo J. M.
Published: February 2004

In order to distinguish among the effects of interatomic distance, B–O–B angle and chemical substitution in perovskites, a study has been made of the variation of Néel points of several samples in the system Ca1−xSrxMnO3 as functions of x and pressure. It is found that throughout the region 0<x<0.5, TN increases with increasing x, and hence with increasing lattice parameter. However, dTN/dP>0 throughout this region, which indicates just the opposite dependence on cell size. This apparent contradication is compatible with Goodenough's hypothesis regarding the effect of covalence of the nonmagnetic A-site cation on the magnetic interaction. In addition, it is found that application of hydrostatic pressure reduces the weak ferromagnetic moment. This is most marked at intermediate values of x, where a 25% decrease in moment can be obtained upon application of a hydrostatic pressure of 8 kbar. Increasing x at atmospheric pressure also decreases the weak ferromagnetic moment. These findings indicate that orthorhombic distortion decreases with pressure as well as with x and that variation of the Mn–O–Mn angle influences TN less than variation in chemistry.
Synthesis and Characterization of Single Crystal (Re, Pb)MnO3 Perovskites
Authors: Janes D. L. , Bodnar R. E.
Published: February 2004

Compounds of the type (RE)2/3Pb1/3MnO3, with RE=La3+, Pr3+ and Nd3+, were grown as single crystals several mm on a side from molten flux by a slow cooling. X-ray studies showed the materials to be nearly cubic perovskites with a0∼3.9 Å. These compounds are soft ferromagnets with magnetizations of 70–80 emu/g at 55°K. The lanthanum compound has a Curie temperature of 330°K, but Tc can be lowered by as much as 150° by introduction of unpaired 4f spins via Pr3+ and Nd3+. There appears to be a negative coupling between the Nd3+ or Pr3+ and the Mn ions, although the resultant magnetization at 55°K does not reflect a completely antiferromagnetic configuration of moments.
Mathematical Analysis of ``Permanent'' Magnet Suspension Systems
Authors: Borcherts Robert H.
Published: February 2004

A mathematical analysis of ``permanent'' magnet suspension systems as related to high-speed ground transportation has been calculated for a ceramic magnet roadbed-ceramic magnet vehicle system as well as a ceramic magnet roadbed-superconducting magnet vehicle system. The approximation of the ceramic magnet by a current sheet around the magnet makes it possible to investigate the performance of a magnetic suspension system in terms of lift force as a function of magnet separation and thickness. For each of these an optimum width is found beyond which the net lift force is reduced. Calculations are also made assuming a higher-density current sheet for the vehicle magnet, i.e., a superconducting magnet, and similar results are also obtained. Finally, these two systems along with the magnetic induction system utilizing a diamagnetic conducting roadbed, are compared in their performance, i.e., lift force, ground clearance, magnet weight, and possible losses.
Cast Permanent Magnets of the Co5RE Type with Mixtures of Cerium and Samarium
Authors: Nesbitt E. A. , Chin G. Y. , Gallagher P. K. , Sherwood R. C. , Wernick J. H.
Published: February 2004

Previous studies of the effect of composition on the magnetic properties of arc-cast alloys in the vicinity of Co3.5CuFe0.5Ce yielded maximum energy products from 4 to 10×106 G Oe, residual inductions from 4000 to 6400 G, and intrinsic coercive forces from 5000 to 12 000 Oe. The partial substitution of samarium for cerium in alloys in this vicinity results in an improvement in permanent magnet properties, with maximum energy products from 4.6 to 12.3×106 G Oe, residual inductions 4300 to 7150 G, and intrinsic coercive forces from 4000 to 13 400 Oe. Freezing points of these alloys were determined using differential thermal analysis. The dependence of magnetic saturation on temperature was determined for several of these alloys from room temperature to 1.5°K. The saturation increases gradually with decreasing temperature without anomaly. The temperature coefficient of saturation magnetization between 25° and −100°C was found to be 0.1%/°C for the alloy Co3.5CuFe0.5Ce, 0.06%/°C for the alloy Co3.5Cu1.35Fe0.4Sm. Intermediate values were obtained for mixtures of Ce and Sm. The temperature coefficient of saturation magnetization was reduced to practically zero by the partial substitution of Gd for Ce.
Static Magnetization Curve of Uniaxial Ferromagnets
Authors: Muller M. W. , Yang M. H.
Published: February 2004

We examine the domain nucleation threshold of saturated ellipsoidal specimens from the point of view of stability. When the nucleation is unstable (as it is for most orientations of the applied field), the nucleation mode grows spontaneously into a fully developed domain structure, and a complete hysteresis loop can be calculated. We carry out a complete calculation for a thick uniaxial platelet.
Initial Observations: Cobalt-Mischmetal-Samarium Permanent Magnet Alloys
Authors: Benz M. G. , Martin D. L.
Published: February 2004

Several authors have reported successful fabrication of high-energy-product (20 MG Oe) Co5R-type cobalt-samarium permanent magnets. In earlier papers we have described preparation of such magnets by a liquid-phase sintering process sequence which includes powder preparation, magnetic alignment, pressing, sintering, and magnetizing. We have also described preparation of ternary compositions, primarily Co–Sm–Pr, by this process. In an effort to utilize more abundant rare-earth metals, the cobalt-mischmetal-samarium system has been studied. Initial results indicate that a substantial fraction of the samarium may be replaced with mischmetal while retaining high magnetic properties (BHmax=20 MG Oe).
Cast Permanent Magnets of Cobalt, Copper, and Cerium: Process and Performance Characteristics
Authors: Cullen Thomas J.
Published: February 2004

A process for casting magnets of the cobalt, copper, rare-earth type weighing as much as several pounds has been developed. The characteristics of the alloys are highly reproducible between castings. The uniformity of performance within a casting is excellent. The residual induction of a typical cerium alloy casting is 5600 G. The coercive force is 4800 Oe. The variation of induction with temperature in the region of a load line of 2, is 0.08%/°C from room temperature to 100°C.
Exchange-Enhanced Generalized Susceptibility of Palladium
Authors: Diamond Joshua B.
Published: February 2004

The static, wave-vector dependent spin susceptibility of the 4d electrons of Pd interacting via screened Coulomb forces is calculated within the framework of a model which includes quantitative effects of band structure and of momentum dependence of the interaction matrix elements. For the band structure we use a tight-binding scheme adjusted to fit Fermi surface data and apw calculations. The exchange enhancement is obtained in the rpa by summing the ladder diagrams describing particle-hole multiple scattering via a short-ranged potential. The calculated susceptibility may be fit at long wavelengths to yield a Yukawa form in real space. For a given Stoner enhancement factor (S) the resulting range parameter is approximately 3 times as large as that obtained from a single spherical band model for Pd with a contact-exchange interaction. With S = 20, the present calculation gives a range parameter of 4 Å, in reasonable agreement with Pd (Fe) neutron-scattering data [T. J. Hicks, T. M. Holden, and G. G. Low, J. Phys. C 1, 528 (1968)].
Localized Moments on Fe Impurities in Nb–Mo Alloys: Mössbauer-Effect Absorber Study
Authors: Swartzendruber L. J.
Published: February 2004

Mössbauer-effect spectra for absorbers of Mo and Mo0.8Nb0.2 alloys containing 57Fe impurities have been obtained in an external field as a function of temperature. The spectra for the Mo–Fe alloys are in close agreement with the results of Kitchens et al.1 for (57Co) Mo sources. The Mo0.8Nb0.2 spectra show directly the presence of two distinct moment values for the Fe impurities, one with a moment of 1.9 μB and one with a moment of less than 0.6 μB, with the two moments present in roughly equal proportions. This result gives support for a discontinuous formation model of the type proposed by Jaccarino and Walker.2 The saturation hyperfine field of 115 kG for Fe in Mo is apparently reduced to 90 kG for moment bearing Fe in Mo0.8Nb0.2. There appears to be no quenching of the Fe moment for Fe–Fe pairs such as that observed for Co–Co pairs in Mo–Nb alloys.
Low-Temperature Electrical Resistivity and Magnetoresistance of Dilute Solutions of Fe in Cu–Ni Alloys
Authors: Gärtner H. , Harvey A. R. , Zrudsky D. R. , Peterson D. , Legvold S.
Published: February 2004

The low-temperature (1 K<T<100 K) electrical resistivity and the longitudinal magnetoresistance (B≤90 kG) of Cu1−xNix alloys with x = 0.06, 0.12, 0.23, 0.35, 0.46, and 60 ppm≤Fe≤1100 ppm have been measured. The excess resistivity Δρ, taken as the resistivity of the Fe-bearing samples minus the resistivity of their Fe free equivalents, depended linearly on ln T for an order-of-magnitude in T, indicating that Fe in Cu–Ni alloys forms a Kondo system. In every case the Fe-bearing samples showed a negative contribution to the magnetoresistance correlating with the depth of the minima in the electrical resistivity. The results are interpreted in terms of small magnetic Ni clusters associated with Fe sites.1
Magnetic Properties of Dilute Pd(Gd) Alloys
Authors: Guertin R. P. , Foner S. , McNiff E. J. , Praddaude H. C.
Published: February 2004

Magnetic moment measurements on Pd1−xGdx alloys for 0.002≤x≤0.05, 1.5≤T≤4.2 K, and 0≤B0≤150 kG are reported. The saturation moment per Gd atom is independent of Gd concentration and is equal to that of Gd3+ within experimental error. An empirical formula is given which fits the magnetic isotherms to better than 1% over the entire field range.
Liquid-Phase Homoepitaxial Growth of Rare-Earth Orthoferrites
Authors: Shick L. K. , Nielsen J. W.
Published: February 2004

Homoepitaxial deposits of several rare-earth orthoferrites have been grown from xPbO:B2O3 solutions. Single-crystal epitaxial layers to 1-mm thick of Sm0.55Tb0.45FeO3, TbFeO3, and YbFeO3 have been grown on oriented (001) substrates of the respective materials. Deposition rates of approximately 4 μ/h are utilized during growth. Selection of an appropriate solvent is important for successful growth. Depositions from 20PbO:B2O3(wt) result in excessive heterogeneous nucleation and polycrystalline layers when grown above 990°C. Crystallization of rare-earth borate interferes with orthoferrite growth when carried out at lower temperatures. The solvent 50PbO:B2O3 (wt) has been found to minimize heterogeneous nucleation and eliminate crystallization of rare-earth borate. The growth method is described in detail, including design of the platinum boat, temperature profiles and their effects on growth, temperature programming, and the experimental difficulties encountered in utilizing the highly volatile highly corrosive PbO:B2O3 solvents.
Spin Flopping in LiCuCl3·2H2O
Authors: Forstat H. , Bailey P. T. , Ricks J. R.
Published: February 2004

Single crystal magnetic measurements of LiCuCl3·2H2O in the liquid-helium temperature region revealed the presence of a spin-flop state above 9 kG. By means of the magneto-caloric effect it has been possible to trace out the antiferromagnetic-spin-flop boundary. A least squares fit of the data indicated that this boundary could be represented by Hc=9.36+0.078T+0.173T2 with Hc in kG. The spin-flop-paramagnetic and antiferromagnetic-paramagnetic boundaries were obtained by specific-heat measurements. Calculations gave the anisotropy field (HA) as 3.0 kG, and the exchange field (HE) as 11.0 kG. All of the above measurements were made with the external field parallel to the easy axis of magnetization.
Properties of M2+ (M2+ = Ni, Mn, Cu, Co) and F− Substituted Orthoferrites
Authors: Robbins M. , Pierce R. D. , Wolfe R.
Published: February 2004

Polycrystalline rare-earth orthoferrites (LnFeO3) in which some Fe3+ has been replaced by M2+ (M2+ = Ni, Mn, Cu and Co) have been prepared. Charge compensation was effected by the substitution of F− for O= to give the general formula LnFe1−xM+2xO3−xFx. X-ray techniques and magnetic measurements of Curie points were used to verify the absence of second phases. Materials of the type LnFe0.8Ni0.2O2.8F0.2 (Ln = Y, La–Yb) exhibited room-temperature magnetic moments which were of the order of four times larger than those of the unsubstituted orthoferrites, implying marked increases in canting angle. Curie temperatures decreased about 20°C. Where Ln = Sm the limit for Ni2+ – F− substitution was x ≈ 0.2. For Ln = Ho the substitution limit was x ≈ 0.3. In both cases increasing Ni2+–F− substitution resulted in increasing magnetic moment. When the substituted orthoferrites contained Cu2+ and F− no large changes in magnetic moment or Tc were observed. For Mn2+–F− substitution the Curie temperature decreased about 30°C and the magnetic moments varied irregularly for different rare-earth orthoferrites. Co2+–F− containing orthoferrites were prepared and were single phase with respect to x-ray measurements, however a large magnetic impurity developed during high temperature magnetic measurements. We were unable to prepare pure Fe2+ containing orthoferrites. The enhanced moments obtained with Ni2+–F− substitution would result in useful decreases in ``bubble'' domain diameter in single crystals of these materials.
Anisotropic Magnetization of Ytterbium Orthoferrite
Authors: Clark A. E. , Belson H. S. , Callen Earl
Published: February 2004

The magnetization of single-crystal ytterbium orthoferrite (YbFeO3) was measured from 4.2° to 300°K in fields1 up to 110 kOe. At high temperatures a small spontaneous moment appears along the c direction. The magnetization is proportional to field along the a, b, and c directions. As the temperature is lowered, the field-dependent magnetization parallel to a increases rapidly and approaches a saturation value of 3.2 μb at 4.5°K. Along the c direction, an anomalously small increase in spontaneous magnetization with decreasing temperature is observed below 100°K. In addition, below 2.3°K the magnetization becomes temperature independent but still retains a large field dependence. From this we predict that either: (1) the magnetic unit cell is larger than the crystallographic cell, yielding Yb magnetic sublattices canted from the c direction; or (2) the Yb3+ ground state is not a Kramers doublet widely separated from the remaining levels. The high-field magnetization along the b directions is markedly smaller than that along a. It appears that the driving force for the magnetization reorientation is a huge rare-earth anisotropy, which tends to align the Yb spins along the a axis, along with the net iron magnetization. It is possible to explain our recent low-temperature magnetostriction measurements on YbFeO3, where a sizeable change in length is found for H ∥ a and H ∥ c, whereas a small magnetostriction is observed for H ∥ b.
Spin-Orbit Induced Spin-Flip Scattering by a Local Moment
Authors: Yafet Y.
Published: February 2004

The spin-orbit-induced spin-flip cross section presented to a conduction electron by a 3d impurity having a local moment is calculated. The Anderson model is treated in Hartree-Fock (HF) approximation. The difference from the nonmagnetic case is that the total angular momentum j is not a good quantum number so that one cannot use its eigenstates to calculate the scattering phase shifts. Instead, the HF Hamiltonian has been diagonalized to first order in the spin-orbit coupling Hso and the resulting eigenfunctions have been used to find the phase shifts. To maintain the spherical symmetry of the Hamiltonian when Hso is present it is found necessary to retain the matrix elements of the Coulomb interaction that connect four different orbitals m, m′ to m∓1, m′±1. It is also necessary to abandon the simplification of assuming a single value for the exchange integral and to keep the actual dependence of the matrix elements on m and m′. The calculated cross section depends on an enhanced spin-orbit coupling and on the local moment parameters.
Gd3+-Gd3+ Interactions in Rare-Earth Hydroxides
Authors: Cochrane R. W. , Wu C. Y. , Wolf W. P.
Published: February 2004

Using the method of paramagnetic resonance of isolated ion pairs, we have measured the isotropic exchange interactions between nearest and next-nearest Gd3+ neighbors in Y(OH)3 at 77°K. Defining Hex = JijSi·Sj, we find Jnn = 1134×10−4 cm−1 and Jnnn = −60×10−4 cm−1. Comparable anisotropic interactions are closely accounted for by magnetic dipole coupling. Similar experiments on Gd3+ in isomorphic Eu(OH)3 give Jnn = 1035×10−4 cm−1. Taken together, these results allow a comparison with the interactions in pure Gd(OH)3, obtained from an analysis of susceptibility and specific heat measurements. General agreement is found. On the other hand, comparison of the present results with earlier exchange parameters of Gd3+ in isostructural La-, Eu-, and GdCl3 shows a striking trend for which there is no theoretical explanation.
Far-Infrared Laser Study of Magnetic Resonance in DyPO4
Authors: Prinz G. A. , Wagner R. J.
Published: February 2004

Magnetic resonance has been observed in the Ising system, DyPO4, utilizing a unique far-infrared laser-magnetic resonance spectrometer. Spectra were obtained at 220.23 μ (45.407 cm−1) and 171.67 μ (58.25 cm−1) at 4.2°K in applied fields (H∥C) sufficient to saturate the spin system. The radiation source was a pulsed-discharge gas laser (using H2O or D2O). The spectrometer employed a 90-kOe superconducting solenoid and a dual-beam detection system which electronically integrates the pulsed signals and records directly the percentage transmission. The resonance yields three super-imposed hyperfine patterns, due to even isotopes (I = 0); Dy(161) and Dy(163) both (I=52). All six lines of Dy(163) were resolved yielding a spin Hamiltonian constant (A/g∥) = 37.8×10−4 cm−1, while for Dy(161) only four components could be seen, giving (A/g∥) = 27.5×10−4 cm−1. The observed linewidth for all lines was 25 Oe. The slope of the splitting between the two I = 0 resonances observed at 53 340 Oe and 67 590 Oe yields g∥ = 19.3. The data indicates there is little departure from a linear Zeeman splitting at these high fields, and by extrapolation the antiferromagnetic zero field splitting is obtained. The zero field splitting agrees with the earlier spectroscopic results [J. C. Wright and H. W. Moos, Phys. Lett. 29 A, (1969).] only if an effective field of 1.3 kOe is added to the total dipolar field in the saturated state, indicating additional long-range forces. The full version of this paper is being prepared for publication.
Magnetic Resonance and Electromagnetic Wave Propagation in Materials Having g-Factor Anisotropy
Authors: Rado George T.
Published: February 2004

A method is presented for deriving from quantum mechanics a classical equation of motion of the magnetization which differs from the Landau-Lifshitz equation in that it remains valid for materials in which the g factor is anisotropic. Previous attempts to obtain such an equation of motion either involved only the Zeeman part Hz of the total Hamiltonian or lacked a sound basis. We illustrate the method with a Hamiltonian consisting of ``one-ion'' anisotropy and Hz, and also with a Hamiltonian consisting of anisotropic exchange (expressed in the molecular field approximation) and Hz. Next we use the classical equation of motion derived from the later Hamiltonian to calculate, in the magnetostatic limit, the resonance frequency and the small-signal complex susceptibility of the Ising-like antiferromagnet DyPO4 if it is exposed to a saturating static magnetic field. By solving this equation of motion in conjunction with Maxwell's equations, we then treat the ``dimensional resonances'' arising from electromagnetic propagation effects. Approximations appropriate to the far-infrared laser resonance experiment of G. A. Prinz and R. J. Wagner [Phys. Lett. 30 A, 520 (1969), and (to be published)] lead us to an explicit expression for the field-dependent transmission through a monocrystalline DyPO4 slab whose thickness is comparable to the wavelength. The experimental fact that the transmission near resonance can exceed that at very large fields is shown to provide stronger evidence for propagation effects than the asymmetry of the resonance. We further show that propagation effects are more difficult to avoid in the far infrared than in the microwave region, and that these effects cannot usually be avoided at any wavelength in a transversely excited Ising-like system. The full version of this paper is being prepared for publication.
Orientation Dependence of Spin-Wave Spectra in Single-Crystal YIG Films
Authors: Besser P. J.
Published: February 2004

Spin-wave resonance in epitaxial films of yttrium iron garnet (YIG) has been studied as a function of the orientation of the static field H relative to the film plane and the crystallographic axes. The positions, intensities, and linewidths of the main branch and the higher branches have been measured in (100) and (110) disks with thicknesses from 0.37 to 1.15 μ. Higher branches which are spaced more than ∼100 Oe from the main branch in perpendicular resonance (H⊥film, θ = 90°) disappear between θ = 90° and θ = 0°, while those which are closer are present over the entire range of θ. In a 0.37-μ, (100) film the first higher branch disappears at θ = 49°. It reappears one degree on either side of this critical angle and is spaced ∼50 Oe from the main line at these points. The linewidth of the first higher branch reaches a maximum of 7.1 Oe near θ = 49° and decreases to values of 2.0 and 2.4 Oe at θ = 90° and 0°, respectively. The main branch linewidth increases monotonically from 1.8 to 7.1 Oe between θ = 90° and θ = 0°. Anisotropy of the spin-wave spectrum is observed in parallel resonance (H ∥ film, θ = 0°) of both (100) and (110) films. In a 0.9-μ, (110) sample the spacing between the first two branches is almost identical in the [110] and [111] directions, but is ∼50% less in the [100].
Antiferromagnetic Solid Oxygen
Authors: Wachtel E. J. , Wheeler R. G.
Published: February 2004

The lowest-temperature phase of solid oxygen is antiferromagnetic and blue in color. A review of the electronic properties of the diatomic molecule indicates that the ground state 3Σg− of the lowest configuration is paramagnetic with the spin degeneracy lifted by spin orbit and dipolar forces. This splitting approximately 4 cm−1 leaves the Ms = 0 state lowest. Excited states of this configuration, the 1Σg+ and 1Δg, lie at 13 120 cm−1 and 7882 cm−1. All higher configurations lie above 30 000 cm−1. Both of the above mentioned properties of alpha oxygen can be understood on the basis of a ``single molecule'' but slightly perturbed due to neighboring molecules. The exchange and the orbital overlaps between molecules are responsible for both the antiferromagnetism and the blue color. The latter arises from single photon absorption inducing transitions within the lowest configuration on two neighboring molecules. The magnon spectra and magnon-exciton side band spectra justify this model.
57Fe Hyperfine Fields in Iron-Cobalt Alloys
Authors: Wertheim G. K. , Buchanan D. N. E. , Wernick J. H.
Published: February 2004

The perturbation of the 57Fe hfs field by cobalt atoms in the first four coordination spheres of bcc iron has been obtained by using satellite line positions known from NMR data as constraints in the analysis of Mössbauer spectra. We assume that the perturbation decreases with increasing iron-cobalt distance and that the effects of multiple neighbors are additive. Solutions violating the first assumption were also investigated but were found to be no better than those reported here. The results clearly show that the NMR satellite at Δν/ν = +1.3% is due to a third-neighbor cobalt in agreement with the proposal of Mendis and Anderson. The perturbation due to a near neighbor cobalt is insensitive to outer shell assignments and is (+3.5±0.3) %. That due to a second neighbor is (+3.0±0.6) % but is less well determined. The large probable errors in these two coefficients arise from strong correlation between them in the least-squares analysis. The coefficient for the fourth-neighbor shell was assigned the NMR value. The data also show small isomer shifts due to cobalt neighbors.
Electronic and Magnetic Structure of Dilute Iron-Base Alloys
Authors: Sauer W. E. , Reynik R. J.
Published: February 2004

Mössbauer spectroscopy has been used to characterize changes in the electronic and magnetic structure of iron resulting from dilute additions of vanadium, chromium, manganese, nickel, molybdenum, tungsten, rhenium, aluminum, and silicon. The data showed that both the isomer shift and the hyperfine field depend linearly upon the bulk composition of the alloy and the local atomic configuration. This model separates the metallurgical dependence of these electronic and magnetic parameters into a long-range composition-dependent component and a short-range composition-independent component. The long-range component of isomer shift in iron-chromium alloys agrees with the value predicted by x-ray diffraction measurements of the lattice parameter dilation. The short-range component correlates qualitatively with the electronegativity of the diluents.
Mössbauer Study and Molecular Field Theory of the Magnetic Properties of Fe–Al Alloys
Authors: Huffman G. P.
Published: February 2004

Mössbauer studies of the antiferromagnetic alloys, Fe1−xAlx (x≈0.33 to 0.47), have been made from 1.8° to 295°K. Supplementary low-temperature susceptibility measurements were also carried out. The alloys have the CsCl structure and are best described as two interpenetrating simple cubic lattices, one of pure Fe, the other having composition Al1−cFec (c=1−2x). Fe atoms on the Al1−cFec sublattice (FeA) have nearly the full moment of pure Fe, while most atoms on the Fe sublattice (FeF) have zero moment. The hyperfine fields at both sites have a T2 temperature dependence for (T/TN)2≲0.6. At T = 0, the FeA field consists of a −145-kG core polarization field plus an RKKY conduction electron polarization (cep) field, while the FeF field is given primarily by an RKKY cep field. The RKKY cep fields and Néel temperatures increase linearly with c. A molecular field theory is outlined which considers RKKY exchange interactions between Fe spins out to fifth nearest-neighbor distances and a direct d electron exchange interaction between nearest-neighbor spins. The theory explains the observed properties of the antiferromagnetic state and predicts with reasonable accuracy the magnetic phase diagram of the whole Fe–Al system.
Measurements of Pressure Dependence of Magnetic Moment in Iron-Aluminum Alloys
Authors: Noakes J. E. , Sato H. , Arrott Anthony
Published: February 2004

A method of measuring the change in magnetic moment with pressure for spherical samples at a relatively low pressure level is described. Special attention is paid to the problem of avoiding the effects of sample motion with respect to the detector coil to insure the accuracy in such a highly sensitive measurement. The results of measurements on iron-aluminum alloys up to 30 at.% Al are presented here. There is a sharp increase in the pressure dependence of the magnetic moment with the aluminum concentration. This is interpreted as favoring a competing antiferromagnetic interaction with increasing pressure.
Analysis of Diffuse Elastic Neutron Scattering from Magnetic Alloys
Authors: Garland J. W. , Bennemann K. H.
Published: February 2004

In many magnetic alloys the local moment μn on site n depends nonlinearly on its local environment. A simple one-dimensional model is used to illustrate the profound effect on diffuse elastic neutron scattering of this nonlinear dependence on local environment. It is shown how to extend the analysis of Marshall1 to include such a nonlinear dependence. It is shown why the cloud model of Kouvel et al.2 is preferable to the analysis of Marshall for a dilute concentration of clouds in Cu:Ni alloys, and the cloud model is derived from our general formulas. The case in which μn depends on the moments μn′ is also considered.