bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrathin magnetic films of Fe/Ag(100): An approximate realization of a 2-d Heisenberg ferromagnet? (abstract)
Authors: Koon N. C. , Prinz G. A. , Volkening F. A. , Das B. N. , Aylesworth K.
Published: July 2002

We have measured the transmission Mossbauer spectra of Fe/Ag(100) single crystal multilayers with large thicknesses (∼100 ML) of intervening Ag as a function of both temperature and magnetic field applied perpendicular to the surface. In contrast to earlier work on samples with less silver between layers, ordering temperatures are much reduced and the initial relaxation effects observed on cooling down appear to set in with a very small or zero order parameter.1,2 Measurements with a magnetic field applied perpendicular to the film surface showed a single well defined order parameter, which rules out superparamagnetizatism as the source of the observed relaxation effects. As an example, a sample measured earlier with 2.4 ML of Fe and 4 ML of Ag exhibited a well defined order parameter in zero field with a transition temperature near 200 K. In the present case a 3.0 ML Fe sample with ∼100 ML of intervening Ag exhibited relaxation effects setting in near 100 K, with a nonzero order parameter below ∼60–70 K. Measurements in a magnetic field showed clear evidence for long range spin–spin correlations even at temperatures above 100 K. Our interpretation is that for the films with relatively thin (∼4 ML) Ag layers between the Fe, there is sufficient coupling between layers to drive an essentially 3-d magnetic transition. With a very thick Ag interlayer separation, the characteristics of the film become much more like an ideal 2-d Heisenberg model, which builds up very long range spin–spin correlations at low temperatures, but whose actual magnetic ordering transition is depressed to T=0 (finite T with anisotropy).3 Presumably the transition which we estimate to be at 60–70 K is driven by the fact that the perpendicular anisotropy is not zero, but in fact is larger than the demagnetizing field.
Spin and orbital moments in actinide compounds (invited)
Authors: Lebech B. , Wulff M. , Lander G. H.
Published: July 2002

The extended spatial distribution of both the transition-metal 3d electrons and the actinide 5f electrons results in a strong interaction between these electron states when the relevant elements are alloyed. A particular interesting feature of this hybridization, which is predicted by single-electron band-structure calculations, is that the orbital moments of the actinide 5f electrons are considerably reduced from the values anticipated by a simple application of Hund’s rules. To test these ideas, and thus to obtain a measure of the hybridization, we have performed a series of neutron scattering experiments designed to determine the magnetic moments at the actinide and transition-metal sublattice sites in compounds such as UFe2, NpCo2, and PuFe2 and to separate the spin and orbital components at the actinide sites. The results show, indeed, that the ratio of the orbital to spin moment is reduced as compared to the free-ion expectations. In addition there is qualitative agreement with theory, although the latter predicts values of both components that are larger than those found by experiment. Because L and S are opposed in the light actinides, and L is usually greater than S, the reduction of L can result in a situation for which L−S≂0. This almost occurs in UFe2. However, neutrons are capable of observing the individual components at finite wave vector (Q), although the total component (observed at Q=0) may indeed be close to zero.
Powder metallurgical processing of Tb0.27Dy0.73Fe2−x (0<x≤0.5) by hydrogen decrepitation
Authors: Jones D. G. R. , Fairclough J. P. , Abell J. S. , Harris I. R.
Published: July 2002

This paper reports on the characterization of giant magnetostrictive alloys, of the general formula Tb0.27Dy0.73Fe2−x (0<x≤0.5), at various stages during their powder processing by hydrogen decrepitation, in order to try to optimize the process. Mass spectrometer degassing studies on hydrogenated material have shown that disproportionated material exhibits two desorption peaks and is not degassed completely with respect to hydrogen until above 600 °C. The RFe2 (R=Tb0.27Dy0.73) hydride powder appears to degas at a lower temperature; however, it is thought that a high-temperature desorption peak is not present due to oxidation. Metallographic studies have shown that there is much less rare-earth-rich grain-boundary phases in sintered compacts of initial composition RFe1.5 than in cast RFe1.5 and also reveal the microstructure of hydrogenated and disproportionated RFe1.5. Wavelength dispersive x-ray analysis seems to indicate preferential loss of Dy during sintering.
Magnetostriction in pure and Tb-doped amorphous Gd2Cu
Authors: Arnaudas J. I. , del Moral A. , de Groot P. A. J. , Rainford B. D. , El Khadi M.
Published: July 2002

Magnetostriction, parallel (λ∥) and perpendicular (λ⊥) to the applied magnetic field, and thermal expansion measurements have been performed in a-Gd2Cu and a-(Gd0.98Tb0.02)2Cu. The volume (ω=λ∥+2λ⊥) field-induced magnetostriction peaks at the magnetic transition temperatures. The anisotropic magnetostriction (λt =λ∥−λ⊥) in a-Gd2Cu is non-null and also shows a peak; in the doped compound the peak no longer exists and λt increases down to low temperatures. The strain dependencies of the isotropic and longitudinal two-ion spin-exchange-correlation functions explain the results for a-Gd2Cu and the thermal dependence of ω in a-(Gd0.98Tb0.02)2Cu. The strong single-ion random anisotropy of Tb ions accounts for the λt behavior in the doped compound.
Spontaneously dimerized ground states in 1D frustrated Heisenberg antiferromagnets
Authors: Pimpinelli Alberto
Published: July 2002

We give an explicit proof that the completely dimerized (’Kekulè or nearest-neighbor resonant valence bond) state is the degenerate ground state of a linear chain S =1/2 Heisenberg antiferromagnet, with competing couplings extending up to 2n nearest neighbors, if the relation J1 = 2n, J2 = 2n − 1,...,J2n = 1, holds between the various exchange integrals. The previously known result for J2 =1/2 J1 is then just a special case corresponding to n=1. We study in detail n=2 (fourth-nearest neighbors) and show that this state is the ground state of the Hamiltonian when J1 − 2J2 + J3 = 0, J3 = 2J4, in a finite interval of values of the parameter J4. A rigorous lower limit for the extension of this interval is found to be 0 ≤ J4 ≤1/4 J1. A comparison with ground-state configurations of the equivalent classical Heisenberg and Ising models is made.
Critical behavior of the S=1 antiferromagnetic Heisenberg chain (abstract)
Authors: Bonner Jill C.
Published: July 2002

The spectral properties, and associated critical singularities, of spin-1 Heisenberg antiferromagnetic chains are of high current interest, both theoretical and experimental. Theoretical studies using the single mode approximation and also a Monte Carlo calculation indicate that the spectrum has a gap at k=π, as predicted by Haldane. They also predict the unusual feature that the excited state continuum is asymmetric about the mid-point of the Brillouin zone, k=π/2. However, neither approach is sufficiently accurate to predict reliably the magnitude of the gap at k=0 which was not discussed by Haldane. Our approach is to use static (spectral) calculations of excited states of chains of N=4 through 14 spins for all values of k, a novel feature not associated with previous finite-size calculations, to discuss the dispersion spectra of the S=1 Heisenberg chain. In addition, we have calculated dynamical properties, in particular S(q,ω), exactly for finite chains up to N=14 spins. This allows us to estimate the gap at k=0 to good accuracy, and we find that to within this accuracy the gap at k=0 is twice the Haldane gap at k=π. This result is important in connection with very recent neutron scattering and magnetization measurements. In addition, we have calculated the correlation length and critical singularities associated with dynamical quantities.
Anisotropy and spin reorientation of a single crystal of PrCo3Ni2
Authors: Ballou R. , Michellutti B. , Voiron J.
Published: July 2002

Magnetization measurements have been performed between 1.5 and 750 K on a single crystal of PrCo3Ni2 along the a, b, and c axes of the hexagonal structure. This compound orders magnetically below 635 K. Large anisotropy of the magnetic properties are observed. At low temperature, the b axis is the easy axis, whereas a progressive spin reorientation takes place between 155 and 185 K. This behavior has been confirmed by torque measurements. At higher temperature the c axis is the easy axis. Moreover, between 1.5 and 60 K, the magnetization process in the hard direction presents a smooth metamagneticlike behavior. Results are discussed in terms of competition between the anisotropies of 3d and 4f electrons.
Calculated magnetic moments of Nd2Fe14B
Authors: Nordström Lars , Johansson Börje , Brooks M. S. S.
Published: July 2002

A self-consistent spin-polarized band-structure calculation has been performed for the technically important permanent magnet compound Nd2Fe14B. In contrast to earlier calculations, the localized 4f states on the Nd sites are treated in a consistent way. They are not allowed to contribute to the bonding, but they produce a local exchange field, felt by the valence electrons, which is calculated from first-principles local density theory. Assuming a Russel–Saunders coupled Nd 4f moment of 3.3μB/atom, the total magnetic moment is calculated to be 38.1μB/formula unit, to be compared with the experimental value 37.1μB/formula unit [Givord, Li, and Perrier de la Bathie, Solid State Commun. 51, 857 (1984)]. The calculated local Fe moments are quite different on the different crystallographic sites, varying from 2.1μB to 2.9μB/atom.
Investigation of the domain wall dynamic by the inelastic light scattering method (abstract)
Authors: Kreines N. M.
Published: July 2002

Light scattering by a moving domain wall (DW) has been investigated by means of Brillouin–Mandel’stam spectroscopy for the first time.1,2 The proportional to the DW velocity frequency shift due to Doppler effect has been observed in the light scattering spectra in the weak ferromagnets. The DW velocity (till 13 km/s) and the intensity of the scattered light as a function of pulsed magnetic field are determined from the spectra at different temperatures. As the DW velocity approaches that of sound (transversal or longitudinal) the scattered light intensity increases dramatically due to phonons emitted by the DW (Cherenkov radiation). Regions of the nonstationary DW motion are observed wherein scattering spectra have complicated character. The inelastic light scattering directly by the excited sound soliton (or phonons) has been observed. The space and time spreading of this soliton was investigated.3 The mean free path l of the acoustic phonons was determined. For YFeO3 at T=2 K l≂300 μm for transversal sound with ν≂12 GHz. The temperature dependence of the DW mobility was studied.
Perpendicular magnetic anisotropy in Pt/Fe multilayers
Authors: Katayama T. , Suzuki Y. , Nishihara Y. , Sugimoto T. , Hashimoto M.
Published: July 2002

Perpendicular magnetic anisotropy (K⊥) and related magnetic properties in Pt/Fe multilayers [compositionally modulated multilayer films (CMFs)] are investigated. It is found that Pt/Fe CMFs with about an 18-Å Pt layer and Fe layer below 5 Å become perpendicularly magnetized films. Surface magnetic anisotropy (Ks) and volume anisotropy (Kv) are 0.46 erg/cm2 and about − 2 × 107 erg/cm3 at room temperature, respectively. The Ks is almost equivalent to that of Pt/Co CMFs and increases with decreasing temperature. The absolute value of Kv, however, is much larger than that of the Pt/Co system. The effective magnetic anisotropy (K⊥ eff) of [Fe(4.7 Å)/Pt(18.5 Å)]80 CMF changes from positive to negative at T≂200 K with decreasing temperature.
Effect of oxygen on orientation of polycrystalline Fe films prepared by ECR-magnetron sputtering
Authors: Takebayashi S. , Shimokawa K.
Published: July 2002

An ECR-magnetron sputtering system with permanent magnets has been developed and used to prepare Fe-O films. Oxygen was introduced into an Ar sputtering atmosphere during deposition of Fe onto the glass substrate. X-ray diffraction patterns of these films showed a drastic increase in intensity of the Fe(200) diffraction peak, and a rapid decrease of the Fe(110) peak. The Fe(110) single peak is observed in the film prepared without oxygen. In the films prepared with oxygen, iron oxides were not detected through x-ray diffraction, x-ray photoelectron, or magnetic properties measurements, implying that, in sputter-deposited Fe films, a small amount of oxygen brings about a [100] orientation of the Fe grains.
Growth mode and interface formation of Fe and Co films on ZnSe(001) (abstract)
Authors: Jonker B. T. , Prinz G. A.
Published: July 2002

The incorporation of magnetic layers in semiconductor heterostructures is an increasingly active area of study. We are interested in the growth of ferromagnetic layers on diluted magnetic semiconductor (DMS) substrates as a means of magnetically polarizing the DMS medium, a necessary condition for these materials to exhibit the extraordinary magneto-optic and transport properties for which they are known. Since interfacial effects are expected to play an important role in thin-film heterostructures, we have examined the growth of Fe and Co films on ZnSe(001) to determine the mode of film growth, the formation of the interface, and the structure of the overlayer at the 1–10 monolayer level. The results obtained should apply to the growth of such overlayers on the ZnSe-based DMS compounds for modest magnetic ion concentrations as well, such as (Zn,Mn)Se, (Zn,Fe)Se, and (Zn,Co)Se. The ZnSe(001) samples were grown by molecular-beam epitaxy on GaAs(001) substrates, passivated with a Se coating in a manner analagous to the As passivation of GaAs, and transferred to a surface analysis system.

The Se coating was thermally desorbed in UHV at 150–200 °C just prior to metal deposition. The metal films were deposited from miniature electron-beam sources and characterized in situ with Auger electron diffraction, RHEED, and x-ray photoelectron spectroscopy. The coverages were determined by x-ray flourescence measurements. The coverage dependence of the forward scattering peaks in the Auger electron diffraction scans due to the occupation of second and third monolayer sites provides information on the initial mode of film growth. The angular position of these peaks provides a measure of the out-of-plane lattice spacing, and hence the structure of the overlayer. We find that the growth of Fe(001) on a ZnSe(001) epilayer is predominantly 2D in nature at a substrate temperature of 175 °C, while growth directly on the oxide-desorbed GaAs(001) bulk substrate shows more significant 3D clustering. While the growth of Co on GaAs(001) results in a nearly ideal bcc single-crystal structure, growth on ZnSe(001) epilayers yields a multicrystalline structure. Little interaction with the ZnSe is observed with XPS.
Field-included noncollinearity of the magnetic moments in Gd2Co14B
Authors: Kohashi T. , Ono M. , Date M. , Yamagishi A. , Zhong X. P. , Wang Q. , Yang F. M. , Radwanski R. J. , de Boer F. R.
Published: July 2002

Magnetization measurements at 4.2 K have been performed on magnetically aligned Gd2Co14B and Gd2(Co,Al)14B samples in fields up to 57 T, applied parallel to the alignment direction. At high fields, the antiparallel ferrimagnetic configuration is broken up. Values for the intersublattice molecular-field coefficient and for the molecular field acting on the Gd moment, derived by means of mean-field analysis of the observed magnetic isotherms, are presented.
Small-angle neutron scattering in spin reorientation transitions of (ErxNd1−x)2Fe14B hard pseudoternaries
Authors: del Moral A. , Arnaudas J. I. , Algarabel P. A. , Ibarra M. R. , Schweizer J.
Published: July 2002

Critical small-angle neutron scattering has been performed on (ErxNd1−x)2Fe14B hard intermetallics around the spin reorientation transition (SRT) temperatures, TSR. The measured scattered intensities show squared Lorentzian type behavior (for k ≤ 12 × 10−3 Å−1) and the magnetic correlation lengths show well defined anomalies at TSR, related to the scattering within the magnetic domain walls, which transform at the SRT.
Effect of partial substitution of Tb on the spin reorientation of the Er2−xTbxFe14B system
Authors: Obermyer R. T. , Pourarian F.
Published: July 2002

Numerous articles have been published concerning spin reorientation in R2Fe14B (R=rare earth) compounds. Previous measurements have suggested the presence of axial, conical, and planar regions in the magnetic phase diagrams. In the present systems the magnetization versus temperature (10–300 K) has been measured for aligned Er2−xTbxFe14B samples for x=0, 0.2, 0.5, and 1.5 in applied fields of 0–15 kOe. Transitions were determined from the inflection point as obtained from the derivative of the magnetization versus temperature. Earlier theoretical predictions of the magnetic phase diagrams for Er1.8Tb0.2Fe14B indicate that both plane-to-cone and cone-to-axis transitions should be present. Spin reorientations were observed at 194 K for plane to cone and at 212 K for cone to axis, in agreement with the predicted values. There was a weak-field dependence in the transition temperatures, and the strength of the transitions steadily decreased with increasing field. In the case of Er1.5Tb0.5Fe14B, a cone-to-axis spin reorientation was observed at 78 K, again in agreement with the predicted value. Er0.5Tb1.5Fe14B was axial through the 10–290-K temperature range. Thermal hysteretic effects for the transition temperatures were observed upon heating and cooling in an applied field. Detailed results of the spin reorientation for Er2−xTbxFe14B are discussed in terms of the second-order crystal-field effect and exchange interaction.
Extended x-ray-absorption fine-structure study of the position of Zr within the unit cell of Sm2Co17
Authors: Rabenberg L. , Barrera E. V. , Maury C. E. , Allibert C. H. , Heald S. M.
Published: July 2002

Extended x-ray-absorption fine-structure spectroscopy (EXAFS) has been used to determine the position of Zr within the unit cell of Sm2Co17. Induction-melted Sm2Co17:Zr ternary alloys, aged at 1180 °C, then quenched, consisted of intimately mixed H2:17 and R2:17 having Zr in solid solution as well some regions of R2:17 that were poor in Zr. EXAFS spectroscopy of these specimens indicates that the most probable position for Zr is a site having 2 Sm near-neighbor atoms and 11 Co atoms distributed over three different interatomic distances. This is consistent with a direct substitution of Zr for Co in the Co site in the mixed planes (12j in P63/mmc or 18f in R3m).
Hydrogenated hard magnetic alloys from fundamental to applications (invited)
Authors: Fruchart D. , Miraglia S.
Published: July 2002

The powerful magnet materials based on rare earth (R) and magnetic 3d metals (M=Fe,Co) exhibit a large affinity to absorb hydrogen. Hydrides of the anisotropic materials R2M17 and R2M14X with X=B, C, and RM12 with M=Fe1−xM′x, have been systematically investigated. First, the impact on the fundamental properties (structure, magnetization, anisotropy, easy axis, etc.) have been analyzed by several techniques (x-ray and neutron scattering, magnetization, Curie temperature, etc.), and in many cases the hard magnetic characteristics are shown to be dramatically reduced. These effects are discussed in terms of M-M and M-R exchange interactions, d-f overlaps, and 3d and 4f (CEF) anisotropies. Since magnetoelastic effects upon hydrogenation are large, isotropic and anisotropic cell expansion terms are analyzed. Second, the role of microstructure is of great importance, since the coercivity is very large. Hydrogenation effects at a macroscopic scale have also been investigated. Further restoring heat treatments are needed in order to improve the hydrogen decrepitation route used for magnet processing.
Structure and magnetic properties of rapidly quenched (NdPr)-Fe-(BC) alloys (abstract)
Authors: Zhao Hang , Xu Xiaorong , Yao Guolan , Yang Fuming
Published: July 2002

It had been reported that substitution of C for B enhances the magnetocrystalline anisotropy of Nd2Fe14B1−xCx compounds.1 This paper will study the structure and magnetic properties of (NdPr)15Fe77(B1−xCx)8 alloys changing with x=0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 0.9, and 1.0. It is found that the Curie temperature and welting point of the (NdPr)15Fe77(B1−xCx)8 alloy decrease linearly with increasing C content, indicating that B can be replaced by C continuously (x≤0.9). (NdPr)2Fe14X (X=BC or B1−xCx) phase which maintains the Nd2Fe14B-type structure is formed in quenched ribbons (x≤0.9), and (NdPr)2Fe17Cx with the Th2Zn17-type structure and Nd2C3 phase are formed in the ribbon of x=1. The substitution of C makes the grains fine and homogenized. The best effect of fining grains (size ≪0.1 μm) is obtained in the ribbon for x=0.6, and the values of Ms, Br, i Hc, and (BH)max are 12.5 kG, 7.04 kG, 18.5 kOe, and 8.79 MGOe, respectively (maximum field of 70 kOe). This subject was partly helped by the Magnetism Laboratory of Institute of Physics, Academia Sinica.
Substitution effect of Y3+ with Ca2+ in YBa2Cu3O7−δ (abstract)
Authors: Soga Y. , Yamamoto T. , Saito T. , Shinagawa K. , Tsushima T.
Published: July 2002

The oxide superconductor YBa2Cu3O7−δ is known to have a rather free entrance of oxygen into the CuO chain between the BaO layers, and the oxygen deficiency δ is closely related with the critical temperature Tc. (1) When Y3+ site is substituted with Ca2+, the position of the oxygen vacancy is not on the Cu–O plane, but on the CuO chain as well as in the case of YBa2Cu3O7−δ. (2) When this substitution is done, electric neutrality is kept by the release of a half O2− for one Y3+ substitution with one Ca2+. (3) Therefore, the concentration of the positive hole is constant for this substitution, and Tc of the system is kept at the rather high temperature of about 86 K. (4) The relation between Tc and the hole concentration is in good agreement with the non-Ca2+ containing YBCO, and it can be said as a conclusion that the substitution of Y3+ with Ca2+ does not give any effect on the mechanism of the superconductivity of this system. (5) The crystallinity of the system is improved by the substitution to be better sintered and to have more narrow width of the critical temperature.
Design and performance considerations in high areal density longitudinal recording (invited)
Authors: Tsang Ching
Published: July 2002

In this work we discuss various design and performance issues in very high areal density longitudinal magnetic recording. We show that low flying height and small read gap are the two most important factors for high linear resolution, while small write gap and good P1–P2 alignment are important for high-track density operations. Next, we discuss the on-track signal-to-noise performance in terms of peak shift and peak jitter. For nonlinear peak shift, the proximity-effect transition shift is found to be significant. For peak jitter, major sources of jitter are discussed, and an analytical description of jitter due to head and electronics noise is developed. Applying this result shows a signal-to-noise advantage.
Effects of read/write misalignment and asymmetric side reading on magnetoresistive head off-track performance
Authors: Lee J. K. , Wallash A. , Poon A. L.
Published: July 2002

In this paper we discuss spin stand techniques for studying and measuring the magnetic misalignment and asymmetric side reading in a magnetoresistive (MR) recording head. The concept of a magnetic center for the MR head is introduced and discussed. Two procedures for finding the magnetic center are described. One method uses dual microtrack profiles, and the other uses peak jitter measurements. Finally, calculated error rates and microtrack profile slope data are combined to determine the read/write offset which results in optimum off-track performance.
Magnetic properties of amorphous B-rich R-Fe-B alloys
Authors: Gangopadhyay S. , Tsoukatos A. , Hadjipanayis G. C.
Published: July 2002

The magnetic properties of amorphous and crystallized B-rich R-Fe-B alloys with R=Y, Pr, Nd, and Gd have been studied from 4.2 to 300 K using ac susceptibility and SQUID magnetometry. The amorphous samples with R=Y, Pr, and Nd show spin-glass-type transitions with ordering temperatures at about 150, 200, and 250 K, respectively. The magnetization of the samples is found to decrease from light to heavy rare earths to yttrium corresponding to μFe values from 1.35 to 0.74 μB. The magnetic properties can be explained by a noncollinear arrangement of the rare-earth and Fe magnetic moments. Crystallized samples consist of multiple phases with R1.1Fe4B4 as the majority phase; for the 12% Nd sample Fe2B is believed to be one of the minority phases while for 13% Nd the minority phase is R2Fe14B.
Magnetization study of YBa2Cu3Oy high-temperature superconductors: The effect of copper depletion and silver addition
Authors: Meisner G. P. , Taylor C. A.
Published: July 2002

Meissner effect measurements were used to characterize the effect of copper depletion and silver substitution on the bulk superconducting properties of YBa2Cu3Oy. A strong copper composition dependence of superconductivity was observed: A 20% depletion of copper from the samples resulted in an 88% decrease in the strength of superconductivity in YBa2Cu3Oy as measured by the Meissner effect. Because flux trapping and field penetration were likely to be significant in the high-Tc materials, it was difficult to determine the volume fraction of superconducting material in the samples. Variations in the lattice constants were consistent with copper composition-induced disorder on the orthorhombic lattice which became more pseudocubic, i.e., a≂b=c/3, with copper depletion. The addition of silver had essentially no effect on superconductivity or the lattice constants of YBa2Cu3Oy.
Magnetic and thermodynamic properties of nonsuperconducting (Y,Pr)Ba2Cu3O6
Authors: Kebede A. , Rodriquez J. P. , Perez I. , Mihalisin T. , Myer G. , Crow J. E. , Wise P. P. , Schlottmann P.
Published: July 2002

The concentration dependence of the room-temperature lattice constants and the concentration and temperature dependence of the specific heat C(T) and the magnetic susceptibility have been measured on the tetragonal, nonsuperconducting phase of Y1−xPrxBa2Cu3O6. The effective paramagnetic moment μeff is independent of x and has an average value of 2.7±0.2 μB. For PrBa2Cu3O6, the Néel temperature TN for Pr moment ordering is 10.5 K as compared to TN=17 K for the orthorhombic PrBa2Cu3O7 phase. The TN is depressed rapidly with Y doping in the oxygen-depleted compound and goes to zero at x≊0.4–0.5. At low temperatures and x≤0.5, C(T)/T vs T2 is nonlinear and shows a rapid decrease of C(T)/T indicative of strong magnetic correlations.
Dimensional crossover in the magnetism of MBE-grown Fe(110)/Ag(111) multilayers
Authors: Gutierrez C. J. , Qiu Z. Q. , Wieczorek M. D. , Tang H. , Walker J. C.
Published: July 2002

Fe(110)/Ag(111) heterostructures, composed of fixed 3 monolayer (ML) Fe bilayers and variable-thickness Ag bilayers, were grown by molecular-beam epitaxy and investigated by transmission Mössbauer spectroscopy in the temperature range from 4.2 to 300 K. We found that as the Ag layer is thick enough (≳17 ML) to magnetically isolate the neighboring Fe layers, a quasilinear temperature dependence of the Mössbauer hyperfine field results due to the two-dimensional spin-wave excitations. As the Ag layer is reduced to 4 ML, a dimensional crossover in the spin-wave excitations is induced by the magnetic interaction between neighboring Fe layers which makes the magnetic temperature dependence change from a two-dimensional T-linear relation to a three-dimensional T3/2 dependence.
Two-dimensional spin-wave excitations in MBE-grown Fe(110)/Ag(111) multilayers
Authors: Qiu Z. Q. , Gutierrez C. J. , Wieczorek M. D. , Tang H. , Mercader R. C. , Walker J. C.
Published: July 2002

It is well known that two-dimensional spin-wave excitations result in a linear temperature dependence of the magnetization in a quasi-two-dimensional ferromagnetic system. However, it has been shown also that magnetic relaxation from small islands inside a film can also result in a similar linear temperature dependence. In this paper, it is found that comparative Mössbauer measurements with and without a weak magnetic field can clearly distinguish these two different mechanisms: The linear temperature dependence of the magnetization is unaffected by the external field if 2D spin-wave excitations are responsible for the linear behavior, while the linear slope of the temperature dependence of the magnetization is reduced by the external field if magnetic relaxation is involved.
Preparation and structural characterization of epitaxial Co/Pd(111) superlattices
Authors: England Craig D. , Engel Brad N. , Falco Charles M.
Published: July 2002

High-quality epitaxial fcc Co/Pd(111) superlattices have been prepared on single-crystal GaAs(110) substrates using molecular-beam epitaxy. An ultrathin (6 Å) buffer layer of bcc Co(110) was used to promote epitaxial growth on the GaAs substrate. A thick (≊ 500 Å) buffer layer of fcc Pd(111) was then deposited to provide a smooth surface for superlattice growth. In addition, the Pd buffer layer isolates the superlattice from chemical interaction with the GaAs substrate. Co/Pd superlattices with ultrathin (<20 Å) Co layers are found to have a perpendicular easy axis of magnetization. The source of the perpendicular magnetization in these superlattices is attributed to an interface anisotropy, which depends on the interface quality and crystal structure. The structure of the superlattices has been investigated using a wide range of techniques, including reflection high- energy electron diffraction, low-energy electron diffraction, scanning tunneling microscopy, Rutherford backscattering spectroscopy, and a variety of x-ray-diffraction techniques. Details of the structure, including grain size, coherence length, and interfacial roughness are discussed. The magnetic properties of these superlattices are presented elsewhere in these proceedings.
The search for circular dichroism in high-T
c superconductors (abstract)
Authors: Lyons K. B. , Kwo J. , Dillon J. F. , Espinosa G. P. , McGlashan-Powell M. , Ramirez A. P. , Schneemeyer L. F.
Published: July 2002

Stimulated by recent predictions of broken time reversal symmetry in cuprate superconductors, we have carried out a study of the polar Kerr ellipticity (the ellipticity of normally reflected light with the incident beam linearly polarized) for various cuprate materials, both superconducting and nonsuperconducting. The technique used employs a rotating half-wave retardation plate in order to discriminate against linear polarization effects. The results reveal a signature of a nonzero polar Kerr ellipticity which appears on cooling near 200 K in a variety of superconducting materials, and which is not observed in the corresponding insulating compounds. In this talk, in addition to summarizing these results, we plan to discuss the measurement technique itself in some detail. Measurements on both thin films and bulk samples will be discussed, together with a variety of tests on unrelated materials, which serve to eliminate various possible experimental artifacts.
Magnetic anisotropies in ultrathin fcc Fe(001) films grown on Cu(001) substrates
Authors: Cochran J. F. , Muir W. B. , Rudd J. M. , Heinrich B. , Celinski Z. , Le-Tran Tan-Trung , Schwarzacher W. , Bennett W. , Egelhoff W. F.
Published: July 2002

Brillouin light scattering and ferromagnetic resonance have been used to measure the magnetic-field dependence of the magnon frequency at room temperature for three- monolayer-thick films of fcc Fe(001) grown by means of molecular-beam epitaxy on Cu(001) substrates. These films exhibit a strong uniaxial magnetic anisotropy which causes the magnetization to be oriented perpendicular to the film plane in zero applied field. Four specimens were grown on copper substrates prepared using a chemical polishing technique which avoided the use of abrasives: This treatment greatly reduced the density of etch pits on the copper surface. The uniaxial anisotropies found for these specimens were very consistent and comparable to that previously measured for the best film grown on a mechanically polished copper substrate. There is evidence that substrate smoothness is improved and, hence, that the uniaxial anisotropy is increased when the substrate is covered with a few monolayers of epitaxial copper before the iron films are grown.
Magnetic frustration in high-T
c superconductors
Authors: Dionne Gerald F.
Published: July 2002

The breakdown of antiferromagnetic order and its coincidence with the onset of the superconducting state (the TN, Tc→0 condition) is explained by the itinerancy of polarons that are created by ‘‘impurity’’ cations or by oxygen vacancies. In both La2−xSrxCuO4and YBa2Cu3Oy systems, the electrostatic balance is maintained in a traditional sense by the formation of Cu3+(d8) ions, here in low-spin (S=0) electron configurations because of tetragonal crystal fields at the Cu sites. Since the spins of the host Cu2+(d9) ions (S=1/2) surrounding the diamagnetic Cu3+ p-type polaron ion will undergo canting that results from the magnetic dilution effects, a dynamic region of frustration will exist within the confines of the polaron cell boundaries. As cells begin to merge through increased polaron concentration, percolation of cells will destroy magnetic order at a threshold determined by the product of cell radius and carrier concentration. Analysis of the cell radii indicate that frustration should set in at a Cu3+ concentration (x≊0.07) that is consistent with the TN, Tc→0 condition observed in both of the above superconductor families. From these concepts, a rationale emerges for a superconductor phase diagram that links the magnetic, insulating, superconducting, and normal metallic regions as functions of temperature and composition.
Ordering by quantum fluctuations in a strongly frustrated Heisenberg antiferromagnet
Authors: Harris A. B. , Berlinsky A. J. , Bruder C.
Published: July 2002

The nature of the ordering of a quantum spin-1/2 Heisenberg antiferromagnet is considered in the presence of the strong lattice frustration associated with the pyrochlore lattice. A field theory indicates that quantum fluctuations lead to a state having dimerization that has long-range static correlations not in 〈S〉 but rather in 〈S(r)⋅S(r+δ)〉. A sublattice ordering is found that is consistent with results obtained using degenerate perturbation theory to analyze the ground-state manifold.
Nuclear magnetic resonance study of Co-CoO films for perpendicular magnetic recording (abstract)
Authors: Yoshida K. , Takayama T. , Yasuoka H.
Published: July 2002

Co-CoO films, which are evaporated in the atmosphere of a small amount of O2, have large perpendicular magnetic anisotropy and show excellent read-write (R-W) characteristics. With heat treatment, the microstructure of a film changed and the R-W characteristics were further improved.1,2 This result indicates that the microstructure of a film changed and the R-W characteristics were further improved.1,2 This result indicates that the microstructure of a film closely relates with the R-W characteristics as it does so for Co-Cr films. We measured the spin-echo spectra of a film having various magnetizations to investigate the relationships between R-W characteristics and film microstructure, because NMR is very sensitive to the slight changes of film microstructure.1 A strong absorption was observed at 223 MHz which is for α-Co. The intensity decreased drastically as the amount of oxygen in the films increased and very broad satellites appeared around 250 MHz, implying that Co atoms having a larger magnetic moment than that of α-Co exist for the films containing oxygen atoms. Further, heat treatment increased the absorption intensity at 223 MHz, which means that the internal stress or the lattice defects were removed. The R-W characteristics were found to relate with the absorption intensity. This result shows that the crystallinity of a film greatly influences the R-W characteristics of Co-CoO films.
Preparation and properties of noninteracting spherical magnetic particles
Authors: Slade S. B. , Berkowitz A. E. , Parker F. T.
Published: July 2002

A method is described for preparing samples of noninteracting spherical magnetic particles with a narrow size distribution. The particles are produced using the spark erosion technique. Any conductive material, including alloys, can easily be used to produce particles with this method; electrodes of nickel are used in this study. The particles are filtered through Nuclepore filters which are suspended in an ultrasonic bath. Particles which have diameters the same as the filter pore size become trapped in the pores, forming a sample of narrowly distributed, isolated spherical particles. SEM micrographs show the distribution of particle diameters is quite narrow, with a typical standard deviation of less than 10%. Magnetization curves of nickel particles trapped in 3 μm filters have been studied. Fits of log-normal distributions of anisotropy fields to the magnetization curves in the approach to saturation have determined the mean magnetocrystalline anisotropy fields to be of the order of 2 kOe. These values are much higher than what is expected for magnetically soft spherical particles, and indicate the particles may be under some form of stress as a result of the production method.
Determination of magnetic parameters from the approach-saturation curve of polycrystalline ferrites by a computational method (abstract)
Authors: Zhu Sheng-chuan , Wen Fan-ping , Chen Hai-ying , Zhang Yao-xi
Published: July 2002

The measurements of saturation magnetization and magnetocrystalline anisotropy for polycrystalline ferromagnetic materials are carried out usually on the basis of the approach to saturation.1,2 We have already suggested a method of determining simultaneously the five coefficients of the approach to saturation expression.3 In order to improve on the measured precision and the determined method, we have done a lot of experimental research on polycrystalline ferrites with various properties, and have performed the computational analysis. The results show that: (i) the curve fitting can be performed successfully over the different regions for H; (ii) the coefficients vary with the H region, so that fitting two or three coefficients step by step is not reasonable; (iii) determining a suitable interval for H values by computational method, the magnetic parameters can be obtained; (iv) the obtained magnetic parameters are in agreement with the data obtained by other experimental methods and theoretical estimations; (v) the physical origins of the coefficients are expounded.
Magnetic properties of field-annealed amorphous Fe-B-Si wound cores
Authors: Song Jae Sung , Im Ho Bin , Yun Mun Soo
Published: July 2002

The effects of annealing with and without a magnetic field on the coercive force and loss of cobalt-free amorphous Fe-B-Si wound cores have been investigated as a function of the annealing temperature and field to improve the properties of the ribbon cores. As-wound cores have square B-H loops with high coercive force and low remnant magnetization due to the presence of magnetically hard regions in the ribbons. The remnant magnetization increases, and the coercive force and loss of wound cores become a minimum when they are annealed without a field at 410 °C due to the removal of stresses and the occurrence of crystallization at higher annealing temperature. Near 180° domains exist in the thermally annealed toroid cores, but the domain walls are not parallel to the longitudinal direction of the ribbon. In the specimen annealed with 10 Oe at 390 °C, the domains are parallel to the longitudinal direction due to the field-induced uniaxial anisotropy. These result in a further increase in the remnant magnetization and a decrease in the coercive force and loss. Total core loss, measured at 1.2 T and 60 Hz, of the wound core annealed at 390 °C with field of 10 Oe was 0.15 W/kg.
The magnetic properties of sub-20-μm metallic fibers formed by continuous melt extraction
Authors: Rudkowski P. , Rudkowska G. , Ström-Olsen J. O. , Zeller C. , Cordery R.
Published: July 2002

Melt-extraction technology has been developed which allows the continuous casting of fine metallic fibers down to less than 5 μm in diameter. Outstanding soft magnetic properties are found both for amorphous and crystalline alloys manufactured by this method. The technique will be discussed, as will the magnetic properties, both in ac and dc driving fields.
Analysis of microwave ferrite devices by using the finite-element method
Authors: Lee Jin-Fa , Mittra Raj
Published: July 2002

In this paper, we describe a numerical approach for solving the problem of H-plane waveguide junctions containing lossy ferrite posts of arbitrary shape. This approach is based upon the use of triangular finite elements and employs a local type of radiation-boundary condition on the waveguide ports to truncate the original domain, which is infinite, into a finite and manageable size region.
Comparison of various methods for the modeling of thin magnetic plates
Authors: Bamps N. , Delince F. , Genon A. , Legros W. , Nicolet A.
Published: July 2002

Computation of the magnetic field in thin plates is particularly difficult because nodes on opposite sides of the plate are very close together and because the elements inside the plate are very flat. Three different methods, together with corresponding variations and combinations were used to compute the magnetic field in the plate: the boundary element method, the finite element method, and use of thin plate transmission conditions. With classical methods, the results are good for all cases, provided the computation of coefficients is sufficiently accurate. For very thin plates, a fine meshing is required to avoid failure of the computation. A new method that avoids those problems was designed. Since the vector potential varies very steeply across the plate, the idea is to replace the real plate by an equivalent double layer of current. The result is a special transmission condition, relating the values of the vector potential and of the tangential magnetic field on both sides of the plate. This condition does not depend on the kind of numerical method used and has been introduced in finite element and boundary element methods. Comparison with previous results shows that this highly economical method is both accurate and valid. This is the case even for very thin plates, where other methods fail, particularly if the meshing is too coarse.
Three-dimensional inductance calculation by using Delaunay–Voronoi complementary grids (abstract)
Authors: Lee Jin-Fa , Mittra Raj
Published: July 2002

This paper presents a new numerical procedure for calculating the inductance of a three-dimensional conductor system. The procedure offers three major advantages over the conventional methods, e.g., 3D method of moments or the finite-element method. These are: (i) The use of a scalar quantity, termed the current potential,1 eliminates the need for a rigorous vector analysis of source currents J. (ii) The need for mesh truncation of the problem domain using artificial boundary operators is obviated because the radiation condition is incorporated in the Green’s function itself. (iii) Delaunay triangulation,2 used in this method, provides an automatic surface patching for arbitrarily shaped conductors employing a triangular grid. The current potential is approximated on this grid and the complementary grid, comprising of Voronoi polygons, is used to impose the boundary condition on Bn, the normal component of the magnetic flux. It is demonstrated that this approach yields a numerically stable and accurate solution for the 3D inductance problem.
Modeling and digital nonlinear control of active magnetic bearing (abstract)
Authors: Caron B. , Charara A. , LeMarquand G.
Published: July 2002

In the first part of the paper we propose a complete nonlinear dynamic model of a magnetic bearing. The bearing is made up to two controlled axes and three passive axes. This model takes into account the changeover of the power between the two coils of each controlled axis in order to control each axis without exciting currents. In this case, the model is not linear anymore. In the second part we compute a linear regulator taking into account the nonlinearity of the model. For that, we use the ‘‘describing function approach’’ (the nonlinearity is approximated by a complex gain which is a function of the magnitude of the input signal). The stability of the control is assumed first by using the frequency response of the linear part of the model and second by using the describing function of the nonlinearity. In the third part, the regulator is implemented on a digital computer with a sampling period of 0.4 ms. The matching between simulation results and experimental trials shows the accuracy of both the model and the control. Trials on the plant show the good performances of the control law especially during the crossing of critical frequencies.
Scanning laser microscope studies of the overwrite characteristics of TbFeCo films using magnetic field modulation
Authors: Wright C. D. , Heyes N. A. E. , Clegg W. W.
Published: July 2002

In the search for a directly overwritable magneto-optic recording system, that based upon the magnetic field modulation method looks quite favorable. This approach also enables submicron domains to be written, so increasing attainable storage densities. Here we report on the results of such field modulation studies performed in situ using our magneto-optical scanning laser microscope (SLM). Bit patterns were recorded in TbFeCo films by continuous laser irradiation coupled with the application of an alternating magnetic field supplied by a small, purpose built electromagnet having a peak field of approximately 4 kOe. The recorded domain structures were then imaged using the SLM in its polar Kerr mode. Recorded patterns were overwritten and the resulting bit patterns observed.
Peak shift analysis for magneto-optical storage
Authors: Madison M. R. , Finkelstein B. I. , Yardy R.
Published: July 2002

This paper explores the thermal induced peak shift in magneto-optical (MO) recording. A complete model is presented that includes the writing and reading on magneto-optical media. Data is shown for the readback signal amplitude and peak shift versus write power and pulse length that corroborate the model. The mark shapes are calculated from a purely thermal Green’s function model.