bluuebunny/crossref_metadata_2025 huggingface dataset:
First-principles theory of ferromagnetic and antiferromagnetic order (abstract)
Authors: Weimert B. , Fritsche L.
Published: July 2002

We outline a theory that aims at a consistent description of spin order on a first-principles basis. The theory merely exploits the antisymmetry of the N-electron wave function and its property of solving the associated N-electron Schrödinger equation. As follows directly from the antisymmetry of the wave function, the exchange-correlation energy is generally lowered if one subjects the system to a departure from paramagnetic order. Moreover, it becomes evident that a consistent nonrelativistic N-electron theory can only yield collinear magnetism. We exploit the proven method of mapping the original N-electron Schrödinger equation onto N equivalent one-particle equations familiar from density functional theory. The occurrence of spin order is shown to be connected to Hund’s rule. One obtains closely resembling criteria for antiferromagnetic and ferromagnetic instability, the latter being identical to Stoner’s criterion. By using this criterion we have studied the 3d- and 4d-metals and also two rare-earth metals as to where spin order must or cannot occur. Potassium proves to transform from a simple metal into an insulating ferromagnet as one expands the lattice beyond a certain threshold. From our point of view, it has to be expected that ferromagnetic order can also exist in molten metals as has, in fact, been observed in recent experiments. We can also show that a consistent N-electron theory yields spin fluctuations that will, in general, contribute to the average magnetization and influence its temperature dependence.
Magnetization and coercive force of pulsed laser deposited MnZn-ferrite single crystal films (abstract)
Authors: Williams C. M. , Harris V. G.
Published: July 2002

The authors have examined magnetic and structural properties for a series of pulsed laser ablated single crystal MnZn-ferrite films. The films were epitaxially grown on (001) MgO with substrate temperatures ranging from 300 to 900 °C in oxygen pressures of 15, 30, 60, and 90 mTorr. Contrary to conventional belief, the stoichiometry of pulsed laser deposited ferrite films does not necessarily reflect that of the target. We found the film compositions vary with substrate temperature as well as with the oxygen pressure during deposition. A comparison of the magnetic, structural, and chemical properties shows (1) the magnetization scales with Fe2+ ion concentration for all oxygen pressures and (2) the coercive force is sensitive to the microstructure. Magnetization values for films deposited at 60 and 90 mTorr were found to exceed those of bulk MnZn ferrites; for some deposition temperatures the magnetization exceeded that of bulk by a factor of 2.5. This increase in magnetization is explained in terms of the excess number of Fe2+ ions on the octahedral site of the MnZn-ferrite spinel structure. Results of extended x-ray absorption fine structure measurements of the films will also be discussed.
Brillouin light scattering from ultrathin iron films (abstract)
Authors: Leng Q. , Zhang P. X. , Grünberg P. A. , Zinn W.
Published: July 2002

The thickness dependence of the magnetization, magnetic anisotropy, coercive forces, and g factors of an ultrathin Fe film are studied by Brillouin scattering and magneto-optic Kerr effect at room temperature. The wedge Fe film was fabricated on a Ag buffer with GaAs[100] substrate by means of molecular-beam epitaxy technique. The epitaxial growth and crystalline quality of the specimen structures were controlled by reflection high-energy electron diffraction, spot profile analysis-low energy electron diffraction (SPA-LEED), and Auger electron spectroscopy techniques. The measured magnetization and thickness dependence [1.5–16 monolayer (ML)] are compared with the theoretical predictions. Two magnetic order phases are identified, paramagnetic for 1.5 ML and magnetic for 2 ML and thicker film. It is found that g factor is 7% smaller in 1.5 ML film than that in the bulk. A nearly linear thickness dependence of coercive force in this film is observed. The establishing of the bulk in-plane anisotropy shows similar behavior as that of the magnetization, but with a delay in thickness. No apparent perpendicular magnetic anisotropy is observed at room temperature in this sample, which may be related to the nearly perfect in-plane lattice matching between the sandwiched Fe layer and Ag buffer and capping layers.
Observation of domain dynamics in giant magnetoresistive Co–Cu-based polycrystalline multilayers
Authors: Hua Susan Z. , Lashmore D. S. , Swartzendruber L. J. , Egelhoff W. F. , Raj K. , Chopra Harsh Deep
Published: July 2002

A high resolution interference-contrast colloid technique has been used to investigate domain dynamics associated with the antiferromagnetic (AF)⇔ferromagnetic (F) magnetization reversal processes in polycrystalline Co–Cu-based magnetic multilayers exhibiting giant magnetoresistance. The magnetization reversal was found to be local and nonuniform in nature: within each grain reversal occurs by nucleation of one or more reversed domains at its grain boundary, and different grains require different critical applied field values for wall nucleation, and their subsequent motion. Significantly, the domain dynamics associated with F⇒AF reversal were found to have different characteristics in comparison with the AF⇒F magnetization reversal. These results indicate asymmetrical giant magnetoresistance loop characteristics at the mesoscopic scale.
Synthesis of melt-spun rare-earth transition-metal intermetallics with Nd3(Fe,Ti)29 structure (abstract)
Authors: Gjoka M. , Kalogirou O. , Psycharis V. , Niarchos D. , Leccabue F. , Watts B. E. , Bocelli G.
Published: July 2002

Single phase ingots of Nd3Fe27.5Ti1.5 were prepared by argon arc melting followed by annealing at 1373 K for 72 h in evacuated sealed quartz tubes and water quenching. Pieces of the ingots were then made into ribbons using the spin-melting technique in argon atmosphere with velocities between 20–40 m/s. The ribbon samples were annealed over a range of temperatures 723–1323 K for times in the range 15–50 min. The as-spun ribbons were found to be amorphous and presented a composition close to the nominal one. After annealing, the samples were crystallized and presented the Nd3(Fe,Ti)29-type structure. From the relative intensities of the diffraction peaks it was concluded that the annealed samples presented texture. This was confirmed from the x-ray diffraction patterns of annealed ribbons after powdering which were similar to those of the bulk samples. The lattice parameters are: a=10.63(4) Å, b=8.57(4) Å, c=9.729(5) Å and β=97.01(2)°. Rietveld analysis on the annealed ribbons gave a composition of 60% Nd3(Fe,Ti)29, 21% Nd(Fe,Ti)12, and 19% α-Fe. Thermomagnetic analysis curves of the as-spun ribbons showed a broad transition between 373–433 K and a secondary one at about 553 K indicating the presence of a 1:12 impurity. After annealing, the samples presented a Curie temperature typical for the Nd3(Fe,T)29 compound, 437 K and two secondary magnetic transitions characteristic for the 1:12 and α-Fe phases, respectively. The room temperature saturation magnetization, Ms, was 145.0 and 138.7 emu/g before and after annealing, respectively. Scanning electron microscopy photographs of the annealed ribbons showed a grain size <1 μm and the microanalysis of the grains gave a mean stoichiometry Nd3Fe24Ti5 for the 3:29 phase. The coercivity of the annealed samples was 1 kOe at room temperature. This relative low value was attributed to the presence of the soft α-Fe impurity.
Oscillation of the Fe and Co magnetic moments at the sharp (1–10) Fe/Co interface and temperature dependence of the near interface moments
Authors: Swinnen B. , Dekoster J. , Meersschaut J. , Demuynck S. , Cottenier S. , Langouche G. , Rots M.
Published: July 2002

By means of time differential perturbed angular correlation spectroscopy, we studied the 111Cd hyperfine fields in molecular beam epitaxy-grown Fe/Co multilayers on (1–10) GaAs as a function of temperature (90–570 K). Together with the known fields for Cd in bcc Fe and in bcc Co, we observed 4 sharply defined satellite fields. They have previously been associated with probe positions near a sharp Fe/Co interface. Whereas the Cd hyperfine field in Fe and its satellites decrease with temperature, the satellites on the Co field increase with temperature as the CdCo field does. We determine values for the near interface moments by using them as free parameters when fitting the experimental satellite fields within a semiempirical model for the hyperfine field. At all temperatures, the Fe and Co magnetic moments near the sharp interface between bcc Fe and the monoclinically deformed body centered Co oscillate as a function of the layer number over 2 layers in Co and 3 in Fe. The near interface magnetic moments temperature dependence scales to one of atoms in bulk Fe and Co, respectively: in the investigated temperature range, all Fe moments decrease as (1−A×T3/2) while the Co moments are temperature independent.
Interaction effects on the time dependence of the magnetization in recording particles
Authors: Bottoni G. , Candolfo D. , Cecchetti A.
Published: July 2002

The influence of the magnetostatic interactions on the time dependence of the magnetization of assemblies of particles, which are used in magnetic recording, with various packing fraction is analyzed. The time effects are studied with measurements of magnetization decay and deduction of magnetic viscosity S, fluctuation field Hf, and activation volume Vac. The magnetostatic interactions among the particles are evaluated with measurement of the ΔM deviations from the Wohlfarth equation and calculation of the “Δ plot area,” which expresses the overall effect of the interactions. The experimental results show a general weakening of time effects with the increase of packing and interactions. The magnetization decay, S and Hf decrease, and Vac increases, when p or Δ plot area increase. The strengthening of the interactions supports the thermal stability of the particle magnetization. The result is discussed in relation to the evolution of the magnetization reversal induced by the presence of interactions, which make the occurrence of a coherent switching mode more probable.
Computation and measurement of the interaction field distribution in recording media
Authors: Della Torre Edward , Vajda Ferenc
Published: July 2002

A method that was proposed for the identification of the parameters of the CMH model assumed that the Preisach function was the product of a Gaussian function of the interaction field and a Gaussian function of the critical field. While the latter function is an approximation that is good only for materials whose coercivity is large compared to the standard deviation of critical fields, the former function is applicable to a wide range of magnetic materials. This paper discusses the theory of the interaction field and measurements of it using an anhysteretic magnetizing process. It is shown that the moving model corrects the apparent deviation from Gaussian behavior.
“Exchange-spring” Nd–Fe–B alloys: Investigations into reversal mechanisms and their temperature dependence
Authors: Lewis L. H. , Welch D. O. , Panchanathan V.
Published: July 2002

In order to investigate factors affecting coercivity a series of two-phase Nd2Fe14B-based nanocomposite alloys with different excess iron concentrations were produced by melt-spinning methods. The constituent grain size was estimated by diffraction methods to be in the range of 150–500 Å, and room-temperature demagnetization curves verify that the alloys studied exhibit a modest remanence enhancement. Isothermal remanence magnetization (IRM) and dc demagnetization (DCD) measurements performed at temperatures in the range 275 K⩽T⩽350 K illustrate that the coercivity and irreversible magnetization develop in a bimodal, incoherent manner from a demagnetized state but upon demagnetization from a saturated state the system evinces collective, exchange-coupled behavior as illustrated by the reversible magnetization Mrev. The temperature dependencies and values of the irreversible susceptibility χirr (DCD) suggest that a moderating phase with a magnetic anisotropy intermediate to the two constituent main phases may be present in the alloys.
Conceptual design of a hoop energy storage system
Authors: Lee Kwan-Chul , Chung Kie-Hyung , Kim Dong-Il , Cho Chang-Ho
Published: July 2002

The hoop energy storage system (HESS) was conceptually designed for the diurnal load leveling of an electric power supply. It consists of an approximately 6 m diam carbon fiber composite hoop with permanent magnetic bearings and a linear motor/generator system with solid-state cycloconverter circuits for the energy conversion between kinetic energy and electric energy. With a vacuum chamber and strong tensile strength structure for nonresistive high-speed rotation, the HESS is expected to have a large capacity of energy storage. The optimization of magnet arrangement through the analysis of the magnet’s characteristics was suggested to reduce the power loss and instability. By the measurement of magnetic-field configuration and numerical calculations with a three-dimensional code (TOSCA), it is proved that NdFeB magnets have sufficient magnetic-field strength to support the levitation of the hoop. This analysis also led to minimum drag force and eddy–current loss to make use of the uniform field. As a result of analysis, construction of the HESS has begun in our laboratory and the fabrication procedure will be reported.
Magnetic levitation/suspension system by high-temperature superconducting materials
Authors: Chen In-Gann , Hsu Jen-Chou , Jamn Gwo , Lin Chin E. , Wu M. K.
Published: July 2002

Recently, with the advance of materials processing techniques, such as top-seeding and melt-texturing (TSMT) method, very large single-grained Y-Ba-Cu-O (YBCO) samples up to several centimeters in diameter can be produced. Each sample is capable of levitating over kilograms of weight. A HTS magnetic levitation (MagLev) transportation prototype has been constructed at National Cheng-Kung University (NCKU) to validate the concept of HTS-MagLev system based on Meissner effect. This HTS-MagLev is an inherent stable levitation system, unlike traditional MagLev system that requires sensors and feedback circuits to dynamically adjust its unstable levitation position. In this report, the results of various magnetic levitation parameters, such as different permanent magnet configurations, relative levitation stability, levitation force, etc., as well as magnetic field intensity and distribution will be discussed.
Correlations between magnetic switching volume and magnetic and nanostructural properties of CoSm//Cr films (abstract)
Authors: Shan Z. S. , Malhotra S. S. , Zhang Y. B. , Liou S. H. , Sellmyer D. J.
Published: July 2002

CoSm films with a Cr underlayer (CoSm//Cr) are promising as future high density recording media.1 It was pointed out that the grain size in media affects the media noise and thermal stability significantly. However, if small grains are exchange coupled to form larger “magnetic grains,” the size of these magnetic clusters will be important in noise considerations. Our previous works23 indicated that for CoSm film with Cr underlayer the magnetic grain size is much larger than the 50 Å nanocrystallites and is about the same size as the Cr grain of ∼240 Å in dimension. In the present work, we report our systematic studies of magnetic switching volume and its correlation with magnetic and nanostructural properties for CoSm films. Switching volumes V* were measured with the “field sweep-rate dependence of coercivity Hc,” interactions among the magnetic grains were estimated with the so-called ΔM method, and the nanostructures were investigated with atomic force microscopy and transmission electron microscopy. The thickness of CoSm layers, which were sputtered in an Ar pressure PAr=25 mT, is 300 Å for all samples. It is found that as the Cr underlayer thickness dCr increases from 0 Å (without a Cr underlayer) to 200 Å, the V* and ΔMmax decrease (Hc increases) rapidly, i.e., from V* from 9.3×10−18 to 5.7×10−18 cm3, ΔMmax from 1.3 to 0.5 (Hc from 1.4 to 2.3 kOe), respectively, and then gradually as dCr approaches 800 Å. As Ar pressure PAr of the deposition of the Cr underlayer varies from 5 to 30 mT, the V* and ΔMmax have their minima (anisotropy Ku has its maximum) at PAr≃9.5 mT. V* shows minima at dCr and PAr values where Hc and Ku have their maxima and ΔMmax has a small value. This behavior can be interpreted with a thermal activation model including interactions between magnetic grains. The relationship between crystallite grain size, magnetic grain V*, and media noise will be discussed.
Approximate micromagnetic model for magnetoresistive heads (abstract)
Authors: Bharthulwar Shridhar
Published: July 2002

Micromagnetic models for magnetoresistive (MR) heads tend to be extremely intensive computationally and time consuming owing to the iterative nature of the solution to the Landau–Lifshitz equation.1 On the other hand, the Stoner–Wohlfarth coherent rotation models2 oversimplify the problem and do not include some of the localized demagnetizing fields (edge effects). Similarly these do not include the effects of the longitudinal permanent magnet fields. These do not address the “anisotropic flux propagation phenomenon.” In this article, a simplified micromagnetic model is presented for SAL biased MR heads; this model converts the effects of demagnetizing fields, the permanent magnet stabilizing fields into an equivalent anisotropy field which is then used to compute the spatial magnetization distribution within the MR film. The effects of the shields, the SAL bias field, and the anisotropic flux propagation, are also incorporated. The calculated transfer curve for the device is compared with the measured curve. An excellent agreement is obtained between the two. The model also gives insights into the effect of exchange coupling on the flux decay inside the stripe, the angle of flux injection into the device, and the effects of demagnetizing fields on the magnetization distribution pattern near the edges. The resulting magnetization pattern also approximately matches the pattern obtained from rigorous micromagnetic models.
Electron and hole excitonic spin population inversions in strained semimagnetic semiconductor quantum wells (abstract)
Authors: Jonker B. T. , Yu W. Y. , Salib M. S. , Petrou A. , Warnock J.
Published: July 2002

The large magnetic field-induced band-edge spin splitting that characterizes semimagnetic semiconductors has been used in the past to create tunable quantum confining potentials resulting in spin dependent localization of carriers. Previous work has demonstrated a field-dependent type I/type II transition for the lower energy spin component of the heavy-hole exciton. We report here a magneto-optic study of Zn1−x−yCdxMnySe/Zn1−zMnzSe multiple quantum wells in which judicious selection of composition results in a structure that exhibits a hole and/or electron spin population inversion under conditions of optical pumping. In these structures, the higher energy component of the spin split heavy-hole exciton exhibits a type I to type II transition with applied field (in contrast to previous work in which the lower energy component exhibited such a transition), preventing fast recombination via the normal interband radiative transition. Consequently, the spin-up carriers forming this exciton, at higher energy than their spin-down counterparts, may relax instead via intersubband transitions. In the conduction band, the short spin flip relaxation times lead to rapid decay of the spin-up electron population. The heavy holes, however, have much longer spin relaxation times in these strained structures, resulting in an optically pumped spin population inversion in the Zn1−zMnzSe barriers. Polarization dependent magnetoreflectivity is used to follow the type conversion of the higher energy component and the formation of the hole spin population inversion.
Crystallization behavior and temperature dependence of the initial permeability of an FeCuNbSiB alloy
Authors: Leu M. S. , Chin T. S.
Published: July 2002

The crystallization behavior of an Fe73.5Cu1Nb3Si13.5B9 alloy was studied by differential scanning calorimetry. Two crystallization peak temperatures at infinitesimal heating rate were obtained to be 769 K (Tx1) and 910 K (Tx2), respectively. The associated activation energies by means of Kissinger’s peak shift method were obtained to be 340 and 428 kJ/mole, respectively. Initial permeability (μi) and Q values versus temperature was in situ investigated in the range from 300 to 873 K by a specially designed setup on toroid ribbon cores. Their variations with temperature reveal corresponding changes in magnetization and nanocrystallization, etc. Anomaly μi peaks at 428 K in the cooling curve instead of at room temperature. By isothermal annealing at 823 K, precipitous maxima in μi and Q appear at about 50 min. This in situ method may be too powerful to work out optimal annealing processes for a specific nanocrystalline alloy.
Domain wall dynamics with nonconcervation of magnetization modulus
Authors: Huang H. L. , Sobolev V. L. , Chen S. C.
Published: July 2002

The influence of the nonconservation of magnetization modulus on the steady-state motion of a planar domain wall in the presence of an external field normal to the anisotropy axis is studied. It is shown that the nonconservation of magnetization modulus gives a noticeable contribution to the domain wall mobility in ferromagnets with narrow line of ferromagnetic resonance and almost does not affect the domain wall mobility in ferromagnets with wide line of the ferromagnetic resonance.
Spherical gradient coil for ultrafast imaging
Authors: Liu Haiying , Petropoulos Labros S.
Published: July 2002

High performance magnetic field gradient coils have always been desirable in today’s ultrafast magnetic resonance imaging (MRI) applications, such as single-shot echo-planar imaging and fast spin echo imaging, as well as MR diffusion imaging and microscopy. We present a Lagrange multiplier technique of a minimum inductance gradient coil with spherical geometry. Based on this minimization approach, we construct a functional F in terms of the stored magnetic energy, the magnetic field and a set of field constraint points which are chosen over the desired imaging volume. Minimizing F, we obtain the continuous current density distribution for the spherical gradient coil. Applying the stream function technique to the continuous current distribution, the discrete current pattern can then be generated. Employing the Biot–Savart law to the discrete current loops, the gradient magnetic field has been re-evaluated in order to validate the theory. Using this approach, we have been able to design a spherical z-gradient coil which is capable of generating a gradient field of 176 mT/m with slew rate of 3422 T/m/s over a 30-cm-diam spherical volume if driven by a 350 V–220 A current amplifier. A prototype of the spherical z-gradient coil has been built. The agreement between the analytical and experimental results is excellent. Initial imaging experiments have been conducted. The results indicate the potential use of such a coil for in vivo and in vitro fast NMR applications.
Micromagnetic simulation of 90° domain walls in thin iron films
Authors: Huo S. , Bishop J. E. L. , Tucker J. W.
Published: July 2002

The micromagnetic structure and energy of 90° domain walls in crystalline iron having a (100) surface and ranging in thickness from 30 to 200 nm have been investigated numerically by minimization of the total wall energy using a slightly modified LaBonte method similar to that described by Aharoni and Jakubovics. For films of thickness in the range of 30–110 nm, both symmetric and asymmetric stable Néel wall structures were obtained. Below 45 nm, the former has the lower energy. Above 110 nm, only the asymmetric structure could be obtained. In addition to the wall energies, we provide measures of the wall width and data on the stray field external to the film for comparison with magnetic force microscopy studies.
Effect of a transverse magnetic field on resonant magnetization tunneling in high-spin molecules
Authors: Friedman Jonathan R. , Sarachik M. P. , Hernandez J. M. , Zhang X. X. , Tejada J. , Molins E. , Ziolo R.
Published: July 2002

The recent observation of steps at regular intervals of magnetic field in the hysteresis loops of oriented crystals of the spin-10 molecular magnet Mn12O12(CH3COO)16(H2O)4 has been attributed to resonant tunneling between spin states. Here, we investigate the effect on the relaxation rate of applying the magnetic field at an angle with respect to the easy axis of magnetization. We find that the position of the resonances is independent of the transverse component of the field, and is determined solely by the longitudinal component. On the other hand, a transverse field significantly increases the relaxation rate, both on and off resonance. We discuss classical and quantum mechanical interpretations of this effect.
Spreading of damage in ferromagnetic thin films: Profiles and temporal evolution
Authors: Lima A. V. , Lyra M. L. , Costa U. M. S.
Published: July 2002

We numerically investigate the critical behavior of layered S=1/2 Ising ferromagnetic thin films with an enhanced surface coupling constant by using the spreading of damage technique through the Metropolis dynamics. We obtain that near the bulk critical spreading temperature, the Hamming distance profile changes its shape from a positive curvature profile for Js/J<1.4 to a negative curvature profile for Js/J>1.4, which is a signature of the reminiscent special surface-bulk transition which occurs in the semi-infinite system. Furthermore, we show that the temporal evolution of the damage front exhibits a typical exponential growth above critically while in the critical region the behavior is diffusive.
Narrow track dual magnetoresistive heads for tape
Authors: Trindade I. G. , Kennedy A. , Bain J. , Kryder M. H.
Published: July 2002

An experimental investigation of a dual magnetoresistive head tested on a Metrum tape test stand, is presented. The dual magnetoresistive (DMR) head has a trackwidth of 4 μm and a gap length of 0.1 μm. Metal particle tape, with a Mrδ of 16 memu/cm2 and a coercivity of 1.8 kOe, was used. A square wave was recorded with an FeAlN thin film inductive head, with a trackwidth of 33 μm and a gap of 1.9 μm. A shielded shunt biased MR head used in quarter inch cartridge drives (QIC), with a trackwidth of 19 μm and a shield to element gap of 0.25 μm was used as a benchmark. The DMR head showed a larger signal to dc erased noise per unit trackwidth than the QIC MR head, at linear densities of 20 to 100 kfci. Isolated transitions and dibits with a bit spacing of 0.3 μm were recorded for time domain measurements. The dibit response of the DMR head, characterized by a unipolar pulse with small side over/undershoots demonstrated the differential character of the DMR head. The QIC MR head PW50, measured directly from the isolated transition response and indirectly from the dibit response, was 1.4 μm. This large PW50 value was essentially write process limited, to first order independent of the read head gap length.
Theory of spin waves in ferromagnetic low-dimensional structures (abstract)
Authors: Cottam M. G. , Jiang L.-J.
Published: July 2002

A Green-function method is employed within a matrix formalism to study the spin-wave excitations in some ferromagnetic low-dimensional structures described by the Heisenberg model. Recent advances in materials growth, together with appropriate etching and/or high-resolution electron-beam lithography, have made possible the fabrication of such structures with dimensions on the nanometer scale,1 and it is of interest to study their spin dynamics. The theory is based on a generalization of earlier work on magnetic films and layered systems.2 Applications are made first to ferromagnetic films that are terminated by one or two truncations perpendicular to the film surfaces. The latter geometry allows discussion of ferromagnetic rods as a particular case. Analytic expressions are obtained for the dispersion relations and spectral intensities of the localized surface (and edge) spin waves and quantized bulk spin waves in these finite structures. The results are illustrated by numerical examples. Further applications are made to interfaces between two films with perpendicular truncations. In general, the two films may be of different materials and have different thicknesses, but two special cases of interest are (i) the interface between two different films of equal thickness and (ii) the step between two films of the same material and different thickness. The dispersion relations and spectral intensities of spin waves localized near the interface or step are derived analytically and the results are illustrated numerically.
Temperature dependence of coercivity in Co-based longitudinal thin-film recording media
Authors: Pan Tao , Spratt Geoffrey W. D. , Tang Li , Lee Li-Lien , Feng Yongchang , Laughlin David E.
Published: July 2002

The coercivity of Co-based (CoCrTa and CoCrPt), disk media was found to depend on temperature. The thinner the film, the smaller the switching unit (KuV), and the larger dHc/Hc dT. dHc/HcdT for CoCrTa (400 Å) on different underlayers or intermediate layers is generally larger than that of CoCrPt (400 Å) on different underlayers or intermediate layers. The temperature dependence of coercivity in Co-based disk media is caused by the temperature dependence of crystalline anisotropy of Co and small switching unit of the media.
Studies of interlayer exchange coupling in Fe/Cu/Fe ultrathin heterostructures
Authors: Kowalewski M. , Heinrich B. , Cochran J. F. , Schurer P.
Published: July 2002

The interlayer exchange coupling has been investigated in trilayer structures where two coupled Fe layers were separated by a modified Cu spacer. The multilayer heterostructures investigated in this article have the following general structure: Fe1/kCu/lXcCu1−c/mCu/Fe2, where XcCu1−c indicates one atomic layer of Cu atoms alloyed with foreign X atoms of concentration c. Fe1 and Fe2 indicate bottom and top ferromagnetic iron layers, respectively. k, l, and m represent the number of atomic layers (AL) yielding a total spacer thickness of (k+l+m) AL. The ultrathin films were grown using a molecular beam epitaxy system. Atoms X (Fe,57Fe,Cr,Ag) were codeposited simultaneously with the Cu atoms to create an alloyed layer. Foreign atoms were introduced in the spacer to study their effect on the direct interlayer exchange coupling between the outside Fe layers mediated by the valence electrons of the Cu spacer. The ferromagnetic resonance, magneto-optical Kerr effect, and Brillouin light scattering techniques were employed to determine the exchange coupling in these structures. Mossbauer spectroscopy was used to identify the magnetic state of the Fe57 atoms in the alloyed Fec57Cu1−c layer. The results have shown that the presence of foreign atoms inside the Cu spacer significantly decreases the bilinear antiferromagnetic coupling between the Fe layers by creating a potential barrier for the valence Cu electrons. More significantly, the exchange coupling showed a very strong dependence on the magnetic state of the foreign atom. For high concentrations of Fe in the spacer, the coupling was found to be ferromagnetic due to partial magnetic ordering of the iron atoms.
Interfaces contributions to the nonlinear magneto-optical response of quantum well states
Authors: Kirilyuk A. , Rasing Th. , Mégy R. , Beauvillain P.
Published: July 2002

The nonlinear optical response of a noble metal overlayer on top of a magnetic film shows an oscillatory behavior dominating the total second harmonic output. The oscillation periods are twice as large as those observed with linear Kerr effect measurements. To explain this effect, we decompose the total nonlinear susceptibility in different interface contributions.
Effects of Cr and magnetic bias on read/write and noise characteristics of CoCrTa/Cr longitudinal thin-film media
Authors: Lal Brij B. , Russak Michael A.
Published: July 2002

The effects of Cr and magnetic bias on read/write and noise characteristics of CoCrTa/Cr longitudinal thin-film media were investigated. Samples of magnetic thin films were prepared by using dc magnetron sputtering at four different substrate-bias conditions: no bias (N), Cr bias only (C), magnetic bias only (M), and both Cr and magnetic bias (CM). Magnetic properties, parametrics, media noise, signal-to-noise ratio (SNR), nonlinear transition shift (NLTS), crystallography, and microstructures were studied. Comparison of read/write characteristics measured with a magnetoresistive head at 72.4 kfci recording density showed improvement in resolution and pulse width for the media sputtered with M and CM bias. When the magnetic layers were sputtered with no bias, it was found that the Cr bias had almost no effect on media noise or SNR. However, the application of magnetic bias individually and substrate bias during both Cr and magnetic deposition reduced media noise about 20% and 30% and increased SNR about 5 and 7 dB, respectively, at 136.4 kfci recording density. NLTS was studied as a function of linear recording density for all these four samples. It was observed that the M and CM bias-sputtered media produce lower NLTS which correlates very well with the corresponding media noise. Co(11.0) and Co(10.0) peaks of magnetic films sputtered with M and CM bias showed broader peak width which produces smaller grain size of magnetic film for reduced transition noise. Transmission electron microscope images did not exhibit significant difference in microstructure between the films prepared by sputtering with or without applying substrate bias.
Magnetic force microscopy study of track edge effects in longitudinal media
Authors: Schabes Manfred E. , Tang Kai , Ranjan Rajiv , Yeh Nan , Wachenschwanz David
Published: July 2002

Magnetic force microscopy (MFM) is used to investigate the micromagnetics at the track edges of recorded bits as a function of linear bit density. Dipolar edge charges are observed in two types of isotropic media but are not detected in an oriented medium. We study track edge percolation and the resulting loss of effective track width as a function of bit density in the range of 10–300 kfci. Analysis of the power spectral density (PSD) of the MFM signal as a function of cross-track location shows that at high bit densities the dominant PSD peak shifts to lower frequencies at the track edges as compared to the track center. This is discussed in terms of track edge percolation that leads to magnetically correlated regions with size larger than the bit length.
Modeling of permanent magnets in three-dimensional space using edge finite elements
Authors: Čingoski Vlatko , Yamashita Hideo
Published: July 2002

A surface integration method for modeling of permanent magnets in three-dimensional space using edge finite elements is presented, where the value of the coercive magnetic force Hc is integrated only over the entire surface area of the permanent magnet. This allows direct computation of the equivalent source current values, which are afterwards assigned to the surface edges of the permanent magnet. The main advantages of the proposed method are: accurate results with less computational effort and improved convergence rate of the iterative solver. Verification of this surface integration method is carried by comparing the results with those obtained by the Biot–Savart law and the traditional current sheet method. Finally, a comparison between the numerical results obtained through the surface integration method and the measured results of a complex electromagnetic device with permanent magnets is given.
Order and disorder in layered ferromagnets
Authors: Rastelli E. , Tassi A.
Published: July 2002

The layered spin systems, characterized by a negligible value of the ratio j=Jz/J between the interlayer Jz and the intralayer J exchange coupling, are currently investigated both theoretically and experimentally. Here we give a calculation based on the Peierls variational theorem that leads to the vanishing of the critical temperature of the quantum XXZ ferromagnet at vanishing j in contrast with the common belief that Tc>TKT≠0 for any nonzero j, where TKT is the Kosterlitz–Thouless phase transition temperature of the 2D underlying lattice. The same approach provides an order–disorder transition temperature which remains finite in the limit of negligible j≠0 when the classical XXZ ferromagnet is considered. This is in qualitative agreement with renormalization group calculation as well as Monte Carlo calculation.
Kondo effect in the paramagnetic phase of CrFe alloy with the spin-density-wave suppressed by pressure or V doping
Authors: Galkin V. Yu. , de Camargo P. C. , Bud’ko S. L. , Saitovitch E. B. , Fawcett E.
Published: July 2002

The pressure dependence of the resistivity ρ(T) of (Cr+2.7 at. % Fe)98V2 is measured, following a previous study of the temperature dependence of ρ(T) in the spin-density-wave (SDW) alloy system, (Cr+2.7 at. % Fe)1−x(V,Mn)x. The minimum in ρ(T) in the V-doped alloys seen at T≳TN for x>1 at. % is found to move to lower temperatures under pressure p in the alloy containing 2 at. % V, decreasing in magnitude, and eventually, as the Neel temperature TN goes to zero at p≈10 kbar, becoming a very shallow minimum followed by a maximum and a rapid decrease as T→4.2 K. This latter behavior in the paramagnetic phase is believed to be due to the Kondo effect, while the pressure dependent minimum in ρ(T) in the SDW phase is a manifestation of impurity resonance scattering.
Low frequency charge dynamics of incommensurate chromium alloys
Authors: Viswanath V. S. , Fishman R. S.
Published: July 2002

Chromium alloys order as spin-density-wave (SDW) state antiferromagnets below the Néel temperature TN. The formation of a SDW with wavevector Q±′ is accompanied by the simultaneous formation of a charge-density-wave (CDW) with wavevector 2Q±′. We present the first results for the dynamics of the CDW within the random phase approximation. Phason modes arising from the translational invariance of the free energy are the only Goldstone modes involved in the charge dynamics of chromium alloys. The charge phasons have a nearly linear dispersion at low frequencies with a velocity which approaches the spin-wave velocity as T→TN. We obtain the neutron and x-ray scattering strengths and comment on the observability of the charge phasons.
High-field magnetization of doped Bi-series oxide superconductors
Authors: Zeng D. C. , Liu Z. Y. , Zhou G. F. , Wang Y. Z. , de Boer F. R. , Qiao G. W.
Published: July 2002

The high-field magnetization measurements of Zn- and Ga-doped sintered Bi-based compounds were carried out in a 40 T high-field facility. The critical current density and the flux pinning force of the Bi-series oxide superconductors were calculated using Bean’s critical state model. It is found that both the critical current density and the flux pinning force of the Zn-doped oxide compounds are decreased slightly with increasing Zn content. However, the critical current density and the flux pinning force of the Ga-doped oxide superconductors diminish significantly with enhancement of dopant content. It seems that the superconductivity of Bi-series superconductors is much more severely destroyed by a trivalent dopant than by a divalent one.
Anomalous magnetization behavior in field-cooling magnetization versus temperature curves in Tl-1223/Ag sheathed superconducting tape
Authors: Tang H. , Wang Y. Z. , Yang Z. Q. , Zhang C. , Hua L. , Su X. D. , Zeng D. C. , Qiao G. W. , de Boer F. R. , Zhou S. H. , Peng H. T.
Published: July 2002

The zero-field-cooling and the field-cooling temperature dependence of magnetization for Tl-1223/Ag superconducting tape were measured by using SQUID magnetometer at fields varying from 5 Oe to 5 T in the temperature range of 5–150 K. In the field-cooling curves, some anomalous behaviors, i.e., the positive Meissner effect in the curve of 5 Oe and the diamagnetic dip effect at fields up to 5 kOe, were observed. The diamagnetic dip minimum temperature shifts to the lower temperature as increasing the external field, and the relative amplitude decreases with increasing applied field. Moreover, the dip broadens with increasing applied field. The anomalous behaviors may be attributed to the flux pinning and the flux depinning of Josephson-junction networks in the sample. The dip effect at lower fields may be ascribed to the intergrain Josephson coupling networks, and that at higher fields may be due to the intrinsic intragrain Josephson coupling networks.
Circular free-electron laser structures (abstract)
Authors: Leupold H. A. , Potenziani E. , Tilak A.
Published: July 2002

Typically free-electron lasers (FELs) have employed electromagnets arranged in linearly periodic arrays to provide the magnetic field necessary to impart a transverse oscillatory acceleration to an electron beam thus causing it to radiate. With the advent of the high energy product permanent magnet materials, it has become practical in some cases to replace electromagnets with permanent magnet arrays. Difficulties with both procedures were obtaining desired magnetic field strengths at short wave lengths, utilization of only a small part of the beam energy, and impracticability of obtaining the desired bore cross sections at high field strengths and short periods. These problems can be ameliorated by permanent magnet arrays that offer unusually high fields and modest structural masses. In such structures, the electron beams follow circular rather than linear paths, the magnetic periodicity is azimuthal, the fields are axial and unidirectional, and the “wiggle” is in the radial direction. Because of its circular path, the e-beam traverses the array many times so that a much larger portion of the electron kinetic energy can be converted to radiation than in the usual linear arrays which the electron beam traverses only once. The circular beam arrays are based on both magic spheres and magic toroid structures in which the same field produces the wiggle and maintains the circular path. An example of the calculated results is that of a sphere in which the ratio of the outer to inner diameters is 3, and the extremes of the periodic field values are 3.4 and 3.7 T causing synchrotron radiation of 2.5×1013 Hz and wiggler radiation of 2.0×1014 Hz.
Mössbauer study of the magnetic properties of RFe6Sn6 compounds (R=Y, Gd, Tb, Dy, Ho, Er, Tm)
Authors: Rao X.-L. , Coey J. M. D.
Published: July 2002

Magnetic properties of RFe6Sn6 compounds (R=Y, Gd, Tb, Dy, Ho, Er, Tm) have been studied by magnetization and Mössbauer spectroscopy. Magnetization curves of isotropic powder samples at room temperature show a linear response up to 1.1 T with a susceptibility of 0.003–0.006 while Mössbauer spectra indicate magnetic order of Fe. These results confirm antiferromagnetic ordering of the compounds below a Néels point of approximately 400 K. The hyperfine fields of Fe at 25 K are ∼22 T, 45% greater than that for the isostructural FeSn although the difference in volume is only 3%. X-ray diffraction and Mössbauer spectra measured on magnetically oriented samples show that strong easy c-axis anisotropy of Fe sublattice dominates the anisotropy of these intermetallic compounds for any of the rare earth elements. A large quadrupole shift (∼0.30 mm/s at room temperature and 0.36 mm/s at 25 K) is consistent with the strong Fe sublattice anisotropy. The first anisotropy constant K1 was estimated to be of order 1.5×106 J/m3.
Magnetoresistance in cosputtered Cu–Ni–Fe films (abstract)
Authors: Chen L. H. , Jin S. , Tiefel T. H.
Published: July 2002

The magnetoresistance effect, especially the giant magnetoresistance (GMR) effect, has received much attention in recent years. In this study, we discuss the magnetoresistance behavior in Cu–Ni–Fe thin films with Cu content varied from 40 to 90 at.%, prepared by the cosputtering of both Cu and Fe50Ni50 targets. Films with low Cu content, for example, Cu50Ni25Fe25 and Cu40Ni30Fe30, exhibit a mixed behavior of GMR and anisotropic magnetoresistance (AMR). The electrical resistivity of these films substantially increases once the field is applied due to the anisotropic magnetoresistance contribution, and then decreases again at higher fields, which is believed to be related to the giant magnetoresistance effect. As a result of a compromise between both the GMR and the AMR effects, the MR ratios of these low Cu content films are only minus 1%–2% both at room temperature and at 4.2 K. However, it is found that the giant magnetoresistance contribution dominates magnetoresistance behavior in films with Cu content higher than 50 at.%. There exists a large drop in resistance at low fields followed by a long tail at high fields in the MR curves for these high Cu content films. The MR ratios of these films show an increasing tendency as temperature decreases, for example, from 3% at room temperature to 11% at 4.2 K for Cu90Ni5Fe5 film. The GMR effect in these high Cu content films is ascribed to the spin-dependent scattering at the two-phase interface and in the ferromagnetic phase(s), similar to that of the GMR in multilayers, although the contribution from the magnetic fluctuations cannot be excluded.
Crossover behavior and 2D magnetism in the weak ferromagnet Phenyl-MnPO4⋅H2O (abstract)
Authors: Visser D. , Carling S. G. , Day P. , Kremer R. K.
Published: July 2002

The material Phenyl–O–MnPO3⋅H2O consists of layers of MnO46− octahedra which form a quasi 2D square lattice. These layers are separated by phenyl -O- groups. Quasi 2D magnetic behavior is observed above TN=13.8 (K), θ=−53(1) K. Below TN, weak ferromagnetism has been established and a crossover in the power law dependence of the magnetization has been observed. The value of β changes from 0.23(2) to 0.40(2) at a reduced temperature of εx0=0.06 and is contributed to a crossover in lattice dimensionality from 2D to 3D. The weak ferromagnetic behavior in this material will be compared with equivalent AMnPO3⋅H2O compounds.
Cu
2
(1,4-diazacycloheptane)
2
Cl
4
: Spin-ladder system with competing interactions (abstract)
Authors: Hammar P. R. , Broholm C. , Reich D. H. , Trouw F.
Published: July 2002

The material Cu2(1,4-diazacycloheptane)2Cl4 consists of well-separated ladderlike chains of Cu atoms that undergo spontaneous dimerization leading to a singlet ground state. Previous measurements12 have shown that for T⩽0.7 K, this singlet ground state exists up to a field HC=6.7. Here we report on heat capacity, Cp(H,T), and neutron scattering studies of this material. For T<8 K, Cp(H=0,T)∝T−3/2 exp{−Δ/T} with an activation energy Δ=9.8 K. Cp(H,T) shows a finite-T transition for H>HC, presumably to a canted state. Inelastic neutron scattering on powder samples at H=0 shows a gap of 0.8 meV and a bandwidth for the excitations of 0.6 meV. These excitations exhaust the total moment sum rule, and hence the measured triplet band dominates the magnetic fluctuation spectrum. Comparison of the data with exact diagonalization of finite spin chains shows that the system appears to be close to a spin ladder, but with an additional, weak interrung interaction.