bluuebunny/crossref_metadata_2025 huggingface dataset:
Giant magnetoresistance studies in evaporated Ni–Fe/Cu and Ni–Fe–Co/Cu multilayers (abstract)
Authors: Haftek E. , Zeltser A. M. , Smith Neil
Published: July 2002

Room temperature giant magnetoresistance (GMR) and magnetic properties of (Ni–Fe/Cu)n and (Ni–Fe–Co/Cu)n multilayers were investigated. Alternating layers of Ni–Fe–(Co) and Cu were electron-beam evaporated in a computer-controlled high-vacuum system at base pressure of ⩽4×10−8 Torr and deposition rates of ⩽2 Å/s. To complement and expand our previous investigation,1 GMR properties were additionally studied here as a function of cobalt content of Ni–Fe–Co films, the number (n) of bilayers, deposition temperature, and type of buffer layer. The Co content was varied from 7 to 17 at. %, and the number of bilayers ranged from n=8 to 20. No significant GMR was observed in the as-deposited multilayers. To produce tangible GMR, these multilayers were annealed between 300 and 360 °C for 2 h in a 150 Oe magnetic field in an argon atmosphere. The GMR effect (ΔR/R) was essentially independent of copper spacer thickness, which varied between 25 and 30 Å. For Co containing multilayers the highest ΔR/R=7.6% was obtained for 17 at. % Co deposited at 100 °C. The ΔR/R in all Ni–Fe–Co/Cu multilayers was sensitive to the deposition temperature, and R–H loops always showed significant hysteresis independent of the type of buffer layer. For application of these materials to very high density reproduce heads,2 the best results were obtained for (27 Å NiFe/25 Å Cu)14–18 multilayers deposited at 160 °C on 70 Å Ta buffer layer. For example, n=17 multilayers annealed at 350 °C exhibited ΔR/R=7.5%, half-width at half-maximum of ∼50 Oe, essentially no anisotropy, and virtually zero hysteresis (Fig. 1). Frequency dependent permeability measurements showed constant permeability between 10 and 200 MHz. Low- and high-angle x-ray diffraction as well as atomic force microscopy were used to investigate the effect of different geometries of multilayers on structure and roughness and to correlate them with GMR properties.
Structure and giant magnetoresistance effect of Fe/Cu and FeCoNi/Cu multilayers
Authors: Imada R. , Fujiwara Y. , Tsunashima S. , Jimbo M. , Matsuura M.
Published: July 2002

We have studied the giant magnetoresistance (GMR) effect and the film structure of Fe/Cu and FeCoNi/Cu multilayers (MLs), where the FeCoNi alloy is in the composition range of the bcc phase. Both MLs show the oscillation of MR ratio similar to each other as a function of the Cu layer thickness. The phase of oscillation is much different from those in Co/Cu and fcc-FeCoNi/Cu MLs. The maximum MR ratios of 6% and 40% were obtained at 77 K, respectively, for (Fe 1.5 nm/Cu 1.4 nm)20 and (Fe56Co30N14 1.5 nm/Cu 1.4 nm)20 MLs. The EXAFS of the Cu K edge as well as the XRD pattern indicates that the structure of the Cu layers changes from bcc-like to fcc phase as their thickness increases. At the Cu thickness of 1.5 nm, the Cu layers, in both MLs, are highly disordered and seem to have a bct structure.
Engineering of ferrite films at the atomic scale (abstract)
Authors: Karim R. , Vittoria C.
Published: July 2002

We have developed an artificial preparation scheme to produce ferrite films in which magnetic and nonmagnetic layers were alternated at the atomic scale. Laser ablation deposition (LAD) was used to deposit layers of lithium ferrite and aluminum oxide to a thickness ranging between 2–3 Å each. In contrast to previous efforts12 to produce magnetic superlattices (SL) in which the intrinsic magnetization of each layer contained in the SL multilayering was not changed or affected, our technique induces direct changes in the magnetization at the atomic scale. For example, with an average substitution of 50% of aluminum into our artificial structure, the average magnetization of the composite was reduced by only 20% from the standard consisting only of lithium ferrite. All spinel ferrites prepared by any other technique quench the moment at room temperature for substitutions of aluminum even as low as 40%. We have made x-ray diffraction, FMR and vibrating sample magnetometer measurements as a function of temperature and will report upon them. It is clear from our measurements that the artificial structure is ordered and cannot simply be classified in terms of previous ferrite models.
Cubic intermodulation coupling in junction circulators
Authors: How H. , Sun L. , Vittoria C.
Published: July 2002

Nonlinear intermodulation coupling in ferrite junctions have been solved analytically for the first time. Primary microwave excitations of the ferrite junction at frequencies f1 and f2 will induce effective rf current at frequency 2 f1−f2 which generate intermodulation field within the ferrite satisfying the boundary conditions at the junction boundary. The static demagnetizing field has been shown to have an adverse effect in increasing intermodulation coupling. Our calculations compare reasonably well with experiments.
B>1
T low mass magnetic field sweep assembly
Authors: Cadieu F. J. , Caldwell Charmane , Griffin John , von Molnar S.
Published: July 2002

When making linear or nonlinear conductivity measurements it is often desirable to apply a high magnetic field locally to a sample. If the whole magnet assembly is integral to the sample probe, then, particularly when making low temperature measurements, a low total mass is desired. We have designed, modeled, and tested a variable magnetic field sweep unit suitable for use over a wide temperature range from cryogenic to ≈200 °C which provides a sweepable uniform magnetic field over an air gap of 2 mm and spatial extent of 4 mm×4 mm. The unit consists of a magnet yoke structure with a mass of 18 g and a spur gear driven rotatable magnet to vary the gap field in a nearly sinusoidal manner as a function of the magnet rotation angle. In the present design, a 20 g SmCo magnet has been used which allows for low temperature operation to ≈10 K, when attached to a cryogenic refrigerator cold finger. The shape of the magnetic yoke structure has been modeled and optimized using three-dimensional magnetic field software. The gap field uniformity can thus be modeled and tested experimentally. In a present working model the field for a 2 mm gap at room temperature B(θ)=1.05⋅cos(θ)(T) where θ is the magnet rotation angle. The field sweep amplitude at 19 K only increased by 3% over the 295 K value. With 0.25-mm-thick permunder pole tips, the field amplitude in a similar unit was increased to 1.20 T over a gap of 1.5 mm.
Structural and magnetic studies on vanadium spinel MgV2O4
Authors: Mamiya H. , Onoda M. , Furubayashi T. , Tang J. , Nakatani I.
Published: July 2002

The magnetic properties and the crystal structure of MgV2O4 and Mg(V0.85Al0.15)2O4 have been studied. Both compounds are the normal cubic spinels with highly frustrated magnetic lattice. Around T2=65 K, MgV2O4 has magnetic orders accompanied with the cubic-tetragonal transition. Below T2, the susceptibility shows complex behavior. In Mg(V0.85Al0.15)2O4, the spin-glasslike state appears. The V51-Knight shift of MgV2O4 has an anomalous temperature dependence, which is not simply related by that of the susceptibility.
Magnetic behavior of the oxygen deficient perovskite PrBaCuFeO5+y
Authors: Blackman J. A. , Trohidou K. N.
Published: July 2002

This article concerns the class of compounds RBaCuMO5 where R is a rare earth (Y or Pr) and M is a transition metal (Fe or Co). The magnetic structure is associated with the Cu and M atoms which are surrounded by five oxygens. The dominant interaction is superexchange which results in antiferromagnetic ordering within bilayers. The interaction between bilayers is believed to be much weaker than the intralayer coupling. Neutron diffraction experiments determine the magnetic structure of these materials and also give evidence that the introduction of extra oxygen (e.g., PrBaCuFeO5+y) destroys the magnetic ordering if R=Pr. It is believed that these extra oxygen atoms tend to go into the plane of the rare earth atoms; they provide an interlayer bridge between the Cu/M planes and most likely stronger coupling than in the compounds free of extra oxygen. We introduce a model based on competition between weak ferromagnetic interlayer coupling and strong antiferromagnetic coupling induced by the presence of the oxygen defects. Monte Carlo simulations are used to investigate the conditions under which magnetic long range order (LRO) is destroyed. We find that different values of Cu–Cu, M–M, and Cu–M interactions are important if LRO is to disappear over a wide range of y.
Influence of ion beam bombardment on the properties of permalloy-based multilayers
Authors: Gutierrez C. J. , Selestino R. , Dail R. W. , Tristan L. , Khater M. , Michel Steve
Published: July 2002

This work demonstrates the feasibility of using low-energy Ar ion beam assist bombardment of permalloy (Py=Ni81Fe19) multilayers during growth to selectively manipulate multilayer component coercivity, with minimal degradation of saturation magnetization characteristics. The enhanced giant magnetoresistance characteristics of ion beam sputtered multilayers with thin “Co-dusting” is also reported.
Magnetic circular x-ray dichroism in transverse geometry: A new tool to study the magnetocrystalline anisotropy
Authors: Dürr H. A. , van der Laan G.
Published: July 2002

The orbital and spin magnetic moments in 3d transition metals are no longer collinear when the electron spins are forced out of their easy direction by an applied magnetic field. This effect provides a new way to measure the anisotropy of the orbital magnetic moment using magnetic circular x-ray dichroism in a geometry where the photon helicity vector is perpendicular to the magnetization direction. As an experimental corroboration we studied the magnetic anisotropy in the Co orbital moment of a Co/Pt multilayer.
Sensitivity study of partial response maximum likelihood and peak detection channels to contamination buildup on air bearing surface of proximity recording sliders
Authors: Liu B. , Soo K. T. , Hu S. B. , Low T. S.
Published: July 2002

This work investigates experimentally the sensitivity of partial response maximum likelihood (PRML) and peak detection channels on the increased head-disk spacing caused by the contamination buildup on the slider’s air bearing surfaces (ABS). Results obtained indicate that the peak detection channel is sensitive to both the Transition Effect (writing at increased head-disk spacing) and the Readout Resolution Effect (reading at increased head-disk spacing). The increased error rate detected in peak detection channel at increased head-disk spacing can be reduced significantly if the spacing is resumed to normal. In fact, the influence of the transition effect on the peak detection channel is more significant than the readout resolution effect. The PRML channel, however, is mostly sensitive to the transition effect only. The resolution effect, conversely, has rather limited impact on the PRML channel. Off-track sensitivity studies shows that peak shift in the peak detection channel becomes more sensitive to off-track positions when the head-disk spacing is increased. The nonlinear transition shift in the PRML channel, on the contrary, is far less sensitive to the off-track of magnetic head, even when the spacing is increased by the contamination buildup.
Magnetic anisotropy in Terfenol-D thin films (abstract)
Authors: Su Q. , Teter J. P. , Wen Y. , Cullen J. R. , Wuttig M.
Published: July 2002

Thin-film Terfenol-D has attracted considerable attention for applications as microactuators and sensors. Depending on applications, the magnetic anisotropy plays a key role in physical property control. In this paper, the magnetic anisotropy of sputter-deposited Terfenol-D thin films on single-crystal Si substrates was studied by both dynamic torque and magnetization measurements. The evolution of magnetic anisotropy in the course of annealing treatment was followed by both types of experiments. It was found that there is a strong growth-induced anisotropy in the as-grown amorphous films, which is attributed to Fe–rare-earth bond orientation anisotropy. In samples treated at high temperatures, this growth-induced anistropy is overshadowed by the elastic anisotropy resulting from the substantial stresses in the films of giant magnetostrictive material. Domain patterns of different kinds of anisotropy were also monitored using a magnetic force microscope. The consequences of the growth-induced and elastomagnetic anisotropies to possible applications to microactuation are discussed.
Magneto-optic measurements in the 1.5–6.7 eV range for thick fcc and hcp Co films (abstract)
Authors: Van Drent W. P. , Suzuki T.
Published: July 2002

As the development of solid-state diode lasers for shorter wavelengths is progressing steadily, the approach for high-density magneto-optical recording at shorter wavelengths becomes more feasible. Therefore, the study of magneto-optical effects in a wavelength range shorter than blue is essential for the exploration of new materials for such high-density recording media. In this study, a novel magneto-optical measurement system covering a wide wavelength range from 185 to 900 nm (1.5–6.7 eV) at temperatures from 80 to 600 K has been built. In order to cover such a wide wavelength range, two lamp sources are used: a high-power deuterium lamp for photon energies above 4 eV and a Xe lamp for less than 4 eV. A “continuous” calibration principle has been utilized to assure a high accuracy for measurement of rotation and ellipticity. The entire measurement is automated and computer controlled by software written in Labview. In Co/Pt-based multilayers, the origin of the high negative Kerr rotation at around 4 eV is generally attributed to the polarized Pt. On the other hand, the texture of these layers is generally strongly fcc. Therefore, the Kerr rotation of fcc Co, rather than hcp Co, should be known to understand these layers, and to be able to estimate the respective contributions from the Pt and the Co layers. In this work, results for thick hcp and fcc Co layers are shown. A 1000 Å Co (111) film showed a maximum Kerr rotation of about −0.5° at 4 eV, which is higher by about −0.1° than the rotation obtained for hcp Co. Further, a positive Kerr rotation of about +0.3° was found at about 6 eV. For 1000 Å fcc Co (100) films Kerr rotations between +0.3 and +0.4° were found at 5.5–6 eV. These values coincide with almost zero Kerr ellipticity. Above this photon energy the Kerr rotation decreases again.
Giant magnetoresistance in amorphous magnetic rare-earth silicon alloys (abstract)
Authors: Hellman F. , Tran M. Q. , Gebala A. E. , Wilcox E. M.
Published: July 2002

We explore the influence of local magnetic moments on transport and magnetic properties in a new class of diluted magnetic semiconductors, amorphous RExSi1−x (RE=Gd, or Tb, 0.1<x<0.15). As a comparison, nonmagnetic a-YxSi1−x was also studied. The Gd, Tb, and Y ions have similar size and electronic structure, except for the 4f electrons in Gd and Tb. For 50 K<T<300 K, the conductivity σ rises linearly with increasing T with nearly identical slopes for these three alloys. Below 50 K, they have dramatically different conductivities. a-YxSi1−x shows the usual metal-to-insulator transition observed in doped semiconductors as the Y content is lowered. For a-GdxSi1−x and a-TbxSi1−x, however, σ rapidly decreases such that compositions that are metallic in a-YxSi1−x are insulating for a-GdxSi1−x and a-TbxSi1−x. Applying a magnetic field H has the usual relatively small effects on σ of a-YxSi1−x but produces a large increase in conductivity for the a-GdxSi1−x alloys. For example, in a-Gd0.12Si0.88, σ increases by 60 times at T=2.4 K and H=6 T. This giant negative magnetoresistance drops as T rises, disappearing above 50 K. Magnetization M as a function of H shows no hysteresis. M(H/T) for different fields and temperatures even for a single sample do not fit a Brillouin function, showing no paramagnetism or superparamagnetism. The magnetic susceptibility is much smaller than expected for isolated moments, indicating antiferromagnetic interactions between local moments. However, no Curie, Neel, or spin-glass freezing temperatures are observed. The magnetization and transport data are presumably the result of an interaction between nearly localized conduction electrons and the local moments of the Gd(J=7/2) and Tb (J=6) ions, i.e., a magnetic polaron forms that increases the conduction electron effective mass, causing it to localize. The amorphous nature of these alloys makes conduction significantly different than in the usual crystalline diluted magnetic semiconductors.
Interaction and dimensional effects in micromagnetic models
Authors: El-Hilo M. , O’Grady K. , Chantrell R. W.
Published: July 2002

In this article the effects of dipolar and exchange interactions on the remanence ratio and coercivity in granular solids are examined using a generalized three-dimensional Monte Carlo model. This model allows the final configuration of magnetic moments to be calculated as a function of concentration, temperature and magnetic field. The results show that for a system of polydispersed grains having their anisotropy axes oriented at random, both the remanence ratio and coercivity increase with increasing exchange interaction. In addition the magnetization loops exhibit regions where the reversal slows down and gives rise to distinct quasi-bimodal behavior. These effects are due to the fact that the exchange interactions are not uniform throughout the system. This model can account for many features observed in a wide range of real materials.
Coercivity and transmission electron microscopy study of nanocomposite Sm–Co powders by mechanical alloying
Authors: Chen S. K. , Tsai J. L. , Chin T. S.
Published: July 2002

A nanocomposite Sm2Co17/Co magnet with a composition of Sm7Co93 was prepared by mechanical alloying. A domain–wall pinning mechanism was found to be dominant in the magnet, as investigated using Kronmüller’s micromagnetic theory. Transmission electron microscopy micrographs indicate that the half width between neighboring Co grains is about 40 nm. This value is two times larger than the range factor, r0, obtained from Kronmüller’s model. It appears that crystallographic defects other than the cobalt particles also play an important role in coercivity.
Spin configuration of Gd clusters and thin films (abstract)
Authors: Pappas D. P. , Sherwood T. S. , Reddy B. V. , Khanna S. N. , Popov A. P. , Anisimov A. N.
Published: July 2002

Studies of the magnetism and structure of Gd clusters and surfaces will be presented.1 The spin configuration of a Gd13 cluster and the Gd(0001) surface has been examined using electronic structure calculations as well as a Heisenberg model. Structural calculations show that the ground-state geometry of the cluster has a hcp arrangement with a slightly reduced nearest-neighbor distance compared to bulk and an average moment of 7.8 μB/atom. The Heisenberg model is calculated using a Ruderman-Kittel-Kasuya-Yosida-like interaction. The effects of competing ferromagnetic and antiferromagnetic coupling for the nearest- and next-nearest neighbor interaction, respectively, is investigated. It is shown that for a range of interaction strengths the spins in the cluster assume a canted configuration. This effect leads to lower net magnetization of the cluster, and accounts for the anomalous low moments of Gdn clusters that have been experimentally observed. A similar model of the Gd(0001) surface agrees well with earlier spin resolved low-energy electron diffraction experiments from Gd/W(110), and spin resolved secondary electron spectroscopy studies of the surface magnetism of Gd grown on Y(0001) will be presented.
Magnetic separation using a switchable system of permanent magnets (abstract)
Authors: Watson J. H. P. , Beharrell P. A.
Published: July 2002

Permanent magnets have been used in magnetic filtration, particularly in drum separators, thus, obviating the need for solenoids and electrical power. In the area of high gradient magnetic separation, which uses a matrix magnetized by an external field, the use of permanent magnets has been limited, because the field needs to be switched off periodically, due to the loss of filter efficiency owing to the accumulation of captured material, necessitating periodic cleaning. Cleaning requires the field in the matrix to be reduced, which is not possible with a nonswitchable field source. The solution presented here is to employ a novel arrangement of permanent magnets as the field source that permits the flux density in the matrix to be switched between distinct maximum and minimum levels. As a result, optimum capture is carried out with the field at maximum, then cleaning is accomplished by backwashing the filter with the field at minimum. The aim of this work is to develop a viable filtration system utilizing this principle. Early work has centered on a simple configuration of this system with two-dimensional symmetry and has demonstrated that the basic principles are sound. This apparatus has been incorporated into a separator using a ferromagnetic stainless steel wool matrix and its effectiveness demonstrated using paramagnetic wolframite particles. More sophisticated configurations are being developed with axial symmetry and with more powerful magnet materials, the basic outlines being optimized by means of computer-aided-design. It is anticipated that these subsequent models will provide the basis for a large operating volume, self-contained and fully reusable magnetic separation system.
Specific heat of UNiGe in high magnetic fields
Authors: Hagmusa I. H. , Klaasse J. C. P. , Brück E. , Prokeš K. , de Boer F. R. , Nakotte H.
Published: July 2002

Specific-heat measurements were performed on a single crystal of UNiGe in magnetic fields up to 17.5 T. This compound crystallizes in the orthorhombic TiNiSi structure and undergoes two magnetic phase transitions at 41.5 and 50 K. Besides these transitions, the specific heat also shows a Schottky contribution due to the crystal-field splitting. The effects of an applied magnetic field are compared with results from previous neutron diffraction, magnetoresistance, and magnetization measurements.
Relaxation of written transitions in thin films with perpendicular magnetic anisotropy (abstract)
Authors: Lairson B. M. , Payne A. P.
Published: July 2002

We have measured the rate at which recorded signals decay in Co78Cr17Ta5 alloys, Co70Cr15Ta5Pt10 alloys, and Pd/Co multilayers. At low write frequencies, the magnetization decay resembles that observed by magnetometry. At high write frequencies, the relative decay rate decreases substantially. This is attributed to a change in the demagnetizing energy associated with high linear densities in perpendicular media. A model for magnetization decay is used to deduce the activation volume, the size distribution, and the barrier to flux reversal that exists for the alloy films. A time scale tε is introduced, over which the decay rate is dominated by the initial condition applied by the write head. The observed decay in the multilayers was much lower (<0.5%/time decade) than in the alloys (3%–4%/time decade) at all of the spatial frequencies tested.
Magnetic properties of TM–Co–Fe–Ti spinel oxides (abstract)
Authors: Giordanengo B. , de Melo M. A. C. , Takeuchi A. Y. , Silva P. R. J. , Saitovitch H. , Baggio-Saitovitch E. , Moyo T.
Published: July 2002

Spin glass-like behavior has been recently observed in a ceramic Zn0.5Co0.9Fe1.2Ti0.4O4 spinel oxide.1 This motivated our interest to study how the spin glass-like behavior of magnetic clusters in a diamagnetic matrix correlates to that of nonmagnetic or weakly magnetic clusters in a ferromagnetic matrix, Mössbauer effect, ac susceptibility, field cooled and zero field cooled magnetization, hysteresis loops, and γ–γ perturbed angular correlation (PAC) have been measured for TM0.5−xCo0.9Fe1.2+xTi0.4O4 (x=0.0 and 0.2) spinel oxide with TM=Zn and Cu. The variation of coercive field with temperature has been found to fit a double exponential function form Hc(T)=H1e−β1T+H2e−β2T where Hi and βi are constants for the sample with x=0.0 and TM=Zn. The exponential increase in coercive field can be linked to increased effective clusters arising from ions occupying A and B sites with Fe on both sites. This is in agreement with the Mössbauer measurements which show superparamagnetic fluctuations. The ac susceptibility and Mössbauer spectra for Zn0.5Co0.9Fe1.2Ti0.4O4 show a glass transition temperature (about 320 K). Room temperature PAC measurements on Hf doped samples with TM=Zn showed different interaction frequencies, consistent with magnetic order in the Fe enriched case. Different behavior is observed in the sample with Cu for which the ordering temperature is about 480 K.
Infrared optical properties of La0.7Ca0.3MnO3 epitaxial films
Authors: Boris A. V. , Kovaleva N. N. , Bazhenov̇ A. V. , Samoilov A. V. , Yeh N.-C. , Vasquez R. P.
Published: July 2002

The reflectance of La0.7Ca0.3MnO3 epitaxial thin films on perovskite substrates with a range of lattice constants is studied in the frequency range 50–5000 cm−1. The complex dielectric functions of the bare La0.7Ca0.3MnO3 films are obtained by modeling the measured reflectivity spectra of the two-layer La0.7Ca0.3MnO3/substrate system with separately measured dielectric functions of the bare substrate. The results thus derived indicate that the internal optical phonon modes of the MnO6 octahedra are strongly affected by the substrate-induced lattice distortion.
Cross-tie walls and magnetic singularities on the surface of permalloy films (abstract)
Authors: Lee Y. , Kueny A. , Koymen A. R.
Published: July 2002

An understanding of the surface magnetic microstructure of thin polycrystalline permalloy films is important for the development of improved magnetoresistive sensors. Scanning electron microscopy with polarization analysis (SEMPA) was used to image the surface magnetic domain structure of permalloy films in ultrahigh vacuum. The SEMPA system uses a compact Mott electron spin polarimeter with a Th foil (operating at 25 keV) that has been attached to the back of a hemispherical energy analyzer. Two orthogonal in-plane components of the electron spin polarization were measured to obtain magnetic domain images with excellent contrast. 350 Å Ni83Fe17 films, deposited by Honeywell-Micro Switch using dc magnetron sputtering, were studied. The samples were demagnetized along the easy axis by an ac magnetic field with decreasing amplitude. Using SEMPA, zigzag domain walls separating two large approximately head-on domains were observed. Cross-tie walls were observed with a periodic vortex structure along the straight edges of the zigzag domain walls. The cross-tie walls occur at the points where the magnetization is reversed by 180° across the straight edges of the wall. At high magnification, the elliptical and hyperbolic singularities at the cross-tie walls were clearly observed. In addition, the Néel part and the Bloch part of the cross-tie were distinguished This is a detailed study of cross-tie walls on sputter deposited thin permalloy films using SEMPA and our results are in good agreement with theoretical calculations.
Finite temperature theory of noncollinear magnetism in amorphous Fe
Authors: Uchida T. , Kakehashi Y.
Published: July 2002

Noncollinear magnetism in the vicinity of amorphous Fe has been investigated by developing the finite temperature theory of amorphous metallic magnetism which takes into account the transverse spin fluctuations. The theory is based on the functional integral technique to the degenerate-band Hubbard Hamiltonian with Hund’s rule coupling and the distribution function method for local magnetic moments with structural disorder. It is shown that an isotropic spin-glass is realized in amorphous Fe, by taking into account the transverse components of local magnetic moments. The origin of the spin-glass around amorphous Fe is explained by the nonlinear magnetic couplings between Fe local moments and the local environment effects on the amplitude of Fe local moments. When the d electron number N is increased, the isotropic spin-glass changes to a noncollinear ferromagnet at N=7.40 and finally changes to a collinear ferromagnet beyond N=7.47 at 35 K.
Fixed spin moment calculations of the total energy of Fe16N2
Authors: Jansen H. J. F.
Published: July 2002

We have calculated the total energy of Fe16N2 at the experimental crystal structure as a function of the total magnetic moment. The minimum in the total energy occurs at a value of the magnetic moment which is consistent with all previous calculations. When we decrease the total spin moment, there is a sharp transition to a ferrimagnetic state in which the spin moment on two of the three iron sites remains large. We find no evidence for extremely large values of the spin magnetic moment in this system. Calculations for a similar system without nitrogen gives equivalent results, which shows that nitrogen does not affect the magnetic properties and is only needed to define the structure.
Calculation of the magnetocrystalline anisotropy energy using a torque method
Authors: Schneider G. , Erickson R. P. , Jansen H. J. F.
Published: July 2002

We calculate the torque on a magnetic moment with arbitrary orientation in a solid and use it to calculate the magnetocrystalline anisotropy energy (MAE) of bulk materials by integrating the torque along an angular path connecting the easy and hard magnetization directions. We apply this approach to the calculation of the MAE of elemental ferromagnets Ni and Fe using a tight-binding model with an added spin-orbit-coupling (SOC) term λSOL⋅S. The MAE for both Fe and Ni is computed for a wide range of values of the SOC strength. Our results for the MAE calculated with the torque method agree with the MAE determined from energy differences. The convergence rate as a function of k-vector needed for integrating over the Brillouin zone for the torque method is comparable to the convergence rate of conventional energy difference schemes. We compare the calculated torque as function of angle to the lowest order term in the expansion in anisotropy constants.
High-density recording on advanced magnetic tapes
Authors: Speliotis Dennis , Dugas Matthew , Humenansky John , Babcock Kenneth L. , Peter Klaus
Published: July 2002

The advent of magnetoresistance technology for read transducers should render the trend to higher remanence media counterproductive and undesirable because of demagnetization and nonlinear effects. In this investigation, we used magnetic force microscopy to study the high-density capability of advanced thin-film and particulate tapes beyond the resolution limits of available read heads. At 8900 fc/mm (226 kfci) the particulate media support more stable transitions with higher signal to noise compared with their thin-film counterparts, which supports our argument that high magnetization is detrimental to good high-density recording performance.
Rotational hysteresis in metal particle recording media: Remanent moment vectors
Authors: Templeton T. L. , Arrott A. S.
Published: July 2002

A collective dipole is written into a section of commercial magnetic recording tape containing metal particles by applying a saturating field in the plane of the tape at some particular angle θset with respect to the tape axis. The section is then rotated for many cycles n in a lower magnetic field. The field is reduced to zero at the end of the rotation to the angle θ in the nth cycle. Then the components of the remanent dipole μx(n,θ) and μy(n,θ) are measured. The experiment is then started over beginning with saturation. By plotting the components on a μx,μy diagram, one discovers a remarkable pattern of almost closed ellipses moving with cycle number n such that μcx(n) and μcy(n), the coordinates of the apparent centers of the ellipses, migrate toward the origin. The decay of the two components follows μx,y(n,θ)=μx,y(∞,θ)−Δμx,y(θ)n−p, with small values of p<0.2 in the critical field near in magnitude to the coercive field. The ellipses become centered about the origin after many cycles if the field is near or above the critical field, but not for lower fields. For θset≠mπ/2, the results depend on the sense of rotation (clockwise or counterclockwise) in the field. There are so many more particles in the tape than can be treated in a computer simulation that simulations only begin to suggest the effects observed.
Magnetization reversal models and the temperature dependence of the coercive force in melt spun PrNdFeB magnets
Authors: de Julián C. , Cebollada F. , González J. M.
Published: July 2002

In this work we have studied the temperature dependence of several hysteresis parameters measured in PrNdFeB samples that were obtained from melt spun amorphous precursors crystallized by employing different heating rates. Analysis of the spin reorientation temperature by means of thermal evolution of the remanence and of the demagnetization differential susceptibility revealed the presence in all samples of at least two hard 2:14:1 phases (Nd rich and Pr rich, respectively). The demagnetization curves of most samples exhibited a single magnetization reversal process thus indicating that the hard phases were coupled. Despite the multiphase character of the samples, it was possible to carry out the analysis of the temperature dependence reversal process through micromagnetic and global models, and by accepting a linear variation of the intrinsic properties with the degree of substitution of Nd by Pr. Best fits for both models were obtained when using parameters corresponding to Nd-rich 2:14:1 phases, which indicates that the collective single reversal process is driven by the Nd-rich 2:14:1 grains.
High-speed fiber-optic sensor for magnetic-field mapping
Authors: Rengarajan S. , Walser R. M.
Published: July 2002

A fiber-optic sensor was fabricated with a magnetostrictive multilayer thin-film coating synthesized to have a combination of properties favorable for resolving mm scale inhomogeneities in low level magnetic fields (∼100 mOe) varying at MHz rates. High-frequency detection was achieved by mixing signal and local oscillator fields varying at frequencies f and f+fmr/2, into the mechanical resonance of the sensor at fmr. These sensors are expected to be useful for the noninvasive detection of RF magnetic fields, and for mapping the eddy current induced magnetic-field anomalies in nondestructive test evaluation. Since fiber-optic sensing techniques have high immunity to electromagnetic interference, the sensor reported here is compatible with integration into mapping arrays.
Recording of transitions at high linear density in exchange isolated Pt/CoCrTa multilayer thin films (abstract)
Authors: Lairson B. M. , Liu Wen H. , Ho Kuoksan
Published: July 2002

Multilayer Pt/CoCrTa thin film structures that display little intergranular exchange coupling and nearly 100% remanence in the perpendicular direction were prepared. Written transitions retain good fidelity at more than 300 000 transitions per inch. Magnetic recording of continuous square waves and dibit patterns were characterized using an inductive contact recording transducer, a flying magnetoresistive transducer, and magnetic force microscopy. With the introduction of intergranular exchange isolation into the media, observed media noise in the recording transducers becomes negligible relative to the noise from the recording system electronics. Analysis of magnetic force microscopy images yields a signal to integrated noise ratio of 17 dB at 309 000 transitions per inch written in a 4-μm wide track, which is comparable to a measured signal to total noise ratio of 8 dB at 350 000 reversals per inch measured using an inductive transducer at 100 in./s. These results are contrasted with recording on exchange coupled Pt/Co multilayers and longitudinal media.
Signal-to-noise optimization in narrow vertical and horizontal giant magnetoresistent head sensors (abstract)
Authors: Pohm A. V. , Anderson J. M. , Beech R. S. , Black W. C.
Published: July 2002

An analysis was performed on the optimum distance that vertical or horizontal M–R sensors should extend into a read head gap in order to optimize the signal-to-noise ratio of the amplified signal excluding media noise. The optimum distance depends on the decay length of the signal flux in the gap, sensor resistance, lead resistance, equivalent resistance of the amplifier, gap uniformity, sensor width, signal field sensitivity of the average sensor permeability, and bias field magnitudes. To a first approximation for typical values of the parameters, the optimum distance ranges from 1 to 2.5 times the decay length in the gap. Vertical sensors were found to utilize more effectively a nonuniform gap because the magnetic material can extend under the rear contact. The magnetization at the back edge of a horizontal sensor is essentially pinned by demagnetizing fields. Because sensor current densities are typically high, a 100 °C operating temperature was assumed. Also, because run length limited codes are used, the 1/f noise contribution of the GMR material was not considered in the calculations.
Micromagnetic simulations with eddy currents of rise time in thin film write heads
Authors: Sandler G. M. , Bertram H. N.
Published: July 2002

A dynamic micromagnetic model which includes gyromagnetic effects (via the Landau–Lifshitz equation) and eddy currents has been developed and applied to the rise time problem in thin film write heads. A cross-section of the part of the head covering the coils is simulated with this model, while the rest of the head is modeled as saturable reluctances. The reluctances do not include eddy currents, but they do include leakage, fringing and saturation, and allow for computation of the deep gap field. This model is used to study the details of the dynamics of the magnetization in the yoke as well as the dependence of rise time on geometric and material parameters. It is found that switching starts at the surfaces of the pole and moves inward in “transition zones.” Also, if rise time is defined as the time it takes the deep gap field to reach 7 kOe, the rise time increases with pole thickness but decreases with saturation magnetization and driving current.
Spin–wave instability measurements in Mn substituted single-crystal Zn2Y hexagonal ferrite thin disks at 8.9 GHz (abstract)
Authors: Cox Richard G. , Wittenauer Michael A. , Kabos Pavel , Chen Liang , Patton Carl E.
Published: July 2002

Large magnetocrystalline anisotropy makes hexagonal ferrites attractive for microwave and millimeter wave applications. Critical to such applications is the response at high power levels. Relatively little work has been reported in this area. In order to characterize this response, the minimum microwave field amplitude required to excite spin–wave instability, hcrit, is measured for various field–sample configurations. Plots of hcrit versus static field H, termed “butterfly” curves, are then used to obtain spin wave linewidths and define operational power limits for device applications. Two regimes were studied in this work: (1) saturation of the ferromagnetic resonance (FMR) absorption; and (2) parallel pumping instability for fields well below FMR. The objective was to measure butterfly curves and use the results to determine values of the spin–wave linewidth ΔHk. The measurements were made on a 1.2 mm diam 0.15 mm thick single-crystal Mn substituted Zn2Y (Y-type) hexagonal ferrite c-plane (001) disk with easy plane anisotropy. The static field and the 4 μs wide, 30 Hz pulsed microwave field at 8.9 GHz were applied in the plane of the disk. Standard cavity techniques were used. The low power ferromagnetic resonance (FMR) linewidth was 15 Oe. For resonance saturation, the measured hcrit was 0.29 Oe at the FMR field of 775 Oe. The resonance saturation butterfly curve was asymmetric about its minimum value at FMR. The hcrit for parallel pumping was approximately 5 Oe and independent of H over the range of 150–350 Oe. These data and spin–wave instability theory were used to estimate ΔHk values. For parallel pumping, ΔHk was determined to be 17 Oe for the critical modes at one half the 8.9 GHz pump frequency. For resonant saturation, ΔHk was determined to be 12 Oe for the critical modes at the pump frequency.
Dynamic finite-size effect in the two-dimensional classical XY model
Authors: Oh Suhk Kun , Yoon Chang No , Chung Jean S. , Yu Seong-Cho
Published: July 2002

Explicit dynamic finite-size scaling in the time-dependent correlation functions for the two-dimensional classical XY model is explored. The dynamic scaling method, proposed by us previously, is utilized for finding the time scaling variables of the model. It shows nontrivial dynamic finite-size scaling behavior near and below TKT for the total spin correlation function. Since the model is critical for all temperatures below TKT, the time scaling exponent is markedly different from that of the three-dimensional classical XY model at short times. There also appears to be another dynamic scaling region with a different time scaling exponent at T⩽TKT for a fairly wide intermediate time interval.
Soft a-CoZr-based spin valves (abstract)
Authors: Bailey William E. , Sin Kyusik , Wang Shan X.
Published: July 2002

The materials options to combine high giant magnetoresistance (GMR) ratios with soft magnetic properties have been relatively few, with NiFe providing low-field switching in the great majority of studies. These NiFe layers tend to be hardened appreciably when directly exchange coupled to Co to provide higher GMR ratios, as in (Ta)/NiFe/Co/Cu/Co/NiFe/FeMn spin valves. We propose magnetically soft, amorphous CoZr as a replacement couple to Co in these and other structures. While the high resistivity of amorphous films discourages their use as a single switching element, it favors their use as the drive layer for Co in a spin valve. Our transport simulations, using the Camley–Barnas model modified by Dieny,1 indicate that GMR ratios fall off much less steeply with increasing a-CoZr drive layer thickness than with NiFe drive layer thickness. Since lower coercivities are expected for higher fractions of the drive material, CoZr-based spin valves show promise in combining soft properties and high GMR ratios. We have demonstrated the combination of magnetic softness and high GMR ratio in our own CoZr-based spin valves. A free layer coercivity of 7.2 Oe and GMR ratio of 6.4% were achieved in a nominal structure of NiO(400)/Co(50)/Cu(20)/Co(25)/CoZr(200AA). Differential coercivity structures were deposited as well, using Cr underlayers to induce hardness of the bottom Co. An identical “active layer” structure of the form SiO2/Cr/Co(50)/Cu(20)/Co(25)/CoZr(200AA) demonstrates GMR ratios as high as 4.9%; removal of the interfacial Co layer reduces the GMR ratio to 2.8%, in accordance with our expectations. Discussion of optimization and detailed structural analysis will be presented in the full paper.
Analysis of the Dy contribution to the Faraday rotation of Bi-DyIG film
Authors: Guillot M. , Le Gall H. , Desvignes J. M. , Artinian M.
Published: July 2002

At λ=633 nm, very accurate measurements of the Faraday rotation (FR) on epitaxial garnet films of the composition {R3−xBix}[Fe2](Fe3)O12 with R=Dy, Y, and high Bi content (x≅1.30) are reported, in the temperature range of 10–620 K under magnetic field up to 20 kOe applied along the (111) and the (110) directions. The films which were prepared by standard liquid-phase epitaxy techniques under constant growth conditions present a high crystalline quality. BiDyIG films exhibit a very different temperature and field variation of FR below 140 K whereas BiYIG films data indicate no FR anisotropy. The origin of the FR anisotropy observed when R=Dy is discussed and it is shown that the anisotropy of the magnetization cannot account for the whole FR anisotropy.
Growth and optical characterization of diluted magnetic semiconductor Zn1−xMnxSe/ZnSe strained-layer superlattices
Authors: Jin Caixia , Zhang Baoping , Ling Zhen , Wang Jie , Hou Xiaoyuan , Segawa Yusaburo , Wang Xun
Published: July 2002

Zn 1−x Mn x Se/ZnSe strained-layer superlattices are grown by molecular beam epitaxy on GaAs(100) substrates and characterized by x-ray diffraction and low temperature photoluminescence. For the case where the total thickness of a Zn1−xMnxSe/ZnSe superlattice is well below the critical value, the structure grows pseudomorphically to the buffer layer. A blue shift of 6 meV of the excitonic peak is observed and has been explained by the carrier confinement effect only. For the case of a Zn1−xMnxSe/ZnSe superlattice with a thickness larger than its critical values, we show that it can be treated as free-standing with ZnSe under tension and Zn1−xMnxSe under compression. The strain present can overwhelm the quantum confinement to produce a net red shift of 2 meV of the near band-edge feature. The energy shifts due to the strain and quantum confinement are calculated on the basis of deformation potential theory and Bastard’s method, showing good agreement with the experimental results.
Magnetization reversal processes linked to interphase exchange and dipolar coupling in hard–soft nanocomposite magnets
Authors: Emura M. , González J. M. , Missell F. P.
Published: July 2002

We present simulation results on the behavior of simple nanocomposite hard–soft magnets. In particular, we have investigated the coercive force of single and coupled triads of hard–soft–hard grains having different bulk exchange constants, intergrain exchange coefficients, saturation magnetization, and local easy axis orientations. These systems are described by a micromagnetic model taking into account anisotropy, Zeeman, exchange, and dipolar energies. Their dynamics were followed by a Monte Carlo–Metropolis algorithm.
Temperature dependent exchange anisotropy in single-crystal ferrite thin films (abstract)
Authors: van Dover R. B. , Suzuki Y. , Felder R. J.
Published: July 2002

Single-crystal bilayers of (magnetically soft) (Mn,Zn)Fe2O4 and (magnetically hard) CoFe2O4 can exhibit nearly ideal exchange anisotropy. This is directly related to the extremely high structural quality that can be achieved by epitaxial growth on appropriate substrates. For example, we use (110)-oriented MgAl2O4 substrates with 100 nm CoCr2O4 buffer layers. The paramagnetic buffer layer provides a close lattice match for the ferrites, and is annealed at about 1000 °C to provide excellent crystallinity. Subsequent growth of the ferrite bilayer is executed at 400 °C, which is high enough to permit growth of highly crystalline epitaxial films, yet low enough to avoid substantial intermixing at the interfaces. Measurements at elevated temperatures (290 K<T<1000 K) permit characterization of the exchange anisotropy in regimes of low and moderate crystal anisotropy in the soft layer, as well as directly revealing the blocking temperature and the portion of the overall magnetization contributed by each layer. The results provide additional evidence that the exchange coupling energy across the interface is comparable to that within each layer, and in that sense is nearly ideal. The detailed shapes of the M–H loops are found to be in semiquantitative agreement with micromagnetic theory.
Measurement of the exchange anisotropy in Co/CoO bilayers using the anisotropic magnetoresistance
Authors: Miller B. H. , Dan Dahlberg E.
Published: July 2002

We have developed a new technique to measure the ferromagnetic antiferromagnetic exchange biasing (FAEB) energy with relatively small reversible rotations of the magnetization from the unidirectional easy direction. This new technique uses the anisotropic magnetoresistance to determine the angle the magnetization makes relative to the easy direction as a function of an externally applied magnetic field. This technique, based on reversible magnetization changes, gives FAEB energies roughly twice that of the traditional irreversible magnetization reversal measurements.