bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of density fluctuations on the physical properties of a disordered carbon
Authors: Carmona F. , Delhaes P.
Published: February 2003

A number of properties, chemical composition, density, isothermal transport coefficients, paramagnetic susceptibility, and specific heat, of an anthracene char are investigated during the carbonization process. A simple physical picture for the carbonization process which assumes the existence of density fluctuations is proposed to explain these results. The nonmetal–metal transition which occurs under heat treatment is then interpreted as a percolation process as in Cohen and Jortner’s model. The low-temperature specific heat results are also explained within this picture. Finally, it is shown that the inhomogeneous model proposed for the carbonization of anthracene char is supported by the recent work of Toulouse and Pfeuty who extend the phase transition phenomenology to the critical phenomena observed in the disordered solids.
Pressure and temperature derivatives of the elastic moduli of a machinable glass ceramic
Authors: Gerlich D. , Wolf Marcel
Published: February 2003

The elastic moduli and their temperature and pressure derivatives have been determined for a machinable glass ceramic. The behavior of the thermoelastic properties is similar to the case of crystalline solids, i.e., it possesses negative temperature and positive pressure derivatives. The specific heat evaluated from the elastic data is essentially equal to the measured specific heat over the range 2–20 K. The elastic Grüneisen constant is positive and constant over the range 0–400 K. The thermoelastic properties of the material are mainly determined by the included mica crystallites.
Power deposition in volumetric 235UF6-He fission-pumped nuclear lasers
Authors: Wilson John W. , DeYoung R. J.
Published: February 2003

Pumping gas lasers by the 235U(n,ff) FF reaction has the advantage of depositing large energies per reaction (∼200 MeV), and by using 235UF6, a volumetric energy source is achieved. Factors in 235UF6 pumping of a He gas relating to neutron attenuation and fission product transport within a laser tube are treated in detail. Neutron attenuation is found not to be significant for tube sizes and pressure ranges anticipated for laser applications. Maximum power can be deposited in a laser tube when the tube radius is at least as large as the range of the fission fragments. The maximum power is deposited in the helium gas when the tube radius equals the fission-fragment range and the UF6 partial pressure to total pressure ratio is 0.15 corresponding to a UF6-He mixing ratio of approximately 1 : 6.
Trapped-flux spin-down torques on a spherical superconducting gyroscope
Authors: Holdeman Louis B.
Published: February 2003

Magnetic flux trapped in the rotor of a superconducting gyroscope can cause the rotor to lose energy through dissipation in a readout loop encompassing the rotor and by the generation of eddy currents in the support electrodes. These losses are calculated and shown to be negligible for the rotor in the gyroscope-relativity experiment, a proposed satellite experiment which is to test predictions of the theory of general relativity.
Spin glass to Kondo transition
Authors: Larsen Ulf
Published: February 2003

Recent theoretical results for the resistivity in the spin glass regime of alloys, like AuFe, are reviewed. New results are then presented that extend the theory into the Kondo regime at very low concentrations of magnetic (Fe-) atoms. A sharp transition between a Spin Glass and a Kondo phase is found. The singular point is at zero temperature and leads to observable critical behavior at low temperatures. The transition can be effected by changing the concentration or by applying pressure. The theory is shown to account well for the observed resistivity in a number of different alloy systems over a wide range of concentrations, pressures, and temperatures, including a resistivity maximum in both phases. Special emphasis is on new high-pressure observations in the transitional regime by Schilling and others. These results lead to a better understanding of the competition between spin glass and Kondo effects, and offer an opportunity to unify these hitherto separate physical ideas.
Dynamics of classical X
Y spins far from equilibrium
Authors: Nelson David R.
Published: February 2003

It is possible to understand low temperature spin waves in classical spin systems in terms of a phenomenological ’’fixed-length’’ hydrodynamic theory. The particular hydrodynamic theory appropriate for magnets with an underlying XY symmetry is exactly soluble, and seems to give a good description of equilibrium order parameter dynamics in one, two, and three dimensions. Here, we exploit this exact solubility to study spin wave dynamics far from equilibrium. Spin waves dominate the nonequilibrium equal-time correlation functions. In two dimensions, for example, two point equal-time correlations decay from a state of complete alignment by propagating a cusp-like singularity from short distances toward large. The cusp travels at twice the spin wave velocity, and leads to k-space oscillations in the Fourier-transformed spectrum, ? (k,t). If these oscillations could be detected, they would show up as a time-dependent bull’s-eye pattern surrounding Bragg peaks in a neutron diffraction experiment.
Surface spin waves and surface spin arrangements in helical magnets
Authors: Harada I. , Nagai O.
Published: February 2003

In order to investigate the nature of the spin arrangement in the surface of a semi-infinite helical magnet, we calculate both the surface spin-wave frequency spectra and the classical spin-ordering energies. We present detailed calculations for a (100) surface of a body-centered tetragonal lattice with nearest neighbor and next nearest neighbor exchange interactions (J1 and J2, which are both negative). The possibility that the exchange interactions in the surface (Js2) may differ from those in the bulk (J2) is taken into account. We find the following: In the case 0≳Js2≳J2, the surface spin-wave frequencies are real and positive, and hence the uniform helical spin arrangement is stable. On the other hand, in the case Js2<J2, there exist soft surface spin-wave modes, which indicate that the uniform helical spin arrangement is not stable. In the latter case, it is shown that a nonuniform spin arrangement, characterized by a rearrangement of spins in the surface, has a lower classical spin-ordering energy than the uniform state and hence is stable.
Spin-wave temperature dependence of gadolinium
Authors: Grobsky K. , Harmon B. N.
Published: February 2003

Previous theoretical studies of the temperature dependence of the magnon dispersion curves of rare earth metals have usually considered only the renormalization due to spin-wave interactions and the effect of temperature variations of the magnetocrystalline anisotropy. An important consideration which has not been included in these studies is the possible temperature dependence of the exchange coupling between the localized spins. Taking into account only the shorter conduction electron mean free path and the resulting decrease in the 4f-4f coupling as T increases we can qualitatively understand the general features of the measured magnon dispersion curves. Our results are consistent with recent measurements on Gd alloys which relate the temperature dependence of the magnetic ordering temperature to the conduction electron mean free path.
Behavior of the mean spin on finite clusters in a model for the dilute Heisenberg antiferromagnet
Authors: Halley J. W. , Holcomb W. K.
Published: February 2003

We report a study of the mean spin on finite clusters with 2, 3, 4, 5 and 6 spins in a linearized spin wave model for the dilute antiferromagnet on a square lattice due to Thorpe and Alben. For 2 and 3 spin clusters, the results are compared to the exact results and to the exact solution of the linearized Green’s function equations. We find that, as expected physically, the model treats this property of the small clusters in a manner which is neither self-consistent nor in accord with exact results. We discuss briefly how these results affect the validity of the use of the Thorpe and Alben model for other more global quantities.
Low-field magnetoresistance and domain drag in ferromagnets
Authors: Berger L.
Published: February 2003

Despite common misconceptions, domain walls are too thick to ’’scatter’’ electrons appreciably. However, electrons crossing a wall apply a torque to it, which tends to cant the wall spins. This could be used to measure the conduction electron spin polarization. Most of the low-field resistive anomalies observed in pure Fe, Ni and Co at low temperature are caused by the Lorentz force associated with the internal field B=Ms present inside each domain. The existence of low-resistivity paths extending over many domains accounts for still unexplained magnetoresistance data in iron whiskers. In uniaxial materials, a d.c. eddy-current loop caused by the Hall effect runs around each wall. The field Hz generated by these loops tends to ’’drag’’ the whole domain structure in the direction of the carrier drift velocity. Also, the Joule dissipation of the eddy currents manifests itself as an excess Ohmic resistance. As predicted, this excess resistance decreases as the square of the field, in amorphous Gd25Co75 films, in MnBi films, and in pure bulk cobalt, when the walls are removed by an external field. The excess resistance can also be changed by reorienting the walls.
Theory of field dependent correlation functions for light scattering in magnetic semiconductors
Authors: Safran S. A. , Dresselhaus G. , Lax B.
Published: February 2003

Magnetic field dependent correlation functions for antiferromagnets near the spin flop-paramagnetic phase boundary have been calculated using an RPA technique. The exponents describing the field dependence of the magnetization, the staggered magnetization and the correlation length, which are obtained from these calculations, are compared to those obtained using renormalization groups results. Here, we use our recent results relating the symmetry forbidden one-phonon Raman scattering in magnetic semiconductors to the two-spin correlation function, to calculate the field dependence of the Raman integrated intensity in EuTe. Light scattering experiments are suggested where the exponents related to the spin flop-paramagnetic phase boundary, Hc(T), and the field dependence of the staggered magnetization can be measured from the one phonon–one spin Raman spectrum in EuTe.
Effect of temperature on the antiferromagnetic resonance in FeCl2
Authors: Meyer P. , Brun A.
Published: February 2003

New experiments in the far infrared on the AFMR node in ferrous chloride revealed the appearance of structure of increasing complexity with rising temperature 1.3 K <T<18 K). At temperatures above 8 K we observe on the lower energy side the progressive growth of new peaks distributed in an energy band of about 10 cm−1. We propose an explanation of this structure in terms of a cluster model taking into account only nearest neighbor interactions. The observed peaks would then arise from transitions starting from thermally populated levels corresponding to different spin arrangements of the neighbors.
On the experimental determination of a ferromagnetic surface layer in antiferromagnetic chromium
Authors: Ferguson P. E.
Published: February 2003

The existence of a ferromagnetic surface layer in antiferromagnetic chromium has been confirmed experimentally by detecting surface magnetoplasma waves (SMPW) via the method of attenuated total reflection. The optical configuration consisted of a half cylinder heavy dense flint prism −900 Å MgF2 dielectric thin film–optically opaque chromium film. The dispersion curve for surface plasma waves (SPW) was obtained by measuring the reflectivity as a function of angle of incidence at constant photon energy in the range of 2.0–4.0 eV. Measurement of the normalized reflectivity difference δ due to a low frequency oscillating magnetic field applied perpendicular to the direction of propagation of the SPW indicates the presence of SMPW and therefore a ferromagnetic surface layer. Experimentally δ maximizes at E=3.25 eV and varies linearly with the applied magnetic field to 500 Oe. In the absence of SPMW δ is less than 1% of the δ measured in the presence of SMPW in the same photon energy range.
Magnetic, optical, and magneto-optical behavior of LiTbF4 and KTb3F10 crystals
Authors: Griffin J. A. , Folkins J. , Weber M. J. , Morgret R. , Litster J. D. , Gabbe D. , Linz A.
Published: February 2003

The magnetic, optical, and magneto-optical behavior of the crystals LiTbF4, LiTb0.5Y0.5F4, LiTb0.25Gd0.75F4, and KTb3F10 have been measured. We present the temperature, wavelength, and magnetic field dependence of the Faraday rotation in these materials and the wavelength dependence of the optical absorption. The magnetic ordering found in these crystals and its relation to optical phenomena is also discussed.
Piezoelectric gravitational antennas for use at 3 m °K
Authors: Kos J. F.
Published: February 2003

The design of a gravitational antenna consisting of a relatively large piezoelectric quartz crystal cooled to 3 m °K is presented. Calculations show that the use of the piezoelectric output to detect signals results in a simplification of the apparatus with no loss in sensitivity as compared to that attainable with similar antennas employing magnetic-displacement detectors. For both types of antennas the sensitivity is considerably better than that required to detect thermal oscillations at 3 m °K and compares favorably with that attainable with massive room-temperature antennas.
Effect of substitution of nickel on the magnetic properties of Sm2Co17
Authors: Merches M. , Sankar S. G. , Wallace W. E.
Published: February 2003

Structural, metallographic and magnetic properties of Y2Co17−xNix and Sm2Co17−xNix have been established to ascertain the effect of nickel on the permanent magnet properties of Sm2Co17. The results indicate that the 2 : 17 compositions exist in the concentration limits 0<x<8 in Y2Co17−xNix and 0<x<6 in Sm2Co17−xNix. Metallographic examination of these materials confirms the absence of secondary phases. Magnetic properties were examined in the temperature range 4.2–1200 K in external fields up to 21 kOe. The ordering temperatures of Sm2Co17−xNix and Y2Co17−xNix decrease with increasing concentration of nickel. Examination of saturation magnetization results indicates a smooth variation with the addition of nickel. The easy direction of magnetization at room temperature was established for the samples by examining the x-ray diffraction patterns of aligned powders. The results show that the axial anisotropy exhibited by Sm2Co17 is retained with the substitution of nickel. For a few nickel-rich compositions of Sm2Co17−xNix, thermomagnetic hysteresis effects were observed. The results are interpreted on the basis of a strong competition between the exchange field and the crystal field interaction. Calculations were performed to support this interpretation.
Temperature dependence of coercive force and anisotropy of the Sm(CoFeCuMn)7 permanent magnet
Authors: Sato K. , Tawara Y.
Published: February 2003

Demagnetization curves of Sm(CoFeCuMn)7 with applied fields up to 127 kOe both parallel and perpendicular to the c axis were measured at several temperatures from 4.2 K to 600 K. The ratio of IHc(400 K)/IHc(300 K) was found to be 0.83 which is a factor of two larger than that of Sm2(CoFeMnCr)17. The anisotropy constant was also determined in the temperature range from 4.2 to 480 K. The relationship between IHc(T) and K1(T) was discussed.
Electron microscopy of some rare earth–cobalt alloy magnets
Authors: Mishra Raja K. , Thomas G.
Published: February 2003

Transmission electron microscopy is being used to characterize the microstructural features in some 1 : 5 and 2 : 17 RE-Co magnets. In 2 : 17 compounds containing Fe and Mn, lamellae of hexagonal and other polytypic regions embedded in the rhombohedral matrix are observed. It is shown that the interface between the hexagonal and rhombohedral phases can be described as a stacking fault with fault vector normal to the interfacial plane. APB’s parallel to the prism planes in the rhombohedral structure have also been identified and characterized. In contrast, no planar faults have been observed in the 1 : 5 materials containing misch metal as the major rare-earth component. In these alloys extensive twinning and precipitation of a 2 : 17 rhombohedral phase are the major microstructural features observed. The influence of these microstructures on the magnetic properties are discussed.
Rare earth–cobalt magnets with low reversible temperature coefficient
Authors: Narasimhan K. S. V. L. , Wells M. G. H. , Ratnam D. V.
Published: February 2003

Heavy rare earth substitution for Sm in SmCo5 magnets were carried out with a view of improving the reversible temperature coefficient (RTC) of magnetization. The RTC was found to vary depending on the temperature ranges of interest. Over the range of −50 C to +100 C for Sm0.6Dy0.4Co5 and Sm0.68Er0.08Ho0.08Dy0.08Gd0.08Co5 an RTC of +0.0062% per °C and −0.0045% per °C respectively was obtained with a (BH)max exceeding 11 MGOe.
Coercive force in heat-treated R-Co alloys
Authors: Shibata T. , Katayama T. , Tsushima T.
Published: February 2003

The variation of coercive force as a function of annealing temperature is reported for some compacts of Co5Sm and Co5Pr alloys with the compositions of 63∼64 wt% Co-Sm and 60∼64 wt% Co-Pr, respectively. For the magnets made of these alloys, the curves of intrinsic coercive force against annealing temperature have two peaks; one at around 1080 °C and the other at 1190 °C in the Co-Sm sintered magnets. The curves of coercive force in sintered Pr-Co alloys have also two peaks; one near 1040 °C and the other near 1125 °C. This behavior can be correlated with the peritectic temperatures in the Co-Sm and Co-Pr phase diagrams. It is interpreted in terms of a model which assumes the existence of the pinning sites for the domain walls and their concentration at boundaries between the precipitates and grains of Co5R during the sintering. These precipitates are thought to consist of other R-Co compounds with lower melting points than RCo5.
Valence fluctuation in samarium compounds—A theoretical approach
Authors: Mahanti S. D. , Kaplan T. A. , Barma Mustansir
Published: February 2003

A theoretical approach to the problem of configuration fluctuation in Sm chalcogenides is presented. Properties of the fluctuating valence phase (also referred to as the collapsed phase) are discussed within an essentially localized model (ELM) in which most of the Sm 5d electrons are localized and only a small number (?0.1 electrons per Sm) occupy the free-electron-like states at the bottom of a d-band. We argue that the ELM for the collapsed phase of SmS can account for almost all and, as far as we know, is inconsistent with none of the available experimental data.
Magnetic ordering and configuration mixing in SmSbxS1−x alloys
Authors: Beeken R. B. , Savage Wm. R. , Schweitzer J. W.
Published: February 2003

Magnetic susceptibility measurements of a series of SmSbxS1−x solid solutions are reported. The results for the Sb-rich alloys provide evidence for the disappearance of magnetic ordering in a Sm system which exhibits intermediate valence. The magnetic susceptibility of SmSb where Sm is trivalent (4f5 configuration) shows an antiferromagnetic transition with TN=2.8 K. With increasing substitution of S for Sb there is a rounding of the cusp in the susceptibility at 2.8 K indicating the disappearance of the antiferromagnetic transition. This is attributed to configuration mixing (4f5 and 4f6) where the Sm has homogeneous intermediate valence. An estimate of the intermediate valence as a function of the S concentration is provided by lattice constant measurements.
Susceptibility measurements on the valence fluctuation system (Yb,Sc)Al3
Authors: Majewski R. E. , Edelstein A. S. , Aldred A. T. , Dwight A. E.
Published: February 2003

The susceptibility of YbxSc1−xAl3 (Cu3Au-type structure) for x=0.1, 0.2, 0.5 and 1.0 has been measured from 4.2 to 700 K. In contrast to Ce-Th alloys, the addition of a smaller atom, Sc, to YbAl3 favors the configuration with a larger atomic volume, Yb2+. The susceptibility per mole Yb exhibits a broad maximum which shifts to lower temperatures with increasing x. Room temperature lattice constant measurements indicate that the valance increases gradually from ∼2.7 at x=0.1 to ∼2.9 at x=1.0. Fitting our data to the resonant level-type model of Sales, we find that the width of the resonant level - decreases dramatically with increasing x. All the data approximately satisfies a scaling relationship.
Induced magnetic form factor of Sm in mixed-valence compounds
Authors: Moon R. M. , Koehler W. C. , McWhan D. B. , Holtzberg F.
Published: February 2003

The induced moment form factor of Sm has been measured in a number of compounds using the polarized neutron technique. For integral-valence compounds, experimental and theoretical evidence is presented showing that Sm2+ and Sm3+ usually have large differences in their form factors. For the mixed-valence systems, SmS at high pressure and Sm0.76Y0.24S, the observed form factor is nearly identical to that of the 2+ ion. If we adopt the viewpoint that the Sm ions are fluctuating between 4f5 and 4f6 configurations, then our measurements imply that the induced moment from the 4f5 configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with different J values or that the 4f5 configuration makes a negligible contribution to the form factor. The suppression of the Curie (1/T) terms is consistent with low-temperature susceptibility measurements on these systems. The implications of these observations are examined within the framework of a simple valence-fluctuation model.
Magnetic-, thermal- and transport properties of TmxSe (0.87<x<1.05)
Authors: Ott H. R. , Batlogg B. , Kaldis E. , Wachter P.
Published: February 2003

Various physical properties on single crystals of TmSe with varying but defined chemical composition have been measured. It is found that deviations from stoichiometry strongly influence the low temperature properties of these compounds and certain trends in the behaviour are demonstrated.
NMR in CeAl2: Crystal-field effects and magnetic order in the low-temperature phase
Authors: MacLaughlin D. E. , Hewitt R. R.
Published: February 2003

27Al NMR spectra have been observed in polycrystalline CeAl2 for temperatures between 2 and 295 K. Frequency shifts, quadrupole couplings and linewidths have been obtained in the paramagnetic phase above Tc=3.8 K. The isotropic shift K′ i due to Ce3+ spin polarization indicates a marked decrease of the 27Al hyperfine field Hhf=K′ i/χ with increasing temperature. A non-dipolar contribution to the anisotropic shift scales with the observed K′ i. Both these results can be accounted for only by an anisotropic local moment-conduction electron coupling. Below Tc the NMR line broadens appreciably, attaining a field-independent value at very low temperatures. This observation is consistent with the onset of magnetic ordering, although the absence of a large shift and the magnitude of the linewidth are not understood.
Magnetic excitations in HoFe2
Authors: Rhyne J. J. , Koon N. C.
Published: February 2003

Spin wave and crystal field excitations in single crystal HoFe2 have been studied using inelastic neutron scattering. At 10 K [100] is the easy magnetic direction and at this temperature the acoustic spin wave mode exhibits a 0.60 meV gap due to anisotropy. The acoustic mode is degenerate at the zone boundary with a flat optic mode of energy 8.3 meV. The lack of dispersion in this mode is a consequence of the small (<.01 meV) Ho-Ho exchange energy. A third mode is a highly dispersive optic mode which represents excitations of the iron sublattice spins and has a stiffness parameter almost identical to iron metal. A nearest neighbor linear spin wave model has been applied which represents the data of the three lowest modes well. This model predicts the remaining three allowed modes to be at energies above 200 meV. There is no measurable anisotropy in the spin wave modes with varying propagation directions. At room temperature, higher states of the Ho crystal field multiplet become populated and weaker dispersionless excitations between these levels are observed. The observed excitations are broadened beyond instrumental resolution because of contributions from several levels close together in energy. Crystal field calculations including exchange have been performed and are compared to the energies of the observed optic mode transitions.
Canonical transform theory of spin-waves in strongly anisotropic magnets
Authors: Cooke J. F. , Lindgård P.-A.
Published: February 2003

We have developed a new infinite-order perturbation approach for treating strongly anisotropic magnets. This formalism is based on a canonical transformation of a given system into a new system with an effective two-ion anisotropy which, for example, can be treated by conventional spin-wave techniques. An expression for the spin-wave energy for the general conical magnetic moment configuration with arbitrary single-ion anisotropy has been derived using this formalism. It was found that an analysis of the transverse spin-wave spectra can at most result in a determination of a renormalized exchange integral and two renormalized anisotropy constants. A comparison of the theoretical predictions of this formalism with experimentally determined spin-wave spectra indicate that a substantial part of the large two-ion anisotropy, which had been previously introduced to describe the spin-wave spectra in the heavy rare earth series, actually resulted from the fitting procedure which was used and not from any physical source.
Elastic versus magnetoelastic anisotropy in rare earth–iron alloys
Authors: Cullen J. R. , Rinaldi S. , Blessing G. V.
Published: February 2003

A review is presented of recent results of ultrasonic investigations of rare earth–iron compounds and alloys. The discussion is divided into three sections. The first describes results of sound velocity measurements of single crystal Tb0.3Dy0.7Fe2 as a function of magnetic field strength and orientation with respect to the crystallographic axes. In the second part recent work on polycrystalline and amorphous samples, including evidence for simultaneous propagation of two different types of magnetoelastic waves is presented. A model developed to describe this phenomenon is outlined as well. Finally a transmission experiment in a single crystal (Tb0.3Dy0.7Fe2) is described. Resonant-like transmission is shown to take place when an applied magnetic field is so oriented as to produce almost equal velocities for C44 and C11-C12 modes.
Liquid phase sintering of rare earth–iron (Dy0.7Tb0.3Fe2) magnetostrictive materials
Authors: Croat J. J.
Published: February 2003

A liquid phase powder metallurgy method has been evaluated for the production of dense, thermally stable Dy0.7Tb0.3Fe2 compacts. The method involved the use of a rare earth rich alloy which forms the RFe2 phase and a RFe2-R eutectic. The latter is molten at the sintering temperature employed. Dy0.7Tb0.3Fex alloys ranging in composition from x=1.65 to stoichiometric were ground, aligned, compacted and then sintered between 1130 and 1190 °C. Densities as high as 98% were achieved for compositions between x=1.65 and 1.8. Densities were markedly lower for compositions beyond x=1.8. Although the magnetostriction of the compacts were lower than those found for cast polycrystalline Dy0.7Tb0.3Fe2, the magnetomechanical coupling coefficients (k33) of a number of the sintered compacts appeared to be as much as 50% higher than for the cast materials. Suprisingly, the highest coupling was found for samples with 5 to 15% porosity. When aged in air at 150 °C, the magnetoelastic properties of the liquid phase sintered materials were as stable as cast polycrystalline alloy.
Anisotropic forced magnetostriction of TbIG
Authors: Flanders P. J. , Egami T. , Gyorgy E. M. , Van Uitert L. G.
Published: February 2003

Magnetostriction and forced magnetostriction of TbIG were measured at 77 K in magnetic fields up to 58 kOe in various field strength and direction. It was found that the forced magnetostriction is extremely anisotropic, and both the magnetostriction and the forced magnetostriction involve significant higher order terms with l=6 or 6′, when analyzed in terms of cubic harmonics series. The forced magnetostriction predominates the field dependence of the magnetocrystalline anisotropy. The m4 power law, where m is the relative magnetization, remains unchanged by coincidence.
ΔE-effect and macroeddy-current damping in nickel
Authors: Berry B. S. , Pritchet W. C.
Published: February 2003

Internal friction measurements on nickel reeds have confirmed the macroeddy-current limited relaxation behavior predicted by Zener. The macroeddy-current peak has been observed as a function of frequency, and shown to depend in height and position on the magnetization of the sample. At low frequencies the modulus defect associated with this relaxation causes the ΔE-effect to exhibit a maximum value at an intermediate magnetization in the vicinity of half the saturation value. As the frequency is raised the maximum disappears and the behavior of the ΔE-effect reverts to that observed in resonant-rod experiments. It is emphasized that in this regime, which has frequently been used in the past, the measured ΔE-effect does not provide an equilibrium characterization of the material.
Effects of applied stress on UHV-deposited Fe1−xPdx thin films
Authors: Lee M. H. , Eldridge J. M. , Lee W.-Y.
Published: February 2003

The effect of an externally applied stress on the B-H loops of UHV-deposited Fe1−xPdx thin films depend strongly on composition. A uniaxial compressive stress increasingly applied along the easy axis of fcc films (x≳0.31) causes the usual distortion of the easy-axis loop toward the appearance of hard-axis loop. Similar stresses applied to bcc films (x≲0.31), however, produce no significant changes in the easy-axis loop until x approaches 0.31 despite the fact that the magnetostriction coefficient only ranges over a factor of 2 for x⩾0.12 (λ⩾15×10−6). This locking of the easy axis in bcc films also affects the strain sensitivity which is 0 in the limit of 0 strain, although it approaches the expected value with increasing strain. Several possible origins for this behavior are discussed.
Electron microscope studies of ferromagnetic ordered structures
Authors: Jakubovics J. P. , Lapworth A. J. , Jolly T. W.
Published: February 2003

The magnetic properties of crystallographically ordered ferromagnetic materials are strongly dependent on the presence of crystal defects, because of the effect of defects on the domain structure. Electron microscopy enables the defects and domain walls to be imaged simultaneously, and is therefore capable of giving detailed information about their interactions. In alloys containing Mn, antiphase boundaries (APBs) interact very strongly with domain walls, and tend to act as wall nucleation sites. In MnAl, a possible permanent magnet alloy, in addition to APBs, other defects are also present which tend to behave as wall pinning sites. In some ordered alloys, magnetic anisotropy can be induced by plastic deformation. The resulting changes in the domain structure can be studied by electron microscopy. In spinel ferrimagnets, stacking faults can interact with domain walls.
Domain configurations in single crystal TbFe2 and Tb0.27Dy0.73Fe2
Authors: Sery R. S. , Savage H. T. , Tanner B. K. , Clark G. F.
Published: February 2003

We have observed 71° and 109° walls in single crystal TbFe2 and Tb0.27Dy0.73Fe2 (Terfenol) via the Kerr effect and synchrotron radiation topographs. A Terfenol (110) surface exposed to synchrotron radiation show ’’stripe-like’’ domains with [11̄0]; 109° walls and [001]; 71° walls. Cooling to ∼273 K caused an easy axis switch from 〈111〉 to 〈100〉; no domains were visible. Warming back to ∼293 K brought out mostly 109° walls. ’’Stripe’’ domains were also seen in (110) and (100) TbFe2 surfaces via the Kerr effect. In the (110) sample H fields along [001] and [11̄0] directions created 109° and 71° walls respectively. With H applied at ∼45° to both [001] and [11̄0] directions 71° and 109° walls appeared simultaneously. The domains varied from 5 to 250 μm and fields of ∼3000 Oe away from [001] and [11̄0] directions saturated the sample. Walls in the (100) sample were also 71° or 109° but were only 2 to 15 μm wide.
Néel lines in YIG single crystals
Authors: Labrune M. , Miltat J. , Kléman M.
Published: February 2003

Faraday effect observations of Néel lines belonging to 180° Bloch walls in massive [110] YIG single crystals are reported. 180° walls appear to be bent and Néel lines are seen to be inclined inside the sample and to assume an S shape. When a small steady field is applied normal to the sample, a strong anchoring of Néel lines at the surfaces is observed. After saturation of a 180° wall, Néel lines, upon decreasing the field are seen to nucleate at one surface only. A detailed interpretation of the results must take into account the strong deformation of the sample inherent to the mode of observation.
Computer simulation of wall pinning effect and intrinsic coercivity of thin domain walls
Authors: Chi Mely Chen , Gyorgy E. M. , Alben R.
Published: February 2003

A computer simulation technique is used to study the intrinsic coercivity of a thin (∼ a few atomic layers) domain wall in perfect ferromagnetic crystals, and the domain wall pinning due to the existence of different kinds of defects. A one spin relaxation process is used to minimize the total energy of the system which includes anisotropy, exchange and Zeeman energies. When the applied field is less than the intrinsic coercivity, the equilibrium domain wall remains stationary. At higher fields the wall starts to move through the perfect parts of the crystal. When the domain wall approaches defect sites, it comes to a stop and is pinned by the defects. If the defect is a vacant line then the equilibrium domain wall bows out; the amount at bending of the wall increases with increasing field. The wall breaks free from the defects only when the applied field is increased up to a new critical value. The domain wall energy shows a periodic behavior during the wall motion in the perfect part of the crystal. But the wall energy decreases and forms a deep well around the imperfection. This energy trap results in an increase of coercivity. The increment of coercive force depends on the concentration and the kinds of defects.
Effect of ion-implantation on bubble translation velocity
Authors: Konishi S. , Hsu T. , Brown B. R.
Published: February 2003

Bubble translation velocity has been measured as a function of gradient field drive in both ion-implanted and as-grown garnet films to study the effect of ion-implantation on velocity. The apparent velocity, defined by the ratio of displacement to pulse width, shows a peak then saturates with increasing drive in the as-grown films. In ion-implanted films, the apparent velocity increases above the peak observed in the as-grown films; and in some implanted films, the saturation is not observed. This difference is probably attributable to the suppression of Bloch loop punch-through by the implanted layer and the resultant bubble overshoot effect.
A conductor crossing problem in magnetic bubble memories
Authors: Strauss W.
Published: February 2003

Bubble propagation across a conductor depends, among other considerations, on the magnetostatic energy of the bubble-Permalloy interaction and strain due to patterned metallization layers. Previously neglected, another consideration—bias field distortion in the neighborhood of the crossing—is discussed here. Poles induced by the bias field on a Permalloy element shaped to conform to a conductor produce field components parallel and transverse to the nominal bias. In traversing an isolated crossing at right angles to the conductor length and at a fixed altitude within the magnetic garnet, one calculates for the distortion bias maxima near the conductor edges and a minimum at the center. To obtain a tractable solution for the bias field as a function of coordinates a two-dimensional model was chosen for the conductor crossing: an infinite half space of infinitely permeable Permalloy with a circular depression. At points infinitely removed from the depression which houses the conductor the complex field H=Hx+jHy=jH0; that is Hx vanishes and Hy assumes the value H0 of the nominal bias field. Near the conductor centered at x=0, however, Hx is generally not zero, Hy(0,y) <H0, and Hy(x,y) exhibits a minimum at x=0 for fixed y. Using this model to calculate the bias in the vicinity of a 4-μm wide, 0.5-μm thick conductor, one finds for the bias field, averaged over a 3-μm thick garnet film, extremal values of 0.952H0 and 1.007H0. For current materials with H0?150 Oe the local bias reduction is 7.5 Oe. Because bubbles reside at the edges of Permalloy features, the field perturbations encountered in practice will be smaller than predicted by this model. Were there no forces on a bubble other than those associated with the coordinate dependent bias, a domain situated between maxima would move to the bias minimum and be trapped there. As designers strive for greater information density, this aspect of the conductor crossing problem may assume greater importance because of difficulty in linear scaling of conductor dimensions and the use of materials with higher bias requirements.
Comparison between the propagation margins of asymmetrical chevron and half-disc circuits at 100 kHz
Authors: Dimyan M. Y. , Hubbell W. C.
Published: February 2003

The start/stop straight line as well as complete loop (with 180° corner turns) propagation margins of asymmetrical chevron and half-disc circuits are compared under various operating conditions at 100 kHz. Three circuit periods (14, 16 and 18 μm) and four gaps (1.5, 2.0, 2.5, and 3 μm) are used in this study. Measurements on these circuits fabricated on YSmLuCaFeCe garnet films indicate that for both straight line as well as full loop propagation, the asymmetrical chevron pattern is superior to the half-disc pattern, especially for larger gaps.
Effect of elastic properties of SiO2 spacer on bubble pinning
Authors: Stein B. , Callen H. , Casey M. , Josephs R.
Published: February 2003

Measurements are presented of the bias field margins in bubble devices with sputtered SiO2 spacer layers prepared with different p-etch rates. The bias field margins are increased by SiO2 layers of low p-etch rate. Theory suggests that the bias field margin should increase with increasing SiO2 stiffness (increasing Young’s modulus), and a correlation between stiffness and low p-etch rate is plausible on the basis of known p-etch rate behavior. The demonstrated dependence of bias-field margin on the SiO2 p-etch rate, and of p-etch rate on substrate temperature during sputtering, provide operational criteria for practical device fabrication.
Domains, micromagnetics, and beyond: Reminiscences and assessments
Authors: Brown William Fuller
Published: February 2003

The development of domain theory and micromagnetics from the early 1930’s to the present is reviewed and assessed, with emphasis on the developments of particular interest to the author.
Magnetization and magnetocrystalline anisotropy of PrxSm1−xCo5 alloys
Authors: Heinrich J. P. , Miller A. E.
Published: February 2003

The temperature dependence of the saturation magnetization and the first and second anisotropy constants have been determined for alloys in the system PrxSm1−xCo5. The alloys studied were spheres cut from single crystals grown by the Bridgeman drop technique. Measurements were made on a Foner vibrating sample magnetometer. The anisotropy constants were calculated statistically from hard direction magnetization data by the method of Sucksmith and Thompson. Ms, K1, and K2 for PrCo5 were found to be larger than previously reported values at low temperatures. PrCo5 and Pr0.75Sm0.25 are both magnetically easy come at low temperatures. K2 increases with increasing Sm content in the Pr-rich alloys. However, K2 values for the Sm-rich alloys are anomalously large.
Effect of magnetocrystalline anisotropy on the low temperature heat capacity of HoAl2
Authors: Inoue T. , Sankar S. G. , Craig R. S.
Published: February 2003

Rare earth–aluminum compounds of the composition RAl2 crystallize in Laves phase cubic structure. Low temperature magnetic and thermal properties of these compounds have been investigated thoroughly during the past decade. The occurrence of large magnetocrystalline anisotropy results in significant modifications of magnetic and thermal properties of these materials. In the present study the heat capacity of ferromagnetically ordered HoAl2 was measured in the temperature range 1.5–12 K. An examination of the data reveals that the measured heat capacity Cp can be described by the relation Cp=CN(T)+CM(T)+γT+βT3, where CN is the nuclear term, CM the magnetic term; γT and βT3 are electronic and lattice heat capacities, respectively. A detailed analysis of the magnetic heat capacity term indicates a dependence of the form CM=AT3/2 exp(−Δ/kT) with Δ/k=6.0 K. The exponential dependence of CM is strong evidence of the presence of large magnetocrystalline anisotropy in HoAl2.
Temperature dependence of the first magnetocrystalline anisotropy constant of polycrystalline Ca-In-V and Ca-Zr-YIG
Authors: Simonet W. , Hermosin A.
Published: February 2003

A method to compute the first order anisotropy constant K1 from ferrimagnetic resonance linewidth measurements on polycrystalline samples is discussed and applied to Ca-In-V and Ca-Zr substituted YIG. Experimental accuracy of about ±103 erg/cm3 can be achieved with second-phase free, dense (≳99%) and carefully polished spheres. The temperature dependence of K1 from −196 °C to about Curie point have been measured for both garnet families.
Ultrasonic velocity anisotropy in ferrofluids under the influence of a magnetic field
Authors: Chung David Y. , Isler William E.
Published: February 2003

The propagation of ultrasonic waves in a water-base ferrofluid has been investigated by pulse-echo and continuous wave (CW) methods. The CW methods proved to be well suited for studying the velocity changes (Δv/v) due to a magnetic field. The magnitude of Δv/v was about 10 percent for fields as low as 125 gauss. In order to determine the velocity anisotropy, Δv/v was studied as a function of the relative angle (ϑ) between the magnetic field and the propagation vector. The anisotropy in Δv/v did not show a sin22ϑ dependence as the theory predicts. Also the magnitude of Δv/v was larger than the theoretical value by a factor of 103.
The relationship of switching distribution to tape digital output
Authors: Richards D. B. , Szczech T. J.
Published: February 2003

There are two different definitions of magnetic switching distribution in common use for magnetic recording media. These two parameters are closely related, and this relationship has been determined for a family of experimental recording tapes. In addition, the importance of magnetic switching distribution to peak output at 787 bits/mm is shown for tapes encompassing a wide range of coercive force (Hc) and retentivity (4πMr).
Approaches toward field determined propagation of crossties and Bloch lines
Authors: Schwee L. J. , Anderson W. E. , Liu Y. J. , Lee R. N.
Published: February 2003

Previously time determined propagation of crossties and Bloch lines was described. More reliable methods of propagation are being pursued which are expected to increase the margins of operation. Two new approaches are described and the experimental results presented. Also a new bit stretching detector for a larger output signal is described. Anisotropic or isotropic Permalloy can be used. With isotropic Permalloy shift registers can bend around corners. Permalloy is deposited to a thickness of about 370 Å, then the Permalloy is masked and gold is plated through the photomask. The substrate is remasked and the Permalloy is etched to form serrated strips. The substrate is placed above a stripline and connections are made to the gold pads. This allows the use of Bitter solution for experimental observation. A complete device will require a dielectric layer with vias opened to the plated gold and a final metalization to form the stripline and other connections.
Domain wall mobilities in YFeO3
Authors: Tsang Ching H. , White Robert L. , White Robert M.
Published: February 2003

Using a transit time technique, we have measured domain wall velocities versus applied field (H) for Bloch, Néel and the head-to-head walls in bulk, single-crystal bars of yttrium orthoferrite from 150 °K to 600 °K. Fields as high as 100 Oe were applied and wall velocities exceeding 106 cm/sec were recorded. At low field all three walls display a well-defined linear v-H region, with a common mobility (μ) of 1.2×104 cm/sec Oe at 300 °K. Velocities for the three walls become field independent abruptly at Vs=4.2×105 cm/sec and maintain this velocity until H exceeds ∼60 Oe whereupon the velocity again increases. We demonstrate that the wall velocities ’’saturate’’ at Vs because of strong interactions between the walls and the bulk acoustic phonons. As a function of temperature (T) the wall mobility varies at 1/T2. In investigating various damping mechanisms for wall motion, we find that the dominant mechanism is a four-magnon process. Not only does this lead to a mobility that varies as 1/T2, but also provides the correct order of magnitude. We calculate a relaxation time due to four-magnon processes to be 4×10−10 sec at 300 °K, to be compared with 1.8×10−10 sec computed from domain wall mobilities. Contribution to relaxation from magnon-phonon processes are estimated to be negligible.
Elliptical distortion mechanism for ballistic overshoot of bubbles
Authors: Malozemoff A. P. , De Luca J. C.
Published: February 2003

A theory for ballistic overshoot of gradient propagated magnetic bubbles is described. The overshoot arises from the relaxation of transient elliptical distortion and is predicted to be of order nμγ−1r−10(r10+r20) where n is the number of vertical Bloch lines on the flanks of the bubble, μ the linear mobility, γ the gyromagnetic ratio, r0 the static bubble radius, r10 the change in average bubble radius at the end of the drive and r20 the elliptical distortion at the end of the drive. If one assumes all Bloch lines punch through to form vertical Bloch lines, the theory can account within 50 percent for the observed ballistic overshoot of many as-grown films as a function of gradient pulse time,strength and bias compensation. Furthermore new data on the detailed time dependence of bubble behavior during overshoot shows a rough correlation between displacement and elliptical distortion.
Bubble state stability in rotating fields in garnets and under Permalloy propagation patterns
Authors: George Peter K.
Published: February 2003

Two experimental techniques are employed to study the rotating field state stability in ion-implanted garnets: (1) current-access-rocking in the presence of a rotating in-plane field and (2) field-access-rocking between and under Permalloy bars. The results of these experiments show that stable states do exist in a rotating field and that the state transition boundaries are perturbed only slightly by the presence of the Permalloy. Areas that are explored include the frequency dependence and garnet to Permalloy spacing dependence of the state transition boundaries. The results show that both 0 and 1 states are stable for a broad range of drive field conditions and Permalloy to garnet spacings.