bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth of 3’’ diameter Gd3Ga5O12 crystals
Authors: Brandle C. D.
Published: February 2003

As the bubble technology has progressed from the research stage into pilot plant operation, the need for larger diameter substrates has developed. Today, typical 3 in. diameter crystals of GGG weigh between 9000 and 10 000 grams and are grown from charges of 12 000 grams or more. The growth of 3 in. diameter GGG crystals with low defect densities 1(?/cm2) presents several problems which are not encountered during the growth of small diameter crystals. The first problem encountered is one of proper scaling of the growth furnace and growth parameters. The second problem is that of melt composition to insure a uniform lattice parameter throughout the entire crystal. A third area which can present problems is the size and mass of the charge required for 3 in. diameter crystals. Each of the parameters and their relationship to crystal imperfections is discussed.
Garnet film magnetic property control during growth and processing
Authors: Davies J. E. , Galli G. , Suits J. C.
Published: February 2003

High quality 2 in. diameter gallium and germanium substituted garnet films have been grown by the liquid phase epitaxy (LPE) dipping technique for magnetic bubble domain applications. The factors governing control and reproducibility of the film properties such as LPE melt composition, growth conditions and strategies will be reviewed. With the trend toward smaller bubbles requiring thinner films, control of the most critical parameter, the bubble collapse field, becomes more difficult. This problem is alleviated by a post-growth etching processing which involves controlled reduction in the film thickness by a phosphoric/sulphuric acid etch. Before device fabrication the films are ion implanted to suppress hard bubbles. Any elevated temperatures encountered during device fabrication will cause an annealing of the implanted layer which affects the bubble collapse field. New data will be presented to show how this depends on the thermal conditions (temperature, time) and how, by selection of the film implantation conditions, the effects may be minimized.
Bubble properties of garnet films prepared from molybdate fluxes
Authors: Bonner W. A. , LeCraw R. C. , Pierce R. D. , Van Uitert L. G.
Published: February 2003

Magnetic bubble garnet films have been prepared from a flux composed of lithium and rare earth molybdates using liquid phase epitaxial techniques. In general, the quality and magnetic properties of 〈111〉 films prepared on GGG substrates compare favorably with the properties of similar garnets prepared from PbO based fluxes. A notable exception, however, is a persistent, unusually low value of the growth induced anisotropy (Ku). Typically, Y3−xEuxFe5−yGayO12 films grown at ∼950 °C containing as much as Eu=0.6 and Ga=0.9 have a moment (4πMs) ?400 G and Ku?103 ergs/cm3 when grown from the molybdate flux. Films prepared on 〈100〉 substrates exhibit comparably low Ku values and K1 values of ∼−2.3×103 ergs/cm3. As these films are virtually contaminant free with regard to the flux components, and PbO and B2O3 additions have essentially no effect on Ku, our present understanding of the nonrandom rare earth ion distribution responsible for growth induced anisotropy in garnets may not be adequate to explain these data. Other anomalies which have been observed in these films and will be discussed are the occurrence of triangular bubble domains and square lattice bubble arrays.
(YSmLu)3(FeGa)5O12 for 1 to 3 μm-diameter bubble devices
Authors: Kestigian M. , Smith A. B. , Bekebrede W. R.
Published: February 2003

Investigations of small-bubble-diameter crystalline garnet LPE magnetic films have shown that (YSmLu)3(FeGa)5O12 has some important advantages relative to other garnet compositions for use in bubble devices. In the present study, this material is compared with (YSmLuCa)3(FeGe)5O12, a material widely used for device applications because of its desirable bubble properties. It is observed that in the gallium garnet, the growth process is much easier to control and is not subject to the inhomogeneities noted in the analogous calcium germanium composition. Furthermore, a detailed comparison of the properties of these two materials indicates that their small-bubble-diameter properties are very similar. These data include the Curie temperature and bubble-shift velocity, as well as the temperature dependence of stripe width, collapse field, anisotropy, magnetization and characteristic material length. The most noticeable difference revealed by these measurements is the expected lower Curie temperature of the gallium garnet; however, this has a relatively small effect on the temperature dependencies of stripe width and collapse field at device operating temperatures.
Mössbauer study of glassy alloys (Fe-Mo)80B20
Authors: Chien C. L. , Hasegawa R.
Published: February 2003

Glassy alloys of (Fe-Mo)80B20, containing 2 to 4 at.% of Mo have been studied by 57Fe Mössbauer spectroscopy. It is shown that a small amount of Mo included in the alloys has a pronounced effect in reducing the Curie temperature and in increasing the crystallization temperature, thereby making the glassy state more stable. The values of TC=585 K, 535 K and 480 K have been determined for samples with 2, 2.4 and 4 at.% of Mo. At low temperatures, the effective hyperfine field shows a temperature dependence of Heff(T) =Heff(0)(1−BT3/2...), due to spin wave excitations. The values of B= (28,35,39) ×10−6 K−3/2 have been determined for these samples. However, all three samples share nearly the same value of B3/2=B (TC)3/2 of about 0.4 The sample containing 2.4 at.% of Mo has a crystallization temperature of 760 K. Crystallization phases of (Fe-Mo)3B and (Fe-Mo) alloys have been observed after crystallization.
NMR Knight shifts and linewidths in the Ni-Pd-P and Ni-Pt-P metallic glasses: Composition and temperature dependences
Authors: Hines W. A. , Kabacoff L. T. , Hasegawa R. , Duwez P.
Published: February 2003

NMR Knight shift and linewidth measurements are reported for the 31P nuclei in the metallic glasses (Ni0.50Pd0.50)100−xPx (where x=16 to 26.5) and (NiyPd1−y)80P20 (where y=0.20 to 0.80), and both the 31P and 195Pt nuclei in the metallic glass (NiyPt1−y)75P25 (where y=0.20 to 0.68). The results are discussed in terms of the amorphous structure, electronic structure, and stability of transition metal+metalloid metallic glasses.
Specific heat and susceptibility of Ni-based amorphous alloys with dilute Fe
Authors: Onn David G. , Antoniuk Thomas H. , Donnelly Thomas A. , Johnson Wayne D. , Egami Takeshi , Prater John T. , Durand Jacques
Published: February 2003

The low temperature specific heat CP and magnetic susceptibility of the amorphous alloys FexNi80−xP14B6 (x=3,5,7) have been determined. For x=3,5 their behavior is consistent with a spin-glass regime, while for x=7 a mictomagnetic regime is observed. The amorphous alloys Ni78P14B8 and Ni80P12B8 show no bulk magnetic order, but CP at low temperatures contains a constant contribution attributed to magnetic clusters, the concentration of which is consistent with previous magnetization measurements.
Saturation of low temperature electrical resistance in the amorphous (Fe1−xNix)75P16B6Al3 alloy system
Authors: Rapp Ö. , Grindborg J. E. , Rao K. V.
Published: February 2003

We have measured the electrical resistance of amorphous ferromagnetic Fe75P16B6Al3 and Fe60Ni15P16B6Al3 down to 20 mK. It is found that the resistance saturates and reaches a constant value below about 80 mK in contrast to earlier low temperature studies on amorphous ferromagnets. The present results are in reasonable agreement with recent theories.
Microstructure and magnetism in amorphous rare earth–transition metal thin films
Authors: Dirks A. G. , Leamy H. J.
Published: February 2003

We have examined UHV vapor deposited and sputtered films of various amorphous alloys by transmission electron microscopy (TEM). All of the films exhibited an anisotropic microstructure composed of columnar regions surrounded by a low density network. The structure is produced by shadowing of the incident vapor by the growing film. Films produced by vapor deposition from separate rare earth (RE) and transition metal (TM) sources may possess an easy axis of magnetization in the film plane that is not present when the substrate is rotated or when the film is produced by sputtering. All of the films possess a high tensile stress which may be reduced and eventually converted to a compressive stress by vapor deposition in an oxygen containing ambient or by negative bias sputtering. The appearance of compressive stresses is coincident with the development of a perpendicular easy axis in Gd-Co. A perpendicular easy axis is also induced in vapor deposited and zero volt bias sputtered Gd-Co films upon heating within the TEM. Our results indicate that perpendicular anisotropy may be produced by the formation of an inhomogeneous structure via removal of ferromagnetic Gd, either by oxidation or preferential resputtering.
Electron microscopy of i
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u ion beam sputtered amorphous Gd-Fe alloy thin films
Authors: Gill H. S. , Chen M. , Judy J. H.
Published: February 2003

We have investigated the physical microstructure and Lorentz magnetic structure of in-situ ion beam sputtered and RF sputtered amorphous Gd-Fe alloy thin films. Auger electron spectroscopy was used to determine the composition profile and thickness of the films. Films with 18 to 30 at.% Gd were deposited onto amorphous carbon substrates up to 500 Å inside a modified RCA-EMU4 electron microscope. Two diffused electron diffraction halos were observed indicating amorphous phase separation. Using high-resolution dark-field microscopy with a JEOL (JEM-100C) electron microscope, we observed coherent scattering regions of approximately 8–20 Å diameter. In conclusion, amorphous Gd-Fe alloy films exhibit amorphous phase separation/quasi-amorphous structure, in-plane magnetization ripple and crosstie walls, and stress-induced anisotropy.
Influence of inert gas incorporation on uniaxial anisotropy of sputtered GdCo thin films
Authors: Burilla C. T. , Bekebrede W. R. , Kestigian M. , Smith A. B.
Published: February 2003

A study was made of the gas content and uniaxial anisotropy of GdCo films as a function of bias voltage and gas pressure. Unlike previous studies of GdCo, in this investigation, a multi-target sputtering system was used, making it possible to keep the Gd/Co ratio (and hence 4πM) relatively constant while the bias and pressure were varied. For low bias voltages, the anisotropy was found to increase with bias as has been reported in the literature. However, unlike previous studies, we found that the anisotropy reached a peak value at ∼200 V and then decreased with bias at higher voltages. When argon was used as the sputtering gas, it was found that the gas content in the film also peaked at ∼200 V; with neon, the gas content did not peak but decreased with bias at voltages over ∼100 V. Samples deposited with an argon pressure of 50 mTorr show an approximate proportionality between anisotropy and gas content, suggesting that the presence of the gas is at least partly responsible for the generation of growth-induced anisotropy in GdCo films.
Annealing effects on magnetic properties of amorphous GdCo, GdFe, and GdCoMo films
Authors: Katayama T. , Hasegawa K. , Kawanishi K. , Tsushima T.
Published: February 2003

Long term thermal stability of the amorphous R-T films is a fundamental requirement for the sake of practical use. Detailed experiments of the annealing effects were made on the amorphous GdCo, GdFe, and GdCoMo films prepared by means of the D.C. biased r.f. sputtering technique. It is found that annealing at various temperatures reduces Ku, but it enhances Ms and A in all of the films. Moreover, it is found also that the changing rates of Ms and Ku decrease with the increase of Mo concentration in the amorphous GdCoMo films.
Easy axis and easy plane magnetization in D.C. sputtered amorphous YCo3 films
Authors: Krishnan R. , Suran G. , Sztern J. , Jouve H. , Meyer R.
Published: February 2003

The presence of a transient layer growth of about 500 Å thick in D.C. sputtered amorphous YCo3 films has been detected by a study in the range 4–300 K of ferromagnetic resonance in films of thickness in the range 0.05 to 1.0 μm. The surface mode observed in films e≳0.2 μ transforms into the uniform one for e=0.05 μ. A model of exchange coupled layers explain the results observed. From relaxation process and Bloch law dependence of 4πMS, it is seen that cobalt in the transient layer at the substrate film interface could have an ionic character and in the rest of the film a metallic one. We propose that the transient layer might have a higher Ar content leading to a strong in plane anisotropy. This was verified by subjecting films to an annealing at 200 °C for 24 h in pure Ar atmosphere which indeed induces a very strong in plane anisotropy KU=−5.105 ergs cm−3 at 300 K irrespective of film thickness. This in our opinion is the first direct demonstration of the effect of Ar to induce KU<0.
Magnetostatic wave terminations
Authors: Collins J. H. , Hastie D. M. , Owens J. M. , Smith C. V.
Published: February 2003

Magnetostatic waves (MSW) propagating in low line-width (ΔH<0.5 Oe) yttrium iron garnet (YIG) films grown by liquid phase epitaxy (LPE) constitute a potential basis for analog signal processing directly at microwave frequencies (1–20 GHz). MSW device techniques are complementary to those utilized in surface acoustic wave (SAW) devices at VHF/UHF. A necessary prerequisite for a high performance MSW analog signal processing technology is a low insertion loss (<20 dB) delay line with low passband ripple (<2 dB). Shorted, narrow microstrip transducers readily provide the low insertion loss; thus, the major problem is that of terminating unwanted waves within the YIG samples. Unterminated magnetostatic forward volume wave (MSFVW) delay lines can easily have passband ripple in excess of 20 dB. By measuring the frequency periodicities of these ripples in the passband and the corresponding group delays, all sources of reflection have been identified. Terminations consisting of magnetic recording tape, ferrite powder slurries, and surface-abraded YIG have been tested on the YIG end regions. The MSFVW passband ripple has been reduced to less than 2 dB, over the 500 MHz band from 2.7 to 3.2 GHz, in a reproducible manner.
Ultrasonic studies of spin glasses: Cu Mn
Authors: Hawkins G. F. , Thomas R. L.
Published: February 2003

Measurements have been made of the temperature dependence of the velocity of longitudinal ultrasonic waves in three single crystal specimens of Cu Mn (C=5,10, and 15 at.% Mn). Two types of effects have been studied in the temperature ranges of the susceptibility maxima (T0). (1) Observations of the change in sound velocity at a given temperature under application of a 11 kG transverse magnetic field show positive deviations above T0, negative deviations below T0, with no change in sound velocity at T0, to within experimental uncertainty. (2) Measurements of the temperature dependent sound velocity in zero magnetic field yield a small, broad minimum for the 5 at.% sample, implying the existence of a correspondingly round maximum in the specific heat. No minima were observed for the other samples. These results are compared with those of similar measurements on a Au Fe crystal, for which no zero field minimum was observed, and on three Au Cr polycrystalline specimens, which showed much larger minima near T0.
Effect of melt atmosphere on the magnetic properties of a [(SiO2)45(CaO)55]65[Fe2O3]35 glass
Authors: O’Horo M. P. , Levy R. A.
Published: February 2003

Mossbauer and magnetization measurements have been made on a [(SiO2)45(Ca0)55]65[Fe2O3]35 glass equilibrated at 1550 °C in a variable oxygen partial pressure atmosphere. The concentration of Fe2+ cations, as determined by Mossbauer analysis, is observed to increase with lower pO2 pressures at the expense of the fraction of Fe3+ cations tetrahedrally coordinated. The Fe2+ doublet is found to display the largest isomer shift and quadrupole splitting. For the two types of ferric cations there appears to be a general tendency for the Fe3+ cations with lower coordination to have a lower isomer shift but a substantially larger quadrupole splitting. Room temperature magnetization data indicate all the samples to contain superparamagnetic particles which increase in abundance with lower pO2 pressures. The superparamagnetic behavior is attributed to the formation of ferrite precipitates which occur during quenching.
Remanent magnetization of CoO⋅Al2O3⋅SiO2 glass: A logt law
Authors: Rechenberg H. R. , Bieman L. H. , Huang F. S. , de Graaf A. M.
Published: February 2003

Cobalt and manganese aluminosilicate glasses have magnetic properties very similar to those of spin-glass alloys, demonstrating that spin-glass ordering is not restricted to RKKY coupled spins. In the present paper we report long-time relaxation effects observed in the remanent magnetization of a 33.5 at.% Co-aluminosilicate glass (composition 83.1 mole% CoO, 15.5 mole% Al2O3, 1.4 mole% SiO2). Such effects have also been seen recently in the spin-glass alloy AuFe by Guy and by Nieuwenhuys and Mydosh. As in the case of the alloy, the remanent magnetization follows a logt law. This behavior will be discussed in terms of a superparamagnetic blocking model.
Optical studies of MnO⋅Al2O3⋅SiO2 glasses
Authors: Wong Y. H. , Thomas D. , Thomas R. L. , Danielson P. S.
Published: February 2003

Optical spectra of Mn-aluminosilicate glasses containing 4.8, 5.9, 7.7 and 14.3 at.% Mn ions are reported in the temperature range 1.7 K<T<300 K. The absorption peaks for the 4.8 at.% glass are similar to those observed in crystalline MnO, and are identified as being due to Mn++ in a cubic field. The spectra of the higher concentration samples show a single broad band, which we interpret as being due to Mn+++ in a cubic field. A strong, concentration-dependent λ−4 background indicates the existence of small domains. These domains are comparable in size to those postulated to explain the spin-glass behavior in the Mn-aluminosilicate glass system.
Amorphous magnetic order
Authors: Coey J. M. D.
Published: February 2003

A review is given of the collinear and random magnetic structures which may be found in magnetically-concentrated amorphous solids. General consequences of a non-crystalline lattice on the atomic moments, exchange interactions and single-ion anisotropy are presented. Magnetic structures with one and two magnetic subnetworks are then described, taking examples from the literature of each type. Some discussion is also given of the low energy excitations and the behavior at the spin freezing or ordering temperature. What happens to the familiar forms of magnetic order—ferromagnetism, ferrimagnetism, antiferromagnetism—in the absence of a crystalline atomic lattice? This article sets out to give an overview of magnetic order in non-crystalline solids in an attempt to answer the question. The magnetism of two classes of disordered metals have received particular attention during the past five years; amorphous ferromagnets, notably the Metglas-type alloys, and spin glasses, typically dilute solutions of transition-metal impurities in a non-magnetic crystalline matrix. The subject-matter here will be magnetically-concentrated amorphous materials, with the accent on non-collinear spin structures. A coherent picture of amorphous magnetism will be presented, but it must be admitted that not all the magnetic structures discussed have been equally well established. Some of them are directly analogous to those found in crystals. Others are peculiar to amorphous (or disordered) solids, whereas one crystalline magnetic structure seems impossible in a noncrystalline lattice.
Random anisotropy in amorphous ferromagnets
Authors: Alben R. , Becker J. J. , Chi M. C.
Published: February 2003

Random anisotropy is present in all amorphous magnetic materials, and depending on its strength, it can dramatically affect the magnetic behavior. We describe the effect of strong random anisotropy in materials such as amorphous Tb-Fe and Dy-Fe at low temperature and also examine the role which weak random anisotropy might play in even ideally homogeneous soft materials. Much of our analysis is based on the simple model proposed by Harris, Plischke and Zuckerman for a ferromagnet with random-axis uniaxial anisotropy. We describe computer simulation results for this model and then develop scale length arguments which allow us to describe fluctuations in the magnetization direction. For a perfectly isotropic distribution of anisotropy axis we find that the conventional ferromagnetic ground state is unstable. The new ground state has large frozen in fluctuations but probably has a considerable moment and is, therefore, not spin glass-like. This system does not support domain walls of the conventional type. For a system with both a macroscopic anisotropy axis and random anisotropy, we can have domain walls. We present a theory for the intrinsic coercivity which gives values of about 10−6 Oe for Fe-metalloids and 0.2 Oe for Gd-Co-Mo. This indicates that inhomogeneities of larger than atomic scale are limiting the behavior of present materials. A model illustrating aspects of magnetic resonance behavior is also described.
Magnetoelastic effects in metallic glasses
Authors: O’Handley R. C. , Chou C.-P.
Published: February 2003

This paper reviews data in the literature and reports some new results on magnetoelastic (ME) effects in transition metal/metalloid glasses. All of the ME phenomena are presented in a coherent formalism based on the type of spin interaction from which they arise. Included are isotropic ME effects on mean interatomic spacing (e.g., anomalous thermal expansion and volume magnetostriction) and on the elastic modulus, as well as anisotropic ME effects on mean atom spacing (e.g., magnetostriction) and on the elastic constants (e.g., ’’morphic’’ and ’’ΔE’’ effects).
Magnetic excitations in amorphous ferromagnets
Authors: Mook H. A.
Published: February 2003

Neutron scattering techniques have been used to measure the static structure and magnetic excitations in amorphous magnets. Sum rules and computer models are used to discuss the relationship between the static disorder and the shape of the excitation spectrum. Polarized beam measurements of χ''(Q,E) are compared to analytical theories and computer calculations for the magnetic excitations in amorphous ferromagnets.
Ferromagnetic behavior of disordered MnPt films produced by rf sputtering
Authors: Severin C. S. , Chen C. W.
Published: February 2003

The state of atomic order and the variation of magnetic behavior with atomic order in MnPt are studied. X-ray and neutron diffraction experiments show that quenching still results in appreciable degrees (≳0.8) of order and only through sputtering can completely disordered MnPt be prepared. Magnetization measurements conducted on the quenched powder and on the sputtered films established ferromagnetism in the disordered MnPt. The deduced magnetic moment is 3.25±0.30 μB per Mn atom in the sputtered films at 0 °K. This value is compared with those deduced for Mn in other alloys. The difficulties in invoking the Bethe-Slater criterion or a segregation of constituent atoms to explain the onset of ferromagnetism in the disordered MnPt are discussed.
Low-field magnetic properties of sputter deposited amorphous Fe80B20
Authors: Egami T. , Dahlgren S. D.
Published: February 2003

Amorphous Fe80B20 alloy was obtained by sputtering deposition onto a copper substrate cooled by liquid nitrogen. At room temperature, the sample on the copper substrate was under tension because of the differential thermal expansion, so that the magnetic remanence was up to 90% of the saturation, and the dc coercivity was 22 mOe. These results, together with high crystallization temperature, make the sputter deposited Fe80B20 a very attractive soft magnetic material.
The influence of quenching procedures on the kinetics of embrittlement in a Fe40Ni40B20 metallic glass
Authors: Chi G. C. , Chen H. S. , Miller C. E.
Published: February 2003

The kinetics of mechanical stability (embrittlement), stress relief, and crystallization of Fe40Ni40B20 metallic glassy ribbons obtained by different melt spin quenching processes were investigated. Ribbons made with higher rate of quenching exhibit better mechanical stability over those made with lower quenching rate. The thermal stability, temperatures, and activation energies for crystallization of all ribbons, however, are identical. The rates of stress relief are higher for ribbons made with higher quenching rate. Values of activation energy for stress relief are about the same for all ribbons. These activation energies for stress relief are also close to those values reported for structural relaxation.
Positive muon investigation of internal fields in ferromagnetic materials
Authors: Graf H. , Kündig W. , Meier P. F. , Patterson B. D.
Published: February 2003

The μSR (muon spin rotation) technique enables one to measure the local magnetic field Bμ at the site of an interstitial positive muon in solids. Various contributions to the field Bμ in ferro- and antiferromagnetic substances are discussed. The recently reported measurements on Fe, Co, Ni, Gd, and Dy are compared, and the resulting information about the muon stopping sites and muon diffusion is presented. The μSR data in Gd allow a determination of the temperature dependence of the angle between the easy axis of magnetization and the hexagonal axis. Several models which attempt to explain the hyperfine fields at the muon by the increase of spin density due to charge screening are analyzed.
Magnetoelastic excitations in Y-Fex films
Authors: Sylvester J. G. , Ambrosio A. , Blackstead H. A. , Miller A. E.
Published: February 2003

Conventional microwave resonance, pulse-echo, and spin-phonon spectrometer experiments on vacuum evaporated microcrystalline and amorphous films of YFex(x?2) demonstrate the existence of significant dynamic magnetoelastic coupling. For microcrystalline specimens, multiple transverse phonon signal peaks are observed at the rf frequency as a function of static field applied parallel to the film normal. The applied magnetic fields (0–20 kOe) for which the phonon signals are observed coincide with resonance absorption signals. In addition, the resonance data contain well-resolved structure with as many as 8 peaks. To date, no high field multiple peak structure has been observed in amorphous films. Pulse-echo and spin-phonon spectrometer experiments have also shown that phonons are excited with parallel rf and dc fields in the film plane. A maximum longitudinal signal is observed with this field configuration; the signal intensity varies approximately as cos4φ, where φ is the angle between the rf and dc fields. With the fields in the film plane, a maximum transverse phonon signal is observed at φ=90°.
Sublattice magnetizations and textures in SrFe12O19 ceramic compacts
Authors: Vogel R. H. , Evans B. J.
Published: February 2003

57Fe Mössbauer spectroscopy has been used to determine the degree of orientation and magnetic hyperfine fields in SrFe12O19 ceramic compacts with known bulk magnetic properties. The controversy regarding the assignment of the spectrum to the different sites has been resolved and at 300 K the hyperfine fields for the 4f2, 4f1, 2a, 12k, and 2b sites are 515, 490, 490, 414, and 426 kOe, respectively. This assignment is slightly different from that in BaFe12O19. The 2b site quadrupole splitting of 2.6 mm s−1 is also larger than in BaFe12O19 in agreement with the relative anisotropy fields. Oriented specimens with BΓ and Hc of 3700 G and 3600 G, respectively, still show significant deviations from complete alignment, suggesting that the intrinsic material properties of these ceramic compacts are superior to those reported for individual crystallites of SrFe12O19.
Magnetic hyperfine parameters of iron containing minerals in coals
Authors: Russell P. E. , Montano P. A.
Published: February 2003

Mössbauer spectroscopy of 57Fe is used to study the hyperfine parameters of iron minerals in coals and shales. Mössbauer spectroscopy at low temperatures using a large external magnetic field permits the identification of the electronic ground state of the iron ions in the minerals. The hyperfine parameters of pyrite, iron sulfates and clay minerals are studied in order to characterize and identify the iron minerals in the coals. The full understanding of the magnetic behavior of the iron containing minerals is of importance for potential magnetic separation processes. We have also observed the presence of a Mössbauer spectrum with parameters very close to those of pyrite, but with a smaller electron density at the 57Fe nucleus. This particular spectrum was associated with coal samples rich in a pyrite type called ’’framboidal’’. This led us to the study of in situ ’’amorphous’’ pyrite. All the coal samples studied were from West Virginia seams.
Spin wave propagation near the surface of a ferromagnet with surface anisotropy
Authors: Stakelon T.
Published: February 2003

It is shown how the non-collinear spin ground state that generally results from an additional anisotropy energy at the surface of a ferromagnet alters the propagation of exchange dominated spin waves with wavevectors normal to the surface. These changes in propagation occur in a narrow region near the surface and for long wavelength spin waves are equivalent to changes in the spin wave boundary condition. These new boundary conditions show that the non-collinear ground state can substantially influence the spin wave resonance spectrum of thin ferromagnetic films.
Neutron scattering studies of spin-glass alloys
Authors: Murani A. P.
Published: February 2003

Neutron diffraction investigations of binary alloys such as Cu-Mn, beginning with the earliest reported measurements in 1957, show no evidence of any long range magnetic order except for the broad short range order peaks at the (1 1/2 0) type positions, attributed to both atomic and magnetic short range ordering in alloys containing more than about 10–15 at.% Mn. The current interest in spin glasses is focused on the theoretical possibility of a magnetic phase transition to the spin glass state in the dilute random substitutional alloys. Recently neutron scattering measurements have been attempted to study the dynamics of spins in the alloys with a view to understanding the nature of the processes leading to the spin glass state. Small angle neutron scattering measurements on Au-Fe alloys with Fe concentrations below the percolation threshold for ferromagnetism show a series of q-dependent maxima in the total scattering intensity suggesting a continuum of freezing temperatures. Other measurements employing energy analysis by the time-of-flight technique at somewhat larger q’s on the same and other alloys such as Cu-Mn, Pd-Mn show a broad quasi-elastic paramagnetic spectrum at high temperatures whose energy width decreases with decreasing temperature, apparently goes through a minimum especially at the higher q’s but always remains finite as T→0 K. The intensity of the quasi-elastic scattering, however, diminishes progressively below some temperature with a simultaneous continuous increase of the elastic peak intensity. Although the implications of the recent phase transition theories of spin glasses to the dynamics of spins at finite scattering vectors are not entirely clear, the present neutron scattering results appear not inconsistent with the alternative picture of a continuous process of freezing of spins occurring at least over a finite temperature range.
The electrical resistivity of dilute magnetic alloys: Kondo effect vs spin glass behavior
Authors: Tsay Yeun C. , Shen L.
Published: February 2003

Based on a theory and statistical technique developed previously by the authors, we discuss the electrical resistivity of dilute magnetic alloys in the concentration range from the extreme dilute limit to the lower margin of the spin glass concentration. Our results for resistivity indicate that in this concentration range, the Kondo effect and the mechanism that leads to a spin-glass state coexist and compete with each other. The theory agrees with recent experiments in various aspects. The application of our model to more concentrated spin-glasses is also discussed.
Application of the superposition model of the crystal field to rare-earth garnets
Authors: Nekvasil V. , Roskovec V. , Zounová F.
Published: February 2003

The superposition model (SM) analysis of the crystal field parameters (CFP) in 14 Nd3+, Eu3+, Tb3+, Dy3+ and Er3+ garnets is presented. The ratio of the average values of exponents defining the radial dependence of the intrinsic parameters in these garnets is t6/t4≲0.5 in contrast to the results of Newman and Stedman, who obtained t6/t4≳1 in their earlier study of rare earth garnets. The possibility of using the SM for the quantitative prediction of the CFP’s by extrapolation from one garnet structure to another is demonstrated on Tb3+. Starting with the CFP’s reported for TbAG we extrapolate them to TbIG. From the CFP’s obtained in this way the CF effect on magnetization is calculated and compared with the high field measurements.
Transport properties of the ferromagnetic semiconductor HgCr2Se4
Authors: Goldstein L. , Gibart P. , Selmi A.
Published: February 2003

The spinel ferromagnetic semiconductor HgCr2Se4 (Tc=110 K) has been grown from the vapor. Annealing under Hg atmosphere gives n-type material. The Hg annealed HgCr2Se4 specimens are degenerated semiconductors with n=1017–1018 cm−3, μ4.2?300–1200 cm2/V sec. The resistivity increases by two orders of magnitude at Tc. This resistivity peak, due to spin disorder scattering, can be suppressed by a magnetic field, Δρ/ρ=−0.94 for 10 kOe at Tc. Selenium annealing gives p-type at room temperature. At low temperature these selenium annealed samples become n-type n?1015 cm−3, μ4.2=100 cm2/V sec. These features are explained assuming a two carrier conduction.
Magnetic properties of VSe2: Inferences from the study of metal-rich V1+δSe2 compounds
Authors: Thompson A. H. , Silbernagel B. G.
Published: February 2003

The intrinsic magnetic properties of VSe2 in the incommensurate and commensurate charge denity wave states have been examined by magnetic susceptibility and NMR observations. VSe2 synthesized at high temperatures is slightly metal-rich and since the excess vanadium atoms are magnetic, their contribution to the magnetic properties must be extracted from those intrinsic to VSe2. This separation has been accomplished by studying a series of materials of the form V1+δSe2 (0.01<δ<0.2). The resulting intinsic susceptibility is temperature independent at high temperatures, has a cusp near 120 K and has a mild discontinuity near 70 K. We associate these temperatures with the onset of incommensurate and commensurate charge density waves respectively. We suggest that a susceptibility maximum, observed near 110 K, is an artifact of the separate temperature dependences associated with VSe2 and the excess vanadium species.
Effect of H2 absorption on the magnetic properties of rare-earth transition metal compounds
Authors: Buschow K. H. J. , Sherwood R. C.
Published: February 2003

Rare-earth transition metal compounds are known to be able to (reversibly) absorb large quantities of hydrogen gas at moderate pressures. This absorption of H2 gas leads to substantial changes of the magnetic properties. These include hydrogen induced transitions from Pauli paramagnetism to ferromagnetism. Examples also exist of the reverse effect, i.e., where in ferromagnetic compounds the 3d moment disappears upon hydrogen absorption. The changes in magnetic properties, together with experimental results of X-ray diffraction and Mössbauer-effect spectroscopy are discussed in terms of charge transfer and changes in interatomic distances. The metastable character of the ternary hydrides often leads to a loss of the long range atomic order upon hydrogen absorption. This affects not only the absorption capacity (after repeated cycling) but also influences the magnetic behavior. Examples are shown where the atomic disorder has led to pronounced thermomagnetic history effects.
Effects of dilute Pt on the Gd3+ ESR lineshape in ScH1.9
Authors: Venturini E. L.
Published: February 2003

Gd3+ electron spin resonance (ESR) at 9.8 GHz has been studied from liquid helium to liquid nitrogen temperatures in a metallic scandium dihydride host containing 0.5 atomic percent Gd and from 0 to 1 atomic percent Pt. The sample without the Pt has a residual or zero-temperature linewidth of 180±5 Oe and a g-factor of 1.993±0.003. The addition of nonmagnetic Pt impurities has little effect on the slope of the ESR linewidth versus temperature or on the position of the line center (g-factor), but it drastically reduces the residual linewidth. For a sample containing 0.25 atomic percent Pt the residual linewidth is 71±3 Oe, less than 40% of that for the sample without Pt. A careful analysis of the ESR lineshape shows that it is neither Lorentzian nor a combination of Gaussian and Lorentzian components. We can accurately describe the experimental shape using a Lorentzian function with a single center and a half-width uniformly distributed between a minimum and maximum value. Using this model for the Gd3+ ESR lineshape, we find that the addition of nonmagnetic Pt impurities sharply reduces the maximum half-width of the distribution while producing a much smaller change in the minimum half-width.
Crystal fields and magnetic properties of ErH3
Authors: Flood D. J.
Published: February 2003

Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3–4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie–Weiss behavior from 4.2 K to 2 K, and intercepts the negative temperature axis at ϑ=1.05±0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77±0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5±1.2) ×10−6 Weber m/kg T. The saturation moment is 3.84±0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. Using the wave functions and energy level diagrams of Segal and Wallace (Ref. 4), the first excited state is estimated to be 95 K above the ground state.
Neutron diffraction measurements of the spatial extent of the Ce moment in the Kondo system Y : Ce
Authors: Edelstein A. S. , Stassis C. , Kline G. , Beaudry B. , Maglic R.
Published: February 2003

Neutron diffraction measurements made down to 2.2 K on single crystal samples of the Kondo system Y:Ce have determined that the spatial extent of the impurity magnetic moment is the same as that of the Ce3+ ion. The Kondo temperature TK is probably greater than 6 K. The magnitude of the moment determined by neutron measurements agrees with that obtained from magnetization measurements in the temperature range 2.2 to 30 K.
Magnetic susceptibility studies of NdFeTiO5 single crystals
Authors: Yaeger I.
Published: February 2003

The principal magnetic susceptibilities of NdFeTiO5 have been measured from 77.3 K down to 4.2 K. An antiferromagnetic ordering was observed at TN=18.5±0.5 K. This is interpreted as a cooperative ordering of the Fe3+ spin system. The Nd3+ ions stay paramagnetic below TN with their moments polarized due to an interaction with the ordered Fe3+ spins. The main features of the temperature dependence of the magnetic susceptibility are accounted for in the mean field approximation in terms of an interaction between the ordered Fe3+ spins and the electrons occupying the lowest lying Kramers’ doublet of the Nd3+ ions.
Magnetic structure of disordered Ni-Mn alloys as revealed by high field twist deformation
Authors: Caporaletti O. , Graham G. M.
Published: February 2003

This work reports observations of a twist deformation in Ni-Mn alloys caused by a high longitudinal magnetic field. In this system, the anomalous magnetization vs concentration curve suggests that antiferromagnetic Mn-Mn nearest neighbour exchange interactions cause a spin canted structure in the alloy. The twist is believed to arise from this canting of the atomic magnetic moments. Cylindrical samples of Ni-5 at.% Mn and Ni-25 at.% Mn were examined for evidence of high field twist at 4.2 K. Even though the saturation magnetization of the 5 at.% Mn alloy is approximately four times larger, twist was only observed in the 25 at.% Mn alloy. A surface magnetostatic mechanism is proposed to account for the formation in the 25 at.% Mn alloy of an azimuthal component, of the order of 1% of the saturation magnetization. The results, then, appear to confirm the idea of non-collinearity of the atomic moments, the twist being just a macroscopic manifestation of this magnetic structure.
Magnetic configurations in a two-sublattice system with strong antisymmetric exchange and fourth order uniaxial anisotropy in vanishing external field
Authors: Acquarone M.
Published: February 2003

Fully general solutions to the equilibrium equations of a system of two equal magnetic moments coupled by isotropic, anisotropic and antisymmetric exchange, together with second- and fourth-order uniaxial anisotropy, in vanishing external field, are given. They depend critically upon the fourth-order anisotropy and the antisymmetric exchange, and comprise planar, axial and ’’conical’’ phases. The latter, stable over a wide range of t, the interaction parameters, are of interest in connection with some measurement on the Y and W hexaferrites.
Local environment effects in disordered alloys
Authors: Cable J. W.
Published: February 2003

The magnetic moment of an atom in a ferromagnetic disordered alloy depends on the local environment of that atom. This is particularly true for Ni and Pd based alloys for which neutron diffuse scattering measurements of the range and magnitude of the moment disturbances indicate that both magnetic and chemical environment are important in determining the moment distribution. In this paper we review recent neutron studies of local environment effects in Ni based alloys. These are discussed in terms of a phenomenological model that allows a separation of the total moment disturbance at a Ni site into its chemical and magnetic components.
Local environment effect on atomic moments in ferromagnetic transition-metal alloys
Authors: Miwa H. , Hamada N.
Published: February 2003

Effects of atomic environment on distribution of magnetic moments in ferromagnetic transition-metal alloys are discussed on the basis of an itinerant-electron theory in the Hartree-Fock approximation. The significance of the relation between the atom in question and its environment is demonstrated with emphasis on the two types of free energy curve as a function of the atomic magnetic moment. A method for the self-consistent calculation of magnetic moment distributions in concentrated alloys is presented. Some results of the method applied to Fe-V and Fe-Co alloys are shown to be in good agreement with experimental measurements.
Self-consistent embedded-cluster model for magnetic impurities: β′NiAl
Authors: Ellis D. E. , Benesh G. A. , Byrom E.
Published: February 2003

Substitutional transition metal impurities occupying either A or B sites in AB intermetallic compounds with CsCl structure have been treated by a self-consistent molecular-orbital method. The electronic energy levels and charge and spin densities of MX8 clusters representing a central atom and its nearest neighbors have been calculated in the Hartree-Fock-Slater spin-polarized model, using a numerical discrete variational method. The crystal lattice is included through a self-consistent empirical pseudopotential scheme. Results for Fe, Ni, and Co impurities in β′-NiAl are discussed in connection with experimental resistivity, Mössbauer isomer shift, and NMR data.
Energy band structure of itinerant ferromagnet in the disordered phase
Authors: Liu S. H.
Published: February 2003

The electronic structure of an itinerant ferromagnet is studied by the cluster-Bethe-lattice method in both the ordered and the disordered phases. In the ordered phase the energy bands are spin split as expected. In the disordered phase the propagation of an electron wave in the lattice is characterized by two complex wave vectors. These may be interpreted as two spin split ’’bands’’, and the spin disorder gives rise to the finite line-width or life-time of both bands. The result explains the lack of sensitivity of the photoemission spectrum of nickel to magnetic ordering, and the large spin disorder electrical resistivity in nickel and iron.
Coupling of localized spins in itinerant electron antiferromagnets
Authors: Moyer C. A. , Kelly J. R. , Arajs S.
Published: February 2003

We obtain an effective spin Hamiltonian for a system of impurity spins embedded in an itinerant electron antiferromagnetic matrix. Each spin couples to the polarization field of the spin density wave via an s-d exchange interaction with the conduction electrons. The impurity spins also interact among themselves via modulation of the existing spin polarization. The result is a generalized RKKY interaction whose long range oscillations reflect the Fermi wavenumber as well as the spin density wave periodicity.
Effect of vanadium on the P-C and P-I magnetic transition in chromium alloys
Authors: Gudmundsson H. , Hedman L. , Rao K. V. , Åström H. U.
Published: February 2003

Our experimental studies of electrical resistivity and magnetic susceptibility on Cr+2at.%Si and Cr+3at.%Fe doped with 0–2 at.% vanadium provide evidence that the depression in TN for the same change in impurity concentration is significantly different depending on whether a para-to-incommensurate (P-I) or para-to-commensurate (P-C) spin-density-wave structure is obtained at the onset of antiferromagnetic ordering. This we explain by assuming the observed empirical correspondence between the effect of pressure and the decrease of electron to atom ratio on TN. However, the reason for the two different rates of dTN/dP at the P-I (∼6 K/kbar) and P-C (∼30 K/kbar) type transitions in chromium alloys remain totally unexplained.
Low frequency modes in rare earth systems with singlet ground states
Authors: Huber D. L.
Published: February 2003

We outline a general theory for the low frequency spin dynamics in rare earth ferromagnets and antiferromagnets with singlet ground states. The analysis applies to the paramagnetic phase of systems with degenerate excited states. It is assumed that the characteristic frequencies associated with the intramultiplet transitions are much less than the intermultiplet frequencies. Quantitative studies are carried out treating the static correlations in the molecular field approximation. We calculate the low frequency component of the zone center dynamic susceptibility for singlet-triplet model and the effective g-factor and relative intensity of the Γ5(2) EPR in TbP. Critical anomalies in the EPR of singlet ground state ferromagnets are pointed out. Contact is made with the theories of Buyers and Copper et al.
Magnetic anisotropy of Zn-Cr alloys at very low temperatures
Authors: Moberly L. A. , Symko O. G.
Published: February 2003

The temperature dependent magnetization of Zn-Cr single crystals was measured from 2 K down to 10 mK using a SQUID magnetometer. A large anisotropy is observed for the magnetization along the parallel and perpendicular axes of the crystal. For low concentrations impurity, -impurity reactions do not dominate the behavior of the system, although they are still present even in the 10 ppm impurity, concentration range. The anisotropy in the magnetization, M∥-M⊥, is negative which is in contrast with Mn in Zn system where the anisotropy is positive. The results are analyzed in terms of crystal field splittings and exchange. These results have important implications for the Kondo effect in this system.