bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic Properties and Electrical Conduction of Copper-Containing Sulfo- and Selenospinels
Authors: Lotgering F. K. , van Stapele R. P.
Published: January 2006

CuCr 1+x Rh 1−x Se 4 with −1 < x ≦ 1 and CuCr1+xTi1−xS4 with 0 < x ≦ 1 show p-type metallic conduction and are ferromagnetic, while CuCr1+xTi1−xS4 with x<0 show n-type metallic conduction and no magnetic ordering at 4°K. From the magnetic and electrical properties, which indicate that Cu is monovalent in these compounds, a model for the electronic structure of copper-containing sulfospinels is deduced. Ferromagnetism and p-type conduction are attributed to holes in a broad valence band, n-type conduction to electrons in a broad conduction band. Susceptibility measurements on CuCo2S4, CuTi2S4, CuCoTiS4, CuRh2S4, and CuRh2Se4 are reported. The main contribution to the temperature-independent susceptibility observed is attributed to the Van Vleck susceptibility of Co3+ or Rh3 ions in the low spin state t26.
Model Calculations for the Study of Direct and Superexchange Interactions
Authors: Ellis D. E. , Freeman A. J.
Published: January 2006

Investigations of the two-atom M2+-M2+ and the three-atom M2+-(F−)-M2+ systems have been made in order to study both direct and superexchange mechanisms. In these model calculations all electrons are treated explicitly within the framework of the unrestricted Hartree-Fock procedure; the Fock matrix is evaluated directly by Monte Carlo integration techniques. The effective exchange parameter J is determined at several internuclear distances, and spin densities are exhibited for the ferromagnetic and anti-ferromagnetic states of the three-atom system. The AF state lies lower in energy beyond a calculated critical bond length. Localization of the uhf eigenfunctions and the significance of the Mott transition for magnetic ordering are discussed.
Systematics of the Right-Angle Nearest-Neighbor Exchange Interaction for d3 Ions
Authors: Huang Nai Li , Orbach R.
Published: January 2006

In the chromium chalcogenide spinels the strength of the ferromagnetic nearest-neighbor exchange interaction increases as the anion is changed from O2−→S2−→Se2−. This tendancy was first revealed by the experiments of Baltzer et al. In order to explain this behavior, various mechanisms contributing to the right-angle exchange coupling are examined. Both the superexchange and the direct exchange interactions between the magnetic ions are taken into account. For the four-electron, three-center model in which a ligand orbital forms a π-bond with one magnetic ion, but is orthogonal to the other, it is demonstrated that: (1) An increase in the Cr3+ ligand covalency along the series O2−→S2−→Se2− leads to an enhancement of the ferromagnetic interaction. (2) There is a reduction in the antiferromagnetic direct cation-cation interactions, because the Cr3+ ions are further apart. (3) For those ligand orbitals which form π-bonds with both magnetic ions, the antiferromagnetic interaction tends to become stronger in going from O2−→S2−→Se2−. The last effect is shown to be overcome by a corresponding increase in the first (ferromagnetic term) described above. Larger covalency, leading to a reduction in the magnitude of the electron d-function energy, also reduces the third (antiferromagnetic) mechanism.
Electric Multipole Interactions between Rare Earth Ions
Authors: Wolf W. P. , Birgeneau R. J.
Published: January 2006

Electrostatic coupling between rare earth ions is important for nonradiative optical-energy transfer processes and for effective spin-spin interactions in ionic salts. In calculating such effects is it generally assumed that the electric quadrupole-quadrupole term is much larger than the corresponding interaction between higher degree multipole moments because successive terms of a given parity are each a factor r2/R2∼100 smaller than the preceding one. (r is the radius of 4f-electron orbit and R is the ionic separation). In this paper we point out that the relative importance of the higher-degree terms may be enhanced by electrostatic shielding and induced-moment effects similar to those affecting single ion crystal field terms, and we derive the Hamiltonian operators for the l-l′ multipole interactions up to sixth degree. Comparison with the observed single ion crystal fields also suggests that electric multipole interactions might be relatively important throughout the rare earth series and not only for the larger light ions, as is often assumed.

For pairs of Kramers ions at low temperatures (S′ = 12) the various multipole interactions will contribute in second order to different terms of an effective spin Hamiltonian of the form Si′ · K · Sj′, and the resulting interaction tensor K will generally be quite anisotropic, subject only to symmetry. There is also a corresponding anisotropic contribution to the magnetic g tensor, whose principal values and axes may thus be different from those of the single ions. Detailed multipole calculations are prohibitively complicated in the general case, and unless it can be shown that the higher degree terms are in fact negligible the observable interaction parameters in any particular case must therefore be treated as strictly empirical quantities, restricted only by symmetry.

A more detailed discussion of these effects is being published elsewhere.
Some Properties of Ferromagnetoelectric Nickel Iodine Boracite
Authors: Kaczér J. , Shalnikova T. , Hauptman Z.
Published: January 2006

Synthetic and very pure single crystals of nickel iodine boracite (Ni3B7O13I, NIB), up to 10 mm in their largest dimension grown by a chemical transport process were investigated, using a self-compensating recording balance under quasistatic conditions. Below 64°K ferromagnetic behavior in agreement with the results of Ascher, Rieder, Schmid, and Stössel was observed. Rotation diagrams showed a slight hexagonal component in the (111) plane of the crystal. In the (100) plane, however, large tetragonal torsion moments were recorded having frequent discontinuities and very large rotational hysteresis in fields of about 5 kOe. In a temporally and spatially constant magnetic field, a discontinuous but on the average linear decrease of the torsion moment was observed, taking about 30 min for it to vanish completely.

The above behavior can be only partly explained on the basis of ferromagnetic-domain wall motion. The influence of ferroelectric-domain processes coupled through ferromagnetoelectric interaction seems to be of considerable importance for the relaxation of magnetization and is therefore assessed and discussed.
Statistical Mechanics of Uniaxial Antiferromagnet with First- and Second-Neighbor Exchanges
Authors: Tanaka T. , Libelo L. F.
Published: January 2006

Low-temperature neutron-scattering study together with the zero-field AFMR in MnF2 furnished a reliable set of values of first- and second-neighbor exchange integrals and the uniaxial anisotropy energy. It is the purpose of this paper to see to what extent one can understand the high-temperature properties such as the Néel temperature and the anisotropic susceptibilities in terms of the known parameters obtained at low temperatures. For this purpose the cluster-variation method is used in which up to the two-spin correlation is taken into account. One of the main tasks of this method is to diagonalize effective one- and two-spin Hamiltonians. It is noted here that the anisotropy energy could be as big, at least, as the second-neighbor exchange energy. It is, therefore, not justifiable to treat the anisotropy energy as a small perturbation. In this paper diagonalization of the effective two-spin Hamiltonian is achieved for the actual spin, S = 52, and the Néel temperature and the anisotropic susceptibilities are calculated. Calculations of the sublattice magnetization and spin-flop field are in progress.
Theory of Anomalous Magnetic and Transport Phenomena in Eu-Chalcogenide Alloys
Authors: Yanase A. , Kasuya T.
Published: January 2006

Anomalous magnetic and transport properties of Eu0.99R0.01Se, in which R is Gd or La, are calculated for the magnetic impurity state model. An excess electron is trapped on a central impurity atom, R, and forms a magnetic impurity state, extending mostly onto the nearest neighbor Eu2+ ions and aligning these spins through s-f exchange interaction. Satisfactory agreement is obtained between calculated and experimental results for magnetic properties. For transport properties, a calculation based on the hopping-type impurity conduction model in which the activation energy is determined by the s-f exchange interaction agrees well with experiment for higher temperatures. The magnetic field effect is also calculated. For low temperatures or high magnetic fields the impurity-band-conduction mechanism becomes dominant. The calculation on this model explains the experiment satisfactorily.
Domain-Wall Mobility in Si-Doped YIG
Authors: Enz U. , van der Heide H.
Published: January 2006

Domain-wall mobility measurements have been carried out on picture frames made from single crystals of the composition Y3Fe5−2x3+(SiFe2+)xO12 in the temperature range from 77° to 300°K. The geometry of the picture frames was such that a wall parallel to the 110-plane was favored. For x = 0.04 and 220°< T <300°K the mobility R can be described by R = R0 exp(−E/kT), with E ≈ 0.4 eV and R0 ≈ 6×108cm sec−1Oe−1. The lowest mobility observed is 1 cm sec−1Oe−1. The values of R0 and E are consistent with loss measurements and can be understood as resulting from a diffusion aftereffect involving electron transitions between Fe2+ and Fe3+ ions. Below 220°K this heavily impeded wall motion could not be traced further, and a faster mode with R ≈ 103 was observed. The fast mode could also be observed above 220°K in applied fields surpassing a threshold field Ht. We identify this fast mode with the one observed by Wanas. The transition between the two modes can be described by Janak's theory of diffusion-damped wall motion.
Domain-Wall Dynamics in Fe from Pulsed NMR Experiments
Authors: Stearns M. B. , Overhauser A. W.
Published: January 2006

The dependence of free induction decay and echo amplitude of NMR signals in Fe on rf field strength B1 pulse length τ, and frequency ω has been determined by an extensive series of measurements The observed variation of free-induction decay amplitude with B1τ and ω cannot be explained by a rigid oscillation of domain walls, as usually supposed. Such a model fails, even though proper account is taken of enhancement-factor distribution caused by B1 orientation, and by spatial arrangement of spins perpendicular to the central plane of each wall.

We propose a different model where each domain wall is considered immobile along its periphery. Wall oscillation then resembles that of a drumhead. This modification alters significantly the enhancement-factor distribution. Furthermore, the average enhancement factor of each wall is proportional to its area. Accordingly, a distribution in wall areas is also postulated. The predictions of this model are in excellent agreement with free-induction decay spectra.

Enhancement factors of impure samples are smaller than those of pure ones by as much as a factor of three, indicating that the average wall area in such specimens is smaller by a similar factor. This supports the view that impurities augment domain nucleation.
Ferromagnetic Domains in Single-Crystal Permalloy Platelets
Authors: DeBlois R. W.
Published: January 2006

Thin single-crystal Permalloy platelets with up to 30% iron have been grown nonepitaxially by the reduction of mixed Ni-Fe halides. Those with weak negative magnetocrystalline anisotropy, 〈100〉 edges, and (100) surfaces display a “tulip” closure-domain pattern that reveals the mode of formation of the more complicated echelon and chevron patterns observed in pure nickel platelets. The tulip pattern may appear as a modified form of the Néel-strip closure structure that Bates and co-workers have studied extensively to obtain quantitative information concerning low-field domain processes. The platelets are thus useful for extending this work to smaller, thinner, and more nearly ideal specimens. Platelets in a region near 5000 Å thick and 5% iron have 〈100〉 domains and stripes even though K1<0.
A Magnetically Pulsed Neutron Beam for Time-of-Flight Measurements of Inelastic Scattering
Authors: Mook H. A. , Wilkinson M. K.
Published: January 2006

In many types of experiments, time-of-flight neutron spectrometers have the advantage of higher counting rates than crystal spectrometers, especially in view of the fact that multiple counters may be used. A method of magnetically pulsing a neutron beam has been developed that avoids the necessity of using high-speed rotating equipment.

The magnetic intensity in a Bragg reflection depends on the orientation of the atomic magnetic moments relative to the scattering vector, and it becomes zero when the moments are directed along the scattering vector. The direction of magnetization in ferrite crystals can be switched rapidly with an external magnetic field so that these crystals can serve both as a neutron monochromator and pulser. Furthermore, the neutron pulse-width and repetition rate can easily be varied by changing the current pulse that produces the external field.

The nuclear contribution for the (111) or (331) reflections can be made very small for 7Li-Ti ferrite of the correct composition. Large magnetic scattering intensities can be obtained in each case. The pulsed magnetic field is produced by switching the energy stored in a pulse-forming network into a magnet-drive coil by means of a silicon controlled rectifier. The pulse risetime is about 1 μsec and the pulse length in the present instrument is 12 μsec.
The Lattice Dynamics of Ferromagnetic and Paramagnetic Nickel
Authors: deWit G. A. , Brockhouse B. N.
Published: January 2006

The dispersion relations of the lattice vibrations in the paramagnetic and ferromagnetic phases of nickel have been measured along the major symmetry directions by neutron spectroscopy at 400°C, above the Curie temperature of 358°C, at 300°C, and at room temperature. The latter results are in substantial agreement with those of Birgeneau et al. The average of the ratios of the frequencies at equivalent positions in reciprocal space for different temperatures are: 〈ν(400°C)/ν(22°C)〉 = 0.962±0.003 〈ν(300°C)/ν(22°C)〉 = 0.976±0.004, with a standard deviation of a single ratio from the mean of 0.023. Some branches, such as the Λ(0ξ1) and T(ξξξ) exhibit larger average frequency shifts. Miiller et al. have observed an average frequency ratio of 0.970±0.004 for similar measurements on palladium, an element which resembles nickel in many ways. This fact suggests strongly that the frequency shifts arise largely from anharmonicity, and not from magnetic effects. Broadening of the neutron groups at 300°C and 400°C was observed; the average widths are greater than at room temperature by roughly 45%. For palladium no significant broadening was observed. Thus the broadening may be of “magnetic” origin.
Inelastic Scattering of Neutrons by Virtual Magnon States in Dilute Alloys
Authors: Kroó N. , Pál L.
Published: January 2006

The effect of weakly bound magnetic impurity atoms (Mn, Er) on the spin-wave spectrum of Fe has been studied by incoherent inelastic neutron scattering. Resonances of magnetic origin have been observed in both cases. In Fe-Mn the angular distribution of the total scattered intensity has a maximum, indicating that SMn≠0 and the density distribution of magnetic electrons of iron atoms near the Mn impurity has changed. In Fe-Er, a low-lying virtual level of long lifetime was found.
Spin-Wave Dispersion Relation in Fe-Ni Alloys
Authors: Menzinger F. , Caglioti G. , Shirane G. , Nathans R. , Pickart S. J. , Alperin H. A.
Published: January 2006

Spin-wave dispersion in Permalloys of composition Fe0.2Ni0.8 and Fe0.5Ni0.5 was investigated by neutron-scattering methods. Values of D in the expression for the spin-wave energy, ħω = Dq2(1−βq2), were found to agree with spin-wave resonance measurements, but the quartic coefficients β were an order of magnitude smaller.
Critical Scattering of Thermal Neutrons in Some Ferromagnetic Metals
Authors: Bally D. , Grabcev B. , Popovici M. , Totia M. , Lungu A. M.
Published: January 2006

The small-angle critical scattering of 1.25-Å-wavelength neutrons in Fe, Ni, and Co has been studied. The temperature shift of the maximum scattered intensity in Fe has been determined. The shape of the instantaneous spin correlation function has been deduced from angular distribution measurements of scattered neutrons in Fe and Co. The temperature dependences of the correlation range and magnetic susceptibility for Fe and Co are presented.
Neutron-Diffraction Study of MnSn2
Authors: Corliss L. M. , Hastings J. M.
Published: January 2006

MnSn 2 undergoes an abrupt transition at ∼74°K from an alternating-sheet antiferromagnetic structure (+ − + −) to a double-sheet structure (+ + − −). The propagation vector as well as the AF spin axis are directed along [110] in both structures. Neutron-intensity measurements on bulk samples confirm the sharpness of the transition as well as the hysteresis observed in the susceptibility and resistivity measurements of Kouvel and Hartelius. No change in the magnitude of the ordered AF moment is observed in passing through the transition. In the neighborhood of 90°K, however, weak satellite reflections appear, indicating the presence of a small modulated spin component. The extra reflections vanish abruptly at the transition and exhibit the same hysteresis shown by the main AF component. The diffraction data are consistent with a modulated spin of amplitude ∼0,5 μB oriented parallel to the AF spin axis [110] and propagated perpendicular to this direction [110] with a wavelength of ∼31 Å. This result is discussed in relation to the model of Kouvel and Jacobs.
Mixed Antiferromagnetic-Ferromagnetic State of Ordered Pt3Fe
Authors: Bellarby P. W. , Crangle J.
Published: January 2006

Previous neutron-diffraction and direct magnetic measurements have established that the stoichiometric alloy Pt3Fe exists in a chemically ordered state which has a simple antiferromagnetic structure. Small additions of excess iron lead first to a simple antiferromagnetic state, with the appearance of a second coexisting antiferromagnetic state at lower temperatures but with a single undistorted, cubic unit cell; and second to an incipient ferromagnetic state probably associated with clustering of the excess iron atoms. A model is developed here to explain the observed magnetic susceptibility and magnetization of the incipient ferromagnetic state. Single-domain ferromagnetic clusters are assumed to be acted upon by molecular fields associated with two sublattices in the antiferromagnetic matrix, and ferromagnetically with each other. The results of the calculation agree well with experiments.
Magnetism and Crystal Structure of Zirconium Compound with Laves Structure
Authors: Kanematsu K.
Published: January 2006

Magnetic studies were performed on intermetallic compounds with Laves-type structure, (Zr, Nb)Fe2, (Zr, Mo)Fe2, Zr(Fe, Mn)2, Zr(Fe, V)2, and Zr(Fe, Co)2. The crystal structure of all the compounds except Zr(Fe, Co)2 changes from the MgCu2 type to MgZn2 type with decreasing ZrFe2 content. The magnetism changes from ferromagnetism to paramagnetism with change from MgCu2 type to MgZn2 type, and it is concluded that the MgCu2 structure favors ferromagnetism and MgZn2 structure does not. Zr(Fe, Co)2 has a MgCu2 structure over the entire composition range and the magnetism changes from ferromagnetism to paramagnetism with decreasing ZrFe2 content.
Weakly Coupled Moments in the Strong Antiferromagnet, MnSn2
Authors: Kouvel J. S. , Jacobs I. S.
Published: January 2006

The tetragonal compound MnSn2 orders antiferromagnetically (in state I) at TN ≈ 325°K and transforms abruptly to another antiferromagnetic state (II) at Tcrit ≈ 73°K. Its initial susceptibility (χ), after decreasing from a peak at TN, rises (following a Curie law) to a much higher peak at 86°K (Tpeak), then drops precipitously at Tcrit. Pulsed-field measurements at 4.2°K show a steep increase in magnetization (σ) marking a II→I transition at Hcrit ≈ 120 kOe, above which σ approaches saturation at ∼1 μB/Mn (compared to ∼2.3 μB/Mn for the average sublattice moment). With increasing temperature, Hcrit decreases and the approach to saturation in state I becomes more gradual. Above Tcrit (even above Tpeak), the saturation effect persists, but only after σ vs H undergoes an inflection point near 45 kOe. These unusual variations in χ vs T and σ vs H for state I are consistent with a simple model in which the moments of superparamagnetic clusters of ∼55 Mn atoms are weakly coupled to each ordered sublattice. This superparamagnetic moment decreases with increasing c in isomorphous Mn1−cFecSn2, vanishing at c ≈ 14.
X-Ray and Susceptibility Study of the First-Order Magnetic Transformation in Mn3Pt
Authors: Krén E. , Szabó P. , Pál L. , Tarnóczi T. , Kádár G. , Hargitai C.
Published: January 2006

The first-order transformation from triangular to collinear antiferromagnetic structure (AF-AF transformation) has been investigated in the Mn3Pt1−xRhx and Mn3+yPt1−y systems by x-ray diffraction and magnetic methods.

At the AF-AF transformation temperature Tt a discontinuous change in the lattice parameter is observed with a magnitude depending on concentration but with always the same lattice parameter value of the collinear phase. This implies the existence of a critical lattice parameter value for the AF-AF transformation. Below the Néel temperature TN an anomaly in the thermal expansion occurs indicating exchange interactions dependent on the interatomic separation. This anomaly differs in sign for the collinear-paramagnetic and triangular-paramagnetic transitions. The susceptibility shows an abrupt change at Tt and a maximum at TN in the triangular phase. No maximum occurs for the collinear phase. The observed transition temperatures are in agreement with the earlier neutron diffraction data.
Magnetic and Chemical Order in Pd2MnAl in Relation to Order in the Heusler Alloys Pd2MnIn, Pd2MnSn, and Pd2MnSb
Authors: Webster P. J. , Tebble R. S.
Published: January 2006

The chemical and magnetic structures of the alloy Pd2MnAl, determined from magnetic and neutron diffraction measurements, are described, and compared with results recently obtained for the alloys Pd2MnIn, Pd2MnSn, and Pd2MnSb.

Pd 2 MnSn and Pd2MnSb have the Heusler (L21) structure, and are ferromagnetic with Curie temperatures of 189°K and 247°K, and magnetic moments, confined to the Mn atoms, of 4.23μB and 4.40μB, respectively. Pd2MnIn may be obtained in ordered (L21) and partially disordered (B2) forms by slow-cooling or quenching, respectively. The ordered alloy is antiferromagnetic with a Néel temperature of 142°K and a magnetic moment of 4.3μB per Mn atom. The magnetic moments are aligned in ferromagnetic (111) planes, with neighbouring planes oriented antiparallel as in NiO. In the partially disordered regions the Pd atoms remain ordered, but disorder occurs between the Mn and In atoms resulting in a B2, CsCl type, structure. The Mn atoms are still antiferromagnetically aligned, but on a magnetic unit cell of reduced size consistent with the change in chemical order.

Pd 2 MnAl has the B2 chemical structure, and a magnetic structure identical to that of the B2 regions of Pd2MnIn. The alloy has a Néel temperature of 240°K, and a magnetic moment of 4.4μB/Mn atom.
Spin Waves in Narrow Bands
Authors: Roth Laura M.
Published: January 2006

A variational method is used to improve upon the random-phase-approximation result for the energy of a spin wave in the fully aligned ferromagnetic state of a narrow s band. A calculation for long-wavelength spin waves in a narrow s band has been reported, and the effect of including correlation is to restrict the range of parameters in which the ferromagnetic state is stable. In this paper we generalize this result to the case of finite wavevector, and present numerical results for the case of a simple cubic lattice with nearest-neighbor interaction.
Temperature Dependence of the Spin-Wave Energy in the Itinerant Electron Model of Ferromagnetism
Authors: Mathon J. , Wohlfarth E. P.
Published: January 2006

The long-wavelength noninteracting spin-wave energy for metals at low temperatures is expressed as ħωq = Dq2 = (D0+D1T2)q2. The dependence of the coefficient D1 on the density-of-states function, the number of electrons per atom n and the effective short-range interaction energy I is discussed. The variation of D with T2 comes from the change with temperature of the relative occupation ζ and the chemical potentials of the ± spin subbands as well as from the direct asymptotic expansion of the Fermi distribution functions occurring in the expression for D. Although the calculation is based essentially on the random phase approximation this is here used in an improved version due to Izuyama and Kubo where these distribution functions replace the thermal averages 〈akσ*akσ〉 which would occur in a strict r.p.a. formulation. The general relation for D1 which is obtained reduces to a very simple form giving D = D0ζ/ζ0 in the limit zgr;0→0, where ζ0 is the value of ζ at 0°K.

The general relation for D1 is evaluated numerically for a single tightly bound energy band corresponding to a simple cubic lattice structure. Although the values of D1 obtained are in general negative as normally expected, positive values may also occur for some values of the parameters n and I. The experimental value of D1 for iron2 is in good agreement with the calculated values for reasonable values of the parameters. It is suggested that other experimental data should also be analyzed in terms of D1.

A full account of this work is being published in Proc. Roy. Soc. (London) (1968).
Dipolar Corrections to the Magnon Energy in the Band Theory of Ferromagnetism
Authors: Morkowski Janusz
Published: January 2006

The magnon energy spectrum in the itinerant d-electron model of ferromagnetism is calculated, taking into account the dipolar interaction of magnetic electrons. The model of band ferromagnetism is considered in which the intra-atomic interband exchange provides the dominant interaction of magnetic electrons. The case of two degenerate bands is discussed and the effective mass approximation for the one-electron energy is adopted.

In the long-wavelength limit, the energy of magnons is given by a formula closely resembling the one derived by Holstein and Primakoff for the Heisenberg model of ferromagnetics. The differences consist, first, in replacing the Heisenberg exchange parameter by a quantity which depends not only on the exchange splitting of spin − and spin + electrons, but also on the applied magnetic field, and second, in the presence of small corrections due to the magnetic form factor of the d electrons.
Phase Transition of Ferromagnetic Fermi Liquid
Authors: Izuyama Takeo
Published: January 2006

Critical phenomena of itinerant electron ferromagnets are investigated theoretically at T > Tc on the basis of a simple model in which the electron Coulomb repulsion is assumed to be effective only in an atomic cell. The ladder approximation or RPA used in our previous papers is now improved by taking into account the anomalous broadening of single-particle energy spectrum due to the scattering of individual particle motion by the critical spin fluctuations. The conservation law for the local magnetization is satisfied by the present treatment. The final results for the wavenumber dependent susceptibility and for the mass-operator of one-particle Green's function are expressed in compact forms. χ(q, 0) for small values of q is examined.
Stability of the Ferromagnetic State in the Band Model
Authors: Edwards D. M.
Published: January 2006

An exact expression is given for the coefficient C in the energy CK2 of a spin wave of small wavevector K in a ferromagnetic cubic metal, assuming complete spin alinement in the ground state. This is given in a form which applies to a one-band model with short-range interparticle interactions and is compared with the well-known r.p.a. formula. The main feature of the new result is a modified superexchange term which, in particular, takes account of two-particle correlations of the Kanamori type and of magnon-electron scattering. A necessary condition for the ferromagnetic state to be stable is C > 0 and this is investigated for some special cases. Some doubt is thrown on Nagaoka's demonstration of stability in an infinite crystal for the case of a narrow, almost half-filled, band.
Relation of Spin-Fluctuation-Induced Mass Renormalization to Nuclear Magnetic Relaxation in Transition Metals and Alloys
Authors: Doniach S.
Published: January 2006

General formulas are derived for the enhancement of low-temperature electronic specific heat and nuclear relaxation rate by spin fluctuations occurring in an electron gas near a magnetic instability. It is pointed out that this enhancement will occur near a transition to an antiferromagnetic state of the spin density wave type despite the fact that the bulk (q = 0) susceptibility will remain finite.
Correlation and Magnetic Effects in Narrow Energy Bands
Authors: Pratt George W. , Caron Laurent G.
Published: January 2006

Correlation and magnetic effects in narrow bands are studied using a new type of self-consistent cluster treatment of Hubbard's Hamiltonian. Here the dynamics consist of electron hopping between lattice sites and Coulomb repulsion between electrons only on the same site. The environment of the cluster is regarded as a particle reservoir, one such for each spin. Hopping between the cluster and reservoirs is described by fermion source terms. It is required that the thermodynamic average of the particle currents of each spin within the cluster equal the corresponding particle exchange between cluster and reservoir. Electron motion in the system is examined for varying ratios of the strength of the hopping to the Coulomb repulsion. This is done as a function of electron density, spin, temperature, and external magnetic field. A Mott transition is found under dynamical conditions close to those predicted by Hubbard when there is one particle per site. For 〈n〉 ≠ 1, no metal insulator transition exists although discontinuous changes in the cluster-reservoir coupling occur. The magnetic susceptibility of the cluster reveals antiferromagnetic behavior for 〈n〉 = 1, ferromagnetic behavior for 〈n〉 > 1, and partial ferromagnetic character for n < 1. This appears to agree with Nagoaka's results.
Interaction of Magnons with Longitudinal and Transverse Phonons (H in a [111] Direction)
Authors: Mielnicki J.
Published: January 2006

An analysis of the parallel pumping of magnetoelastic waves near the magnon-longitudinal-phonon and magnon-transverse-phonon crossover regions is presented. Equations of motion describing magnon-longitudinal-phonon and magnon-transverse-phonon interactions have been derived when H0 is along [111] crystallographic direction and transverse phonons are not essentially polarized along H0. Expressions describing the shape of the critical rf field vs dc field are obtained for both cases. The wavevector of the magnetoelastic waves that are excited by parallel pumping changes discontinuously with the d.c. field strength. Additional remarks on the influence of the elastic anisotropy on the discussed interactions are presented.
Magnetoelastic Two-Port Devices: Nondispersive Variable Delay Line
Authors: Kirchner E. K. , Olson F. A. , Bennett G. E.
Published: January 2006

Nondispersive variable delay in a two-port device using magnetoelastic waves in YIG has been developed. Performance is based on generating pure elastic waves at one port and shaping the internal magnetic field for the desired delay characteristic. Transverse elastic waves generated by a CdS thin-film shear-wave transducer, or generated by a longitudinal-wave transducer and a longitudinal-shear mode-conversion surface, provide the delay and are converted to magnetoelastic waves via a magnetic field gradient at the output coupling loop. Nondispersive delay is obtained by steepening the field profile of the axially magnetized YIG rod so that any dispersive contribution from the magnetoelastic region is minimized. Variable delay is obtained by mechanically translating the position of the profile field and the coupling loop. Operating bandwidth is determined by the frequency response of the transducer and the amplitude of the step-magnetic field, and has exceeded 50% in experimental tests, A two-port nondispersive variable delay device has been realized giving delay continuously variable from 1 to 6 μsec with less than 0.03 μsec dispersion over any 100-MHz band between 0.9 and 1.5 GHz.
Observation of Magnetoelastic Coupling by Spin-Wave Resonance
Authors: Weber R.
Published: January 2006

The method of spin-wave resonance has been used to study the magnetoelastic coupling of spin waves and elastic waves over the frequency range from 57 GHz to 62 GHz in several nickel-iron films of thicknesses of the order of 5000 Å. The film compositions have varied from 81Ni-19Fe (zero magnetostriction) to 65Ni-35Fe. For the latter composition, values have been obtained for the velocity of transverse sound-waves in the film (vt ≈ 3×105 cm/sec), for the magnetoelastic coupling coefficient (b ≈ − 6×107 erg/cm3), and for the phonon relaxation time (τ ≈ 2×10−10 sec) at 77°K. A calculation combining the equations of motion for the magnetization and the strain-wave equations under the appropriate boundary conditions for thin films, has provided fair theoretical agreement with the experimental data. However, there still remain points of disagreement concerning the width of the magnetoelastic interaction as a function of the wave-vector. The strength of the observed interaction has been found to be directly proportional to b, while its width in k-space has been found to be strongly dependent on τ−1.
Shock-Induced Demagnetization of YIG
Authors: Shaner J. W. , Royce E. B.
Published: January 2006

Measurements have been made of the shock-induced reduction in the component of the magnetization of polycrystalline YIG along the direction of an applied field. This reduction is induced by the passage of an explosively generated shock wave in a direction perpendicular to the applied field. The data indicate that the passage of the shock wave through the sample induces a uniaxial magnetic anisotropy in the material. This magnetic anisotropy arises from the stress anisotropy associated with the shock-wave compression of a material having a nonzero yield strength. The induced magnetic anisotropy makes the shock-propagation direction an easy direction of magnetization, and the observed reduction in the component of the magnetization along the applied field arises mainly from the rotation of the magnetization toward the easy direction. Experiments were performed at three bias fields, each at three widely spaced shock pressures. At 0.09 Mbar, the induced anisotropy energy may be described by a term in sin2θ. The coefficient of this term is in agreement with the value estimated from the usual magnetoelastic coefficients. At 0.20 and 0.44 Mbar, the contribution from a term in sin4θ is observed to dominate that from the sin2θ term, and the coefficient of the sin4θ term increases several-fold over its low-pressure value. A possible origin of this increase is discussed.
Improvement in the critical current density of ex situ powder in tube processed MgB2 tapes by utilizing powder prepared from an in situ processed tape
Authors: Nakane Takayuki , Kitaguchi Hitoshi , Kumakura Hiroaki
Published: January 2006

We dramatically improved the critical current density, Jc, in applied magnetic fields, B, of MgB2 tapes fabricated via an ex situ powder-in-tube (PIT) process using an Fe sheath and MgB2 powder prepared in lab. The starting powder for this technique was obtained from the core of an MgB2 tape prepared via an in situ PIT process using a mixture of MgH2 and amorphous boron. The highest Jc at 10 T was 6.1×103A∕cm2. This value exceeds not only that of conventional ex situ MgB2 tapes made from commercial MgB2 powder but also that of in situ MgB2 tapes fabricated using MgH2(3.4×103A∕cm2). These experimental data indicate that ex situ MgB2 tapes have great potential to upgrade the Jc-B property if the quality of the starting powder is improved.
Magnetoelastic Interaction in a Thin Magnetic Film
Authors: Ignatchenko V. A. , Kuzmin E. V.
Published: January 2006

Magnetic and acoustic excitation of coupled magnetoelastic oscillations in a thin magnetic film were investigated. The amplitudes and effective relaxation parameters of the resonance peaks of the magnetic and acoustic components of magnetoelastic oscillations were calculated. The following were analyzed in detail: (1) generation of hypersound in the nonmagnetic rod by means of a thin magnetic film sputtered onto the end of the rod (the conversion m→p); (2) generation of spin waves in the magnetic film by means of hypersound wave which is incident from the rod (the conversion p→m); and (3) double conversion m→p→m (echo-spectrum).
Nuclear Spin-Lattice Relaxation of Several Nuclei in Rb2MnCl4·2H2O
Authors: Taylor C. E. , Cowen J. A.
Published: January 2006

Rb 2 MnCl 4 ·2 H 2 O becomes antiferromagnetic below TN = 2.24°K. Measurements of the nuclear spin-lattice relaxation time Tln have been made as a function of temperature from 1.6° to 0.32°K for four nuclei. The resonance lines studied are characterized by the following frequencies at 1.1°K: 18.1 MHz for 1H, 8.53 MHz for 35Cl, 3.22 MHz for 85Rb, and 3.89 MHz for 87Rb. Standard rf pulse techniques were used and the measurements were made in zero external magnetic field. The results, discussed in terms of magnon relaxation mechanisms, suggest the following conclusions: (1) the relaxation is not due to paramagnetic impurities, (2) the temperature dependence of Tln, qualitatively the same for all four lines, is influenced by a large energy gap in the magnon spectrum, and (3) the dominant relaxation mechanism is probably a two-magnon Raman process, though lack of detailed information on the magnon spectrum prevents a quantitative comparison with theory.
Proton Spin-Lattice Relaxation in Paramagnetic Crystals in High Fields and Low Temperatures
Authors: Jeffries C. D. , Gunter T. E.
Published: January 2006

For nuclei in diamagnetic atoms a shell-of-influence model of the nuclear relaxation rate T1n−1 in dilute paramagnetic crystals at low temperature T and high fields H, predicts that 1/T1n ≈ 3/10(gβ/H)2〈1/r6〉[sech2(gβH/2kT)/T1e], where r is the separation between the nucleus and the paramagnetic ion, and T1e is the relaxation time of the ion. The factor sech2(gβH/2kT) ≈ 4 exp(−gβH/kT) arises from the fact that nuclei are relaxed only by mutual ion-nuclear spin flips, which are proportional to the ion population in the upper Zeeman level. The over-all consequence is that the nuclear relaxation time becomes exponentially longer at high fields and low temperatures. The above expression well fits our data for T1n for protons in (Nd0.01, La0.99)2Mg3(NO3)12· 24H2O over the ranges 10≤H≤50 kOe, 0.5≤T≤3°K. At 19.5 kOe, T = 0.5°K, we find T1n = 40 h; dynamically induced proton polarization can be maintained for very long times.
Nuclear Relaxation Rates in Single-Domain Cobalt
Authors: Kaplan N. , Jaccarino V. , Lewis R. T.
Published: January 2006

Recent studies indicate that the “intrinsic” nuclear spin-lattice relaxation rate, 1/T1, in the domains of bulk ferromagnetic metals is obtained only when an external field Hex is applied which exceeds the demagnetizing field and no walls remain. This has motivated the present work, on small, single-domain Co particles with the following results: (1) At low fields and at all temperatures between 1.3° and 300°K a very strong field dependence of T1 is found; T1 appears to be constant when Hex ≳ 4 kOe. (2) At all temperatures 1/T1 decreases monotonically with increasing particle size. (3) Studies above 4 kOe show that, unlike in the bulk, T1T is not constant. However, (d/dT) (1/T1) approaches a constant value for T ≳ 60°K which is independent of particle size. This is indicative of a contribution to the relaxation rate, not present in the bulk particles, arising from paramagnetic-like surface atoms that are exchange-coupled either to each other or to the cobalt host. No effects of superparamagnetism were found even for the smallest (∼110 Å) particles.
Indirect Nuclear Spin Relaxation in Heavy Rare Earth Metals
Authors: Sherrington D.
Published: January 2006

Relaxation through the Suhl-Nakamura effect is investigated in some heavy rare earth metals. Basal-plane ferromagnets have large magnon energy gaps and perturbation theory leads to estimates for the low-temperature relaxation which are not explicitly temperature dependent. Numerical estimates in Tb, Dy are found to be in reasonable agreement with experiment. In spiral structures the unperturbed magnons have no gap and account is taken of the strong coupling of low-momentum nuclear and electronic spin modes leading to temperature-dependent expressions for an effective gap and for the relaxation.
Torque Measurements of the Anisotropy of Energy and Magnetization of Nickel
Authors: Aubert G.
Published: January 2006

An analysis of experimental methods and the design of an appropriate torquemeter made it possible to improve by several orders of magnitude the accuracy of anisotropy measurements. The experiments performed on nickel between 100°K and the Curie point allowed, for instance, the determination of the eighth-order anisotropy constant K3 within a few percent. The anisotropy constants K1, K2, and K3 are found to be negative in this whole temperature range. An explanation is given of the main discrepancies in published data between the numerous determinations of anisotropy constants by static methods. The influences of both anisotropy of energy and anisotropy of magnetization are separated and measured as functions of applied magnetic field and temperature in the above-mentioned range. This interpretation of torque measurements has been confirmed by a direct determination of anisotropic magnetization of nickel at room temperature, and an absolute measurement of the saturation magnetization of nickel at 20°C has given 55.01±0.05 emu/cm3, a value higher by more than one percent than that usually quoted. Experimental evidence is given that the determinations of anisotropy constants from static measurements and from ferromagnetic resonance experiments may lead to fundamentally different results. Finally, a new phenomenological model of ferromagnetic anisotropy is proposed, introducing an effective field to describe spin-orbit interaction. This model seems to give a better agreement with experiments than the previous ones.
The Status of the Theories of Magnetic Anisotropy
Authors: Yosida K.
Published: January 2006

Microscopic considerations on magnetic anisotropy are described, and through discussion of the origins of magnetic anisotropy in some typical cases, the present status of the theories of magnetic anisotropy is reviewed.
Magnetostriction
Authors: Callen Earl
Published: January 2006

This paper reviews work since 1961 on magnetostriction, including both its sources and its temperature and magnetic field dependence beyond saturation (forced magnetostriction). A statistical theory of the magnetostriction of ferromagnets is in fine agreement with experiments on nickel and on heavy rare-earth metals, which are reviewed in detail. A different mechanism or two has been invoked to describe the behavior of iron. The paper sketches the origins of magnetoelastic coupling, particularly in rare-earth metals, and describes a phenomenological spin Hamiltonian approach to nonmetals. The theory is then extended to ferrimagnets, whose magnetostrictive behavior is described. Experiments on spinels and garnets are reviewed. The meager literature of the magnetostriction of antiferromagnets is surveyed, and the review ends with a brief description of oscillatory magnetostriction in diamagnets.
Neutron-Scattering Studies of Critical Magnetic Phenomena
Authors: Riste T.
Published: January 2006

This paper reviews neutron-scattering studies of critical magnetic fluctuations. By comparison with theoretical predictions, it is concluded that the spatial dependence of critical fluctuations and the temperature laws of their parameters are rather well in hand. For the time dependence of the fluctuations the situation is less clear, although two experiments, which are referred to, give evidence for a vanishing decay parameter at Tc, in agreement with theory. The question of the simultaneous existence of spinwaves and critical fluctuations is qualitatively discussed, and experiments bearing on this question are mentioned.
Temporal Fluctuations of Moment in Paramagnets by Neutron Diffraction
Authors: Collins M. F.
Published: January 2006

The inelastic scattering of neutrons by temporal fluctuations of spin in a paramagnet is reviewed. The scattering cross section is directly proportional to the imaginary part of the generalized frequency and wavelength-dependent susceptibility of the scattering material. For localized spins the theory of the scattering is discussed both in terms of exact solutions for the susceptibility in the few cases where this is known and in terms of the moments of the susceptibility. Experimentally the technique provides good measurements of exchange constants at high temperatures for insulators and for rare earth metals where the magnetic electrons are localized. In transition metals the form of the susceptibility is modified by the itinerance of the magnetic electrons and this is reflected in the experimental data. Exchange and correlation effects will enhance the low-frequency component of the susceptibility over that predicted for a noninteracting electron gas. The experimental data available appear to confirm the existence of such enhancement effects.
Magnetic Structures and Phase Transformations in Mn-Based CuAu-I Type Alloys
Authors: Pál L. , Krén E. , Kádár G. , Szabó P. , Tarnóczi T.
Published: January 2006

Investigations of the magnetic properties in the ordered CuAu-I-type manganese alloys are reviewed. The formation of the ordered phase and the possible magnetic structures are discussed. New experimental results for the Mn-Ni, Mn-Pd, and Mn-Pt systems are reported.

Neutron diffraction shows the basic antiferromagnetic structure to be the same for all the alloys investigated. In MnPt a magnetic-structure transformation has been observed. Below 800°K the magnetic moments turn from the basal plane gradually to the tetragonal axis. The variation of the transition temperature in the Mn-Pt system as well as that of the Néel temperature in the Mn-Ni, Mn-Pd, and Mn-Pt systems with concentration were measured and are theoretically analyzed.

The dependence of lattice parameters on concentration and temperature was determined. The lattice parameters suffer a significant but continuous change near the Néel temperature which is attributed to Mn-Mn exchange interaction.

Magnetic susceptibility measurements support the neutron-diffraction results. The increase in susceptibility, observed by several authors at lower temperatures, is found to be a result of contaminations.
Magnetization and g Factor Measurements of Nickel-Palladium Alloys
Authors: Fischer G. , Herr A. , Meyer A. J. P.
Published: January 2006

Saturation magnetization measurements of Ni-Pd alloys show for a Pd concentration range c, 0–50 at. %, a linear decrease of the mean atomic moment according to M = MNi−0.11c, with MNi = 0.616 μB. The extrapolated moment for Pd is 0.51 μB. Ferromagnetic resonance measurements at 35 GHz show a hyperbolic increase of the g factor with Pd addition. The calculated spin moments vary linearly as Msp = 0.56−0.17c, indicating 0.39 hole in the 4d band of Pd. The g factor of Pd would be 2.58, indicating a non-negligible orbital contribution.