bluuebunny/crossref_metadata_2025 huggingface dataset:
Using small-angle neutron scattering to probe the local magnetic structure of perpendicular magnetic recording media
Authors: Wismayer M. P. , Lee S. L. , Thomson T. , Ogrin F. Y. , Dewhurst C. D. , Weekes S. M. , Cubitt R.
Published: June 2006

Small-angle neutron scattering (SANS) has been used to measure the local magnetic structure of perpendicular media consisting of granular CoCrPt–SiOx thin films. The dimensions of the magnetic structures determined by SANS are consistent with the physical grain sizes suggested by transmission electron microscopy measurements, but yield additional information on magnetic structure within the grains, including the existence of a much smaller magnetic core. The results are similar to those recently obtained on longitudinal magnetic recording media, but with the addition of strong interference terms due to the narrower distribution of grain sizes in these samples.
One-step synthesis of magnetic hollow silica and their application for nanomedicine
Authors: Wu W. , DeCoster M. A. , Daniel B. M. , Chen J. F. , Yu M. H. , Cruntu D. , O’Connor C. J. , Zhou W. L.
Published: June 2006

Magnetic nanoparticles are usually present in the form of magnetic carriers and used in nanomedicine and biosystem. In this paper, magnetic hollow silica (MHS) nanoparticles were fabricated by a one-step synthesis of Fe3O4 nanoparticles and then coating of silica on nanosized spherical calcium carbonate under alkaline conditions, in which nanosized calcium carbonate (CaCO3, 25–60nm) was used as a scarified template, tetraethoxysilane as a precursor, and Fe3O4 nanoparticles (∼5nm), formed in the initial reaction stage, as magnetic agents. The as-synthesized nanoparticles were immersed in a weak acetic acidic solution to remove CaCO3, forming MHS carriers. The nanostructures of the MHS carriers were characterized by scanning electron microscope, transmission electron microscope, and x-ray diffraction. Superconducting quantum interference device measurement exhibited that the MHS nanoparticles were superparamagnetic. Toxicity was tested for MHS carriers using rat brain microvascular endothelial cells. The cells treated with concentration lower than 50μg∕ml of the MHS nanoparticles showed no significant toxicity. After modification by silane coupling agent, the MHS carriers have strong absorption for ibuprofen in nanomedicine field.
Near-field optically assisted hybrid head for self-aligned plasmon spot with magnetic field
Authors: Nakagawa Katsuji , Kim Jooyoung , Itoh Akiyoshi
Published: June 2006

Magneto-optical hybrid head is one of the key devices for the optically assisted magnetic recording. However, there has not been enough research about the hybrid head. Here, we present a near-field optical advanced hybrid (NOAH) head, which makes a self-aligned plasmon spot with a magnetic field. Since a near-field optical intensity distribution is strongly affected by the presence of the recording medium, the shape of the electrode is analyzed by a finite difference time domain method with the TbFeCo film as the recording medium. We calculate the dependences both for the optical and magnetic characteristics on a gap distance between the electrode and the recording medium. The NOAH head makes the sharp distribution of light intensity (full width at half maximum ∼40nm) with the magnetic field ∼29Oe∕mA in the case for 5nm in gap distance. We also calculate the dependence of the plasmon effect on electrode materials. It is revealed that the material which has a high extinction coefficient shows a high peak intensity. A fundamental experiment for the writing on a Cu phthalocyanine dye film is also reported for testing the method to use a glass slider.
Studies of cell toxicity and binding of magnetic nanoparticles with blood stream macromolecules
Authors: Macaroff Patrícia P. , Simioni Andreza R. , Lacava Zulmira G. M. , Lima Emília C. D. , Morais Paulo C. , Tedesco Antonio C.
Published: June 2006

In this work, we investigated the interaction between different molecular-coated magnetite-based nanoparticles stabilized as biocompatible magnetic fluids (BMFs) and both serum albumin proteins (HSA and BSA). The investigation was focused on the fluorescence quenching of the tryptophan residue proteins after their binding with the molecular-coated magnetic nanoparticles (MNPs) at specific sites. The results showed distinct association of the BMFs with HSA and BSA, a fact that directly affects the application, biodistribution, and clinical trial therapy using colloidal stabilized MNPs.
Structure, ferromagnetic resonance, and permeability of nanogranular Fe–Co–B–Ni films
Authors: Pasquale Massimo , Celegato Federica , Coisson Marco , Magni Alessandro , Perero Sergio , Kabos Pavel , Teppati Valeria , Han Suk Hee , Kim Jongryoul , Lim Sang Ho
Published: June 2006

The static and microwave magnetic properties of soft nanogranular (Fe0.7Co0.3)71B22Ni films with a 2T saturation magnetization are presented as functions of thickness, ranging from 136to235nm. Microwave permeability values from 60 to 260 are measured and ferromagnetic resonance frequencies up to 6.8GHz are found using a vector network analyzer, connected to coplanar/microstrip structures loaded with the ferromagnetic films. Topographic and magnetic structures down to 20–40nm were observed by atomic/magnetic force microscopy. The combination of reasonable values of the permeability and high ferromagnetic resonance frequency makes these nanogranular materials useful in the development of inductive microwave components.
Effect of annealing on magnetotransport and structural properties of Co∕Cu multilayers with Ta buffer layer
Authors: Mohanan Senthilnathan , Grob Andreas , Herr Ulrich
Published: June 2006

The main aim of this study is to investigate the effect of annealing on magnetotransport and on structural properties of Co∕Cu multilayer with Ta buffer layer, prepared by dc magnetron sputtering. The heat treatment of the sample at 250°C up to 20h induced a prominent change in giant magnetoresistance, which correlates with the corresponding changes in remanent magnetization. In order to investigate the origin of the changes in the magnetotransport properties, the microstructure of the samples has been studied by x-ray diffraction. The structural modifications were elucidated by comparing x-ray spectra calculated using a nonideal step model with measured spectra and they are related to the changes in magnetotransport properties. A correlation between the changes in remanence and interlayer exchange coupling has been established using a modified Stoner-Wohlfarth [Philos. Trans. R. Soc. London, Ser. A 240, 599 (1948)] model.
Correlation between magnetism and magnetocaloric effect in the intermetallic compound DyNiAl
Authors: Singh Niraj K. , Suresh K. G. , Nirmala R. , Nigam A. K. , Malik S. K.
Published: June 2006

Magnetization studies carried out in polycrystalline sample of DyNiAl show the presence of two magnetic transitions, one at 15 K and the other at 30 K. The low-temperature transition is attributed to the onset of antiferromagnetic ordering, while the other one corresponds to the ferro-para transition. Thermomagnetic irreversibility found in the temperature dependence of magnetization data is attributed to the domain-wall pinning effect and also to the magnetic frustration. Magnetocaloric effect is found to be negative in the antiferromagnetic phase and positive above the Néel temperature.
Engineering of spin filtering in double epitaxial tunnel junctions
Authors: Tiusan C. , Greullet F. , Sicot M. , Hehn M. , Bellouard C. , Montaigne F. , Andrieu S. , Schuhl A.
Published: June 2006

Double-barrier fully epitaxial crystalline magnetic tunnel junctions employing a class of artificial antiferromagnetic (AAF) subsystem are elaborated by molecular-beam epitaxy. Our specific AAF subsystem is constituted by an Fe(10nm)∕MgO(0.7nm)∕Fe(20nm) trilayer stack where the antiferromagnetic coupling between the two Fe layers occurs by spin-polarized tunneling of electrons across the three, atomic-layer thin MgO insulating barrier. In our junctions, the efficiency of spin filtering is validated by the tunnel magnetoresistance effects of about 140% at room temperature and a high output voltage up to 500 mV at 1.3 V.
Microscopic theory of spin-wave excitations in ferromagnetic nanorings
Authors: Nguyen T. M. , Cottam M. G.
Published: June 2006

A microscopic theory is developed for the spin-wave excitations in ferromagnetic nanorings where the physical dimensions (inner and outer diameters and height) may be of the same order. Both the dipole-dipole and the exchange interactions are included in the Hamiltonian, together with single-ion anisotropy terms and an external magnetic field assumed to be applied either parallel or perpendicular to the ring axis. The equilibrium configurations of the systems are obtained by minimizing numerically the energy functional with respect to the spin orientations. The nanorings can be in bottleneck, twisted, bidomain, or vortex types of configuration. A Green’s function method is employed to calculate the dispersion spectra and other spin-wave properties. Applications to real systems, such as nickel nanorings, are made by choosing the parameters appropriately.
Spin-transfer-induced excitations in bilayer magnetic nanopillars at high fields: The effects of contact layers
Authors: Chen Wenyu , Kent Andrew D. , Rooks M. J. , Ruiz N. , Sun Jonathan Z.
Published: June 2006

Current-induced excitations in bilayer magnetic nanopillars have been studied with large magnetic fields applied perpendicular to the layers at low temperature. Junctions investigated all have Cu∕Co∕Cu∕Co∕Cu as core layer stacks. Two types of such junctions are compared, one with the core stack sandwiched between Pt layers (type A), the other with Pt only on one side of the stack (type B). Transport measurements show that these two types of junctions have similar magnetoresistance and slope of critical current with respect to field, while A samples have higher resistance. The high-field bipolar excitation, as was previously reported [Özyilmaz et al., Phys. Rev. B 71, 140403(R) (2005)], is present in B samples only. This illustrates the importance of contact layers to spin-current-induced phenomena. This also confirms a recent prediction on such spin-wave excitations in bilayers.
In situ transport measurements of plasma-oxidized MgO magnetic tunnel junctions during the annealing process
Authors: Anderson G. I. R. , Hindmarch A. T. , Marrows C. H. , Hickey B. J.
Published: June 2006

The postdeposition annealing of magnetic tunnel junctions has become standard practice in junction fabrication to improve barrier properties. We have performed tunneling measurements at elevated temperatures during the annealing process on plasma-oxidized, MgO barrier, magnetic tunnel junctions. The data were interpreted in terms of the Hartman model which accounts for an asymmetric barrier profile. The results could not be interpreted solely by the temperature smearing of the Fermi distribution and so the electrode/barrier/electrode structure had been altered during the annealing. While the barrier height at one interface improved, the other was degraded. Our experiments lead us to believe that this is due to the diffusion of Mn to the barrier/pinned electrode interface. We show that through transport and tunneling magnetoresistance measurements it is possible to monitor physical processes such as material migration during the annealing process.
Magnetic and transport properties of NiMnAl thin films
Authors: Vovk Andriy , Yu Minghui , Malkinski Leszek , O’Connor Charles , Wang Zhenjun , Durant Eden , Tang Jinke , Golub Vladimir
Published: June 2006

The magnetic and transport properties of Ni2MnAl thin films prepared using pulse laser deposition were investigated. It was shown that the films are granular and multiphase. Contrary to the data reported earlier we observe nonmonotonic temperature dependence of resistance with minimum in the vicinity of 100 K for the films deposited on substrates held at 773 K and negative magnetoresistance (MR) values of about 2.5% at 5 K and 1.6% at RT in magnetic field of 50 kOe. These values of MR are the highest reported up to date for Ni2MnAl films. Large negative MR in our films arises from a nonhomogeneous structure and its origin is the same as for granular films.
Nd
–
Fe
–
B
films with perpendicular magnetic anisotropy
Authors: Yamasawa Koji , Liu Xiaoxi , Morisako Akimitsu
Published: June 2006

The dependence of magnetic properties on the film thickness has been studied for Nd–Fe–B films with perpendicular magnetic anisotropy. The W underlayer was used for preventing oxidation of Nd. It is found that the perpendicular coercivity decreases with the increase of the film thickness, although the thick film still has perpendicular magnetic anisotropy. A laminated structure composed of [Nd–Fe–B∕W]n has been proposed to prepare thick Nd–Fe–B with higher coercivity. The laminated structure can suppress the grain growth in Nd–Fe–B films while still keeping the perpendicular anisotropy.
Composite domain walls in flat nanomagnets: The magnetostatic limit
Authors: Youk H. , Chern G.-W. , Merit K. , Oppenheimer B. , Tchernyshyov O.
Published: June 2006

We discuss the structure of the so-called “vortex” domain walls in soft magnetic nanoparticles. A wall of this kind is a composite object consisting of three elementary topological defects: two edge defects with winding numbers −1∕2 and a vortex with a winding number +1 between them. We provide a qualitative model accounting for the energetics of such a domain wall.
Magnetization in insulating phases of Ti4+-doped SrFeO3−δ
Authors: Srinath S. , Mahesh Kumar M. , Sahner K. , Post M. L. , Wickles M. , Moos R. , Srikanth H.
Published: June 2006

SrFeO 3 − δ belongs to the Ruddlesden-Popper class of systems exhibiting interesting electronic and magnetic properties. Ti4+-doped, oxygen-deficient SrFeO3−δ(SrFexTi1−xO3−δ; x=0.9 and 0.7) samples show canted antiferromagnetic spin ordering, in contrast to the undoped sample (x=1) which is known to exhibit a metallic behavior with a helical spin arrangement. ac susceptibility χac (f,T) at 10⩽T⩽300K shows a frequency (100Hz⩽f⩽10kHz)-dependent temperature maximum, reminiscent of a spin-glass behavior. Increasing the Ti4+ content reduces the irreversibility, indicating a decrease in the frustration, which results in a diluted helical spin structure. Hysteresis loops can be associated with a gradual reorientation of spins in the field direction, indicating a canted type of spin arrangement.
Effect of Al on the magnetic properties of Pr3(Fe1−xAlx)27.5Ti1.5 (x=0.1, 0.2, and 0.3)
Authors: Venkatesh S. , Ravi  , Markandeyulu G. , Rama Rao K. V. S.
Published: June 2006

The effect of Al on the structural and magnetic properties of Pr3(Fe1−xAlx)27.5Ti1.5 (x=0.1, 0.2, and 0.3) compounds has been investigated by x-ray-diffraction, magnetization, and Curie temperature measurements. All these compounds have formed in monoclinic symmetry with A2∕m space group with Pr(Fe,Ti)12 as secondary phase and traces of α-Fe. The monotonic decreases in MS both at 300 and 80K are discussed on the basis of electron transfer from the 3p band of Al to the 3d band of Fe near EF. The variations in TC are explained using the Friedel approach. In all the compounds, the easy magnetization direction is away from the b axis.
Rate-dependent light-induced thermal hysteresis of [Fe(PM-BiA)2(NCS)2] spin transition complex
Authors: Enachescu Cristian , Tanasa Radu , Stancu Alexandru , Chastanet Guillaume , Létard Jean-François , Linares Jorge , Varret François
Published: June 2006

The light-induced bistability under permanent irradiation (LITH: light-induced thermal hysteresis), due to the competition between light irradiation and self-accelerated relaxation, shows crucially rate-dependent hysteresis. Major and minor hysteresis loops for [Fe(PM-BiA)2(NCS)2] spin crossover compound have been measured. Using appropriate master equations, distribution data from the first-order reversal curves diagrams of the purely thermal hysteresis, and parameter values previously derived from relaxation curves, we could simulate the LITH experimental data. The tunneling contribution to the relaxation rate revealed to be a key factor for the final agreement.
Magnetoresistance of nickel nanocontacts fabricated by different methods
Authors: Wei H.-X. , Wang T.-X. , Clifford E. , Langford R. M. , Han X.-F. , Coey J. M. D.
Published: June 2006

Nickel nanocontacts have been fabricated by focused ion-beam (FIB) milling of e-beam patterned planar contacts, FIB milling of conical-shaped nanoperforations in a silicon nitride membrane, and nanoimprinting using an atomic force microscope. Their sizes ranged from 1 to 30 nm. Magnetoresistance of up to 3% is developed in a field of a few millitesla. This is interpreted in terms of ballistic magnetoresistance across a wide domain wall whose structure is determined by dipolar interactions at the contact.
Small-angle neutron-scattering studies of reentrant spin-glass behavior in Fe–Al alloys
Authors: Martín Rodríguez D. , Plazaola F. , del Val J. J. , Garitaonandia J. S. , Cuello G. J. , Dewhurst C.
Published: June 2006

Fe–Al alloys around the concentration of 30 at. % Al show very interesting magnetic properties. Alloys with this concentration present magnetic inhomogeneities and their evolution with temperature is also interesting because they show a reentrant spin-glass behavior at low temperatures. Small-angle neutron-scattering measurements were performed in Fe69.5Al30.5,Fe70Al30, and Fe70.5Al29.5 alloys in the temperature range of 4–300 K. Even though the change in Fe content is very small, the response variation is quite large. The results show that the samples have magnetic inhomogeneities that evolve with temperature. Only in the Fe-richest sample the random-field model can be applied.
Spin-wave excitations in epitaxial ultrathin FeCo with zero magnetocrystalline anisotropy
Authors: Sperl M. , Kipferl W. , Dumm M. , Bayreuther G.
Published: June 2006

Spin-wave theory predicts reduced thermal spin-wave excitations due to a magnetic anisotropy. Recent results show that a strong uniaxial in-plane anisotropy in ultrathin Fe∕GaAs(001) films indeed stabilizes the ferromagnetic order versus thermal spin excitations [Kipferl et al., J. Appl. Phys. 97, 10B313 (2005)]. In order to study whether a fourth-order in-plane anisotropy has a similar effect epitaxial Fe70Co30∕Au(001) samples with zero magnetocrystalline anisotropy were studied and compared to results for Fe∕Au(001). The temperature dependence of the spontaneous magnetization MS for T<0.5Tc can be well described by the Bloch [Z. Phys. 61, 206 (1930)] T3∕2 law in the entire thickness range. However, for each thickness B is reduced in Fe70Co30∕Au(001) compared to Fe∕Au(001) films despite the reduced anisotropy. This means that the effect of a weak in-plane anisotropy is overcompensated by the enhanced exchange interaction.
Half-metallic ferromagnetism with high magnetic moment and high Curie temperature in Co2FeSi
Authors: Wurmehl Sabine , Fecher Gerhard H. , Ksenofontov Vadim , Casper Frederick , Stumm Ullrich , Felser Claudia , Lin Hong-Ji , Hwu Yeukuang
Published: June 2006

Co 2 Fe Si crystallizes in the ordered L21 structure as proven by x-ray diffraction and Möβbauer spectroscopy. The magnetic moment of Co2FeSi was measured to be about 6μB at 5 K. Magnetic circular dichroism spectra excited by soft x-rays were taken to determine the element-specific magnetic moments of Co and Fe. The Curie temperature was measured with different methods to be (1100±20)K. Co2FeSi was found to be the Heusler compound as well as the half-metallic ferromagnet with the highest magnetic moment and Curie temperature.
Magnetic nanowires for acoustic sensors (invited)
Authors: McGary Patrick D. , Tan Liwen , Zou Jia , Stadler Bethanie J. H. , Downey Patrick R. , Flatau Alison B.
Published: June 2006

Tiny hairlike sensors or cilia play a very important role in detection for many biological species, including humans. This research took inspiration from the packaging and transduction processes of the inner ear’s cochlea and cilia to design acoustic sensors. Specifically, this work uses nanowires of magnetostrictive materials as artificial cilia to sense acoustic signals. Anodic aluminum oxide (AAO) templates with hexagonal spacings were fabricated using a two-step anodization process as well as nanoimprint assisted self-assembly and were characterized using atomic force microscopy. Patterned microelectrodes were also fabricated at the backside of several templates using photolithography. Ni, Co, and Galfenol (Fe1−xGax0.1⩽x⩽0.25at.%) nanowires were fabricated using electrochemical deposition into nanoporous AAO templates where the pores had various geometries and some had large-area ordering as dictated by nanoimprinting. High aspect ratio nanowires with diameters varying from 10 to 200 nm and lengths up to 60μm were fabricated in arrays and were collectively and individually characterized using scanning electron microscopy. Galfenol thin films, fabricated electrochemically using a Hull cell, were characterized using x-ray diffraction and energy dispersive x-ray spectroscopy to determine the optimum current density for deposition. The magnetic response of millimeter-scale cantilevered beams to dynamic bending loads was also measured and compared to constitutive and free-energy models. A giant magnetoresistive sensor behind the beam measured the magnetic response of mechanical excitation applied to the tip of each rod and validated the models. Potenial applications of these nanowire cilia include sonar arrays, underwater cameras, and medical devices.
Exchange bias between magnetoelectric YMnO3 and ferromagnetic SrRuO3 epitaxial films
Authors: Martí X. , Sánchez F. , Fontcuberta J. , García-Cuenca M. V. , Ferrater C. , Varela M.
Published: June 2006

Orthorhombic YMnO3 (YMO) epitaxial thin films were deposited on SrTiO3 (STO) single-crystal substrates. We show that the out-of-plane texture of the YMO films can be tailored using STO substrates having (001), (110), or (111) orientations. We report on the magnetic properties of the YMO(010) films grown on STO(001) substrates. The dependence of the susceptibility on the temperature indicates that the films are antiferromagnetic below the Néel temperature (around 35 K). Orthorhombic YMO(010) films were also deposited on an epitaxial buffer layer of ferromagnetic and metallic SrRuO3 (SRO). The magnetic hysteresis loops of SRO show exchange bias at temperatures below the Néel temperature of YMO. These results confirm that the YMO films are antiferromagnetic and demonstrate that magnetoelectric YMO can be integrated in functional epitaxial architectures.
Magneto-optic vector magnetometry of CoFeCrSiB amorphous ribbons
Authors: Životský O. , Postava K. , Foldyna M. , Pištora J. , Kraus L.
Published: June 2006

A magneto-optic method suitable for investigation of soft magnetic ribbons is proposed that combines the internal magnetic field induced by a current flowing through the ribbon axis with the external magnetic field generated by an air coil. The method eliminates effects of demagnetizing factor and shape anisotropy on measured hysteresis loops. Both in-plane magnetization components are measured by means of magneto-optic vector magnetometry. The method is applied to study magnetic properties of CoFeCrSiB amorphous ribbons showing asymmetric giant magnetoimpedance effects. A coherent rotation of magnetization and in-plane easy axis inclined from the ribbon axis is observed. It corresponds to helical magnetic anisotropy originating from the preparation processes.
Electrical and magnetotransport properties of Nd-based manganite nanoparticles
Authors: Krishnamoorthy C. , Sethupathi K. , Sankaranarayanan V. , Nirmala R. , Malik S. K.
Published: June 2006

Electrical and magnetotransport properties of nanocrystalline Nd0.7Sr0.3MnO3 sample having an average particle size of 45 nm have been studied. The resistivity in paramagnetic regime follows Mott’s variable range hopping mechanism with an average hopping distance of about 21 Å. The observed magnetoresistivity (MR) has best been described by assuming that canted spins and defects are distributed all over the volume of the nanoparticle. The MR could be quantitatively best fitted to spin-dependent hopping model, together with phase-separation phenomenon. In this model, hopping barrier is proportional to the angle between the magnetic moments of the clusters. The hopping barrier height is minimum when the moments are parallel to each other and is maximum when the moments are randomly oriented. The fit yields a small cluster size of about two to three lattice constant dimensions in the paramagnetic (PM) phase and of about four to five lattice constants in the ferromagnetic (FM) phase. The results indicate that FM phase contributes to MR at low fields, whereas PM phase contributes at relatively high fields.
Magnetization-induced second-harmonic generation in epitaxial magnetite thin films Fe3O4∕MgO(100)
Authors: Rzhevsky A. A. , Krichevtsov B. B. , Rata A. D. , Chang C. F. , Sutarto R. , Tjeng L. H. , Schneider C. M.
Published: June 2006

The surface magnetic and structural properties of magnetite thin films have been probed by nonlinear second-harmonic generation optical method in high- (centrosymmetric, Oh) and low- (noncentrosymmetric, C1) temperature phases. A model taking into account the cubic magnetocrystalline anisotropy and magnetic symmetry reduction due to a magnetically modified surface layer is supposed to describe the azimuth variations of the nonlinear response. The metal-isolator transition (Verwey) manifests itself in an increase of nonlinear response (∼25%) and a decrease in magnetic contrast (∼50%).
Slater-Pauling rule and Curie temperature of Co2-based Heusler compounds
Authors: Fecher Gerhard H. , Kandpal Hem C. , Wurmehl Sabine , Felser Claudia , Schönhense Gerd
Published: June 2006

A concept is presented serving to guide in the search for materials with high spin polarization. It is shown that the magnetic moment of half-metallic ferromagnets can be calculated from the generalized Slater-Pauling rule. Furthermore, it was found empirically that the Curie temperature of Co2-based Heusler compounds can be estimated from a seemingly linear dependence on the magnetic moment. As a successful application of these simple rules, it was found that Co2FeSi is, actually, the half-metallic ferromagnet exhibiting the highest magnetic moment and the highest Curie temperature measured for a Heusler compound.
Comparison of time-resolved x-ray magnetic circular dichroism measurements in reflection and transmission for layer-specific precessional dynamics measurements
Authors: Guan Y. , Bailey W. E. , Kao C.-C. , Vescovo E. , Arena D. A.
Published: June 2006

We present experimental techniques to measure magnetization precession of individual layers in a “spin-valve” trilayer. Precessional motions of individual Ni81Fe19 and Co93Zr7 layers have been separated in Ni81Fe19∕Cu∕Co93Zr7 using ±45ps time-resolved x-ray magnetic circular dichroism (tr-XMCD) at Fe and Co edges. We compare the efficacy of two experimental configurations in this paper. Pulsed-field tr-XMCD measurements in reflectivity are compared with resonant-field tr-XMCD measurements in transmission. Despite the order of magnitude larger angles of precession excited in pulsed-field reflectivity measurements, data quality is found to be superior in resonant-field transmission measurements. Relative roles of sample preparation and timing jitter in the different techniques are discussed.
Analysis of magnetization processes in composite media grains
Authors: Richter H. J. , Dobin A. Yu.
Published: June 2006

Composite grains, consisting of a subgrain with high anisotropy field coupled with a subgrain with zero anisotropy, are analyzed using a two-spin model. An analytical expression is given for the exchange coupling between the subgrains that minimizes the field required to reverse the magnetically hard layer. The results of the two-spin model are compared with those of a spin chain that represents an “exchange spring magnet.” The limitations of the two-spin model are worked out.
Analysis of the performance of permanent magnetic stepping motor with trapezoid stator tooth
Authors: Liu C. P. , Li Y. C. , Liu K. H. , Wu K. T. , Yao Y. D.
Published: June 2006

With the rapid development of automation techniques, the claw-poled stepping motors are widely used, especially in optical scanners. Most research on stepping motors has concentrated on electrical properties, but a few papers have reported on the performance in relation to the magnetic-field distribution. The purpose of this study is to analyze the effect of the shape of the stator tooth and to search for the optimal parameters using a magnetic-circuit simulation technique. Through numerical calculations, the optimal performance conditions can be obtained by varying the ratio between the two bases of the trapezoid teeth between 1∕5 and 1∕3, and varying the ratio between the heights of individual teeth in relation to the separation between the two yokes from 11∕15 to 13∕15.
Molecular-beam epitaxy growth and magnetic properties of BeTe with Cr doping
Authors: Viloane Ko , Sreenivasan M. G. , Teo K. L. , Liew T. , Chong T. C.
Published: June 2006

We report the growth and magnetic properties of BeTe with Cr doping grown on GaAs(001) substrate at Te-rich condition by solid-source molecular-beam epitaxy. Ferromagnetisms were observed for films with high Cr concentration, x. The Curie temperatures TC for x=0.13 and x=0.17 are 175±5K, while the TC for x=0.27 is 250±5K. Structural properties were studied by in situ reflection high-energy electron diffraction, atomic force microscopy, and x-ray diffraction. Be1−xMnxTe and Cr1−δTe films were also grown and compared with the Be(Cr)Te films. We show that the ferromagnetic ordering of Be(Cr)Te films can be attributed to precipitations of NiAs Cr1−δTe.
Influence of eddy currents on the effective damping parameter
Authors: Hrkac G. , Kirschner M. , Dorfbauer F. , Suess D. , Ertl O. , Fidler J. , Schrefl T.
Published: June 2006

A method for the calculation of the effective damping parameter as a function of particle size and electric conductivity is presented. An extended three-dimensional finite element/boundary element micromagnetic model is used to solve the Landau-Lifshitz-Gilbert equation together with the eddy current diffusion equation. From the change of energy with time an effective damping parameter is derived from which the net eddy current contribution is calculated. It is shown that the eddy current effects are reduced with an increasing applied field and are increased for materials with a low resistivity. Furthermore it is shown that the size of the particles, which designates the coherence of the spin structure, is essential for the eddy current net contribution.
Disorder-induced carrier localization in ultrathin strained SrRuO3 epitaxial films
Authors: Chopdekar Rajesh V. , Takamura Yayoi , Suzuki Yuri
Published: June 2006

In this study we explore the influence of disorder on the properties of epitaxial SrRuO3 thin films. Disorder includes interface effects from misfit strain and bulk effects from cation substitutions and interstitial defects. Isostructural perovskites, including LaAlO3, SrTiO3, DyScO3, GdScO3, and BaTiO3, were used as growth platforms to place films under either biaxial tensile or compressive strain. In addition, films on (001)SrTiO3 were codeposited with FeOx, CoO, SrTiO3, and SrFeOx of concentrations up to 10% to compare the disorder distributed throughout the film with the disorder confined to surfaces and interfaces. Pure SrRuO3 films exhibited metallic behavior with low residual resistivity (<0.1mΩcm), and underwent a ferromagnetic transition at a suppressed Curie temperature (TC) of 145K. The addition of impurity oxides raised the film’s residual resistivity and affected the low-temperature magnetotransport. Interface effects did not seem to play a dominant role in carrier localization, but a few percent of oxide impurities increased the low-temperature resistivity by an order of magnitude. Higher concentrations pushed the resistivity minimum to temperatures comparable to TC.
Interaction effects within Langmuir layers and three-dimensional arrays of ε-Co nanoparticles
Authors: Sachan M. , Walrath N. D. , Majetich S. A. , Krycka K. , Kao Chi-Chang
Published: June 2006

Magnetostatic interactions are studied in two types of nanoparticle assemblies. Highly ordered monolayers of ε-Co nanoparticles are prepared from Langmuir films and self-assembled three-dimensional (3D) arrays formed by slow precipitation of colloidal dispersions of same nanoparticles. The structure of assemblies is found from electron microscopy and small-angle x-ray scattering. The differences in magnetic behavior are interpreted in terms of the particle moment, interparticle separation, and coordination number. Preferential in-plane magnetization is observed due to interactions within a monolayer. 3D arrays show stronger interactions compared to monolayers due to an increased number of nearest neighbors.
Size-dependent magnetic properties of ordered Li0.5Fe2.5O4 prepared by the sol-gel method
Authors: Kang Kun Uk , Hyun Seong Wook , Kim Chul Sung
Published: June 2006

Size-dependent magnetic properties of ordered Li ferrites prepared by the sol-gel method have been studied by x-ray diffraction, vibrating-sample magnetometer, and Mössbauer spectroscopy. X-ray-diffraction patterns were analyzed using Rietveld refinement. Samples annealed below 500°C have the cubic spinel structure with a small amount of α-Fe2O3 and γ-Fe2O3. As the annealing temperature increases over 500°C, hematite and maghemite phases disappear and the single-phase spinel structure (space-group P4332) with ordered cation distribution on the octahedral site is seen. Particle sizes determined by x-ray analysis range from 37nm to 87nm, respectively, as the annealing temperature increases from 450°C to 700°C. The saturation magnetization is ∼46emu∕g for the sample annealed below 500°C and increases up to 63.3emu∕g for the sample annealed at 700°C, while the coercivity of the samples has a maximum value of 152.5Oe at 500°C. Mössbauer spectral analysis revealed that there was 29% γ-Fe2O3 in the sample annealed at 450°C. The ordered cation distribution by the local symmetry reduction of sublattice gives a reasonable interpretation about the magnetic properties of nanosize particles.
Electron-spin-resonance study of gadolinium borosilicide: A rare-earth ladder compound
Authors: Mori Takao
Published: June 2006

Electron-spin-resonance (ESR) measurements were utilized as a microscopic probe to gain more information about the magnetic transition in the REB50 compounds. A gadolinium phase (Gd3+=S87∕2) was synthesized and used for this measurement. GdB50 will not form due to the large size of gadolinium, but we have found that with the addition of a small amount of Si, the lattice constants expand and an isotypic borosilicide is formed which can accommodate the gadolinium phase. This phase has a ladderlike arrangement of the gadolinium atoms and although a broad peak in the specific heat has been observed previously at T∼4K, the transition could not be discerned in the susceptibility due to an inevitable low-temperature tail. However, the ESR absorption intensity was found to exhibit a sharp drop at TN=7K, clearly indicating the magnetic transition. The g factor also exhibited a temperature dependence, decreasing rapidly below 30K, indicating the growth of short-range correlation. The temperature dependence of the ESR linewidth was significantly different from that of a three-dimensional antiferromagnetic system, being indicative of the exchange narrowing of a one-dimensional classical spin system. These results indicate that the Gd ions magnetically form pairs along the bond-alternating ladder which is the direction of the second-nearest metal-metal spacing.
A fast Fourier transform on multipole algorithm for micromagnetic modeling of perpendicular recording media
Authors: Liu Z. J. , Long H. H. , Ong E. T. , Li E. P.
Published: June 2006

In this paper, a fast algorithm for the rapid calculation of the demagnetization field in micromagnetic modeling is presented. The method is applied to finite element micromagnetic modeling to study the switching phenomenon of perpendicular recording media. The method is shown to provide high efficiency in the numerical simulation with reduced memory requirement. With significant improvement in the computing speed, the practical application of recording physics based simulations can be extended to the investigation of perpendicular magnetic recording processes in various aspects, for instance, a large number of micromagnetic simulations can be carried out to build microtrack model and to predict the performance of perpendicular head and media combinations.
Magnetic properties of hexanuclear manganese antiferromagnetic clusters {Mn6}
Authors: Han K. H. , Kim B. J. , Suh B. J. , Belesi M. , Moon D. , Lee K. , Lah M. S.
Published: June 2006

We report a study of magnetic properties of a series of hexanuclear manganese metallamacrocycles {Mn6}. The magnetic properties are characterized by antiferromagnetic (AFM) coupling constants J1∕kB≈−4.0K, between the nearest neighbors, and J2∕kB≈−1.2K, between the next-nearest neighbors. The temperature dependence of H1 NMR linewidth is observed to be consistent with the AFM singlet ground state with a total spin ST=0. H1 nuclear spin-lattice relaxation rate 1∕T1 shows a strong enhancement at low temperatures with a peak around T≈J1, as commonly observed in AFM molecular rings and clusters.
Effect of grain size on the soft magnetic properties of FeCoV∕CoNbZr multilayers
Authors: Contreras M. C. , Calleja J. F. , Matarranz R. , Presa B. , Corrales J. A. , Pan G.
Published: June 2006

Experimental characterization of macroscopic effective anisotropy field and coercive field for nanocrystalline (FeCoV∕CoNbZr)16 multilayers was carried out with transverse susceptibility. The multilayer structure consisting of a FeCoV ferromagnetic nanogranular layer covered with a CoNbZr amorphous layer was prepared by rf sputtering. All multilayers had the same thickness of CoNbZr (3 nm) with FeCoV layer thickness varying between 6 and 30 nm. The properties of the optimized multilayer structured films are related to an appropriate grain size of FeCoV where the CoNbZr layer acts as a very effective grain refiner and reduces the average grain size. The experimental values of the coercive force and the macroscopic uniaxial anisotropy were related to the grain size. The values were analyzed according to a two dimensional random anisotropy model which takes into account the uniaxial anisotropy.
Micromagnetic simulation of spin transfer torque switching by nanosecond current pulses
Authors: Apalkov Dmytro , Pakala Mahendra , Huai Yiming
Published: June 2006

Magnetization reversal by spin transfer torque due to current flowing in magnetic tunnel junction nanopillar with 200nm×125nm elliptical cross section is studied by micromagnetic simulation assuming the Slonczewski model for spin transfer torque. The magnetization dynamics during the switching is found to be highly inhomogeneous with formation of complex domain structure and spin wave generation even for artificially increased exchange length. The Oersted field influence on onset of precession leading to switching is described in terms of spin wave generation. The switching time as a function of current is found to have nonmonotonic behavior, which is not completely understood but is found to be related to the presence of Oersted field.
Containment can losses in permanent-magnet pump drives
Authors: Smith A. C. , Peralta-Sanchez E.
Published: June 2006

The integrated “pump-motor-containment” concept is a development in the chemical process industry. Both the stator and rotor members are isolated using stainless-steel containment cans. During normal operation, however, losses are generated in the stator can due to the stator excitation and the permanent magnets mounted on the rotor. A layer model has been developed to analyze the pseudothree-dimensional current flow in the stator can and hence predict the can losses. This represents an application of this particular analytical approach. The layer model is validated using a 10 kW experimental pump motor and a subsequent design study has demonstrated that the stator can losses are a significant factor in ensuring the integrity of the containment system. It has also been shown that the thickness and overhang length of the stator containment should be minimized to reduce the losses subject to mechanical constraints.
Experimental and computational analysis of the angular dependence of the hysteresis processes in an antidots array
Authors: Pigazo F. , García Sánchez F. , Palomares F. J. , González J. M. , Chubykalo-Fesenko O. , Cebollada F. , Torres J. M. , Bartolomé J. , García Vinuesa L. M.
Published: June 2006

We have experimentally characterized the magnetization processes of a square array of micron sized circular antidots lithographed on a Fe(001)/GaAs film with its diagonals along the Fe magnetocrystalline easy axes (100). Both the anisotropy and the angular dependence of the magnetization reversal were measured by means of magnetooptic techniques. The coercivity of the loops along the easy and in-plane hard axes of the array increases approximately 2.5 times with respect to that measured in the continuous film region, and the first order anisotropy constant remains equal to that of bulk Fe. The magnetization reversal takes place in two steps for all the loops measured out of the easy and hard axes. We have simulated the magnetization reversal using two different micromagnetic models. In the first one, assuming that the reversal takes place fully inside the array, we have observed that the reversal nucleates at the magnetic inhomogeneities occurring at the antidot boundaries and resulting from magnetostatic energy minimization. In our second model we artificially introduced a domain wall outside the antidot region that governs the magnetization reversal showing a qualitative agreement with the angular dependence of coercivity.
Effects of an Os layer on the magnetic properties of CoFe∕IrMn
Authors: Peng Tai-Yen , Lo C. K. , Chen San-Yuan , Yao Y. D.
Published: June 2006

The uses of Os as an antidiffusion and buffer layer in IrMn exchange coupled CoFe film were investigated. For the purpose of antidiffusion, the inserted Os layer showed a distinct improvement of S>0.9, with HC slightly increasing by 1.6 times for the CoFe∕Os∕MnOs multilayer after 400 °C annealing, even though the Os thickness was as thin as 0.3 nm. Furthermore, with a 0.3 nm Os barrier, the 350 °C annealed CoFe∕Os∕IrMn∕CoFe showed almost the same magnetic behavior as the as-deposited state, while the Hex of the upper part of the CoFe∕Os∕IrMn changed from 100 to 190 Oe. In addition, as a buffer layer, the Os buffer layer could enhance the diffraction peak intensities of IrMn(111)∕Os(002) and CoFe (111), and the Hex of CoFe∕IrMn was proportional to the Os thickness. A 120 Oe of Hex was achieved by using an 11 nm Os buffer layer in a CoFe10nm∕IrMn15nm bottom type film. These results show that Os has the potential to be an antidiffusion and buffer layer in a magnetic multilayer.
Effect of ferromagnetic/antiferromagnetic interfaces on the magnetic properties of La2∕3Sr1∕3MnO3∕Pr2∕3Ca1∕3MnO3 superlattices
Authors: Niebieskikwiat D. , Hueso L. E. , Salamon M. B. , Mathur N. D.
Published: June 2006

The magnetic properties of a series of ferromagnetic/antiferromagnetic (FM/AFM) superlattices are studied. The multilayers are composed of five La2∕3Sr1∕3MnO3∕Pr2∕3Ca1∕3MnO3 (LSMO/PCMO) bilayers, grown by pulsed laser deposition using SrTiO3 as the substrate. The evolution of the magnetic moment with the AFM PCMO layer thickness (0⩽tA⩽7.6nm) is analyzed and is found to be strongly affected by the strain fields introduced by the FM/AFM interfaces. The effect on the LSMO is the appearance of a magnetic dead layer of ∼1.5nm, close to the interface with the PCMO. On the contrary, the magnetic moment of PCMO is remarkably enhanced for thicknesses tA around 3.5nm, where nanometric FM droplets would find the optimal strain conditions to be accommodated within the AFM PCMO spacer. The implications of these findings on the magnetoresistance response are discussed.
The effect of particle size distribution on the usage of the ac susceptibility in biosensors
Authors: Nutting Joseph , Antony Jiji , Meyer Daniel , Sharma Amit , Qiang You
Published: June 2006

Magnetic nanoparticles in a liquid have two relaxation times, Néel relaxation τN and Brownian relaxation τB. For particle size larger than 25nm, τN quickly becomes much larger than τB and can be ignored. τB has a relaxation period from 10−1to10−5s, and related to the particle’s hydrodynamic volume, which includes coatings and biomolecules attached to the magnetic nanoparticle cores. This causes the imaginary part of the ac magnetic susceptibility to display a maximum at a frequency f=1∕2πτB, and can be used to create a sensor capable of detecting biomolecules. Because this is based on particle size, a size distribution will broaden the curve and reduce the sensitivity. Although the magnetic nanoparticles may have a narrow size distribution, this may not be true once coatings have been added and biomolecules have bonded to the magnetic cores. Our group has examined the effects of normal and lognormal size distributions on the ac magnetic susceptibility using several theoretical measurements, and we have found that the effect of size distributions on the ability to use τB and the ac magnetic susceptibility as the basis of a biosensor is not significant.
A quantitative model for the nonlinear response of fluxgate magnetometers
Authors: Geiler A. L. , Harris V. G. , Vittoria C. , Sun N. X.
Published: June 2006

An analytical nonlinear hysteresis model was developed to describe the hysteresis loops of different magnetic materials, which was capable of fitting Permalloy films as an example. As a case study this model was applied in the sensitivity analysis of the fluxgate magnetometers where accurately capturing the nonlinear behavior of the magnetic core is crucial. Straightforward analytical equation of the output voltage signal as well as its harmonic decomposition was obtained for the fluxgate magnetometer as a function of hysteresis loop parameters. Sensitivity values predicted by the model were found to be consistent with the measured values published previously. The model allows the harmonic content distribution in the output signal of the fluxgate magnetometer to be studied analytically as a function of various device parameters, and thereby very useful in the selection of core materials in fluxgate devices.
Behavior of three-phase induction motors with variable stator coil winding pitch
Authors: Deshmukh R. , Moses A. J. , Anayi F.
Published: June 2006

A pulse-width modulation (PWM) inverter and sinusoidal output voltage supplies feeding four different chorded three-phase induction motors were tested for low-order odd voltage harmonic components and efficiency at different loads. The total harmonic distortion due to the third, fifth, and ninth harmonics was lowest in a motor with 160° coil pitch energized by both sinusoidal and PWM voltages. The efficiencies of the motor with the short-chorded winding were as much as 5% and 16% higher than that of the full-pitched motor under sinusoidal and PWM excitation, respectively, due to harmonic cancellation.
Pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As magnetic tunnel junction
Authors: Chiba D. , Kita T. , Matsukura F. , Ohno H.
Published: June 2006

We have investigated the pulse-width and magnetic-field dependences of current-induced magnetization switching in a (Ga,Mn)As∕GaAs∕(Ga,Mn)As magnetic tunnel junction. Critical current to induce magnetization switching shows a linear dependence on pulse width from 10to1000μs. The magnetic-field dependence appears to indicate that the observed current-induced magnetization switching proceeds through metastable magnetization structures.