bluuebunny/crossref_metadata_2025 huggingface dataset:
Source of anomalies in the temperature dependence of ferromagnetic resonance and susceptibility data in the FeAg heterosystem
Authors: Lubitz P. , Rubinstein M. , Christodoulides J. A. , Chrisey D. B.
Published: May 2004

Nanometer size Fe particles in an Ag matrix were prepared by pulsed laser deposition and were characterized from 4 to 300 K using ferromagnetic resonance (FMR) at 9.46 GHz and superconducting quantum interference device magnetometry. The magnetic behavior changes systematically from superparamagnetic to ferromagnetic with increasing Fe concentration, increasing deposition or postdeposition annealing temperature, or decreasing measurement temperature; FMR linewidths and anisotropy fields also increase. At low temperatures a dramatic further increase in linewidth occurs, accompanied by a strong downward shift in the resonance field for all orientations, and zero field cooled susceptibility decreases sharply. These phenomena indicate the development of much larger anisotropy than in bulk Fe. For samples annealed in H gas above 600 K, this anisotropy is largely absent, indicating that the anomalous behavior is induced by a surface oxide layer on the Fe, as in the exchange bias effect, despite the absence of oxygen in the nominal composition.
Magnetic switching and reversal process in a tip ring structure
Authors: Liou Yung , Chen Dong-Cheng , Yu Chwen , Chen J. S. , Lee Shang-Fan , Yao Y. D. , Tzeng C. C. , Chen T. Y. , Cheng K. W. , Ma Ron
Published: May 2004

Patterned Permalloy submicron-size structures have been fabricated by e-beam lithography in the shape of a ring with a tip. A tip was intentionally added into the ring as a geometrical defect to interrupt the continuity of the magnetization, which aligns along the ring, in order to pin the domain wall. Magnetic switching and reversal process have been measured by the magnetoresistance measurement. The switching field about 260 Oe was obtained. At the remanent state, there was a 0.21% difference in the magnetoresistance between the angles of 90° and 0° that was equivalent to the domain wall magnetoresistance. By applying an external field, the domain wall moved along the ring under a lower field (100 Oe), which is smaller than the switching field. A drop (0.24%) in the resistance between the angle of 70°–120° has been observed that means the domain wall was moving into the voltage measuring region during the rotation.
Velocity of transverse domain wall motion along thin, narrow strips
Authors: Porter D. G. , Donahue M. J.
Published: May 2004

Micromagnetic simulation of domain wall motion in thin, narrow strips leads to a simplified analytical model. The model accurately predicts the same domain wall velocity as full micromagnetic calculations, including dependence on strip width, thickness, and magnitude of applied field pulse. Domain wall momentum and retrograde domain wall motion are both observed and explained by the analytical model.
Fabrication of nanometer scale magnetic tunnel junction arrays and their enhanced magnetoresistance
Authors: Haraichi Satoshi , Wada Toshimi
Published: May 2004

We have fabricated nanometer size magnetic tunnel junction arrays to investigate spin-dependent transport in the Coulomb blockade regime at relatively high temperatures and developed a prototype of a high-density magnetic memory. Four by four nanocontact-hole arrays have been successfully fabricated in a sputtered SiO2 interlayer using an electron beam direct write lithography process. Tunnel junctions with diameters of 20 nm deposited in the nanocontact holes consist of Ni/NiO/Co/NiO/Ni double barrier structures. The magnetoresistance ratio in the Coulomb blockade regime reaches around 100% and 65% at the off state and the on state, respectively.
Magnetostriction and elasticity of body centered cubic Fe100−xBex alloys
Authors: Clark A. E. , Wun-Fogle M. , Restorff J. B. , Lograsso T. A. , Petculescu G.
Published: May 2004

Magnetostriction measurements from 77 K to room temperature on oriented (100) and (110) disk samples of Fe93.9Be6.1 and Fe88.7Be11.3 reveal substantial increases in λ100 compared to iron. For the 11.3% alloy, λ100=110 ppm, a sixfold increase above that of α-Fe. For the 6.1% alloy, λ100=81 ppm, ∼40% and ∼170% greater than λ100 of comparable Fe–Ga and Fe–Al alloys, respectively, for H=15 kOe. Large differences exist between the values of λ100 and λ111 (λ100>0, λ111<0) and their temperature dependencies. Elastic constants, c11, c12, and c44, from 4 to 300 K were obtained on the same Fe–Be alloys. From these measurements, the magnetoelastic energy coefficients b1 and b2 were calculated. While the magnitudes of the magnetostrictions λ100 and λ111 are widely different, the magnitudes of b1 and b2 are within a factor of 2. The Fe–Be alloys are highly anisotropic magnetostrictively, elastically, and magnetoelastically. For Fe88.7Be11.3 at room temperature λ100/λ111, 2c44/(c11−c12), and b1/b2 are −6.6, 3.55, and −1.86, respectively.
Magnetoresistance monitoring of processes that affect thin film exchange bias
Authors: Negusse E. , Idzerda Y. U. , Suci P. A.
Published: May 2004

A simple method where the relative change in exchange bias, introduced by external process, is measured by taking only two magnetoresistance readings at a selected field value is presented. Modeling of this measurement process showed that optimal performance can be achieved by engineering the device parameters so that the maximum in the magnetoresistance occurs near the exchange bias field and the width of the magnetoresistance (MR) curve is twice the field value of this maximum MR. Our analysis further showed that the sensitivity can be enhanced by operating the sensor at an operation field close to the exchange bias field.
Giant magnetostriction behavior at the Curie temperature of single crystal Gd5(Si0.5Ge0.5)4
Authors: Han M. , Jiles D. C. , Snyder J. E. , Lograsso T. A. , Schlagel D. L.
Published: May 2004

We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single crystal sample of Gd5(Si0.5Ge0.5)4 prepared by the Bridgman method. TE and MS were measured along the c axis by the strain gauge method and the temperature was controlled using a closed cycle helium refrigerator. From the TE measurements the magnetic structural phase transition temperature was found to be 259.5 K on cooling and 261.5 K on heating. The abrupt change in strain and the temperature hysteresis indicate that it is a first order transition. MS measurements were conducted at 15, 258, and 265 K. At 15 K, the magnetostriction amplitude was 3–4 ppm, whereas at 258 K it was 100 ppm. At 265 K, which is just above the Curie temperature, a giant magnetostriction of 2000 ppm was found. This unusual behavior is due to the fact that the external magnetic field can increase the transition temperature above 265 K, resulting in a first order magnetic/structural phase transition. The results reveal that giant magnetostriction in Gd5(Si0.5Ge0.5)4 only occurs as a result of the magnetic/structural transformation.
Microscopic mechanisms of magnetization reversal (invited)
Authors: Safonov Vladimir L.
Published: May 2004

Two principal scenarios of magnetization reversal are considered. In the first scenario all spins perform coherent motion and an excess of magnetic energy directly goes to a nonmagnetic thermal bath. A general dynamic equation is derived which includes a tensor damping term similar to the Bloch–Bloembergen form but the magnetization magnitude remains constant for any deviation from equilibrium. In the second reversal scenario, the absolute value of the averaged sample magnetization is decreased by a rapid excitation of nonlinear spin-wave resonances by uniform magnetization precession. We have developed an analytic k-space micromagnetic approach that describes this entire reversal process in an ultra-thin soft ferromagnetic film for up to 90° deviation from equilibrium. Conditions for the occurrence of the two scenarios are discussed.
Magnetic properties of helimagnetic YMn2−x(Fe,Co)xO5−δ (0.0⩽x⩽1.0) for the room temperature ferroism
Authors: Shim In-Bo , Yeom Jai Hoon , Choi Kang Ryung , Kim Chul Sung , Shin Hyun Jung , An Sung Yong
Published: May 2004

The structural, electrical, and magnetic properties of the YMn2−x(Fe,Co)xO5−δ (0.0⩽x⩽1.0) pseudobinary oxides have been studied utilizing x-ray diffraction, atomic force microscopy, and magnetic measurements. Our interest in these oxides begins with the hypothesis that both ferroelectric and ferromagnetic properties coexist (a condition known as “ferroism or multiferroism”) in these oxides. Pure and (Fe,Co)-doped YMn2O5−δ series have been successfully prepared by means of the sol–gel process. The samples with YMn1.8Fe0.2O5−δ and YMn1.8Co0.2O5−δ composition display a room temperature spontaneous polarization (ferroelectric) and magnetization (ferromagnetic). The ferroism responses in (Fe,Co)-doped YMn2O5−δ series present an opportunity to create and implement spintronics devices that actively couple the ferromagnetic and ferroelectric order parameters.
Correlation between spin-glass-like behavior and lattice distortion in itinerant Ca1−xSrxRuO3 metal
Authors: Lin B. N. , Ku H. C.
Published: May 2004

The long-range ferromagnetic order was observed only for x⩾0.25 in the Ca1−xSrxRuO3 (0⩽x⩽1) metallic system, while short-range magnetic correlation of ferromagnetic double-exchange interaction persists for x<0.25. Temperature, field, and time dependent magnetization m(T,Ba,t) for exchange-enhanced paramagnetic Ca0.8Sr0.2RuO3 near the boundary show similar electronic-inhomogeneous, spin-glass-like behavior as in Sr-free CaRuO3. X-ray Rietveld analysis and x-ray absorption near-edge structure data indicate that weaker double-exchange coupling strength Jde in the Ca-rich regime is closely related with stronger RuO6 tilting and distortion, which results with weaker Ru 4d–O 2p hybridization, narrower bandwidth W, and stronger on-site Coulomb repulsion Udd.
Variational Monte Carlo study of a spin one quantum antiferromagnet on a fractal lattice
Authors: Ghosh Dipan K. , Warrier Latha S.
Published: May 2004

A variational Monte Carlo calculation is made on a spin one Heisenberg antiferromagnet on a fractal lattice. Ground state energy and spin correlation functions are computed for a Sierpiński gasket containing up to 123 sites. The results indicate approximate validity of Marshall sign rule for a tripartite lattice. Correlation functions decay fast with distance, suggesting a disordered ground state.
Angular dependence of switching properties in single Fe nanopillars
Authors: Brown G. , Stinnett S. M. , Novotny M. A. , Rikvold P. A.
Published: May 2004

The continued increase in areal densities in magnetic recording makes it crucial to understand magnetization reversal in nanoparticles. We present finite-temperature micromagnetic simulations of hysteresis in Fe nanopillars with the long axis tilted at angles from 0° to 90° to the applied sinusoidal field. The field period is 15 ns, and the particle size is 9×9×150 nm. The system is discretized into a rectangular pillar of 7×7×101 spins each with uniform magnetization. At low angles, reversal begins at the endcaps and proceeds toward the center of the particle. At 90° reversal proceeds along the entire length of the particle (save at the ends). The switching field was observed to increase over the entire range of angles, consistent with recent experimental observations. A second, lower-resolution micromagnetic simulation with 1×1×17 spins, does not agree with experiment, but shows behavior very similar to that of the Stoner–Wohlfarth model of coherent rotation.
Spin transfer excited regular and chaotic spin waves in current perpendicular to plane spin valves
Authors: Zhu Xiaochun , Zhu Jian-Gang , White Robert M.
Published: May 2004

Spin polarized current induced spin wave excitations have been studied with utilizing the spin transfer modified Gilbert equation in micromagnetic modeling. An elliptical shaped spin valve at deep submicron dimension is modeled. It is found that stable spin waves with extremely narrow linewidth are excited in the free layer when the perpendicularly injected direct current is slightly above a threshold. The spin transfer effect yields self leveling of the generated spin waves and results in spin waves with stable amplitude for each excited mode. At high current level, the excited spin waves become chaotic in nature, causing a pronounced 1/f-like spectral content in the magnetoresistive output.
Magnetic properties of bulk Mn12Pivalates16 single molecule magnets and their self assembly on functionnalized gold surface
Authors: Abdi A. Nait , Bucher J. P. , Rabu P. , Toulemonde O. , Drillon M. , Gerbier Ph.
Published: May 2004

Magnetic properties of oriented Mn12 Pivalates16 (Mn12Piv16) single molecule crystals have been studied. Steps in the hysteresis loops, which are characteristic of quantum tunneling of magnetization, are observed at regular intervals of the magnetic field. Self-assembling of monolayers of this molecular material on a surface is then achieved. A process is described by which Mn12Piv16 clusters are grafted onto the crystalline Au(111) surface by means of organosulfur SH–(CH2)10–COOH ligands in order to hold them firmly. This self-assembled monolayer was then characterized by Scanning tunneling microscopy and X-ray photoelectron spectroscopy so as to provide the topographic structure, the quality, and the stability during annealing to high temperature.
Quasiperiodic magnetization dynamics in uniformly magnetized particles and films
Authors: Serpico Claudio , d’Aquino Massimiliano , Bertotti Giorgio , Mayergoyz Isaak D.
Published: May 2004

We study Landau–Lifshitz–Gilbert (LLG) magnetization dynamics in uniformly magnetized uniaxial particles and films subject to circularly polarized electromagnetic fields. Rotational invariance of the system and the introduction of an appropriate rotating reference frame permit one to reduce the problem to the study of an autonomous dynamical system on the unit sphere. Quasiperiodic magnetization dynamics correspond to limit cycles of this reduced dynamical system. A perturbation technique based on the Poincaré–Melnikov method is applied to predict the existence, the number, the shape, and the stability of these limit cycles for small value of the damping constant in the LLG equation.
Magnetic anisotropies of ultrathin Fe films on Au(111) as function of iron thickness and gold capping
Authors: Toulemonde O. , Petrov V. , Abdi A. Nait , Bucher J. P.
Published: May 2004

Iron thin films between 1.5 and 3.75 ML grown on a reconstructed Au(111) surface have been studied by in situ magneto-optic Kerr effect in the polar and longitudinal geometries. In films with Fe coverage less than 2.8 ML, we observe a perpendicular magnetic anisotropy tendency at room temperature. With further iron deposition, the film exhibits a continuous spin reorientation from out-of-plane to in-plane. Distinct changes in the magnetic properties as a function of substrate quality and gold capping layer thickness may be related to different state of the film structures.
Substrate and thickness effects on structure and transport properties of La2/3Ca1/3MnO3 films
Authors: Wu X. S. , Cai H. L. , Xu J. , Tan W. S. , Hu A. , Jiang S. S. , Hase T. P. A. , Tanner B. K. , Xiong G.
Published: May 2004

La 0.67 Ca 0.33 MnO 3 (LCMO) thin films with the thickness varying from 200 to 500 Å have been deposited by magnetron sputtering on sapphire, SrTiO3, yttrium-stabilized ZrO2 substrates. A metal-to-insulator transition behavior was observed for all samples with the transition temperature and width varying with the substrate and the film thickness. A percolation model has been used to obtain the physical parameters on low-temperature metallic transport behavior and the adiabatic thermal small polaron hopping progress. The surface of the deposited films was measured by atomic force microscopy and grazing incidence x-ray specular and diffuse scattering. Fits to the data show the presence of a mixed layer between the substrate and the LCMO films the extent of which depends on the substrate used.
Magnetic measurements on ferromagnetic behavior in the bulk II–VI diluted magnetic semiconductor Zn1−xCrxTe
Authors: Pekarek T. M. , Arenas D. J. , Crooker B. C. , Miotkowski I. , Ramdas A. K.
Published: May 2004

Magnetic measurements on the ferromagnetic behavior in the bulk II–VI diluted magnetic semiconductor Zn1−xCrxTe were made on two x=0.0033 single crystals taken from different regions of the same boule. Ferromagnetism was verified in both samples by an Arrott plot analysis with a transition temperature at 365 K (well above room temperature). For both samples at room temperature, the coercive field is ∼0.0100 T and the remanent magnetization is 23% of the saturated value. The similarity in the observed ferromagnetic behavior between the two samples suggests that a stable CryTez or possibly ZnxCryTez precipitate phase is responsible, although a Cr-rich region in the bulk Zn1−xCrxTe itself cannot currently be conclusively ruled out as the source.
Binding state and microstructure analyses of Co-doped TiO2 thin film
Authors: Yao X. F. , Zhou T. J. , Gai Y. X. , Chong T. C. , Wang J. P.
Published: May 2004

Multilayered Co-doped TiO2 thin films with postannealing treatment were investigated. Based on x-ray photoelectron spectroscopy, Co(II) binding state was found after annealing. Its intensity increases with the annealing temperature. Postannealing in an ultrahigh vacuum system is an efficient way to drive Co atoms to diffuse into TiO2. It is interesting to find that samples with partial cosputtering structure have a much stronger Co(II) peak than those of pure multilayer structure. Transmission electron microscope patterns show that the annealed films have a polycrystal rutile-TiO2 phase. The saturation magnetization of annealed sample at 150 K is 1.325 uB per Co atom, which is close to the value expected for low-spin Co(II).
Magnetic domain structures of focused ion beam-patterned cobalt films using scanning ion microscopy with polarization analysis
Authors: Li Jian , Rau Carl
Published: May 2004

Studies of magnetic domain distributions in patterned magnetic materials are of pivotal importance in the areas of ultrahigh density magnetic recording, MRAM design, and miniaturized magnetic sensor arrays. Scanning ion microscopy with polarization analysis (SIMPA) is used to perform in situ topographic and magnetic domain imaging and focused ion beam (FIB) patterning. For FIB-patterned 30 nm thick Co films, it is found that rectangular Co bars of sizes between 10–30 μm exhibit S type, whereas circular shaped magnetic elements show C type micromagnetic magnetization patterns. It is shown that SIMPA provides a simple way to directly identify different micromagnetic domain patterns.
Multistep switching phase diagram of ferromagnetic ring structures
Authors: Kläui M. , Vaz C. A. F. , Bland J. A. C. , Heyderman L. J. , David C. , Sinnecker E. H. C. P. , Guimarães A. P.
Published: May 2004

We present a study of the phase diagram for the multistep switching occurring in micrometer-sized ferromagnetic ring structures. The transition from the double to the triple switching is experimentally determined as a function of the geometrical parameters. We find that triple switching involving the vortexcore state is present only for a small range of geometrical parameters, since it requires a large film thickness, a large ring width, and a small inner diameter. The double switching prevails for other geometries. The phase boundary between the switching regimes can be explained by the results of micromagnetic simulations.
Zero-temperature phase diagram for strongly correlated nanochains
Authors: Luo Yan , Verdozzi Claudio , Kioussis Nicholas
Published: May 2004

Recently there has been a resurgence of intense experimental and theoretical interest on the Kondo physics of nanoscopic and mesoscopic systems due to the possibility of making experiments in extremely small samples. We have carried out exact diagonalization calculations to study the effect of the energy spacing Δ of the conduction band on the ground-state properties of a dense Anderson model nanochain. The calculations reveal for the first time that the energy spacing tunes the interplay between the Kondo and Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions, giving rise to a zero temperature Δ versus hybridization phase diagram with regions of prevailing Kondo or RKKY correlations, separated by a free spins regime. This interplay may be relevant to experimental realizations of small rings or quantum dots with tunable magnetic properties.
Micromagnetic simulation of magnetization reversal in small particles with surface anisotropy
Authors: Scholz W. , Suess D. , Schrefl T. , Fidler J.
Published: May 2004

In order to study the effect of surface anisotropy, we have developed a three-dimensional finite element micromagnetic model. The finite element mesh of the particles is split into a core region with bulk properties and a surface region (shell) of variable thickness with modified material parameters. We have studied the magnetization reversal in FePt nanoparticles. The effect of surface anisotropy strongly reduces their coercivity. An Fe–oxide shell with low anisotropy in the plane of the surface has been considered as well as surface anisotropy with high anisotropy constants and axis orientation perpendicular to the surface.
Relaxation in epitaxial Fe films measured by ferromagnetic resonance
Authors: Kuanr Bijoy K. , Camley R. E. , Celinski Z.
Published: May 2004

We have measured the ferromagnetic resonance (FMR) frequency and field linewidths of thin epitaxial Fe films (grown using molecular beam epitaxy) using network-analyzer FMR and conventional FMR. From the linewidths, we determined quantitatively the intrinsic and extrinsic contribution to the Gilbert damping. The observed broadening of the linewidth for thinner Fe films is consistent with the two-magnon scattering mechanism proposed by Arias and Mills. The decrease of the linewidth as a function of film thickness is fit to a power law. This fit allows a determination of the intrinsic contribution to the linewidth. We find a frequency width=0.147 GHz and field linewidth=47 Oe.
The effect of high magnetic field on phase stability in Fe-Ni
Authors: Nicholson D. M. C. , Kisner R. A. , Ludtka G. M. , Sparks C. J. , Petit L. , Jaramillo Roger , Mackiewicz-Ludtka G. , Wilgen J. B. , Sheikh-Ali Askar , Kalu P. N.
Published: May 2004

Identically prepared samples of Fe0.85Ni0.15 were annealed either in the ambient magnetic field or in a field of 29 T. Room temperature x-ray powder diffraction measurements that were performed after magnetic annealing showed that the ratio of the volume of the γ to α phase is decreased in the field-annealed sample by a factor of 2. First-principles calculations of the magnetic structure in the presence of a magnetic field are used to compute the resulting change in free energy. Analysis in terms of the phase diagram calculated with and without a magnetic field is in substantial agreement with the measurements.
Magnetic properties and microstructure of low ordering temperature L1 FePt thin films
Authors: Sun A. C. , Kuo P. C. , Chen S. C. , Chou C. Y. , Huang H. L. , Hsu J. H.
Published: May 2004

Polycrystalline Fe52Pt48 alloy thin films were prepared by dc magnetron sputtering on preheated natural-oxidized silicon wafer substrates. The film thickness was varied from 10 to 100 nm. The as-deposited film was encapsulated in a quartz tube and postannealed in vacuum at various temperatures for 1 h, then furnace cooled. It is found that the ordering temperature from as-deposited soft magnetic fcc FePt phase to hard magnetic fct L10 FePt phase could be reduced to about 350 °C by preheating substrate and furnace cooling treatment. The magnetic properties measurements indicated that the in-plane coercivity of the films was increased rapidly as annealing temperature is increased from 300 to 400 °C, but it decreased when the annealing temperature is higher than 400 °C. X-ray diffraction analysis shown that the as-deposited FePt thin film was a disorder fcc FePt phase. The magnetic measurement indicated that the transformation of disorder fcc FePt to fct L10 FePt phase was started at about 350 °C, which is consistent with the analysis of x-ray diffraction patterns. From scanning electron microscopy observation and selected area energy disperse spectrum analysis, the distributions of Fe and Pt elements in the films became nonuniform when the annealing temperature was higher than 500 °C due to the formation of the Fe3Pt phase. After annealing at 400 °C, the in plane coercivity of Fe52Pt48 thin film with film thickness of 100 nm is 10 kOe, Ms is 580 emu/cm3, and grain size is about 12 nm.
Quantum fluctuations of the total spin in molecular nanomagnets: Evidence from torque and specific heat
Authors: Carretta S. , Santini P. , Liviotti E. , Magnani N. , Guidi T. , Caciuffo R. , Amoretti G.
Published: May 2004

Macroscopic quantum phenomena arising from S-mixing in molecular nanomagnets are studied. While in the lack of an external magnetic field such effects are usually negligible, when a field of suitable intensity and direction is applied total-spin fluctuations can be amplified to the point that they dominate the macroscopic behavior at low temperature. In particular, the behavior of torque and heat capacity as a function of the field strength is dominated by S-mixing, and the former is characterized by peculiar peak-like structures. Our calculations show that the Mn-[3×3] grid is particularly suited to observe these effects.
Experimental determination of Lévy flight distributions of the energy barriers in spin glasses
Authors: Gorham N. T. , Woodward R. C. , St. Pierre T. G. , Stamps R. L. , Walker M. J. , Greig D. , Matthew J. A. D.
Published: May 2004

Zero-field magnetic viscosity measurements at low temperatures were made on a series of Pd40Ni40−xFexP20 (x=10–20) samples in order to determine the apparent magnetic-moment-weighted energy barrier distributions for these spin-glass alloys. The distributions observed are best described by a stretched exponential in the form of a truncated Lévy flight distribution. This form suggests a hierarchical landscape of apparent energy barriers arising from interactions between randomly oriented magnetic clusters within the material. The degree of stretching of the exponential form of the energy barrier distributions is found to increase with decreasing iron concentration.
Dielectric responses of the layered cobalt oxysulfide Sr2Cu2CoO2S2 with CoO2 square planes
Authors: Okada S. , Terasaki I. , Ooyama H. , Matoba M.
Published: May 2004

We have studied the dielectric responses of the layered cobalt oxysulfide Sr2Cu2CoO2S2 with the CoO2 square planes. With decreasing temperature below the Néel temperature, the resistivity increases like a semiconductor, and the thermopower decreases like a metal. The dielectric constant is highly dependent on temperature, and the dielectric relaxation is systematically changed with temperature, which is strongly correlated to the magnetic states. These behaviors suggest that carriers distributed homogeneously in the paramagnetic state at high temperatures are expelled from the antiferromagnetically ordered spin domain below the Néel temperature.
Magneto-optic indicator film observations of domain motion in magnetostrictive materials under stress
Authors: Shull R. D. , Quandt E. , Shapiro A. T. , Glasmachers S. , Wuttig M.
Published: May 2004

The magneto-optic indicator film technique was used to image the magnetic domain structure in a Tb40Fe60/Fe50Co50 multilayer as either a stress or magnetic field was applied 90° to the material’s easy axis. These observations showed that the magnetization vectors rotated under application of the stress, and recovered upon removal of that stress, in general without domain wall motion. However, magnetization rotation under field application was observed to include wall motion, and the magnetic state was found to have been changed upon removal of the field contrary to expectations. In both cases, both inhomogeneous and homogeneous rotations were observed.
Spin dependent transport effects in Cu nanowires
Authors: Gillingham D. M. , Müller C. , Bland J. A. C.
Published: May 2004

In this work we investigate quantum transport in Cu nanowires created by bringing macroscopic Cu wires into and out of contact under an applied magnetic field in air. Here we show that a 70% magnetoconductance effect can be seen in a Cu nanowire in a field of 2 mT at room temperature. We propose that this phenomenon is a consequence of spin filtering due to the adsorption of atmospheric oxygen modifying the electronic band structure and introducing spin split conduction channels. Since bulk Cu is not magnetic it may provide a new perspective for spintronics.
Angular dependence of the microwave excitation by direct current
Authors: Zhu Xiaochun , Zhu Jian-Gang
Published: May 2004

Excitation of magnetization precession by direct current injection in a current perpendicular to plane spin valve device at deep submicron dimension is simulated with utilization of spin transfer modified Gilbert equation in micromagnetic modeling. The simulation shows that the transient time for the excited microwave to reach steady state is significantly shorter if the spin current polarization direction is near 90° angle with respect to the local magnetization as compared with the transient time if the excitation angle is 0°.
Interlayer perpendicular domain coupling between thin Fe films and garnet single-crystal underlayers
Authors: Chun Y. S. , Krishnan Kannan M.
Published: May 2004

The magnetic behavior and domain structure of Fe films grown on yttrium–iron–garnet (YIG) underlayers were studied to elucidate their interlayer coupling, as a function of the Fe layer thickness, using magnetic force microscopy (MFM) and magneto-optic Kerr effect (MOKE). The YIG layer, grown by liquid phase epitaxy on single crystal gadolinium–gallium–garnet (GGG) substrates, showed a characteristic stripe domain structure in MFM images. Fe layers with thickness values of (5, 10, and 20 nm) were separately deposited on the YIG layer using ion beam sputtering system at room temperature. MOKE measurements of Fe/YIG/GGG with different Fe thickness showed that Fe films preferred to be magnetized perpendicular to the surface when the thickness was less than 20 nm. Stripe domain structure was observed for Fe thickness of 5 and 10 nm, but not for 20 nm film. Micromagnetic simulations of these Fe/YIG bilayers show that the volume exchange stiffness term at the interface plays a dominant role, when compared with the bilinear and biquadratic coupling contributions, in determining the interlayer coupling.
Numerical and analytical study of fast precessional switching
Authors: d’Aquino M. , Scholz W. , Schrefl T. , Serpico C. , Fidler J.
Published: May 2004

The switching process of magnetic recording NiFe thin films is considered. First, it is illustrated through numerical micromagnetic simulations that precessional switching process can be reasonably considered a quasiuniform process, while in conventional switching process, domain nucleation and wall motion are involved in the magnetization reversal dynamics. Second, we used analytical uniform mode theory of precessional switching to predict the duration of the applied field pulse. We verified that the uniform mode theory provides reasonably good indications on the quasiuniform precessional switching dynamics.
Thickness dependence of Co anisotropy in TbFe/Co exchange-coupled bilayers
Authors: Lin Chao-Cheng , Lai Chih-Huang , Wei D. H. , Hsu Y. J. , Shieh Han-Ping D.
Published: May 2004

The orientation of element-specific moments was determined by using x-ray magnetic circular dichroism spectroscopy to explore exchange anisotropy of TbFe/Co bilayers. Perpendicular anisotropy of 15 Å Co was induced by TbFe through exchange coupling, resulting in the out-of-plane Co moments. With increasing the thickness, Co moments were gradually tilted to the in-plane orientation because of increasing planar anisotropy. In the bilayer with thick Co, interfacial Fe moments were unidirectionally aligned in the plane, leading to in-plane exchange bias of Co. The coercivity and exchange bias field of Co in the bilayers exhibited a strong dependence on Co thickness.
Transformation strain by chemical disordering in silicon carbide
Authors: Li Ju
Published: May 2004

Swelling is observed in radiation-induced amorphization of SiC, which can be attributed to both structural and chemical disordering. By first-principles calculations, an attempt is made to separate the two factors by creating complete chemical disorder with no initial structural disorder in a 64-atom supercell. By relaxing all stresses and internal forces, significant transformation strains, both hydrostatic and shear, are observed. The relaxed configurations are found to be metallic. Softening of the bulk modulus is found to correlate closely with volume expansion, regardless of whether the expansion is caused by structural or chemical disordering, or with no disorder at all. It is postulated that partial chemical disordering contributes significantly to the internal residual stresses and macroscopic swelling of amorphous SiC.
Generalized magnetostatic modes around large magnetization motions
Authors: Bertotti Giorgio , Bonin Roberto , Mayergoyz Isaak D. , Serpico Claudio
Published: May 2004

The notion of magnetostatic mode is extended to far-from-equilibrium conditions. A general formulation for the calculation of mode frequencies and associated magnetization patterns is obtained by linearizing the Landau–Lifshitz–Gilbert equation around previously derived analytical solutions for spatially uniform driven magnetization motions. Analogies and differences with respect to Walker modes are discussed.
In situ study of temperature dependent magnetothermoelastic correlated behavior in ferromagnetic shape memory alloys
Authors: Sullivan Matthew R. , Ateya Daniel A. , Pirotta Steven J. , Shah Ashish A. , Wu G. H. , Chopra Harsh Deep
Published: May 2004

This study reports the first in situ observation of temperature-dependent micromagnetic and twin structure in oriented single crystals of Ni–Mn–Ga Heusler alloys. Micromagnetic measurements were made over a temperature interval of 50 to −35 °C covering both forward and reverse martensitic transformation. Magnetic domains in the martensite phase were found to be uniformly spaced (25–30 μm); the direction of the domain walls conforms to the changing direction of the magnetic easy axis as they traverse from one twin to another. The martensite twins could be reoriented in applied fields as low as 1300 Oe.
Atomic force microscope lithography in perovskite manganite La0.8Ba0.2MnO3 films
Authors: Li Run-Wei , Kanki Teruo , Tohyama Hide-Aki , Zhang Jun , Tanaka Hidekazu , Takagi Akihiko , Matsumoto Takuya , Kawai Tomoji
Published: May 2004

Atomic force microscopy (AFM) lithography has been investigated in La0.8Ba0.2MnO3 (LBMO) films. Unexpectedly, AFM lithography can be performed in LBMO film not only under a positive sample bias, but also under a negative sample bias. Under a positive sample bias voltage, grooves can be obtained directly with poor controllability and reproducibility. However, under a negative sample bias, controllable nanosized patterns were obtained with excellent reproducibility and a high etching selectivity. It is expected that various nanodevices will be fabricated by AFM lithography and etching techniques with perovskite manganites.
Precessional switching of a 3×1 micrometer elliptical element in a crossed-wire geometry
Authors: Krichevsky A. , Freeman M. R.
Published: May 2004

Switching dynamics of a thin-film 3×1 micrometer elliptical permalloy element in a crossed-wire geometry was investigated for different pulsed field amplitudes using time resolved scanning Kerr microscopy. We observed coherent switching of magnetization by fast-risetime hard axis magnetic field pulses. The combined action of easy- and hard-axis fields resulted in reliable half-select switching of the sample magnetization. Varying the delay between the pulses one can control the postswitching oscillations of magnetization or eliminate them altogether achieving an ultrafast half-select switching of risetime of just 180 ps. Our results agree quantitatively with Landau–Lifshitz–Gilbert-based micromagnetic simulations.
Stability and shrinkage of a cavity in stressed grain
Authors: Wang Hua , Li Zhonghua
Published: May 2004

A cavity in a stressed grain may change its shape through surface diffusion and volume through lattice diffusion. A suitable thermodynamic potential is established, from which a dimensionless loading parameter that controls the cavity evolution is introduced. Above a critical value of this parameter the cavity collapses to a crack; below that the cavity will stay stable. Then, the shrinkage rate, thereby the healing time, of a spherical cavity is derived as a function of external applied pressure and internal gas pressure. The effects of the applied pressure and the internal pressure on the rate process of the cavity are discussed.
Anisotropic pinning effect on a Nb thin film with triangular arrays of pinning sites
Authors: Wu T. C. , Kang P. C. , Horng Lance , Wu J. C. , Yang T. J.
Published: May 2004

A superconducting Nb thin film with triangular arrays of pinning sites has been fabricated to investigate the commensurate vortex lattices. Vortex-vortex interactions in this pattern film make the observed anisotropy in the pinning properties. Periodic minima appear in the magnetoresistance curves at equal field intervals that can be associated with matching effect. The transport properties at near critical temperatures show anisotropic pinning properties related to the configuration of pinning centers. The critical current is depressed when the driving force is applied along the short diagonal in the rhombic unit cell of pinning array. It implies that the pinning potential along the short diagonal creates a moving channel of vortices in triangular arrays of defects. The pinned and interstitial vortices can be movable in an incommensurate vortex row and a straight unhindered one-dimensional path.
Thin-domain modeling of mass transport in microchannels, with application to diffusive mixing
Authors: Stay Matthew S. , Barocas Victor H.
Published: May 2004

Because of the significance of diffusion on the microfluidic length scale, diffusion-based mixers are a valuable and popular component in lab-on-a-chip designs, with many variations emerging. The design of such mixers, however, is limited by existing simulation tools, which are often slow and poorly suited to the special geometry of microchannels. In this article, we derive the necessary conditions for a reduced-dimension model of flow and diffusion in a thin (i.e., height much less than width) channel. We subsequently show that the resulting mass transport equation can be solved in a streamfunction-pressure coordinate system, providing a simpler, more accurate, and more stable solution than is possible using standard methods in the x–y coordinate system. Simulations by this method were fast, requiring less than 30 min even for complex domains; the bulk of the time was spent on noncomputational tasks such as mesh generation. The approach was applied to three problems: a simple mixer based on the T sensor of Yager and co-workers, and then two variations on the base case in which obstructions were introduced to increase the available diffusion time. Although the obstructions changed the flow pattern significantly, there was little effect on the outlet concentration profile. It was observed from the model equations that if the flow rate is unchanged, the improvement in mixing can be expressed in terms of an upper bound based on the increase in pressure drop. In the case studied, the pressure drop change was small, so the mixing was not improved very much.
First-principles study of electronic and magnetic structure in layered cobaltite Na0.75CoO2
Authors: Okabe H. , Matoba M. , Kyomen T. , Itoh M.
Published: May 2004

The electronic structure of Na0.75CoO2 has been calculated by the LDA+U method, considering the location of Na ions. The magnetic moments of Co ions are strongly influenced by the bond distances between the Co and Na ions, and are unusually distributed. This implies the occurrence of a charge disproportionation state (2Co3.25+→Co3.25+δ+Co3.25−δ) in this material. The magnetic ground state of Na0.75CoO2 is therefore a spin-glass-like, short-range ordered state resulting from randomly distributed magnetic Co ions.
Effect of spin disorder on magnetic properties of nanostructured Ni-ferrite
Authors: Zhang Y. D. , Ge S. H. , Zhang H. , Hui S. , Budnick J. I. , Hines W. A. , Yacaman M. J. , Miki M.
Published: May 2004

This work presents a structural and magnetic study of NiFe2O4 nanoparticles. Powder samples with different particle sizes were made by annealing the fine powder at different temperatures. X-ray diffraction, transmission electron microscopy, and low temperature SQUID magnetometry experiments were carried out to characterize the samples. Results support a previously proposed model that the NiFe2O4 nanoparticle has a core/shell structure due to surface disorder. However, a detailed analysis of the data reveals that although the structural disorder in the surface of the particle is the major cause for forming a shell, the superexchange coupling is the driving force for the magnetic configuration, which creates a magnetically immobilized region near the surface region with thicknesses significantly larger than that of the structurally inhomogeneous layer on the surface. Also, the ease of magnetization of the core can be significantly affected. Since the crystalline and magnetic disorders commonly exist in the surface region of magnetic nanoparticles, one has to wisely utilize the exchange coupling in the development of nanomagnetic materials.
Evaluation of Faraday-rotation characteristics with CdMnCoTe and CdMnFeTe films
Authors: Imamura M. , Ahn J. Y. , Okada A. , Takashima K.
Published: May 2004

Magneto-optical properties with quaternary CdMnCoTe and CdMnFeTe films deposited on quartz glass substrates using molecular beam epitaxy equipment are studied at an optical wavelength of 635 nm. The magnitude of the Faraday rotation θF in the CdMnCoTe films was compared with that in the CdMnFeTe films, and the largest θF has been obtained for the Cd0.603Mn0.371Co0.026Te film. It is shown that a large θF was also observed in the Cd0.575Mn0.372Fe0.053Te film and its optical properties were poor compared to those in the Cd0.603Mn0.371Co0.026Te film. A few problems that occur for measurements of the Faraday rotation are indicated.
On the bowing parameter in Cd1−xZnxTe
Authors: Zelaya-Angel O. , Mendoza-Alvarez J. G. , Becerril M. , Navarro-Contreras H. , Tirado-Mejı́a L.
Published: May 2004

Cd 1−x Zn x Te thin films were prepared on 7059 Corning glass substrates using an rf magnetron sputtering system and CdTe(1−y)+ZnTe(y) targets. The concentration (x) of Zn in the films did not coincide with the relative weight (y) of the ZnTe powder in the compressed targets. Values of x were in the range 0–0.30 as determined from x-ray diffraction patterns. The band gap energy (Eg) of the Cd1−xZnxTe samples was calculated from the photoreflectance spectra measured on the films. The position of the experimental points in the Eg versus x plot show a deviation from the phenomenological quadratic relation Eg=Eg0+ax+bx2 calculated within the virtual crystal approximation (VCA). The depart of the Eg values for higher Zn concentrations from the expected VCA model is probably due to a larger clustering of Zn atoms and/or a percolation phenomena. We obtain fitted values for the parameters a and b within the VCA approach. Comparison with data reported by other authors is made.
Interfacial reaction of eutectic AuSi solder with Si (100) and Si (111) surfaces
Authors: Jang Jin-Wook , Hayes Scott , Lin Jong-Kai , Frear Darrel R.
Published: May 2004

The dissolution behavior of Si (100) and (111) dies by eutectic AuSi solder was investigated. On the Si (100) surface, the dissolution primarily occurred by the formation of craters resulting in a rough surface. The dissolution of the Si (111) resulted in a relatively smooth surface. The morphology of the Si (100) surface during a AuSi soldering reaction exhibited more time-dependent behavior and the etching craters on a Si (100) surface grew larger with time whereas Si (111) did not significantly change. This difference was ascribed to the surface energy differences between Si (111) and (100) surfaces that resulted in the two- and three-dimensional dissolution behaviors, respectively. This difference plays an important role in the formation of voids during the AuSi die bonding. The etching craters on Si (100) act as a AuSi solder sink and the regions surrounded by etch pits tend to become voids. For Si (111), flat surfaces were observed in the voided regions. Cross section analysis showed that no solder reaction occurred in the voided region of the Si (111) surface. This suggests the possibility of the formation of a thin inert layer in a potentially voided region prior to assembly. To achieve void-free die bonding, different parameters must be adjusted to the Si (100) and Si (111) surfaces with the AuSi alloy.
Nonlinear damping of high-power magnetostatic waves in yttrium–iron–garnet films
Authors: Scott Mark M. , Patton Carl E. , Kostylev Mikhail P. , Kalinikos Boris A.
Published: May 2004

The nonlinear decay of continuous-wave magnetostatic surface wave and backward volume wave signals has been measured for propagation in a narrow 6.9 μm thick yttrium–iron–garnet film strip, with excitation frequencies and wave numbers between 5470 and 5630 MHz and 47 and 216 rad/cm, respectively. The results show: (1) that the onset of nonlinear damping is a threshold effect, and (2) that a nonlinear decay model with two damping terms, one linear and one quadratic in the wave power, are needed to model the measured response.
Jet-cooled laser spectroscopy of the cyclohexoxy radical
Authors: Zu Lily , Liu Jinjun , Tarczay György , Dupré Patrick , Miller Terry A.
Published: May 2004

The laser-induced fluorescence and laser-excited dispersed fluorescence spectra of the cyclohexoxy radical has been observed under two sets of free-jet-cooling conditions, characterized by rotational temperatures of approximately 1 and 100 K. Although five conformers of cyclohexoxy are possible, it appears that all presently observed spectral bands can be accounted for by a single one. All cold spectral bands are assigned to the B̃–X̃ electronic transition of the cyclohexoxy radical. Transitions to both a′ and a″ B̃ state vibrational levels are observed and allowed due to a substantial pseudo-Jahn–Teller effect in the X̃ state. Hot bands are also observed, which we attributed to transitions to the B̃ state from the low-lying Ã electronic state. Analysis of the spectra yields vibrational frequencies for the X̃, Ã, and B̃ states as well as the energy separations of their vibrationless levels.