bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical Evaluation of Limiting Storage Density by Thermomagnetic Recording
Authors: Minnaja Nicola
Published: February 2004

It is generally accepted that thermomagnetic writing allows in principle a far better density in digital storage than does conventional recording. The aim of this paper is to point out the physical limitations to storage density in thermomagnetic mass memories; some numerical evaluation will be performed in a highly idealized model, namely for a self-supporting homogeneous film with high coercivity, fully saturated in one easy direction, imbedded in a field opposite to the magnetization and heated in a spot by means of a high-energy well-focused beam. Nucleation of a reversed spot during cooling through Curie temperature is the recording phenomenon. For the density limitation the following reasons have been considered: (1) exchange interaction at the boundary between heated and cold region during cooling; (2) domain walls after cooling; (3) thermal conductivity. The approximate evaluation of the effect of exchange near Curie temperature has been possible by means of a new formulation of Brown's equation valid at high temperature. Considerations on thermal conductivity yield the result that the possible density is reduced if the recording speed increases.
Hall Effect and Asymmetric Scattering in Pure Iron and Dilute Fe–Co at 4.2 K
Authors: Majumdar A. K. , Berger L.
Published: February 2004

Hall effect and transverse magnetoresistance have been measured in fields up to 7 T (70 kG) in polycrystals. The Hall data for two samples of pure iron (ρ300/ρ4.2 = 1991 and 507 in small longitudinal fields) and for alloys with 0.25%, 0.50%, 0.92% Co fall roughly on the same Kohler curve as the iron data of Dheer if the resistivity in small longitudinal fields is used to construct the Kohler diagram. This curve is curved downwards at ωcτ ≈ 1, indicating the presence of more than two bands. For pure iron, extrapolation of the tangent of Hall angle to the high-field limit gives tanφH = (−2.2±0.5) × 10−2; a finite asymptotic value is predicted for a compensated magnetic metal, on the basis of asymmetric scattering. Extrapolation to the low-field limit for Fe–Co gives tanφH = (+1.4±0.2) × 10−2; a finite value is also predicted from asymmetric scattering. Contribution to Hall voltage from the nonclassical Kohn-Luttinger mechanism reaches 15% for the 0.92% Co sample, and must be subtracted from data.
Resistivity of Iron from 0.3° to 4.2°K
Authors: Trussell C. W. , Christopher J. E. , Coleman R. V.
Published: February 2004

Measurement of the temperature dependence of resistance and magnetoresistance in single crystals of iron have been made in the temperature range 0.3° to 4.2°K. Samples with residual resistivity ratios of 1000–4000 were used. In the temperature range 1° to 4.2°K measurements have been made in applied magnetic fields up to 50 kOe. The data have been fitted by computer to expressions for the resistance and in all cases both T and T2 terms are present. In the range 1° to 4.2°K a strong linear term is present as was previously reported.1 Below 1°K the T2 term appears to become more dominant and at temperatures less than 0.8°K the data appear to fit an exponential down to the lowest points measured. The coefficients are strongly affected by applied magnetic fields up to 1200 Oe. However, for 50 kOe fields the changes are much smaller than might be expected. The data will be discussed in terms of electron-magnon scattering, electron-electron scattering, and other possible contributions to the temperature dependence of resistance in the helium range of temperature.
Inelastic Scattering from MnF2 in the Critical Region
Authors: Schulhof M. P. , Heller P. , Nathans R.
Published: February 2004

The scattering law S(q, ω) in the anisotropic antiferromagnet MnF2 has been measured over an extended range of ω, q, and temperature in the neighborhood of the transition. The data have yielded values for the relaxation rate of transverse (Γ⊥) and longitudinal (Γ∥) fluctuations, the range of the spin-spin correlations (ξ) and the static staggered susceptibilities.1 From our results we find the relaxation rate at TN linear in q3/2 for both longitudinal and transverse. Above TN for q = 0 the longitudinal Γ∥ ∝ [(T - TN) / TN]0.95±0.05. The relaxation rates, when plotted on a reduced scale Γ∥ (q, ξ) ξ3/2 vs qξ, fall on a single curve indicating the existence of a homogeneous scaling function. This substantiates the prediction of general dynamical scaling.2 The scaling function has a different behavior above and below TN, joining continuously as ξ becomes large. A detailed comparison of the experimental S(q, ω) with theory awaits a more explicit calculation of the behavior of the critical fluctuations in the anisotropic system.
Critical Magnetic Fluctuations in MnF2
Authors: Dietrich O. W.
Published: February 2004

The critical spin fluctuations in MnF2 have been studied near the Néel temperature using a triple axis spectrometer with high resolution. The experimental data were analysed in terms of the static susceptibility and the spectral shape function. The results can be summarized as follows: The longitudinal component χ∥ (q) of the static susceptibility behaves as expected for a Heisenberg magnet, χ∥ (q = 0) ∝ [(T - TN) / TN]−1.35±0.01 and κ1∥ = 0.27[(T - TN) / TN]0.68±0.03 where κ1∥ is the longitudinal inverse correlation length. The corresponding transverse component χ⊥(q) is suppressed by the anisotropy. In terms of the suppressed Néel temperature T⊥ introduced by Moriya (1962) the temperature dependences were χ⊥(q=0)∝[(T−T⊥)/T⊥]−0.85±0.03 and κ1⊥=0.21[(T−T⊥)/T⊥]0.45±0.01. The characteristic widths of the spectral shape functions varied as Γ∥ (q = 0) = 1.5[(T - TN) / TN]0.62±0.02 and Γ⊥(q=0)=1.4[(T−T⊥)/T⊥]0.53±0.05 indicating thermodynamic showing down for the longitudinal component. The exponent 0.62±0.02 of Γ∥ (q = 0) is in bad agreement with the existing theories but is consistent with the similar exponent found in RbMnF3 by Nathans, Menzinger, and Pickart (1968). The q-variation of the transverse width Γ⊥(q) was consistent with the q3/2-law predicted by dynamic scaling.
Critical Spin Relaxation in Anisotropic Ferromagnets
Authors: Krueger David A. , Huber David L.
Published: February 2004

The decay times of the spin correlation functions have been calculated for uniaxial anisotropic ferromagnets which order in the plane perpendicular to the axis. The self-consistent theory when applied to the hydrodynamic region just above the critical temperature leads to coupled nonlinear integral equations which have been solved numerically. In addition to obtaining a temperature dependence in agreement with the dynamic scaling laws, our calculation yields absolute magnitudes of the decay times which are one or two orders-of-magnitude larger than the conventional theory which predicts a temperature dependence inversely proportional to the susceptibility. The dynamic scaling law behavior is followed over a temperature range of about 0.1 (Ty - Tz) / Ty where Ty and Tz are the perpendicular and parallel paramagnetic Curie temperatures, respectively. We have also made a comparison with experimental values of the decay rates in terbium. The agreement, while not precise, is an order-of-magnitude better than that obtained with the conventional theory.
Magnetism in Ni–Cu Alloys
Authors: Beck Paul A.
Published: February 2004

Low-temperature specific heat and magnetization measurements with Ni–Cu solid solutions indicate the presence of magnetic clusters in these alloys between 47% and 62% Cu. Up to 57% Cu the interaction between clusters is sufficient for the alloys to become ferromagnetic at low temperatures.1 The paramagnetic susceptibility of the alloys with higher Cu content is essentially independent of temperature above 200°K; in this temperature range the susceptibility depends on the Cu content. At low temperatures the susceptibility becomes temperature dependent and its value is strongly influenced by impurities, e.g., Fe and Co. The decrease of the ferromagnetic moment with increasing Cu content is not a result of the filling of the d band. Rather, it can be accounted for on the basis of the dependence of the moment associated with Ni atoms on their local atomic environment.2
Magnetic Properties of the System Au4(VFe)
Authors: Sill L. R. , Kimball C. W. , Clark R. H. , Mass W. J. , Darby J. B.
Published: February 2004

The magnetic properties of a series of Au80 (V20−xFex) alloys, where x varied from 1 to 9, were investigated to determine the effect of adding a solute with a localized moment on the magnetic behavior of Au4V. Magnetization measurements were made over the temperature range from 4.5° to 300°K. All specimens were magnetically ordered at 4.5°K. Above the magnetic ordering temperature, the inverse susceptibility vs temperature curves can be represented by an equation which includes a Curie-Weiss term and a Pauli paramagnetism term. The effect of adding Fe to the ordered Au4V alloy is to increase the effective moment and the Curie temperature of the alloys with up to 7 at.% Fe. However, the alloy with 9 at.% Fe did not chemically order under conditions employed for the other compositions, but did order magnetically at about 20°K.
Effect of High Pressure on Antiferromagnetism in Cr Alloys
Authors: Jayaraman A. , Rice T. M. , Bucher E.
Published: February 2004

The pressure dependence of antiferromagnetic ordering in Cr–Ru, Cr–Mn, and Cr–Re alloys has been investigated. At low concentrations, TN drops very rapidly and nonlinearly with pressure up to some point and then exhibits a much slower variation with a distinct break in slope. We attribute this break in slope to a transition from communsurate phase (C) at lower pressures to an incommensurate phase (I) at higher pressures. If we extrapolate the paramagnetic (P) to incommensurate boundary back to 1 atm we obtain values for the Néel temperature (TNI) for P - I transition, if the P - C transition had not occurred first. TNI when plotted against e/a shows a sharp break at pure Cr, in accord with theoretical predictions based on the ``depairing'' effects of impurity scattering.
Electrical Resistivity of the Ni–Cu Giant Moment System
Authors: Houghton R. W. , Sarachik M. P. , Kouvel J. S.
Published: February 2004

We have measured the resistivity of a series of Ni–Cu alloys as a function of temperature between 1.5° and 300°K, and as a function of field from 0 to 50 kG at 4.2°K. Experiments have been completed on nine paramagnetic alloys ranging in composition from 30 to 44 at.% Ni. A minimum occurs at low temperatures1 and the depth of the minimum increases with Ni content. At higher temperatures, the slope of the resistivity-temperature curve decreases with increasing temperature, and in fact, becomes negative for high nickel content. The high-field magnetoresistance is negative in all cases and is proportional to the concentration of giant polarization clouds2 as a function of Ni–Cu composition. These properties may be attributed to the scattering of conduction electrons from polarization clouds acting as large superparamagnetic moments. Results will also be presented for alloys beyond the critical composition for ferromagnetism.
Theory of Localized vs Band Magnetic Semiconductors
Authors: Kaplan T. A. , Bari R. A.
Published: February 2004

We have considered the question of whether the Hubbard Hamiltonian can lead to properties characteristic of two types of semiconductors, depending on the value of the ratio Δ/I of bandwidth to intra-atomic Coulomb integral. In one type there is a transformation, with increasing T, from a magnetic insulating state to a paramagnetic insulating state, and in the other the system goes from a magnetic insulating state to a paramagnetic metallic state. We have applied a new variational single-determinantal approximation which, in contrast to the standard thermal Hartree-Fock approximation, duplicates the exact behavior of the model both in the atomic limit, Δ/I=0, and the band limit Δ/I=∞. Limiting ourselves to well-known types of one-electron states, we have obtained stability boundaries (as determined by the free energy) between various phases. While the boundaries obtained to date have intrinsic interest, we find that further calculations are necessary to completely answer the question raised above.
Cooperative Magnetic Transitions in the Titanium-Oxygen System: A New Approach
Authors: Mulay L. N. , Danley W. J.
Published: February 2004

Semiconductor-to-metal-type transitions were studied by magnetic susceptibility measurements on pure single phases of Ti3O5, Ti4O7, Ti5O9, and Ti6O11. This work represents a more detailed investigation than those reported previously. From this and related studies the following model is proposed to explain the observed phenomena. Below the transition, groups of cations are distributed periodically throughout the lattice. Within these groups the d electrons are delocalized; however, ``constrained-type'' antiferromagnetism sets in between specific neighboring d electrons through homopolar bonding of cations. Neighboring groups interact via thermal excitation of electrons. Above the transition this type of magnetic ordering is modified by changes in crystal structure; here 3d overlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice. The behavior is described by the free-electron gas model. In addition, in all but Ti3O4, a Curie-Weiss law phenomenon is seen to be superimposed on this transition, which may result from relatively small (<11%) interactions between the unpaired spins. In proposing this model, calculations of the effective electronic mass and Goodenough's ideas have been used.
Onset of Magnetism in Vanadium Oxides: 51V NMR Studies of VO
Authors: Warren W. W. , Gossard A. C. , Banus M. D.
Published: February 2004

Vanadium ``monoxide'' VOx exists over a wide homogeneity range (VO0.79–VO1.30) in the cubic NaCl structure. With increasing oxygen content or V-vacancy content, its properties progress from weak temperature-independent paramagnetism to a stronger temperature-dependent paramagnetism, and from metallic to semiconducting conductivity. In the present work we have observed the nuclear resonance of 51V in small single-crystal pieces of pressure-annealed VO0.86, VO1.02, and VO1.23, using cw and spin-echo techniques in fields of 9–50 kOe at temperatures from 1.4°–300°K. In contrast with previously reported measurements, in no compound was there evidence of a sharp metal-to-insulator transition, and nuclear resonance was observable at all temperatures in all the compounds. A Knight shift of 0.4%, independent of composition and temperature, was observed, while the resonance linewidths increased with increasing susceptibility to a value of 5% of the applied field at 1.4°K in VO1.23. These results indicate that the bulk of the temperature-dependent magnetization comes from local moments on a minority of sites whose nuclear resonances are unobservable. These moments, however, cause the broadening of the observed majority-site resonance. A maximum in the transverse relaxation rates observed between 1.4° and 77°K in VO1.23 in a 47-kOe field apparently results from the onset of spin-spin correlations in this range.
Impurity Magnons in MnF2:Co
Authors: Parisot G. , Allen S. J. , Dietz R. E. , Guggenheim H. J. , Moyal R. , Moch P. , Dugautier C.
Published: February 2004

Complete understanding of magnetic impurity modes can be obtained from their energies and wavefunctions. Energies are obtained from absorption, Raman, and fluorescent measurements but wavefunction information from g factors is most useful when the impurity and host g factor are very dissimilar. To this end, impurity modes in MnF2:Co2+ have been studied by far ir absorption and Raman scattering, respectively. The g factor for the s0 mode is measured to be 3.21 and differs substantially from 4.24, the value obtained by EPR in the diamagnetic isomorph MgF2. The s0 energy and g factor can be explained consistently by exchange mixing of the ground state with excited orbital states of Co2+. The g factor for the pair mode at 170.3 cm−1 was found to be 1.61, significantly larger than g (s0) - g (Mn2+) = 1.2. The Ising pair mode energies and g factors have been renormalized by including transverse exchange to second order. The correct energies for the impurity modes are obtained but the g factors of the pair modes do not agree with experiment. Higher-order spin interactions produced by the orbital motion on the cobalt ion do not significantly alter this result.
Two-Magnon Raman Scattering and Infrared Absorption in MnF2
Authors: Thorpe M. F.
Published: February 2004

Recent work has shown that it is essential to include magnon-magnon interaction effects in order to calculate the two-magnon line shape for optical processes in antiferromagnets. These experiments are very sensitive to any structure in the density of states, and so it is necessary to include both J1 and J2 (obtained from inelastic neutron scattering measurements) even though J1 is much smaller than J2. We have calculated line shapes for Raman scattering and infrared absorption in MnF2 and obtained much improved agreement with the experimental results. We show that the sharp structure in the density of states and magnon-magnon interactions are equally important in determining the line shape. Simple arguments based on the Ising model to get the shift in peak position that are valid for RbMnF3, where there is a single sharp peak in the density of states, are no longer correct in MnF2. We conclude that a small J3 exchange and a possible extended range of interaction between light and the magnetic system can only have a small effect on the optical spectra.
Long-Range Magnetic Interactions in Uranium Monopnictides and Monochalcogenides and Their Solid Solutions
Authors: Kuznietz Moshe , Grunzweig-Genossar J.
Published: February 2004

Uranium monopnictides (Uv) and monochalcogenides (Uvi) and their solid solutions have a NaCl-type structure and are good conductors of electricity. Measurements of the magnetic properties of these compounds show that the magnetic interactions between the localized 5f-electrons are of long range. Such long-range interactions are responsible for the type-I antiferromagnetic structure of Uv compounds and the ferromagnetic structure of Uvi compounds, although these magnetic structures could be explained by molecular field theory. A neutron diffraction study of the UP-US solid solutions has revealed two new long-range magnetic structures at intermediate compositions, the type-IA (2+, 2−) antiferromagnetic structure (found also in UAs at low temperatures) and the antiphase (5+, 4−) ferrimagnetic structure. These structures are due to long-range RKKY-type interactions via the conduction electrons. Analogous structures are observed in the lanthanides (e.g., thulium) and their metallic compounds. As the weight of the anions present increases, the importance of superexchange interactions increases and affects the type of magnetic ordering. This effect is more pronounced in lanthanide monopnictides and europium monochalcogenides than in the Uv and Uvi compounds.
Mössbauer Study of Some 2–17 Lanthanide-Iron Compounds
Authors: Levinson Lionel M. , Rosenberg E. , Shaulov A. , Shtrikman S. , Strnat K.
Published: February 2004

We have studied the iron-rich, rare-earth-iron intermetallic compounds R2Fe17 (R = Pr, Gd, Tm, Lu), using the Mössbauer effect in 57Fe. The measured average hyperfine field H(T/Tc) acting on the 57Fe nuclei is a unique, monotonically decreasing function of T/Tc, with no observed irregularity at temperatures corresponding to a suspected ferromagnetic-antiferromagnetic phase transition in these compounds. The onset of paramagnetism occurs at temperatures close to the upper transition points measured by Strnat et al., namely at Tc = 282°, 472°, 271°, and 263°K for Pr2Fe17, Gd2Fe17, Tm2Fe17, and Lu2Fe17, respectively. Low-temperature (≃90°K) spectra indicate the presence of at least four magnetically nonequivalent Fe sublattices, while for T>Tc broadened 2-line absorption spectra typical of 57Fe at noncubic sites in paramagnetic material are observed.
Exchange Enhanced Generalized Susceptibility Function for Paramagnetic Chromium Including Band Structure Effects
Authors: Gupta R. P. , Sinha S. K.
Published: February 2004

We have calculated the exchange enhanced susceptibility function χ(Q) for paramagnetic chromium attempting to include band structure effects in a realistic manner. First, the unenhanced diagonal element χ0(Q, Q) of the susceptibility was calculated from an APW band calculation using the appropriate matrix elements and a Spline-fit interpolation scheme to produce an effective mesh of 128 000 points in the zone. The result exhibited a peak corresponding to the wave vector of the spin-density wave in chromium but differed quantitatively from previous calculations which ignored the matrix elements. Next, within the spirit of the RPA, the exchange enhanced susceptibility was obtained by approximately solving the coupled selfconsistent equations between the different Fourier components of the response to an applied field instead of simply dividing by the usual scalar denominator. This resulted in susceptibility functions which look similar to those obtained from a localized Heisenberg-type Hamiltonian.
Diagonal and Nondiagonal Susceptibility of a Ferromagnetic Electron Gas
Authors: Kim D. J. , Praddaude H. C. , Schwartz Brian B.
Published: February 2004

The spatial dependence of the spin and charge response of a ferromagnetic metal is discussed by introducing the q-dependent spin and charge susceptibility to a magnetic field χmm(q) and χem(q), and the spin and charge susceptibility to a charge potential χme(q) and χee(q). The nondiagonal susceptibilities χme and χem are shown to be very important in ferromagnetic metals, since the effect of an impurity can be represented by the addition of both a charge Ze and spin S. Some of the essential features of neutron scattering experiments about impurities in ferromagnetic metals can be understood by properly including both the diagonal and nondiagonal susceptibilities.
NMR Studies of Localized Spin Fluctuations in Al(Mn) Alloys
Authors: Alloul H. , Launois H.
Published: February 2004

Similar to Cu(Fe) or Au(V), the Al(Mn) system exhibits at low temperatures a T2 negative dependence of its impurity resistivity. We present NMR data, taken on both 27Al and 55Mn. At helium temperatures, the 55Mn resonance has a big negative Knight shift K = −2.05%. Its spin-lattice relaxation rate is frequency independent, and verifies a Korringa relationship. In high field, the 27Al line is split into a main line and two satellites. The first one with ΔK1/ΔKAl = −1.09 is attributed to first nearest neighbors of Mn. The temperature dependence of the Knight shift of the first Al nearest neighbors is much bigger than expected, and can be described by a (T+θ)−1 law, with θ = 2000±300°K. The fact that low-temperature local susceptibilities are in good agreement with Hartree-Fock theory and that the relaxation rate is frequency independent indicate that the localized spin-fluctuation theory is appropriate to describe that system.
Spin-Wave Instability in Nonuniform Fields and the Generation of Amplified Echoes
Authors: Herrmann G. F. , Hill R. M. , Kaplan D. E.
Published: February 2004

Amplified echoes can be described as a manifestation of nonlinear spin-wave interaction in an inhomogeneous magnetic field. In a typical experiment a very small pulse is incident on a ferrimagnetic sample at t=-τ and a large pulse is incident at t=0. An echo is thereupon emitted at t=τ with an amplitude greatly exceeding that of pulse 1. The disturbances associated with the two incident pulses and with the echo can be represented by ``quasi-stationary'' spin waves whose wave vectors vary with time on account of the field inhomgeneity. The wave vectors of the three waves are matched throughout and nonlinear coupling under proper conditions gives rise to unstable solutions in which the pair of spin waves associated with the pulse 1 and the echo grow exponentially at the expense of the spin wave generated by the large second pulse. At t=τ the echo spin wave evolves into a uniform precession and at that time radiates a highly amplified echo. The process is generically related to the Suhl instability in a uniform ferrimagnet in which pairs of opposite spin waves grow exponentially at the expense of the uniform precession mode. A simple mathematical model is introduced to illustrate this behavior.
Critical Reexamination of First-Order Electronic and Nuclear Spin-Wave Instabilities in Antiferromagnets
Authors: Platzker A. , Morgenthaler F. R.
Published: February 2004

The excitation of first-order spin-wave instabilities in an antiferromagnet which may carry a nuclear moment is reexamined. Nuclear, electronic, and mixed magnon pairs can be excited by processes which are analogous to the ferromagnetic case. Various geometries of pumping fields are possible and their critical values are given. Contrary to the assumption of previous authors, we show that when photon pumping fields are applied to a flopped antiferromagnet, the uniform precession is always excited, and its contribution to the threshold amplitude is significant and can never be neglected. This implies that direct magnon excitation in which the linear susceptibility is negligibly small (parallel pumping) does not exist in an antiferromagnet when the weak dipolar effects are ignored. However, when phonon fields are applied, parallel pumping processes are allowed due to the change in symmetry of the effective magnetoelastic pumping field. In particular when a longitudinal wave is propagating parallel (or nearly so) to the dc sublattice magnetizations the effect of the negligibly small linear subsceptibility can be ignored and the process is analogous to parallel pumping in a ferromagnet.
Magnetic Properties and Structure of GdN and GdN1−xOx
Authors: Gambino R. J. , McGuire T. R. , Alperin H. A. , Pickart S. J.
Published: February 2004

GdN is shown to be ferromagnetic but small amounts of oxygen (GdN1−xOx with x≈0.05) cause paramagnetism and short-range antiferromagnetic order to set in. We attribute this to nearest-neighbor interactions having opposite sign.
Neutron Diffraction Study of the Magnetic Structure of BaNiF4
Authors: Cox D. E. , Eibschütz M. , Guggenheim H. J. , Holmes L.
Published: February 2004

The orthorhombic compounds BaMF4 (M=Mn, Fe, Co, or Ni) have recently been found to undergo antiferromagnetic transitions at low temperatures. Susceptibility measurements on BaNiF4 reveal that a broad maximum occurs at about 150°K similar to that observed in a number of layer-type structures. As the temperature is lowered, χ∥ along b decreases rapidly, while χ⊥ goes through a shallow minimum around 50°K. At 77°K, no magnetic order can be detected in powder neutron diffraction patterns of BaNiF4, but at 4.2°K there are magnetic peaks which can be indexed on a unit cell doubled along the b and c directions (A21am orientation). The magnetic structure consists of puckered (010) sheets within which the moments are coupled antiparallel to neighboring moments about 4-Å apart. However, the crystal structure is such that the net interaction between adjacent sheets is zero, and ordering in the b direction depends upon interactions between second nearest sheets 14-Å apart. This is very similar to the situation in K2NiF4 and pronounced two-dimensional behavior is therefore to be expected. The moments are directed along the b axis and the ordered moment is about 2.0 μB per Ni ion. The magnetic symmetry is actually monoclinic, and the simplest magnetic space group is P2a21.
Spin-Wave Correlation Effects in CrBr3
Authors: Silberglitt Richard , Cobb C. H. , Jaccarino V.
Published: February 2004

A first-order (self-consistent) renormalized spin-wave theory has been used to fit the temperature dependence of the magnetization M (T, 0) in the hexagonal ferromagnet CrBr3. However, this theory appears incapable of simultaneously explaining M (T, O) and new measurements we have made of M (T0, H). Reexamining the first-order theory we find that (a) self-consistency corrections are negligible because of the low magnon density and (b) an equally satisfactory fit to M (T, O) can be obtained from a rectangular model with appropriately chosen intralayer JT and interlayer JL exchange constants and reciprocal-lattice volume. This is possible because JT is so large that there is little thermal excitation beyond the kx2+ky2 part of the spectrum. Since large kz magnons are present due to the smallness of JL, this is a strongly interacting system for which correlation effects are known to be important. Using the rectangular model we have treated spin-wave interactions to all orders via the t-matrix and find deviations of up to 40% from the first-order renormalization. Since the first-order renormalization effects on M (T, O) were shown to be large, we may infer from our calculations that the first-order fitting of one thermodynamic function was fortuitous and was made possible by the multiparameter character of the theory.
High-Field Magnetic Susceptibility of MnCO3 and MnO2
Authors: Leduc J. , Stevenson R.
Published: February 2004

The magnetic susceptibilities of polycrystalline MnCO3 and MnO2 have been measured in fields up to 300 kOe in the temperature region 80°–300 K. MnCO3 shows an increase in susceptibility at about 190 kOe at 300°K. The low- and high-field phases obey an antiferromagnetic Curie-Weiss law, and constants θ=61°±9°K, and θ=44°±9°K respectively, MnO2 shows a decrease in susceptibility at about 120 kOe and a subsequent increase at 270 kOe. The low-, intermediate-, and high-field phases again follow an antiferromagnetic Curie-Weiss law with constants θ=266°±57°K, 110°±18°K, and 159°±12°K. The transitions are discussed in terms of a suppression mechanism by which the high fields affect the ligand orbitals, and estimates of the transition fields are made on the basis of this theoretical model.
Magnetoelectric Evidence for the Attainability of Time-Reversed Antiferromagnetic Configurations by Metamagnetic Transitions in DyPO4
Authors: Rado George T.
Published: February 2004

The observation of the linear magnetoelectric (ME) effect in the metamagnetic antiferromagnet DyPO4 is reported. Also presented is a new quantum-mechanical mechanism of the ME effect which is, in contrast to previous mechanisms, specifically designed for DyPO4 and other magnetically ordered materials containing rare-earth ions. These results are shown to provide evidence for the feasibility of isothermal forced switching between time-reversed antiferromagnetic configurations by means of metamagnetic transitions. The optically deduced1 magnetic structure of DyPO4 is shown to require 4′/m′mm′ symmetry so that the ME susceptibility tensor has only one independent component α. It is found experimentally that |α| decreases monotonically with increasing temperature from roughly 2×10−4 Gaussian units at 1.5°K to zero at the Néel temperature of about 3.38°K. Application of a sufficiently large, static, magnetic field Hc along the antiferromagnetic axis (which is the crystalline c axis) causes not only a metamagnetic increase of the magnetization to its saturation value but (due to the transition to 4/mm′m′ symmetry) also an even more abrupt decrease of |α| to zero. The relative sign of α is determined, rather remarkably, solely by the sign of that Hc which caused the most recent metamagnetic transition prior to the measurement of α. This new switching method is potentially applicable to an isothermal storage and retrieval of information in materials which are both metamagnetic and magnetoelectric. All the above-mentioned work is described more fully elsewhere.2 Subsequently, appropriate ME annealing produced a larger value of |α| (≈1×10−3 Gaussian units) so that a non-ME-annealed DyPO4 crystal is not a single antiferromagnetic domain.
Dipole-Exchange Spin Waves in Ferromagnetic Films
Authors: De Wames R. E. , Wolfram T.
Published: February 2004

The effects of exchange on the magnetostatic surface and bulk spin waves of a ferromagnetic film in the regime where the dipole and exchange fields are both important are determined from the solutions of a sixth-order differential equation. The eigenstates are admixtures of bulk and surface waves. With an applied magnetic field parallel to the film surface and for small values of the wave vector parallel to the surface, k, the Damon and Eshbach surface state is split into segments which join adjacent bulk branches. No sharp cutoff of the surface branch occurs as the angle between k and the applied field is increased, instead the character of the branch changes continuously from surface-like to bulk-like. For values of | k |≳104 a number of branches have significant surface character so that no single branch can be identified with the Damon-Eshbach surface state. When the applied field is perpendicular to the surface a new type of surface spin wave occurs below the bulk manifold and is characterized by a complex wave vector normal to the surface. These surface states exist only when the dipole and exchange energies are comparable. By including microscopic perturbations in the boundary conditions it is shown that these states map smoothly into the microscopic solutions for the Heisenberg model. Detailed calculations of the dispersion curves of the bulk and surface spin-wave frequencies and eigenvectors are presented for yttrium iron garnet for both the parallel- and perpendicular-field cases.
Low Field Susceptibility Anisotropy in the Basal Plane of Hematite (α-Fe2O3) and its Dependence on the Remanent Moment
Authors: Schmidt V. A. , Fuller M. D.
Published: February 2004

Measurements of room temperature susceptibility in the basal plane of natural single crystals of α-Fe2O3 in which a high field isothermal remanence has been induced reveal a substantial anisotropy whose minimum is coincident with the direction of remanence. The susceptibility anisotropy appears to be related primarily to the low coercivity fraction of the remanent moment. Sizeable discontinuous changes in the susceptibility are sometimes present as the specimen is rotated. Recent domain wall studies have disclosed the presence in synthetic crystals of 180° walls exclusively. The present work suggests that the magnetization process within the basal plane of natural crystals of hematite is more complex.
Current- and Magnetic-Field-Induced Order and Disorder in Ordered Nematic Liquid Crystals
Authors: Teaney Dale T. , Migliori A.
Published: February 2004

The voltage-magnetic field phase diagram is explored for a nematic liquid crystal aligned between rubbed conductive glass plates. New patterns of long-range order are found for ac excitation and nonzero magnetic field. The variation of conductance with changes of order and with the angle with respect to the nematic director have been measured. An unexpected onset of turbidity is found at a voltage proportional to applied field when current is flowing parallel to the director.
Mechanism of Secondary Emission and Single-Particle Statistics from Low-Density Films of Alkali Halides
Authors: Garwin E. L. , Llacer J.
Published: February 2004

The mechanism of field-enhanced secondary emission has been studied experimentally in detail for KCl low-density transmission dynodes transferred in a dry nitrogen atmosphere. By means of a Kelvin probe located at the center of a collector, dynode surface potentials and their relationship with collector potential, secondary yield, primary energy, and current density have been studied. A unique relationship between yield and surface potential has been found, independent of all other parameters for current densities between 1 and 10 nA/cm2. This study clearly shows that an avalanche process, together with an increase in escape length, is responsible for the high yields obtainable. A study of transient effects reveals that the high surface potential needed for high yield decays rapidly after the primary beam is turned off. A smaller surface potential, with a long decay time constant, can be expected to provide slight field enhancement for single particle bombardment at low rates. The statistical distributions of the number of emitted secondary electrons per primary incident at high (1-MeV) and low (9.5-keV) energies have been measured for low-density films of CsI and KCl at low count rates. The results do not differ very much from those obtained previously from normal density films except that the yields are somewhat higher, particularly after the film has been charged with a dense primary beam. The distributions obtained are non-Poissonian in nature (more nearly exponential), and this is explained by their similarity to normal density films.
Correlation of the Electron-Emitting Properties of Cathodes in Vacuum and in Gas Discharges
Authors: Speros D. M. , Buccilli P. R.
Published: February 2004

Experimentation involving the transients of the starting behavior of gas discharges reveals that this proceeds, in general, in three distinct stages: (1) a time lag for breakdown, termed plateau A, (2) a period of transition due to interaction between discharge and cathode, termed plateau B, and (3) the establishment of a final steady state for both discharge and cathode, termed plateau C. The initial state of plateau B is identified as a state of cathode and discharge that would constitute a steady state if the effects of ionic bombardment and Joule heating were absent. This state is found to obey the following relation, which is analogous to the Schottky equation for cathodes in vacuum: lnI=lnIθ+[d(lnI)/d(V1/2)](V1/2−Vθ1/2),where Iθ=AT2 exp (−∈φ/kT), the Richardson-Dushman thermionic emission; Vθ is constant with a value near the ionization potential of the gas in the diode, and the voltage at which occurs a readily determined rearrangement of the cathodic sheath; the slope d(lnI)/d(V1/2) is a constant dependent on cathode material and the atomic number of the gas in the diode. It is shown that the extrapolated linear dependence of Eq. (1) intersects the ordinate (V=0) at a singular point IR, independent of the atmosphere of the diode (vacuum, gas or type of gas) and identical with the zero-field current of the cathode in vacuum. All experimental observations appear consistent with a theoretical model postulating a Schottky effect at the cathode. A consequence of these results is a firm correlation of cathode behavior in vacuum and in gas discharges.
Analytical Treatment of Radiation and Conduction Losses in Thermal Induction Plasmas
Authors: Eckert H. U.
Published: February 2004

Approximate solutions have been obtained of the Elenbaas-Heller equation for a two-zone model of thermal induction plasmas. Most of the heat production takes place in the external zone, where it is kept in balance by conduction losses. The major portion is lost to the wall, the minor transferred to the internal zone. This heat supply accounts for most of the radiation losses, which are confined to the internal zone. A small fraction of radiation is balanced by direct dissipation. Under these conditions, the distribution of the heat conduction potential assumes a parabolic shape for the inner zone, whereas it is described by Bessel functions in the external zone. A coordinate transformation in this zone accounts for the nonuniformity of the induced electric field and explicitly shows the effect of skin depth upon the profile shape. Matching of the zonewise solutions yields temperature distributions that are continuous to the first derivative and display the characteristic minimum at the axis known from experimental work. Generalized charts are presented by which a particular profile can be calculated from the gas transport properties and the time rate of change of primary magnetic flux. The method is applied to argon plasmas at atmospheric pressure with radiation properties calculated from atomic theory. Simple expressions are derived for the total radiated power and the radiation efficiency of the discharge by a further idealization of the model. They indicate that, at most, 50% of the input power can be converted into radiation as long as the discharge remains attached to the tube wall. It is also shown that large field penetration depth improves the efficiency. The results are in qualitative agreement with available experimental data. A quantitative check will require more complete experiments.
Intensity Correlation Functions of a Non-Q-Switched Laser, Measured by Second-Harmonic Generation
Authors: Mathieu E. , Keller Hj.
Published: February 2004

The intensity correlation function of the light from a non-Q-switched Nd:glass laser has been measured, using an optical gate (OG) to produce second-harmonic light. The resolution time of this optical gate depends on the differences of the group velocities involved and the crystal length of the optical gate; in the described experimental arrangement a time resolution of about 10−13 sec is achieved. The effect of thick crystals on the time dependence of the second-harmonic light intensity is discussed for some special cases. The experimental results are in agreement with the freerunning laser model, namely that a non-Q-switched Nd:glass laser has intensity fluctuations similar to thermal light. These results confirm some earlier results which have been achieved by a different technique.
Demagnetizing Fields in Thin Magnetic Films Due to Surface Roughness
Authors: Schlömann Ernst
Published: February 2004

The magnetic field generated by the magnetization has been calculated for films with arbitrarily rough surfaces for the case in which the magnetization is uniform throughout the film. Simple results are obtained on the assumption that the film surface is not too ``ragged,'' i.e., for the case in which the inclination of the local film surface relative to the mean surface is fairly small (<45°). A rough estimate of the demagnetizing factor for a direction in the plane of the film is given by π〈p2〉/λd, where 〈p2〉 is the mean square deviation of actual film surface from the mean surface, λ the predominant wavelength of the surface roughness, and d the film thickness. The results are discussed in the context of induced uniaxial anisotropy and ferromagnetic resonance.
Effect of Grain Size on the Microwave Properties of Polycrystalline Yttrium Iron Garnet
Authors: Patton Carl E.
Published: February 2004

Data on the parallel pump and subsidiary absorption threshold at 9.1 GHz and effective linewidth ΔHeff at 9.9 GHz have been obtained for polycrystalline YIG materials with an average grain diameter a0 from 1 to 30 μ. The fine-grain samples (1–10 μ) were made by hot pressing. The parallel pump spin-wave line-widths extrapolated to zero wavenumber, ΔHk→0, vary as a0−1, with ΔHk→0=25 Oe at a0=1 μ. The ΔHk for the fine-grain samples (a0 ≈ 1 μ) show an inverse k-dependence. A simple model in which the spin-wave transit time across individual grains limits ΔHk appears to explain the a0−1 dependence but not the observed k-dependences for ΔHk. The high-field ΔHeff is 5.5 Oe for a0=1 μ and varies as a0−1/2. The low-field values are slightly larger, due to a small two-magnon contribution arising from coupling to degenerate short-wavelength spin waves. Even though the high-field ΔHeff contains no two-magnon contribution, it is significantly larger than linewidths in single crystals. A recently proposed magnetostriction mechanism is consistent with the a0−1/2 dependence, but the predicted magnitude of ΔHeff is much smaller than observed.
Magneto—Mössbauer Study of Cu0.5Fe2.5O4
Authors: Nagarajan A. , Agajanian A. H.
Published: February 2004

Saturation magnetization and Mössbauer measurements at different temperatures from 4.2° to 623°K have been made on samples of composition Cu0.5Fe2.5O4 sintered at different temperatures between 1210° and 1350°C and quenched to room temperature. The Mössbauer spectra consist of a superposition of two six-line patterns, which are attributed to iron on B sites and A sites, on the basis of the change in the intensity of the spectra with the substitution of a small amount of Sc in Cu0.5Fe2.5O4. The relative isomer shifts and hyperfine fields also confirm this assignment. The internal magnetic field at the iron nucleus at 300°K is Hβ = 473±2 kOe and HA = 402±2kOe. The isomer shifts relative to metallic iron and the A and B sites are found to be characteristic of the iron in the trivalent state. The saturation magnetization, Curie temperature, and hyperfine fields are found to be practically independent of the sintering temperature. The ratio of the iron site occupancy factor is determined from the intensity ratio of the Lorentzian fits of the two patterns and is compared to the value deduced from magnetization measurements. The various models proposed for this system are discussed in the light of the present work.
Analysis of Saturation Magnetic Recording Based on Arctangent Magnetization Transitions
Authors: Potter Robert I.
Published: February 2004

The minimum transition length for the arctangent magnetization transition is calculated. Readback voltage for the arctangent transition, where the magnetization contains components both in and normal to the coating plane, is calculated via the reciprocity theorem and Karlquist's fringe field equations. Several misconceptions currently existing in the literature are discussed.
Light-Induced Modulation of Broad-Band Optical Absorption in CdS
Authors: Conway Edmund J.
Published: February 2004

The optical transmission of CdS single crystals has been changed using a laser. With ac techniques, these changes have been observed from 530 to 2000 nm in the transmission spectrum, and include both increased and decreased transmission. Comparison between the intensity dependence and the temperature dependence of photoconductivity and of change of transmission suggest a mechanism. The effect is a change of optical absorption produced primarily by the change in hole population of imperfection levels in the band gap. Changes in transmission due to population changes of approximately 1012 cm−3 have been observed.
Mass Spectrometric Study of Argon Afterglow Plasmas
Authors: Bhattacharya A. K.
Published: February 2004

Mass identified measurements of the time dependence of the density of atomic argon ions in decaying plasmas are reported. The measurements yield a mobility μ0(Ar+)=1.55 cm2 V−1·sec−1 for Ar+ in argon at a gas temperature of 300°K, and for the rate of conversion of Ar+ into Ar2+, a value of 1.9×10−31 cm6 sec−1.
Capacitance Energy Level Spectroscopy of Deep-Lying Semiconductor Impurities Using Schottky Barriers
Authors: Roberts G. I. , Crowell C. R.
Published: February 2004

The capacitance-voltage (C-V) relationship of a Schottky barrier diode is predicted for an energy distribution of impurity levels having spatially uniform concentration in an n-type semiconductor. The model applies for forward and reverse bias voltages at modulation frequencies near dc and at modulation frequencies at which one or more of the deep doping levels cannot respond. Effects of partial ionization of impurity species and the effect of electrons in the depletion region are considered. It is predicted that the diode [d(1/C)/dV] versus V relationship exhibits sharp minima when the barrier height minus the applied bias is equal to the energy level relative to the conduction band edge of any of the predominant deep-lying impurities in the semiconductor. The way in which deep lying impurities consequently affect a C-V impurity profile analysis is discussed.