bluuebunny/crossref_metadata_2025 huggingface dataset:
Amplified Ferrimagnetic Echoes
Authors: Kaplan D. E. , Hill R. M. , Herrmann G. F.
Published: February 2004

Multiple pulse echoes of the type originally discovered by Hahn in paramagnetic resonance have recently been observed in ferrimagnets. The behavior of the ferrimagnetic echo differs from the paramagnetic spin echo in two important respects. First, it is possible to obtain very large amplifications, that is, echoes exceeding the signal pulse by many orders of magnitude. Second, with increasing pulse separation τ the echo initially grows in an exponential manner, reaches a peak and then decays to zero. It appears that like spin echo this echo phenomenon depends upon storage of the magnetic excitation among a large number of resonators, which in the present case are provided by spin-wave modes. The specific echo mechanism is, however, quite different, resulting primarily from nonlinear intermode coupling which causes some of the excited modes to grow exponentially at the expense of others through a mechanism similar to parametric pumping. This process operates only under conditions of an inhomogeneous internal magnetic field and amplification is therefore favored in irregularly shaped samples. The amplification and the τ dependence of the echo amplitude are accounted for by this model. The delay and correlation properties of this phenomenon allow application to pulse compression and signal processing. Since neither cooling nor a highly homogeneous magnetic field are required, devices based upon the effect can be small, lightweight, and operate at room temperature.
Ultrasonic Propagation in EuO
Authors: Shapira Y. , Reed T. B.
Published: February 2004

The attenuation of 10 to 90-MHz longitudinal and shear ultrasonic waves propagating along the [100] and [110] directions of EuO single crystals was measured between 63° and 78°K. At zero applied magnetic field, the attenuation was constant above the Curie temperature, TC=70°K, but increased as the temperature was lowered through TC. The increase in attenuation was much greater for the shear modes than for the longitudinal modes. Preliminary experiments at 4.2°≤T≤78°K with 30-MHz longitudinal and 10-MHz shear waves propagating along the [100] direction showed that the excess attenuation at T<TC passes through a maximum near 60°K and is small at 4 2°K. An external magnetic field H≳5 kG removed the excess attenuation at T<TC. No attenuation peak was found at TC. The excess attenuation at T<TC is tentatively interpreted as being due to ferromagnetic domains. The elastic constants at 78°K are (in 1011 cgs units) c11 = 19.2±0.6, c44 = 5.42±0.13, c12 = 4.25±0.85. These lead to an adiabatic bulk modulus B = (9.2±0.6) ×1011 cgs units, and a Debye temperature θ = 350°K.
Critical Behavior of the Electrical Resistivity of Palladium Iron Alloys
Authors: Kawatra M. P. , Skalski S. , Mydosh J. A. , Budnick J. I.
Published: February 2004

The critical behavior of three PdFe alloys containing 3, 6, and 9 at.% Fe have been studied by measuring the electrical resistivity in the neighborhood of the magnetic ordering temperature and computing dp/dT and d2p/dT2 from the data. The critical exponent λ in the electrical resistivity above Tc is quite insensitive to the iron concentration, showing that the critical behavior of these alloys scales with Tc.
Concentration Dependence of the Range of Spin Polarization in Pd(Fe) Alloys
Authors: Kim D. J. , Schwartz Brian B.
Published: February 2004

According to recent neutron-scattering experiments of Low and Holden, the conduction-electron spin polarization around a magnetic Fe impurity in Pd has a very long range ∼10Å. This long range has been attributed to the large exchange enhancement in Pd. Another interesting aspect of this problem, although it has received less attention, is that the range of the spin polarization, the total giant moment per impurity atom, and the Pd high-field susceptibility depend very sensitively on the concentration of the magnetic impurity. For example, neutron scattering shows that an increase in the Fe concentration from 0.26% to only 4.0% reduces the range from 10 to 1 Å. We understand this very sharp dependence of the range by taking into account the increase in spin splitting of the host Pd bands with increasing Fe concentration. The expression for the q-dependent susceptibility for split bands is generalized to include the effects of the detailed structure of electron interactions and is used to obtain a quantitative fit to the data.
Examination of the Field Dependence of the Stoner Enhancement in Pd–Ni Alloys at High Magnetic Fields
Authors: Hein R. A. , Schindler A. I.
Published: February 2004

The paramagnetic susceptibility χ of the Pd–Ni system is a strong function of Ni concentration; the critical composition for ferromagnetism is somewhat less than 2 at.% Ni. This experimental result can be explained either by a uniform Stoner enhancement which increases with Ni concentration, or by an increased local enhancement in the vicinity of the Ni atoms. This latter concept is also useful in explaining several related exchange-enhancement dependent properties. To determine which model is correct, measurements of χ were made at 1.4° and 4.2°K in magnetic fields up to 87 kG. The local enhancement model predicts a field dependent Stoner enhancement (χ decreasing as H is increased). This type of field dependence in χ was indeed found at 1.4°K for all the Pd–Ni alloys examined (0.47, 0.97, 1.66, and 3.6 at.% Ni); the strength of the field dependence of χ was found to increase as the Ni concentration increased. At 4.2°K, the 0.47 at.% Ni sample yielded a field independent χ, while for pure Pd, χ was field independent at both temperatures. It was not possible to relate either χ or the magnetic moment to a simple power series in the magnetic field.
Reinterpretation of Hoffmann's Ripple Theory
Authors: Brown William Fuller
Published: February 2004

Hoffmann's linear theory of ripple in films has the advantage of an analytically simple Green's function; in the rigorous theory, only the Fourier transform is simple. Unfortunately, Hoffmann's treatment of the magnetostatic interaction rests on dubious approximations. It is proposed here that his formula be regarded merely as an empirical modification of the known Green's function for the case of no magnetostatic interactions, and his cy as a parameter to be adjusted for ``best fit,'' in some sense, to the rigorous formula; such an adjustment is possible if the fit criterion is an integrated quantity that can be evaluated from Fourier transforms by Parseval's theorem. Values of cv based on several such criteria agree fairly well with each other and with Hoffmann's value and give a fair fit as judged by the criterion. But the correct spatial variation of the first-order correction for magnetostatic interaction, which can be expressed in terms of tabulated functions, shows little resemblance to Hoffmann's; in particular, at large distances it behaves like a dipole field instead of decaying exponentially.
Determination of Domain Wall Profiles in Magnetic Films
Authors: Harte K. J. , Cohen M. S.
Published: February 2004

To determine the domain wall profile in a magnetic film from a defocused Lorentz micrograph, the electron intensity g(x) (which is inferred from the micrograph) must be translated into a curve of φ(ξ), where φ is the angle between the magnetization M and the wall normal and ξ is the position in the film plane measured from the wall center. To accomplish this task, classical and semiclassical ``inversion'' procedures which yield φ(ξ) from g(x) are presented. The validity of this approach is demonstrated by computer calculations on an artificial wall. An example of the inversion procedure applied to experimental data is given.
Far-Infrared Antiferromagnetic Resonance in Hexagonal YMnO3
Authors: Penney T. , Berger P. , Kritiyakirana K.
Published: February 2004

Hexagonal YMnO3 is an unusual compound. It is ferroelectric, and below 80°K is a triangular compensated antiferromagnet with all spins perpendicular to the polar c axis. The Mn is trivalent on a distorted D3h site. We have calculated the resonant modes of this triangular system. Two modes are degenerate in zero field and resonate at the geometric mean of χ−1 and the uniaxial anisotropy. We have observed these modes at v̄ = 43.0 cm−1 and at 2°K. Using this value and χ⊥ = 1.53×10−4 we find the uniaxial anisotropy is 3.2×107 erg/cm3. In this model the modes are split by H ∥ c with a geff close to the g of the Mn3+ ion. We observe a linear splitting with geff = 1.90±0.1 for H∥ to 54 kOe. Crystal field theory predicts g = 2.0 and a single ion anisotropy of order 107 erg/cm3 in agreement with our data. The temperature dependence of v̄ implies a T3.5±0.5 dependence for M (0) − M (T) in the range 0.2 <T/TN<0.5. The third resonant mode was not observed, presumably because the in-plane anisotropy is small.
Magnetic and Magneto-Optic Properties of FeRh and CrO2
Authors: Stoffel A. M.
Published: February 2004

For polycrystalline FeRh and single-crystal CrO2 the complex index of refraction N and the complex quantity Q (Q=Q0e−iq is the off-diagonal component of the complex permittivity tensor) were determined as a function of wavelength from 425 to 1000 mμ. The largest value for the magneto-optic amplitude Q0 was 0.008 at 700 mμ for FeRh, which corresponds to a Faraday rotation, F = π | NQ |/λ0, of 90 500 deg cm−1 and an absorption coefficient α, I/I0 = e−αx, of 329 000 cm−1 for a normalized Faraday rotation, f = F/α, of 0.28 deg. The largest value of Q0 for CrO2 was 0.025 at 1000 mμ which corresponds to F of 135 000 deg cm−1, α = 102 000 cm−1, and f = 1.3 deg. The wavelength dependence of the complex dielectric constant ε indicates a resonance close to 1000 mμ, which would explain the rather high magneto-optic effect. These values were measured at room temperature for CrO2 and for FeRh just above the first-order magnetic phase transition at 60°C. For both materials the temperature dependence of the coercive force was measured. The coercive force for FeRh increases with decreasing temperature and goes to infinity at the magnetic transition, while Hc for the CrO2 single crystal was temperature-independent.
Temperature Dependence of the Antiferromagnetic Resonance Linewidth in EuTe
Authors: Battles J. W. , Everett Glen E.
Published: February 2004

The linewidth of EuTe has been measured as a function of temperature through and above the Néel temperature. The samples used were single-crystal spheres, the magnetic field lying in a (111) plane along a 〈112̄〉 direction. EuTe is an easy-plane antiferromagnet having an in-plane spin-flop field of 700 G. These measurements at 24 GHz and about 8½ kG were for the low-frequency branch in the spin-flopped condition. The temperature dependence of the linewidth is similar to results obtained on other antiferromagnets. The linewidth rises from 150 G at 0.1 TN to a sharp maximum of 2700 G near, but slightly above TN, dropping to 1000 G at 2.0 TN. The results in the paramagnetic region have been compared with the [T/(T−TN)]¾ dependence predicted by Mori and dependence predicted by Mori and Kawasaki. Fitting this expression to the measured linewidth at 77°K gives good agreement down to 12°K (TN=9.6°K). For the antiferromagnetic region, the linewidth decreases approximately linearly with temperature and is not explained by existing theories.
Antiferromagnetic Resonance in Copper Formate Tetrahydrate
Authors: Seehra Mohindar S. , Castner T. G.
Published: February 2004

It has been suggested 1–3 that monoclinic Cu (HCOO)2·4H2O may closely approximate a two-dimensional antiferromagnetic. Magnetic susceptibility and magnetization measurements have indicated that antiferromagnetic ordering occurs at 17°K and have also shown evidence for weak ferromagnetism.2,3 An additional broad peak in the susceptibility has been observed near 65°K.3,4 Recent ESR measurements5 have shown a unique linear temperature dependence of the paramagnetic linewidth. In order to obtain more information on the magnetic parameters characterizing this crystal, we have made AFMR measurements between 1.4° and 15°K. Resonance transitions have been observed at 9 GHz, from 32 to 36 GHz, from 50 to 80 GHz, and from 107 to 124 GHz with magnetic fields up to 26 kOe. The minimum resonance field for the low-frequency mode in the ac plane is observed at a position (called the z axis) 3° from the c axis. The AFMR modes (frequency vs magnetic field) for the field along the z and b axis suggest that z and b are respectively the easy and intermediate axes for the spins. However, the following differences from the behavior of a normal antiferromagnet are observed: (1) Along the z direction, the high and lowfield branches intersect at a nonzero frequency. (2) Along the b axis a discontinuity is observed at 5.3 kOe at which a weak signal is observed at various frequencies between 9 and 72 GHz; (3) Above 5.3 kOe, an additional resonance is observed in the bc′ plane. We have attempted to fit the data with a two-sublattice model with z as the easy axis and weak moment along the b axis. The model contains isotropic exchange, symmetric and antisymmetric exchange, and g-tensor anisotropy. Good agreement between the calculation and the data is obtained for the low-frequency mode along the z and b axes above 5.3 kOe. From this fit and the static measurements2,3 consistent values of the exchange parameters are obtained. However, below 5.3 kOe the calculated resonance frequencies are considerably lower than the experimental ones. Furthermore, this two-sublattice model is inadequate to explain the discontinuity along the b axis and the additional resonance in the bc′ plane. A detailed paper will be published elsewhere.
Thermal Expansion in Dysprosium Aluminum Garnet
Authors: Philp J. W. , Gonano R. , Adams E. D.
Published: February 2004

We have observed the Néel-point anomaly in the linear thermal expansion coefficient α of dysprosium aluminum garnet using the three-terminal capacitance technique. The shape of the λ peak at 2.53°K is similar to the specific-heat peak reported by Keen, Landau, and Wolf, although our measurements have less resolution since the relative length change from 1°–4°K is only approximately 2×10−7. For |1-T/TN|<0.1 the data are consistent with a logarithmic singularity when TTN. The predominant magnetic interaction is the dipolar coupling of the Dy3+ ions. We use a thermodynamic relation between the expansion coefficient and the specific heat at the λ transition to estimate - ∂ logTN/∂logV=1.0±0.5 in agreement with theoretical expectations for dipolar coupling.
High-Temperature Critical Indices for the Classical Anisotropic Heisenberg Model
Authors: Jasnow David , Wortis Michael
Published: February 2004

Using the vertex renormalized form of the technique developed by Englert,1 high-temperature series expansions for the spin-spin correlation function of the classical anisotropic Heisenberg model are calculated for various lattices and anisotropies through order T−8 (close-packed lattices) and T−9 (loose-packed lattices). These series are combined and then extrapolated to give the high-temperature critical indices γ (zero-field susceptibility), ν (correlation range), and α (specific heat) as functions of anisotropy. The results are consistent with the hypothesis that the critical indices only change when there is a change in the symmetry of the system, e.g., in interpolating between the Ising and isotropic Heisenberg models, indices remain Ising-like until the system becomes isotropic, at which point they appear to change discontinuously. Previous results for the limiting cases are confirmed and extended. Series for the limiting cases (spin-infinity Ising, XY and isotropic Heisenberg models) as well as for the spin-½ Ising model2 have been derived for the B-site spinel lattice. Analysis of these series shows that the anomalies in the critical indices, reported on the basis of six-term expansions,3 disappear with the use of longer series.
Critical Magnetic Scattering from RbMnF3
Authors: Corliss L. M. , Delapalme A. , Hastings J. M. , Lau H. Y. , Nathans R.
Published: February 2004

Critical magnetic scattering of neutrons and sublattice magnetization have been measured for RbMnF3 in the vicinity of the Néel point. The following critical indices have been obtained: β=0.316±0.008,M∼(ΔT)β0.6%<ΔT/TN<5%ν=0.724±0.008,κ1∼(ΔT)ν0.04%<ΔT/TN<18%γ=1.397±0.034χ∼(ΔT)−γ0.04%<ΔT/TN<4%,where M is the sublattice magnetization, κ1 the inverse range parameter, and χ the susceptibility. ν and γ were obtained from the critical scattering assuming (1) the quasielastic approximation and (2) the existence of the parameter η in the cross-section formula as proposed by Fisher and discussed below. For ΔT/TN<0.1% the critical scattering exhibited a pronounced deviation from Lorentzian shape. Excellent agreement was obtained, however, using a cross section of the form (A/κ12+q2)1−η/2, where q is the momentum transfer relative to the (111) magnetic reciprocal lattice point. The best value of η, obtained by least-squares curve fitting, was 0.067±0.01. Analysis of the data for q≫κ1, where resolution corrections were negligible, gave the same result. It is to be noted, in addition, that the best values of ν and γ obey the relation γ=(2-η).
Magnetic Breakdown in Ferromagnetic Nickel
Authors: Zornberg E. I.
Published: February 2004

Using Mueller's interpolation scheme we had previously obtained good agreement with the dHvA measurements on the X pocket Fermi surface of Ni. We have extended these calculations to include the magnetic breakdown effects observed in the same experiment. Our calculations support the results of Ruvalds and Falicov who used k̄·p̄ perturbation theory and group theory to determine band degeneracies in symmetry planes normal to the applied magnetic field. Our model Hamiltonian yields the required accidental degeneracies between bands of opposite spin in these symmetry planes. Our band structure now also gives good agreement with the Fermi surface neck dHvA data.
Photoemission and Optical Studies of Cu–Ni and Ag–Mn Alloys
Authors: Walldén L. E. , Seib D. H. , Spicer W. E.
Published: February 2004

Photoemission and optical studies of transition metals alloyed into noble metals enable us to distinguish between the rigid band model for these alloys and models such as that of the virtual bound state in which the local nature of the transition-metal d state is retained. For the noble-metal-rich Cu–Ni and Ag–Mn alloys we have studied, the results are in agreement with the virtual bound state model. For both systems, no change is observed in the energy separation between the top of the noble-metal d band and the Fermi level. With the addition of Ni to Cu, states are formed above the Cu d states which extend to the Fermi level. For 7–14% Mn in Ag the filled Mn d states do not extend to the Fermi level, rather they are separated from it by about 1.5 eV. The width at half-maximum of the virtual bound states is estimated to be about 1.5 eV for Ni in Cu and about 2 eV for Mn in Ag.
Temperature Dependence of Ms and K1 of BaFe12O19 and SrFe12O19 Single Crystals
Authors: Shirk B. T. , Buessem W. R.
Published: February 2004

Single crystals of BaFe12O19 and SrFe12O19 were grown in a Na2O flux. Static magnetic measurements on the two crystal types were made to reveal Ms and K1 as functions of temperature. It was found that Ms of BaFe12O19 and SrFe12O19 are practically identical over the temperature range 77°K to the Curie point with a room-temperature value of 380 G. Extrapolations to absolute zero reveal that the Bohr magneton numbers are approximately 19.3 and 19.7 per molecule, respectively, while the Curie points are 740° and 750°K, respectively, for BaFe12O19 and SrFe12O19. K1 for SrFe12O19 is 10% higher than that of BaFe12O19 over the entire temperature range. Room temperature values are 3.25×106 and 3.57×106 erg/cc, respectively, for BaFe12O19 and SrFe12O19. Calculations made on the basis of the Stoner-Wohlfarth theory indicate that the intrinsic coercive force of SrFe12O19 will be higher than that of BeFe12O19 by about 10%. Theoretical room-temperature values of coercive force for an isotropic assembly of crystallites are 6000 Oe for BaFe12O19 and 6700 Oe for SrFe12O19.
Magnetic Properties of an Equi-Atomic PtCo Single Crystal Having a [111] Orientation
Authors: Walmer M. S.
Published: February 2004

The equi-atomic alloy of PtCo (77 wt% Pt, 23 wt% Co) is a high energy product, high coercive force permanent magnet material. Polycrystalline magnets of PtCo are isotropic. Measurements of the intrinsic coercive force as a function of crystallographic orientation made on a small PtCo single crystal showed it to be anisotropic. The maximum Hci value was found to occur in the 〈111〉 direction. Subsequently, a larger single crystal was produced in the form of a right circular cylinder for the purpose of making B-H measurements in the [111] direction. Values were obtained for Br, Hc, Hci, and (BH)max from hysteresis graphs of both the single-crystal PtCo magnet with a [111] orientation and the conventional isotropic polycrystalline PtCo magnet.

Values measured were: BrHcHci (BH)maxSingle crystal 〈111〉6500590082009.8×106Polycrystalline6400550075009.2×106.
Anhysteretic Magnetization of an Assembly of Single-Domain Particles
Authors: Jaep W. F.
Published: February 2004

A kinetic analysis of anhysteretic magnetization is presented that departs from previous analyses by including the time dependence of particle switching. Each particle is assigned a switching probability which depends upon the applied field and its period. The growth of anhysteretic remanence is shown to occur in a narrow range of fields immediately below the particle coercivity. In contrast to previous analyses, the kinetic analysis predicts a finite anhysteretic susceptibility for noninteracting particles that is large but not infinite as previously supposed.
High-Energy-Product Permanent Magnets
Authors: Buschow K. H. J. , Luiten W. , Westendorp F. F.
Published: February 2004

Due to its hexagonal structure, the intermetallic compound SmCo5 has an extremely high crystalline anisotropy. The anisotropy field determined on a single crystal is 290 kOe. Approximately 5% of this value is obtained for the coercivity when SmCo5 powder particles are subject to grinding or milling. The saturation magnetization (4πIs) is 9800 G. From this, one obtains a theoretical limit for the energy product of nearly 24×106 G Oe. Previous efforts to realize this value for a SmCo5 fine particle magnet only resulted in a value of 8.1×106 G Oe. The main reason is the low relative density of the pressed-powder magnet body (70%). A new method has been developed to press the powder particles together. Relative densities of 95% were obtained, resulting in a magnet body with an energy product of 18.5×106 G Oe. The corresponding values obtained for the remanence and coercive force are 4πIR=8700 G and BHC=8400 Oe.
Pressure-Effect Measurements Using a Vibrating-Coil Magnetometer
Authors: Menyuk N. , Kafalas J. A. , Dwight K. , Goodenough J. B.
Published: February 2004

A vibrating-coil magnetometer used in conjunction with a helium-gas pressure-generating system has proved to be a versatile new means of studying the effects of hydrostatic pressure on magnetic properties. The sensitivity of this system permits the measurement of paramagnetic susceptibilities on powder samples of the order of 0.1 g. Paramagnetic measurements on MnAs, which undergoes a hexagonal-orthorhombic transition upon application of pressure, have directly established that the transition is accompanied by a change of the manganese ions from a high-spin to a low-spin state. Regions in the pressure-temperature space in which the high-pressure (orthorhombic) phase is metamagnetic and ferromagnetic have been delineated. In addition, the pressure dependence of the magnetic-ordering transition temperature has been determined for a number of materials, and the results are consistent with the predictions of Good-enough where applicable. Finally, it has been established that this instrument can be used to study the effect of pressure on magnetic anisotropy.
Total Intensities of Some Crystal Field Transitions in MnO and MnS Related to the Antiferromagnetism
Authors: Huffman Donald R.
Published: February 2004

In recent years there have been reports of enhanced absorption intensities for crystal-field spectra of transition-metal ions in crystals where appreciable magnetic exchange coupling exists between ions. We report here some oscillator strength determinations for the two lowest-energy transitions of the Mn2+ ion (6A1q → a4T1g and a4T2g) in the antiferromagnets MnO and α-MnS. These broad bands are anomalously intense (f≃10−3) for such spin- and parity-forbidden transitions. The data show an increase of oscillator strength with temperature below the Néel temperature which can be approximately related to the sublattice magnetization. Above TN, there is a leveling off of the oscillator strengths. The large intensities of these transitions and their relation to the magnetism suggest that the violation of ``spin-forbiddenness'' is associated with the antiferromagnetic coupling in these broad, phonon-assisted transitions as well as in the much narrower and weaker magnon sidebands.
Critical Pressure for the Metal-Semiconductor Transition in V2O3
Authors: McWhan D. B. , Rice T. M.
Published: February 2004

The metal-semiconductor transition is found to be suppressed at Pc=25.8±1.0 and 23.0±1.0 kbar for increasing and decreasing pressure. The temperature and volume dependence of the resistivity of the metallic phase is anomalous. The resistivity varies as BT2 at low temperature and rises more slowly at high temperatures. The data are analogous to Sr and Yb in the region where the band overlap approaches zero with increasing pressure. We propose that the metal-semiconductor transition in V2O3 may be driven by the Coulomb attraction between electrons and holes and may be an example of an excitonic phase change.
Infrared Transmission, Magnetic Birefringence, and Faraday Rotation in EuO
Authors: Dimmock J. O. , Hurwitz C. E. , Reed T. B.
Published: February 2004

We have measured the infrared transmission, magnetic birefringence, and Faraday rotation of EuO single crystals in the wavelength range between 2 and 20 μ. These effects have been studied as a function of temperature between 5° and 200°K in magnetic fields up to 9 kG. The most transparent samples have an absorption coefficient less than 0.5 cm−1 in the range between 3 and 9 μ and less than 1.0 cm−1 at 10.6 μ. The transmission decreases gradually at longer wavelengths and is largely independent of temperature and magnetic field except for shifts in the short wavelength absorption edge and effects of domain scattering at low temperature. Below the Curie point at about 70°K these crystals show a strong magnetic birefringence and Faraday rotation. At 20°K and 9 kG the difference in the index of refraction for light polarized perpendicular and parallel to the magnetic field is n⊥-n∥=1.07×10−2 at 10.6 μ and increases gradually to 1.5×10−2 at 2.5 μ. Under these experimental conditions the Faraday rotation is approximately 660 degrees/cm at 10.6 μ corresponding to nr-nl≃3.9×10−3. This rotation increases to about 2.5×104 degrees/cm at 2.5 μ. This corresponds to a specific rotation per unit attenuation in excess of 150 degrees/dB at 10.6 μ and 104 degrees/dB at 2.5 μ.
Magnetism in Two Dimensions
Authors: Lines M. E.
Published: February 2004

A brief review is presented of the present state of knowledge (both theoretical and experimental) concerning quasi-two-dimensional Heisenberg ferromagnets and antiferromagnets, and comparisons are made with typical three-dimensional magnetic systems. The greatly enhanced quantum and short-range order effects in two dimensions expose the limitations of most current statistical theories and should stimulate both theoretical and experimental interest in a number of directions. These include zero-point spin deviations (for antiferromagnets), bound-magnon states, possible new phase transitions to disordered states with infinite susceptibility, critical coefficients, the role of anisotropy in stabilizing long-range order in two dimensions, and the study of propagating spinwaves in the paramagnetic phase.
Ferromagnetic to Canted-Ferrimagnetic Transition in Fe(Pd, Pt)3
Authors: Kouvel J. S. , Forsyth J. B.
Published: February 2004

Magnetization and neutron-diffraction measurements were made on the pseudo-binary Cu3Au-type ordered alloys Fe(PdxPt1−x)3 down to 4.2°K. The magnetic transition reported earlier for compositions near x=0.6 is found to correspond to a rapid but continuous change, as the temperature is lowered below ∼140°K, from pure ferromagnetism (as in FePd3) to a state in which the Fe moments open out into a simple canted structure (with two sublattices arranged identically to those in the pure antiferromagnet FePt3). The Pd, Pt moments are always parallel to the net ferromagnetic Fe moment; these moments vanish for compositions x<0.5. The occurrence and nature of this transition can be readily explained within a molecular field model in which the Pd, Pt moments are induced by and are a rapidly saturating function of the net exchange field from the Fe moments.
First-Order Magnetic Transitions in (Fe, Mn)≥2As
Authors: Rosenberg R. M. , Cloud W. H. , Darnell F. J. , Flippen R. B. , Butler S. R.
Published: February 2004

First-order Fi→AF transitions, with increasing temperature, have been observed in the system Fea−xMnxAs when a=2.00 to 2.35. The transition temperature, Ts, is strongly dependent on the Mn/Fe ratio and can be set over the range <273° to 345°K. Similar transitions, recently reported for Fe1.95−xMnxAs, are limited to Ts<153°K. The transition, which involves no change in the P4/nmm crystal symmetry, is accompanied by a discontinuous decrease in crystal volume with increasing temperature. Saturation magnetization, σ, ΔV/V, and field coefficients, dH/dT, were measured. Typical properties when Ts=286°K are: σ=14 emu/g, ΔV/V=0.58%, dH/dT=3680 Oe/deg, dP/dT (calc)=60 atm/deg, ΔS (calc)=0.23 cal/mole deg. Measurements of σ vs H, parallel and perpendicular to c, the tetragonal axis, show the basal plane to be the preferred direction of magnetization with anisotropy fields near 36 000 Oe. The magnitude of σ, and its unusual temperature dependence, indicate ferrimagnetic ordering in the low-temperature state.
Mechanism for the First-Order Magnetic Transition in the FeRh System
Authors: Tu P. , Heeger A. J. , Kouvel J. S. , Comly J. B.
Published: February 2004

Measurements of the field dependence of the critical temperature (Tcrit) for the first-order antiferromagnetic-ferromagnetic transition in FeRh established that the total entropy change (ΔST) at Tcrit is much larger than the change in lattice entropy (ΔSL). Doping experiments (Pd, Pt, or Ir) showed that ΔST−ΔSL=ΔS varies almost linearly with Tcrit. Apparently the transition involves a change in the electronic density of states and thus in the electronic entropy, S=γTcrit, where γ is the electronic heat coefficient. To test this mechanism, low-temperature specific-heat measurements were made on several compositions in the Fe(Rh, Pd) system. The compositions and γ values are Fe53Rh47 (59 μJ/g·°K2), Fe51Rh49 (60 μJ/g·°K2), Fe48Rh46Pd6 (64 μJ/g·°K2), Fe49Rh51 (16 μJ/g·°K2), and Fe48Rh49Pd3 (31 μJ/g·°K2). The first three are ferromagnetic at T=0; the latter two are antiferromagnetic with Tcrit of 310° and 210°K, respectively. The insensitivity of γ to Fe concentration in the ferromagnetic samples suggests a broad peak in the d-band at the Fermi surface. The resulting entropy changes, ΔS=(γF−γA)Tcrit, agree with Kouvel's measurements and confirm the electron gas entropy as the main driving force for the transition.
Studies of Au4X-Ordered Alloys: Electron and Neutron Diffraction, Resistivity and Specific Heat
Authors: Toth R. S. , Arrott A. , Shinozaki S. S. , Werner S. A. , Sato H.
Published: February 2004

Micro-, chemical and magnetic structures of ordered alloys Au4X, where X stands for various transition elements and their mixtures, have been studied using electron, x-ray, and neutron diffraction. Specific heat and electrical resistivity measurements have been carried out for Au4Ti, Au4(Ti0.5Cr0.5)Au4V, Au4(V0.5Mn0.5)Au4Cr, Au4(Cr0.5Fe0.5), and Au4Mn. The magnetic form factors are given for Au4Cr and Au4Mn.
Magnetic Properties of Fe5Ge3–Mn5Ge3 Solid Solutions
Authors: Austin A. E.
Published: February 2004

Measurement of saturation magnetization and Curie temperature were made of the solid solutions (Fe1−yMny)1.67Ge for 0≤y≤0.4. The Curie temperature decreased linearly from 485° to 326°K while the saturation magnetization, in μβ per mole, decreased from 2.65 to 2.35 for y=0.2 and then increased up to 2.66 for y=0.4. The limit of the solid-solution field is the ordered ternary compound Fe0.9Mn0.9Ge with Curie temperature 241°K and 1.39 μβ per mole. The manganese with a magnetic moment of 3.0 μβ preferentially substitutes for iron with 1.80 μβ in (2a) sites of Fe1.67Ge. The net moment is decreased by the increasing antiferromagnetic component of iron atoms in (2d) sites with its cant angle increasing from 28° to 80° for y=0.2, while for 0.2<y≤0.4, the cant angle remains at 80° and the net moment increases. The ferromagnetic structure of the ordered compound Fe0.9Mn0.9Ge has a manganese sublattice (2a) similar to same one in Mn5Ge3 and an iron sublattice (2d) comparable to that of Fe5Ge3. Hyperfine fields also indicate additivity of the two sublattices from the binary compounds.
Temperature Dependence of the Magnetic Moment and Hyperfine Constant of Nickel
Authors: Kaul Roger , Thompson E. D.
Published: February 2004

The results of measuring the saturated magnetic moment of pure, single-crystal Ni over the temperature range from 4.2°–300°K with an accuracy of 0.1% of the total moment are presented. A Foner-type vibrating sample magnetometer was used with an applied uniform field of 10 kOe in the [111] direction. The magnetic-moment data was extrapolated to zero applied field using the measured, temperature dependent saturated magnetic susceptibility. Using the appropriate thermodynamic equations, the derived magnetization at constant volume is presented and is compared to the NMR hyperfine field results. The temperature dependence of the hyperfine constant in the form A(T)=A(4.2)[1+CnTn], where n=32 and 2, is evaluated near 300°K. The T3/2 coefficient in the temperature dependence of the magnetization at constant volume is found to be (7.8±1.1)×10−6°K−3/2 in zero applied field from 4.2 to 100°K.
Ferromagnetism in VAu4 and Related Compounds
Authors: Asik J. R. , Gambino R. J. , McGuire T. R.
Published: February 2004

VAu4 undergoes an order-disorder transformation at about 750°K from a fcc to a bct (Ni4Mo) structure. In the ordered (bct) structure, VAu4 exhibits a paramagnetic moment with a θ ranging from 41° to 53°K. It undergoes a ferromagnetic transition at Tc∼53°K. Both the paramagnetic moment and saturation moment depend on the heat treatment and thus the degree of crystallographic order. To determine whether VAu4 is an intrinsic ferromagnet, we have made magnetization and susceptibility measurements on alloys containing up to 5% Fe replacing V. For these alloys no enhancement of the paramagnetic or ferromagnetic moment was observed. We have observed the kinetics of the moment formation in VAu4 by high-temperature-susceptibility measurements between 700° and 800°K. Starting with a disordered sample, it only takes approximately 10 min to achieve μeff=1.40 μB. After this initial rapid increase, the rate of growth slows considerably, and if we extrapolate our results, we estimate that it requires approximately 5 h to reach a susceptibility corresponding to 1.73 μB. Above 800°K, a temperature-independent susceptibility χB of 0.71×10−6 emu/gm·Oe is observed. Using this value to correct the susceptibility data below the ordering temperature, one obtains a very good fit to the Curie-Weiss law. If one corrects χB for the core diamagnetism and assumes this value is due solely to Pauli paramagnetism, a value of 23.6 states/eV·(VAu4) is obtained for the density of states in good agreement with Luo, Creveling and Maita's1 determination of the electronic specific-heat density of states, 23.2 states/eV·(VAu4). However, since one expects the orbital paramagnetism to be nonnegligible, this is certainly an overestimate. To further study this material, we have made magnetic measurements on the alloys VAu4(1−x)Ag4x where x=0.025, 0.10, and 0.20. Ferromagnetism was observed in the alloy with x=0.025, with a saturation magnetization of 0.12 μB/VAu4 and Tc=28°K, even though XRDF measurements showed no evidence of long-range order. Hc was found to be 16 kOe for VAu3.9Ag0.1, even though we found Hc>25 kOe for VAu4. Ag was found to inhibit the formation of the ordered phase and narrow the range of temperature over which it occurs. For x=0.10 and 0.20, no ferromagnetism and no time-dependent susceptibility were observed.
Photoemission Studies of the Electronic Structure of Transition Metals
Authors: Eastman D. E.
Published: February 2004

New photoemission studies of the optical densities of states of several transition metals (nickel, cobalt, iron, manganese, and chromium) and copper are reported. Experimental results are compared with theoretical one-electron energy-band densities of states. The principal results of these studies are: (1) New structure is observed which shows encouraging agreement with magnetic one-electron energy-band calculations, and (2) the large anomalous peaks in ferromagnetic Ni, Co, and Fe about 5 eV below the Fermi level which were previously reported by others have been found to be due to spurious surface effects. The two largest peaks observed in the optical densities of states of ferromagnetic Ni and Fe and anti-ferromagnetic Cr agree within 0.15 eV, respectively, with the two largest peaks in calculated magnetic energy-band densities of states for each of these metals.
Optical Studies and Band Structure of Cu–Ni Alloys
Authors: Feinleib J. , Scouler W. J. , Hanus J.
Published: February 2004

This paper reports on recent results of optical studies on the Cu–Ni alloy system. Reflectivity measurements have been made on Cu, 84 Cu-16 Ni, 39 Cu-61 Ni, and Ni from 0.2–10 eV. Infrared absorption in the Cu-rich alloy indicates the existence of a virtual bound state approximately 1 eV below the Fermi level. This absorption is related quantitatively to the model of the state and is compared with other dilute alloys of the transition metals.
Mössbauer Study of Perovskites of Composition SrFe1−x CrxO3−y
Authors: Banks E. , Mizushima M.
Published: February 2004

Single-phase specimens of composition SrFe1−xCrxO3−y with x≤0.75 were prepared by usual ceramic techniques. Specimens with x≥0.05 were found to have apparently cubic perovskite structure. The lattice constant of this system increases sharply to a broad maximum at x=0.30 with increasing chromium concentration. Mössbauer spectra for specimens with x=0.20 to 0.30, composed of two sets of six peaks at room temperature, show that these specimens are magnetically ordered at room temperature. The two sets of six peaks may be attributable to Fe3+ ions and Fe4+ ions, respectively. The spectrum of SrFe0.5Cr0.5O3−y taken at room temperature, consists of two peaks split by electric quadrupole interaction, attributed to Fe3+. The values of isomer shift and quadrupole splitting are +0.16 mm/sec and 0.97 mm/sec, respectively. The effective Bohr magneton number obtained from the slope of reciprocal susceptibility curve vs temperature is approximately 4.0 μB, which is close to the value expected for 0.50 Fe3+, 0.25 Cr3+, and 0.25 Cr6+. Both SrFe0.75Cr0.25O3−y and SrFe0.5Cr0.5O3−y are antiferromagnets with a small residual magnetization. It seems that the usual superexchange interaction cannot explain why the Néel temperature of SrFe0.75Cr0.25O3−y is much higher than that of SrFeO3. Differences in the chemical bonding character may have to be considered.
57Fe Hyperfine Fields in Magnetite (Fe3O4)
Authors: Evans B. J. , Hafner S. S.
Published: February 2004

The 57Fe Mössbauer spectrum in an applied 37 kOe field is interpreted in terms of three superimposed six-line patterns corresponding to at least three distinct species of Fe ions. While the observed hyperfine fields and isomer shifts of Fe3+ at A and B are consistent with the trends in ferrite spinels, the apparent Fe2+ values are rather unusual.
Amplification of Magnetostatic Surface Waves by Interaction with Drifting Charge Carriers in Crossed Electric and Magnetic Fields
Authors: Schlömann Ernst
Published: February 2004

The dispersion relation for propagation of electromagnetic waves along the interface between a low-loss ferrite and a high-mobility, current-carrying semiconductor film has been determined. The waves described by this relation resemble magnetostatic surface waves, except that they grow rather than decay provided that the direct current flowing through the semiconductor film exceeds a certain threshold value. The gain induced by the presence of the semiconductor film is negative at very small wavenumbers, becomes positive as the phase velocity of the guided waves becomes smaller than the drift velocity of the charge carriers, goes through a maximum, and then declines again at very high wavenumbers. Net amplification can be obtained when the maximum gain induced by the semiconductor exceeds the total loss (which is primarily due to relaxation of the ferrite magnetic moment). If the film thickness is large compared to the wavelength, the threshold condition is primarily a requirement upon the power dissipated per unit volume of semiconductor. In the converse case it is primarily a requirement upon the current per unit width of the semiconductor film. For a device using YIG (ΔHk/4πM = 2×10−4) and InSb (mobility = 5×104 cm2/V sec) at room temperature, a film thickness of 3000 Å, and a frequency of 3 GHz, the calculated threshold power and current densities are respectively 2.5×106 W/cm3 and 2.2 A/cm.
Anisotropy Broadening of Ferromagnetic Resonance in Dense NiCo-Ferrites
Authors: Vrehen Q. H. F. , van Groenou A. Broese , de Lau J. G. M.
Published: February 2004

Measurements have been made at 9 GHz of the circularly polarized components of the susceptibility tensor in dense samples of Ni1−xCoxMn0.05Fe1.95O4, where x=0, 0.015, 0.027, and 0.05. The materials were prepared from powders, obtained by spray drying a mixed solution of sulphate salts, and subsequent thermal decomposition of the dried mixture. After isostatic pressing and conventional sintering, porosities below 0.8% were obtained. The susceptibility data were interpreted in terms of the quantities1 S and W, defined by χ+ = Ms/(H−ω/γ+S+jW/2). The variation of S and W over the spinwave region is in quantitative agreement with Schlömann's theory,2 although the predicted singularities were not found. Outside the spinwave region W has a constant value: at high field W is proportional to x (16 Oe per % Co); at low fields the values are higher and not linear in x. The absence of the predicted singularities cannot be explained by taking into account exchange or intrinsic damping. Another possible explanation is that the spinwave energies themselves are influenced by the variations in anisotropy field. For a simple variation of the internal field, as given by Hi=H0+Aq cosq·r, the interaction between spinwaves parallel to q can be calculated (neglecting exchange interaction). The frequency of these spinwaves is broadened over an amount 2γAq. In a random sample Aq can be taken to be independent of the direction of q. In order to express Aq in terms of the anisotropy constants K1 and K2, the rms value of the anisotropy field in this model is put equal to that in a random sample. Therefore ½Aq2 = (16/21) [K12+ (2/11) K1K2+ (5/143) K22]/ Ms2. Using this A value, and summing S and W over-all directions of q, quantitative agreement is found with experiment.3
Magneto-Optical Recording
Authors: MacDonald R. E. , Beck J. W.
Published: February 2004

Magneto-optical recording is often proposed as an alternative to conventional magnetic recording techniques. The interest in this approach is that it offers the potential of large capacity, fast data rates, and short access times for information storage. The practicality of this approach, however, is strongly dependent on the recording medium, a suitable transducer, and optical deflection techniques In this paper, we review the key considerations in terms of writing and reading which are involved in magneto-optical recording and report the progress which we have made in using thin films of gadolinium iron garnet GdIG for this purpose.
Infrared Sensitive Ferrimagnetics
Authors: Teale R. W. , Temple D. W. , Enz U. , Pearson R. F.
Published: February 2004

The relaxation and magnetic anneal of the magnetic anisotropy which are observed in silicon-doped yttrium iron garnet are summarized. These arise from centers in the crystals at which the magnetization is bound to a crystallographic cube body diagonal. Redistribution of centers between the four body diagonals causes the thermal relaxation of the magnetic anisotropy. At low temperatures T≲100°K, this relaxation becomes very slow and if the specimen is exposed to infra-red radiation the absorption of photons redistributes the centers, overriding the thermal effect; the anisotropy can thereby be controlled by radiation. By assuming that the absorption probability for a photon by a center depends upon the angles between (i) M0 and the body diagonal (ii) plane of polarization of the radiation and the body diagonal, we are able to account for the effect upon the anisotropy observed when a sample is exposed to plane polarized radiation while M0 is held in a fixed orientation. Radiation controlled changes of several hundred gauss in the anisotropy field are possible at low temperatures for samples of composition Y3Fe5−αSiαO12 with α≳0.03. The initial permeability and coercive field also show a large sensitivity to radiation at 77°K, but the sensitivity is greatest for α≲0.03 and the observed reduction in μ is not reversible following further radiation exposure in the way that the changes in anisotropy are. The detailed mechanisms of the μ and anisotropy sensitivity may be different, though both show greatest response to radiation of wavelength 1 to 2 μ. The nature of the centers referred to and their relaxation is discussed, but we are not confident that the simplest explanation, in terms of Fe2+ ions and valence exchange on octahedral sites, is yet confirmed. Technical application of the photosensitivity for radiation detection and information storage is mentioned briefly.
Study of Spin Dynamics in Iron with Slow Neutrons
Authors: Collins M. F. , Minkiewicz V. J. , Nathans R. , Passell L. , Shirane G.
Published: February 2004

Inelastic scattering of slow neutrons in iron has been studied over a wide temperature range, from temperatures well below Tc, where spin wave modes are well defined, through the critical region in the neighborhood of Tc to temperatures above Tc where diffusive modes are dominant. Below Tc, in the temperature range 0.005<(1−T/Tc)<0.2, the spin-wave energies vary as (1−T/Tc)0.37±0.03. Slightly below Tc the spin waves become over-critically damped. There is no indication of a peak corresponding to a diffusive mode. Above Tc a well defined hydrodynamic region exists in which the scattering is accurately described by diffusion theory. Both the static susceptibility and the spin-diffusion constant are found to vary as (1−Tc/T)ρ over the temperature range 0.008(1−Tc/T)<0.05, For the static susceptibility the value of ρ is 1.30±0.06, in reasonable agreement with theory; however, for the spin-diffusion constant it is 0.14±0.04 which is considerably smaller than expectations.