bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic structure of vortex and antivortex states in patterned Co elements studied by using scanning ion microscopy with polarization analysis
Authors: Li Jian , Rau Carl
Published: June 2006

Scanning ion microscopy with polarization analysis (SIMPA) is used to investigate the surface magnetic structure of patterned Co elements created in situ by focused ion-beam lithography from thin (30nm) Co films deposited on Si(100) substrates by electron-beam evaporation. The diameter d of the circular-shaped Co elements is varied between 5 and 38μm. Three-dimensional, spin- and spatially resolved SIMPA spin maps directly reveal the nonuniform micromagnetic structure of magnetic vortex and antivortex states. They are dominated by a circular or hyperbolic surface magnetization profile with a wide vortex or antivortex core in the center with a perpendicular surface magnetization component which decreases with increasing distance from the core. Varying d, the vortex wall thickness can be changed from 0.6 to 4.2μm. For Co elements with d<13μm, only single magnetic vortex states are found, whereas for d>13–15μm, magnetic multivortex and antivortex states are observed.
TbFeCo perpendicular magnetic recording media deposited on nanohole arrays of porous alumina layer
Authors: Tofizur Rahman M. , Liu Xiaoxi , Morisako Akimitsu
Published: June 2006

A nonlithographic fabrication method of magnetic nanodot array by using porous anodized alumina formed on a glass substrate is studied. We carried out anodic oxidation of a sputtered Al film at the anodic voltage in the range of 10–30 V, and found that the density of the nanohole arrays increased with the decrease in anodization voltage. On the other hand, hole diameter decreased with the decrease in anodic voltage. Then TbFeCo is deposited onto this porous array by sputtering with a thickness of around 20 nm and subsequently overcoated with 5 nm tungsten (W) for the protection from surface oxidation. The TbFeCo deposited on this porous layer shows complete perpendicular anisotropy. The coercivity increased with the decrease in anodization voltage as well as hole diameter. The coercivity of the TbFeCo deposited on the porous array with a mean hole diameter of around 15 nm is 4.3 kOe. The squareness ratio is also improved with the reduction of the hole diameter. From the angular dependence of coercivity, Hc, it is found that the Hc decreases gradually with the decrease of applied field angle from the perpendicular direction (easy axis) to in-plane direction (hard axis). This indicates that the magnetization reversal in the TbFeCo nanodot array occurs by Stoner-Wohlfarth model.
Design of permanent magnets to chaoize doubly salient permanent magnet motors for electric compaction
Authors: Chau K. T. , Wang Zheng
Published: June 2006

This article proposes and implements a class of chaotic motors for electric compaction. The key is to develop a design approach for the permanent magnets (PMs) of doubly salient PM (DSPM) motors in such a way that chaotic motion can be naturally produced. The bifurcation diagram is employed to derive the threshold of chaoization in terms of PM flux, while the corresponding phase-plane trajectories are used to characterize the chaotic motion. A practical three-phase 12∕8-pole DSPM motor is used for exemplification. The proposed chaotic motor is critically assessed for application to a vibratory soil compactor, which is proven to offer better compaction performance than its counterparts. Both computer simulation and experimental results are given to illustrate the proposed chaotic motor.
Complex oxide-based magnetic tunnel junctions with nonmagnetic insulating barrier layers
Authors: Alldredge L. M. B. , Chopdekar R. V. , Nelson-Cheeseman B. B. , Suzuki Y.
Published: June 2006

Magnetic tunnel junctions composed of the colossal magnetoresistance material La0.7Sr0.3MnO3 (LSMO) and magnetite (Fe3O4) have been fabricated and characterized. The films were grown on (110) SrTiO3 substrates using pulsed laser deposition, with the nonmagnetic spinel Mg2TiO4 (MTO) chosen as the barrier layer in order to provide a well lattice-matched isostructural interface with the Fe3O4 electrode. Junction magnetoresistance (JMR) values of up to −26% were observed for junctions with 50-Å-thick MTO barriers at 70K. Measurements were performed on junctions with barrier thicknesses ranging from 50–120Å. The resistance jumps in JMR correspond well to the switching fields of the LSMO and Fe3O4 magnetic electrodes. The magnitude of the JMR peaks and then begins to decline with decreasing temperature, indicating that prior observations of such temperature-dependent JMR behavior in CoCr2O4 barrier junctions were due to the electrode at low temperatures rather than to magnetic moments in the CoCr2O4.
Thickness dependence of magnetic anisotropy in uncovered and Cu-covered Fe∕GaAs(110) ultrathin films studied by in situ Brillouin light scattering
Authors: Madami M. , Tacchi S. , Carlotti G. , Gubbiotti G. , Socino G.
Published: June 2006

The thickness dependence of magnetic anisotropy in Fe∕GaAs(110) ultrathin films has been investigated by in situ Brillouin light scattering. A simultaneous best-fit analysis of the spin-wave frequency, measured as a function of (i) the intensity of the applied magnetic field, (ii) its direction in the film plane, and (iii) the incidence angle of light, enabled us to determine several magnetic parameters, including the film thickness, and both the in-plane and out-of-plane magnetic anisotropy constants. A marked evolution of the strength as well as of the symmetry of the in-plane magnetic anisotropy has been observed for increasing thickness. The effect of a nonmagnetic Cu capping layer on the magnetic properties of the Fe films has also been studied.
Effects of spin current on ferromagnets (invited)
Authors: Li Z. , He J. , Zhang S.
Published: June 2006

When a spin-polarized current flows through a ferromagnet, the local magnetization receives a spin torque. Two consequences of this spin torque are studied. First, the uniformly magnetized ferromagnet becomes unstable if a sufficiently large current is applied. The characteristics of the instability include spin-wave generation and magnetization chaos. Second, the spin torque has profound effects on the structure and dynamics of the magnetic domain wall. A detailed analysis on the domain-wall mass, kinetic energy, and wall depinning threshold is given.
Mechanical property measurements of thin-film carbon overcoat on recording media towards 1Tbit∕in2
Authors: Lee Ki Myung , Yeo Chang-Dong , Polycarpou Andreas A.
Published: June 2006

In the pursuit of extremely high areal density recording of 1Tbit∕in2, very thin protective carbon overcoats are needed, and reliable measurement of their mechanical properties is critical. Measuring mechanical properties of thin films has been challenging especially as their thicknesses have shrunk to only few nanometer thick, and one is faced with both instrument and technique limitations in extracting reliable information about such films. Two common techniques utilized for the characterization of submicron thin films are the nanoindentation and nanoscratch techniques. In this paper, shallow sub-10-nm nanoindentation and nanoscratch experiments were performed to extract the mechanical properties of reduced elastic modulus, hardness, and shear strength of a 10-nm-thick magnetic medium carbon overcoat.
Thermal and magnetic properties of La-doped Ce1−yLayGe1.80 with <y<1.0
Authors: Lin C. L. , Yuen Tan , Zan J. A. , Li Jing
Published: June 2006

CeGe x with 1.71<x⩽2.0 crystallizes in an orthorhombic structure and exhibits two magnetic transitions: an antiferromagnetic transition at TN=7K and a ferromagnetic transition at TC=4.3K. When x decreases below 1.71, CeGex transforms to a tetragonal structure and exhibits only an antiferromagnetic transition at TN=7K. We have further studied the physical properties of La-doped Ce1−yLayGe1.80 with 0<y<1.0. The magnetization and specific-heat measurements show that with initial La substitution for Ce, TN decreases monotonically while TC increases slightly, and TN and TC merge to become a single ferromagnetic state at TC=4.8K in the vicinity of y=0.22. With further La doping, TC decreases slowly and linearly from 4.8 to 1.75 K at y=0.65. The x-ray measurements show that the lattice parameters remain nearly unchanged in the two magnetic transition regimes with 0<y<0.22 and increase monotonically with further La substitution for Ce (0.22<y⩽1.0). The entropy removal and magnetic moment calculations are also discussed.
Structural dependence of the tunnel magnetoresistance for magnetic tunnel junctions with a full-Heusler Co2Fe(Al,Si) electrode
Authors: Tezuka N. , Okamura S. , Miyazaki A. , Kikuchi M. , Inomata K.
Published: June 2006

We have investigated the crystal structure and magnetic moment of the Co2FeAl and Co2FeSi films deposited onto thermally oxidized Si and MgO(100) single-crystal substrates, and the structural effect on the tunnel magnetoresistance (TMR) of the magnetic tunnel junctions (MTJs) using a Co2FeZ (Z=Al or Si). The structure was changed by the substrate and postannealing temperatures, in which the fully epitaxial and polycrystalline Co2FeAl and Co2FeSi films were obtained with the different disorder structure. The magnetic moment of Co2FeAl films was found to be uninfluenced by the crystal structure. Spin-valve-type MTJs consisting of Co2FeZ(100nm)∕Al(1.2nm)-oxide∕Co75Fe25(2nm)∕IrMn(10nm)∕Ta(60nm) were fabricated (Z=Al or Si) on thermally oxidized Si and MgO(100) single-crystal substrates. The maximum TMR obtained is about 50% at room temperature for MTJs with Co2FeAl films, regardless of the crystal structure of Co2FeAl.
Spin-polarized current-induced magnetization precession and switching for perpendicular anisotropic ferrimagnetic thin films
Authors: Canizo-Cabrera A. , Garcia-Vazquez Valentin , Wu Te-ho
Published: June 2006

We present here a theoretical calculation on magnetization precession and switching induced by spin-polarized current spin-transfer torque using perpendicular anisotropic magnetic multilayered thin films which contain two strongly coupled antiparallel magnetic subnetworks. We develop a modified Landau-Lifshitz-Gilbert equation for this material structure to obtain the final equation governing the dynamics of the net magnetization. We present results of the time evolution of the net magnetization under the influence of a spin current as well as the estimation of the critical current for practical application in magnetic random access memory writing. A relation between magnetization switching time and current is also given.
A method for anhysteretic magnetization and magnetostriction measurement of thin ferromagnetic films as a function of applied isotropic stresses
Authors: Finkel P. , Garrity E. , Lofland S. E.
Published: June 2006

A method for measuring stress-dependent anhysteretic magnetization and magnetostriction in thin-film anisotropic ferromagnetic thin films or wires is described. This method uses a conventional vibrating-sample magnetometer combined with a specially designed loading fixture providing controlled uniaxial stresses on thin films and wires. Stresses are deduced from the characteristic resonant frequency of the sample in the fixture and verified via pulse propagation velocity measurement. Both indirect stress measurements are contactless, relying on remote vibration measurement using a laser Doppler vibrometer. These measurements have been demonstrated for membrane thicknesses down to 50μm and stresses up to 1 GPa, with estimated accuracy better than 0.2%. This method was applied to measure anhysteretic permeability and the magnetization curve of Ni–Fe alloys as functions of stress and temperature. This technique was also shown to be able to measure magnetostriction coefficient as a function of external magnetic field.
Near-field imaging of high-frequency magnetic fields with calorimetric cantilever probes
Authors: Lee S. , Lee Y. C. , Wallis T. M. , Moreland J. , Kabos P.
Published: June 2006

Calorimetric probes for near-field imaging of high-frequency (1–20GHz) magnetic fields were fabricated by depositing patterned metal structures on micromachined, dielectric multilayer cantilevers. In the presence of high-frequency magnetic fields, the metal structures are heated via the generation of eddy currents or via ferromagnetic resonance (FMR). Measurement of the resulting cantilever deflection as a function of probe position produces a map of the microwave power distribution. Comparative measurements from probes with 5 and 10μm Au rings show that the rings are the active area for eddy current generation. Probes with 10μm square permalloy patches function in both the eddy current and FMR imaging modes.
Influence of aluminum substitution on the magnetic couplings in R-T alloys Ho6Fe23−xAlx
Authors: Ostoréro J. , Guillot M.
Published: June 2006

Magnetic properties of Ho6Fe23−xAlx compounds (x=0, 2.5, and 5) are reported in the temperature range of 5–550K under a magnetic field of up to 90kOe. A large decrease of the Curie temperature is obtained (−20K per Al atom) as in Y6Fe23−xAlx. The compensation temperature varies from 185K (x=0) to 280K (x=5); it is associated with a strong increase of low-temperature spontaneous magnetization (+2.3μB per Al atom). In the vicinity of Tcomp field-induced transitions are observed for x=0 and x=2.5. The different results are analyzed within the frame of the Néel ferrimagnetic model: the Ho–Fe and Fe–Fe magnetic interactions and the canting angle are then determined. The presence of the rare-earth Ho “stabilizes” the ferrimagnetism.
Real-time evidence of two-magnon scattering in exchange-coupled bilayers
Authors: Weber M. C. , Nembach H. , Hillebrands B. , Carey M. J. , Fassbender J.
Published: June 2006

Coherent spin waves in exchange biased bilayers have been excited by ultrafast photomodulation of the exchange bias anisotropy. Photoinduced precession in CoFe∕IrMn samples with different IrMn thicknesses, thus, different exchange bias fields, has been studied in real time by time-resolved Kerr effect magnetometry. The extracted effective damping parameter is proportional to the square of the exchange bias field. Two-magnon scattering of the coherent precession of the ferromagnetic layer at local interfacial fluctuations of the exchange bias field can account for the observed increase. Hence, there is a time-domain evidence of two-magnon damping involved in the relaxation of photoexcited spins.
Topological defects in flat nanomagnets: The magnetostatic limit
Authors: Chern G.-W. , Youk H. , Tchernyshyov O.
Published: June 2006

We discuss elementary topological defects in soft magnetic nanoparticles in the thin-film geometry. In the limit dominated by magnetostatic forces the low-energy defects are vortices (winding number n=+1), cross ties (n=−1), and edge defects with n=−1∕2. We obtain topological constraints on the possible composition of domain walls. The simplest domain wall in this regime is composed of two −1∕2 edge defects and a vortex, in accordance with observations and numerics.
Transport properties of ferromagnetic GaMnAs interfaced with paramagnetic ZnMnSe in the form of bilayer structures
Authors: Choi I. S. , An S. Y. , Chung S. J. , Lee S. , Liu X. , Furdyna J.
Published: June 2006

We have investigated the magnetotransport properties of a series of magnetic semiconductor bilayer structures consisting of 300-nm-thick Ga1−xMnxAs and 150-nm-thick Zn1−yMnySe layers. The temperature scan of resistance and an Arrot plot obtained from Hall resistance data showed that the Curie temperature (TC) of the GaMnAs∕ZnMnSe bilayer is higher than that of the bare GaMnAs. Furthermore, magnetoresistance measurement reveals significant enhancement of the magnetic hardness of the GaMnAs system in bilayer form even though the magnetic easy axis of the GaMnAs was not altered by the neighboring ZnMnSe layer. This observation indicates that the magnetotransport properties of GaMnAs can be changed by an adjacent magnetic layer due to the interactions between the two magnetic layers.
Synthesis and magnetic properties of Co-doped ZnO nanowires
Authors: Cui Jingbiao , Zeng Qi , Gibson Ursula J.
Published: June 2006

An electrochemical approach to the synthesis of Co-doped ZnO nanowire arrays on conducting substrates at 90 °C is reported. The structure and properties of as-grown and annealed nanowires were studied by x-ray diffraction, scanning electron microscope, energy dispersive x-ray spectroscopy, photoluminescence, and vibrating-sample magnetometry. The Co-doped ZnO nanowires exhibit a broadened band-edge emission at 386nm with a strong emission band around 550nm. Room-temperature ferromagnetism was observed in the doped ZnO nanowires, which makes them potentially useful as building components for spintronics. Effects of Co concentration and annealing on the magnetic properties were investigated.
Magnetic-field sensors based on iron garnets with in-plane magnetization
Authors: Holthaus C. , Nistor I. , Mayergoyz I. D. , Krafft C.
Published: June 2006

A magnetic-field sensor is proposed that uses thin garnet films with in-plane magnetization and strong magneto-optical activity. The advantages of this sensor are its high intrinsic sensitivity and the possibility of tailoring the field range for linear response. Measurements at 80 Hz yield noise-equivalent magnetic fields of 1.5μT∕Hz and a signal-to-noise ratio of 68 dB.
Computational study of copper ferrite (CuFe2O4)
Authors: Zuo Xu , Yang Aria , Vittoria Carmine , Harris Vincent G.
Published: June 2006

Magnetic properties and electronic structure of copper ferrites in both normal and inverse spinel structures are studied using a principle spin-polarized band structure calculation method with a modified Becke’s three-parameter exchange correlation. The calculated exchange constants show that the Néel configuration may be unstable for both normal and inverse structures. The local magnetic moments are calculated using Mülliken population analysis and show that the normal structure may achieve very high magnetization. The calculated density of states show that copper ferrite in both normal and inverse spinel structure may be half metallic.
Magnetic properties of Sb-doped FePt nanoparticles
Authors: Yan Q. Y. , Kim T. , Purkayastha A. , Xu Y. , Shima M. , Gambino R. J. , Ramanath G.
Published: June 2006

Sb-doped FePt nanoparticles with an average diameter of 8.5nm were prepared by thermal decomposition of platinum acetylacetonate, antimony acetate, and iron pentacarbonyl. Upon annealing to ∼300°C for 30min, nanoparticles with XSb=0.14 and 0.23 show Hc>500mT, and L10 ordering parameter S values of ∼0.83–0.87. Transmission electron microscopy of the annealed assemblies shows no observable nanoparticle coalescence at 300°C. Low-temperature coercivity measurements with a superconducting quantum interference device indicate the presence of particles that exhibit superparamagnetism, probably due to the large particle size distribution or inhomogeneity in Sb incorporation. Our results underscore the necessity to synthesize monodisperse FePt nanoparticles with controlled composition to maximize ferromagnetic behavior.
Role of double exchange interaction on the magnetic and electrical properties of Pr0.8Sr0.2MnO3 ferromagnetic insulating manganite
Authors: Rama N. , Sankaranarayanan V. , Rao M. S. Ramachandra
Published: June 2006

In this work the ferromagnetic state in a ferromagnetic insulating manganite viz. Pr0.8Sr0.2MnO3 was analyzed. The temperature variation of ac susceptibility is seen to show two transitions with a high temperature transition occurring at 158K and a low temperature cusp (Tf) around 90K. The high temperature transition (TC) was found to be independent of frequency while the low temperature transition shifts with frequency indicating a frustrated magnetic state. The relaxation time observed from the critical slowing down spin analysis indicates that this state is that of a cluster glass. Temperature variation of resistivity shows an insulating behavior with two distinct humps corresponding to TC and Tf. Temperature variation of thermopower exhibits a peak at TC and decreases below TC which is attributed to the sudden release of holes from traps, similar to a ferromagnetic metallic manganite. Magnetoresistance is seen to show a change in curvature upon cooling the sample across TC which in conjunction with the peak in resistivity and thermopower suggests that this transition is mediated by a double exchange mechanism. The frustrated state is seen to arise because of a competition between double exchange and superexchange mechanisms. The effect of double exchange mechanism is seen to persist all the way down to 5K as seen from the magnetoresistance analysis. These results clearly suggest that double exchange still plays an important role in dictating the magnetic and electrical properties in ferromagnetic insulating manganites.
Ferromagnetic resonance studies on colossal magnetoresistance films: Effects of homogeneity and light illumination
Authors: Bah R. , Bitok D. , Rakhimov R. R. , Noginov M. M. , Pradhan A. K. , Noginova N.
Published: June 2006

Ferromagnetic resonance (FMR) in the dependence on magnetic field orientation, and effect of the laser light on the magnetic ordering were studied in colossal magnetoresistance films, La1−xSrxMnO3 and La1−xBaxMnO3. The FMR linewidth increased with the decrease in temperature in La1−xSrxMnO3 films, and was nearly temperature independent in La1−xBaxMnO3, that was attributed to the different degree of magnetic homogeneity of the films. Layered structures La1−xBaxMnO3 on silicon with intermediate layer of SrTiO3 were characterized by FMR as highly homogeneous. From the orientation and temperature dependences of the FMR spectra, the effective magnetization, and effective anisotropy fields were estimated. The laser light illumination resulted in the change of FMR line shape and intensity, and was interpreted in terms of optically induced magnetic disordering and effective heating of the spin system.
Self-biased Y-junction circulators using polycrystalline hexaferrite: An accurate electromagnetic analysis
Authors: Guennou Annaïg , Quéffélec Patrick , Gelin Philippe , Mattei Jean Luc
Published: June 2006

The theoretical approach proposed here was aimed at accurately describing the electromagnetic response of self-biased Y-junction circulators made of polycrystalline hexaferrites. The accuracy of circulator response was enhanced by considering the real magnetization state of the material through a calculation of the permeability tensor; this tensor remains valid whatever the strength of the local effective dc field in the ferrite is. Moreover, the hysteresis phenomenon was introduced in the calculation of tensor through the use of the Stoner and Wolhfart [IEEE Trans. Magn. 27, 3475 (1948)] law; as a consequence, the tensor took into account the privileged direction of easy-axis distribution due to the manufacturing process, which allowed us to treat the polycrystalline hexaferrites as being magnetically self-biased. However, the results issued from our approach indicate that this higher accuracy is at the expense of Y-junction circulator performances, which are far from those reported in the literature or obtained with electromagnetic commercial simulators and based on the frequent assumption of saturated hexaferrite, even at the remanent state.
Enhancement of magnetoresistance in [Pd∕Co]N∕Cu∕Co∕[Pd∕Co]N∕FeMn spin valves
Authors: Joo H. W. , An J. H. , Lee M. S. , Choi S. D. , Lee K. A. , Kim S. W. , Lee S. S. , Hwang D. G.
Published: June 2006

The dependences of the stack number and Co and Cu thicknesses on magnetoresistance, and perpendicular exchange biasing in the [Pd∕Co]×N1∕Cu∕Co∕[Pd∕Co]×N2∕FeMn spin valves, were investigated. The magnetoresistance ratio of the spin valves with N1=4 and N2=2 was above 5% for the Cu between 1.5 and 2.5nm, and the exchange-biasing field (Hex) was increased to 600Oe. When the Co thickness over the Cu layer was increased from 0.48to1.28nm, the ratio likewise rose to 7.4% due to the strengthened spin-dependent scattering effect. Moreover, the Co thickness below the Cu layer rose to 9.1%. The maximum ratio of 10.2% was obtained in the structure of the [Pd0.8∕Co0.32]×2∕Pd0.8∕CoFe0.48∕Cu2.25∕CoFe0.6∕[Pd0.8∕Co0.48]×2∕FeMn15 (nanometer) multilayer with Hex=450Oe.
Design of outer-rotor-type multipolar switched reluctance motor for electric vehicle
Authors: Fujishiro Satoshi , Ishikawa Kazumi , Kikuchi Shinki , Nakamura Kenji , Ichinokura Osamu
Published: June 2006

In an electric vehicle (EV) with in-wheel motors, reducing the weight of the motor is a very important problem in order to improve the driving performance. In this paper, we examine the lightweight design of an outer-rotor-type multipolar switched reluctance (SR) motor applied to a prototype EV. We design three SR motors which have different yoke widths and calculate the motor characteristics at a steady rotational speed based on a finite element method. We discuss the optimum relationship between a pole and yoke widths.
Perpendicular giant magnetoresistance composed of [Co∕Pt] multilayer and CoFe∕TbCo
Authors: Lin Meng-Shian , Lai Chih-Huang , Liao Yu-Yi , Wu Zhi-Hao , Huang Sheng-Huang , Jiang Ruo-Fan
Published: June 2006

Perpendicular spin valves composed of [Co∕Pt] multilayer and CoFe∕TbCo bilayer were fabricated. The perpendicular giant magnetoresistance ratio of 4.8% was obtained with current-in-plane measurements. The interlayer coupling increased with increasing the net magnetization of CoFe∕TbCo. We proposed a perpendicular pinning structure which possessed a high perpendicular coercivity (7000 Oe) but zero net moment. Consequently, the interlayer coupling between free and pinned layers can be significantly suppressed. This perpendicular pinning structure can be potentially used for high-density perpendicular magnetic random access memory cells.
Peltier cooling in current-perpendicular-to-plane metallic junctions
Authors: Fukushima Akio , Kubota Hitoshi , Yamamoto Atsushi , Suzuki Yoshishige , Yuasa Shinji
Published: June 2006

Peltier cooling in submicron-sized current-perpendicular-to-plane metallic junctions (CPP-MJs) and enhancement of the Peltier coefficients are reported. The main goal of this study was to measure the temperature dependence of the Peltier coefficient. We fabricated submicron-sized CPP-MJs with Co∕Au and Cr∕Au interfaces. The obtained Peltier coefficients for the Co∕Au and Cr∕Au interfaces had opposite signs, and the magnitudes of both decreased as temperature decreased. A bridge type CPP-MJ having a Ti island connecting Co and Cr pillars was fabricated as a model of an electronic element. It also had a decrement of resistance with current. This indicates that Peltier cooling is effective to control the temperature of an electronic device such as a hard-disk head.
Ferromagnetic resonance studies in ZnMnO dilute ferromagnetic semiconductors
Authors: Lasley-Hunter B. , Hunter D. , Noginov Maxim , Dadson J. B. , Zhang K. , Rakhimov R. R. , Pradhan A. K. , Zhang Jun , Sellmyer D. J.
Published: June 2006

We report on the ferromagnetic resonance studies on ZnMnO films grown by the pulsed-laser deposition technique. ZnMnO films were annealed in different atmospheres. The films grown and annealed in oxygen demonstrate ferromagnetic behavior at room temperature and below. However, annealing in either nitrogen or argon deteriorates the ferromagnetic response of the films. Further annealing the films in oxygen recovers the ferromagnetic response. Our results suggest that oxygen plays a major role for controlling the ferromagnetic properties in ZnMnO films.
Generalized susceptibility and superconductivity in CeMIn5(M=Co,Rh,Ir) and PuCoGa5
Authors: Wang J. L. , Zeng Z. , Lin H. Q.
Published: June 2006

The band structure, Fermi surface, and generalized susceptibility χ(q) in the constant-matrix-element approximation for the heavy fermion series CeIn3, CeMIn5(M=Co,Rh,Ir), and PuCoGa5 are studied by density functional calculations to address the relationship among superconductivity, magnetism, and Fermi surface nesting in these compounds. Within the scenario of magnetically mediated d wave superconductivity, our results indicate that the behavior of χ(q) of the Ce class materials favors superconductivity if 4f electrons are treated as core states, since it exhibits broad plateaus with large magnitude around q=(0.5,0.5,0) and (0.5, 0.5, 0.5). In contrast, when the 5f electrons are treated as valence electrons, the χ(q) of PuCoGa5 exhibits a very sharp peak at q=(0.5,0.5,0) due to the strong nesting of hole and electron Fermi surface from the 16th and 17th bands. Not only the large spin fluctuation energy but also the strong nesting of Fermi surface in PuCoGa5 may contribute to the much higher Tc than that in the Ce series.
Magnetic-flux-induced persistent currents in symmetric-polymer mesoscopic rings
Authors: Zhang R. L. , Peng R. W. , Hu X. F. , Cao L. S. , Zhang X. F. , Wang Mu , Hu A. , Jiang S. S.
Published: June 2006

We investigate magnetic-flux-induced persistent currents in one-dimensional symmetric-polymer (SP) mesoscopic ring, where a defect cluster is embedded into two periodic clusters and the whole cluster possesses mirror symmetry. Under tight-binding approximation, the flux-dependent energy spectra, electronic wave functions, and persistent currents are obtained. It is shown that the energy spectra form band structure and the defect modes appear in the band gap. The persistent current (PC) depends on the magnetic flux, the site energy, and the Fermi level. Particularly, by changing the Fermi level, the SP ring presents metallic and insulative features alternately. Our investigations provide a possible model to explain the anomalously large PC observed in some experiments and contribute to the potential application in quantum-switch devices.
Detection of the domain structure change using magnetotransport for a series of circular Permalloy dots
Authors: Chung T. Y. , Hsu S. Y.
Published: June 2006

The magnetic structure of a magnetic dot depends on its geometrical factors such as thickness and dot size and can be either a multidomain state or a vortex state. The magnetoresistance (MR) of a series of magnetic dots with diameters ranging from 0.3to5μm was measured to investigate the correlation between domain-structure and magnetotransport properties. A dot with a diameter of less than 2μm has a domain of vortex state at remanence and demonstrates similar reversible MR results. Moreover, the behaviors of MR show clear changes corresponding to the domain-structure change from vortex to multidomain states with an increasing dot diameter. Data can be qualitatively described by the anisotropic magnetoresistive effect. Hence, our results show that the magnetotransport can be a tool to detect the magnetic domain structure of a submicron magnetic dot.
Structural and magnetic properties of Nd3(Fe1−xAlx)27.5Ti1.5 (x=0.1–0.4)
Authors: Ravi  , Venkatesh S. , Markandeyulu G. , Prasad Rao K.
Published: June 2006

The structural and magnetic properties of Nd3(Fe1−xAlx)27.5Ti1.5 (x=0.1–0.4) were investigated by x-ray diffraction, magnetization, and Curie temperature (TC) measurements. The unit cell volume increases with the Al substitution, in accordance with the larger atomic radius of Al. The saturation magnetization and the Curie temperature decrease with increasing Al content, and the decrease is attributed to the modification of the density of states of 3d band due to the 3p band of Al and to the magnetic dilution effect. The easy direction of magnetization is along the b axis for x=0.4. The anisotropy field (HA) decreases with increasing Al concentration.
Magnetic energy and radial vector model of hysteresis
Authors: Cardelli E. , Della Torre E. , Pinzaglia E.
Published: June 2006

In this paper we discuss some properties of the radial vector model (RVM) proposed as a three-dimensional extension of the classical scalar Preisach model (CSPM). In particular, we prove that the irreversible model of magnetization of the RVM is consistent with the conservation of magnetic energy and with the second law of thermodynamics. The model continues to have these properties even when a reversible component of magnetization is added to the magnetization.
Dynamics studies in magnetic bilayer Fe∕Au∕Fe(001) structures using network analyzer measurements
Authors: Mosendz O. , Kardasz B. , Heinrich B.
Published: June 2006

Fe ∕ Au ∕ Fe ( 001 ) ultrathin film structures were grown by molecular beam epitaxy on 4×6 reconstructed GaAs(001) substrates. A microwave network analyzer was used to carry out ferromagnetic resonance (FMR) measurements down to microwave frequency of 1GHz. The FMR results indicate that the crystalline ultrathin Fe films are of high crystalline quality. The FMR linewidth is mostly given by the intrinsic Gilbert damping and is only weakly affected by small local variations of the internal anisotropy fields. The study at low microwave frequency allowed one to investigate the spin-pumping mechanism in the configuration of noncollinear magnetic moments.
Magnetization reversal study in submicron half-ring patterned wires with different corner structures
Authors: Yu C. , Chen Y. S. , Yao Y. D. , Cheng K. W. , Lee S. F. , Wu K. T. , Liou Y.
Published: June 2006

Magnetization reversal phenomena in submicron half-ring patterned wires with different corner structures have been investigated. The wires with corner structures of rectangular, semicircle, and triangular shapes were connected in series by two or five half-rings. The variation of domain structures with magnetic field is illustrated by electron transport measurements, simulation, and magnetic force microscopy (MFM) images. The switching behavior depends on the corner structures connecting the half-ring structures. These results are consistent with the domain pattern and magnetic phase shift observation provided by the MFM measurements.
Determination of antiferromagnetic interactions in Zn(Mn)O, Zn(Co)O, and Zn(Mn)Te by inelastic neutron scattering
Authors: Kolesnik S. , Dabrowski B. , Wiren Z. Q. , Kepa H. , Giebultowicz T. M. , Brown C. M. , Lëao J. , Furdyna J. K.
Published: June 2006

The nearest-neighbor magnetic exchange interactions in Zn0.95Mn0.05O, Zn0.95Co0.05O, and Zn0.98Mn0.02Te were investigated by measuring inelastic neutron scattering from isolated exchange-coupled spin-spin pairs. The experiments on Zn0.98Mn0.02Te were carried out at atmospheric pressure and then at 4kbar pressure in order to determine the dependence of the exchange parameter on the spin-spin distance.
Relationships between microstructure and superconducting behaviors of MgB2
Authors: Hon W. M. , Ng D. H. L.
Published: June 2006

Hexagonal MgB2 was successfully produced in a Mg matrix by sintering a powder mixture of 70wt% Mg and 30wt% B. From differential thermal analysis, we found that the reaction between Mg and B occurred at 600°C. The superconducting properties of the sintered samples including magnetic moment (M), magnetization loop, and critical temperature (TC) were measured. When the fabrication temperature was increased from 650to800°C, the lateral size of the hexagonal MgB2 platelets was increased from 0.5to2μm. Both TC and M were increased from 34.7to38.2K and from 6.2to13.7emu∕g, respectively, while the area of the magnetization loop at any fixed measuring temperatures below TC became larger. These increments were due to the increase in fractional volume of the superconducting MgB2 phase in the Mg–MgB2 samples and the reduction of vacancies in the MgB2 platelets.
Electrical resistivity of amorphous Cr74Fe26 thin film: Giant magnetic moment formation
Authors: Öner Yildirhan
Published: June 2006

Electrical resistivity measurements have been carried out on a flash evaporated amorphous Cr74Fe26 thin film with a thickness of 2020Å, in the temperature range of 3–300K. Upon both warming and cooling the sample between 3 and 100K, the resistivity increases with decreasing temperature. After thermal cycling, this was accompanied by an anomaly at about 55K, where the resistivity exhibits a sudden drop. This anomalous behavior becomes more pronounced after each thermal cycling process. The resistivity measurements were repeated in a magnetic field of 120kOe, with no significant change observed except the temperature of the anomaly was shifted from 55 to 65K. The temperature dependence of the initial resistivity curve was analyzed using all possible mechanisms. It was found that the resistivity fits Mott’s [J. Non Cryst. Solids 1 (1968)] law [R(T)∼exp(T0∕T)1∕4] perfectly over the entire temperature range (3–300K). Magnetization measurements were performed under conditions identical to those reported for the resistivity measurements. After a few thermal cycles, irrespective of the presence of the external fields, a giant magnetic moment (approximately 10μB per Fe atom at saturation) was formed for the perpendicular geometry of the sample. The resulting magnetization is highly anisotropic with the highest value for the perpendicular geometry. The magnetization does not exhibit any temperature dependence for temperatures up to 50K. This unusual giant moment may be attributed to the existence of Bose-Einstein condensation on the surface of the glass substrate via triplet pairing of the electrons, as suggested by Vager and Naaman [Phys. Rev. Lett. 92, 087205 (2004)] for thin organic layers on GaAs. As an alternative explanation, originally suggested by Venkatesan et al. [Nature (London) 430, 630 (2004)] and Coey [Solid State Sci. 7, 660 (2005)] for thin films of HfO2 and non-stoichiometric CaB6, an impurity band due to the presence of lattice or bond defects may become spin polarized, thereby causing giant moment formation.
Atypical time-dependent magnetic behaviors of Fe–Bi films
Authors: Hsu Jen-Hwa , Wang Hong-Xian , Sun An-Cheng , Kuo Po-Cheng
Published: June 2006

Films of metastable Fe–Bi solid solution were prepared by dc sputtering on Si(100) substrate. The magnetic properties of Fe–Bi films are generally changing slowly with aging time. The as-prepared films exhibit a large out-of-plane magnetic anisotropy. The natural aging process precipitates out Fe atoms and forms nanograins with dead magnetic layers on the surface, reducing saturation magnetization and eliminating the magnetic anisotropy of the as-prepared films. On the contrary, Co–Bi films do not exhibit this weakly time-dependent behavior.
Tunneling resonances and Andreev reflection through an interaction quantum dot coupled with two half metals and a superconductor
Authors: Feng Jin-Fu , Wu Xiao-Shan , Jiang Shu-Sheng
Published: June 2006

We investigate the transport characteristics of electrons through a quantum dot with two semi-infinite half metal (HM) leads and a superconductor by the use of the equivalent single-particle multichannel network technique that takes into account the multilevel structure and the Coulomb interactions on the dot, pairing potential on the superconducting side and the exchange field on the HM side. The transport properties depend on the interplay between the Coulomb blockade effect and Andreev reflection (AR). It is found that at finite temperatures and small bias the current versus the gate voltage exhibits a series of peaks, due to the AR, depending on the resonances controlled by the charging energy and level spacing of the dot. In the case of single occupation for the quantum dot, the current of the system can be enhanced or suppressed by the exchange field.
A differential magnetic flux position transducer: Analysis, simulation, and test results
Authors: Flores Filho Aly F. , Müller Roberto
Published: June 2006

A magnetic linear position transducer is proposed. It relies on a principle based on the variation of the magnetic flux distribution produced by the linear axial displacement of an ac-excited coil in a proper magnetic circuit. The principle and the correlation between the magnetic flux distribution and the linear displacement are demonstrated by a proposed linear reluctance network analytical model and by experimental results.
Development and processing temperature dependence of ferromagnetism in Zn0.98Co0.02O
Authors: Hays J. , Thurber A. , Reddy K. M. , Punnoose A. , Engelhard M. H.
Published: June 2006

We report the development of room-temperature ferromagnetism (FM), with coercivity Hc=2000Oe and saturation magnetization Ms∼0.01emu∕g, in chemically synthesized powders of Zn0.98Co0.02O processed at 150 °C, and paramagnetism with antiferromagnetic interactions between the Co2+ spins (S=3∕2) in samples processed at higher temperatures 200⩽TP⩽900°C. X-ray diffraction data show a decrease in the lattice parameters a and c with TP, indicating a progressive incorporation of 0.58Å sized tetrahedral Co2+ at the substitutional sites of 0.60 Å sized Zn2+. Diffuse reflectance spectra show three well defined absorption edges at 660, 615, and 568 nm due to the d-d crystal field transitions A24(F)→E2(G),A24(F)→T14(P), and A24(F)→T12(G) of high spin (S=3∕2)Co2+ in a tetrahedral crystal field, whose intensities increase with processing temperature. X-ray photoelectron spectroscopy shows that the doped Co2+ ions in the 150 °C processed samples are located mostly on the surface of the particles and they disperse into the entire volume of the particles when processed at higher temperatures. The observations suggest that the FM most likely results from Co2+ attached to the surface sites and it is lost in well dispersed samples formed at TP>150°C.
Prediction method of inhomogeneous thermal flux loss in a magnet
Authors: Fukunaga H. , Toyota A. , Mine N. , Yamamoto R.
Published: June 2006

Thermal flux loss (FL) is one of the important properties in applications of Nd–Fe–B magnets to electrical and electronic devices. FL is expected to occur inhomogeneously in magnets with a complex shape, because it depends on the strength of the local demagnetizing field. Thus, in terms of their applications, we need to predict the inhomogeneous FL from basic magnetic properties measured for a magnet with a simple shape such as a sphere. In this contribution, we propose a method of predicting inhomogeneous initial flux loss, FLint, in a magnet with a complex shape, and compare the predicted values of FLint with the measured ones for a model magnet. Consequently, the predicted and measured distributions agreed with each other. This result suggests that the proposed method can be applied to the prediction of FLint in magnets with a complicated shape used in electrical and electronic devices.
The ac effect of vortex pinning in the arrays of defect sites on Nb films
Authors: Wu T. C. , Horng Lance , Wu J. C. , Hsiao C. W. , Koláček Jan , Yang T. J.
Published: June 2006

Niobium thin films with spacing-graded array of submicrometer-scaled holes had been fabricated using electron beam lithography through a lift-off technique. The magnetoresistance measurements and current-voltage characteristics were carried out with the external magnetic field applied perpendicular to the film plane, in which commensurable effects were observed in both experiments. The magnetoresistance with positive/negative directions of dc current revealed identical curves except the dips at matching fields separated. Two distinct current-voltage curves, which resulted from the positive and negative applied current directions, respectively, were discerned when the external magnetic field was fixed at the matching field, which is believed to be due to asymmetry pinning potential in the spacing-graded array of holes. In addition, ac current-voltage curve measured at matching field showed a ratchet bump along with another extra peak associated with incommensurable effect.
Magnetoresistance and magnetization switching characteristics of magnetic tunnel junctions with amorphous CoFeSiB single and synthetic antiferromagnet free layers
Authors: Hwang Jae Youn , Yim Hae In , Kim Mee Yang , Rhee Jang Roh , Chun Byong Sun , Kim Young Keun , Kim Taewan
Published: June 2006

To obtain low switching field (Hsw) we introduced amorphous ferromagnetic Co70.5Fe4.5Si15B10 single and synthetic antiferromagnet (SAF) free layers in magnetic tunnel junctions (MTJs). The switching characteristics for MTJs with structures Si∕SiO2∕Ta 45/Ru 9.5/IrMn 10/CoFe 7∕AlOx∕CoFeSiB 7 or CoFeSiB (t)∕Ru 1.0/CoFeSiB (7−t)∕Ru 60 (in nanometer) were investigated and compared to MTJs with Co75Fe25 and Ni80Fe20 free layers. CoFeSiB showed a lower saturation magnetization of 560emu∕cm3 and a higher anisotropy constant of 2800erg∕cm3 than CoFe and NiFe, respectively. An exchange coupling energy (Jex) of −0.003erg∕cm2 was observed by inserting a 1.0nm Ru layer in between CoFeSiB layers. In the CoFeSiB single and SAF free layer MTJs, it was found that the size dependence of the Hsw originated from the lower Jex experimentally and by micromagnetic simulation based on the Landau-Lisfschitz-Gilbert equation. The CoFeSiB SAF structures showed lower Hsw than that of NiFe, CoFe, and CoFeSiB single structures. The CoFeSiB SAF structures were proved to be beneficial for the switching characteristics such as reducing the coercivity and increasing the sensitivity in micrometer- to submicrometer-sized elements.
Comparison of the interfacial structure between MgO and Al–O oxidation layers for perpendicular magnetic tunnel junction
Authors: Chen Tzu-Jung , Canizo-Cabrera A. , Chang Che-Hao , Liao Kuo-An , Li Simon C. , Hou Chun-Kan , Wu Te-ho
Published: June 2006

We discuss the interfacial structure of MgO and Al–O barrier layers and influence on the magnetic properties of perpendicular magnetic tunnel junction (pMTJ) devices. The pMTJs layer structures analyzed were Si-wafer∕Pt∕Gd(FeCo)∕FeCo∕MgO (AlO)∕FeCo∕Tb(FeCo)∕Pt. The deposit of all pMTJs structures was carried out by rf and dc magnetron sputtering systems. Transmission electron microscopy (TEM) clearly showed that the interfacial structure of FeCo∕MgO or AlO∕FeCo in the pMTJs was very smooth and uniform. Hysteresis loops obtained by an alternating gradient magnetrometer (AGM) for the different oxide barrier layers of pMTJ structures showed that the Al–O layer performs better than the MgO layer. An additional discussion on the oxide layer thickness in the TEM and AGM measurements is also presented.
Calibration of triaxial fluxgate gradiometer
Authors: Včelák Jan
Published: June 2006

The description of simple and fast calibration procedures used for double-probe triaxial fluxgate gradiometer is provided in this paper. The calibration procedure consists of three basic steps. In the first step both probes are calibrated independently in order to reach constant total field reading in every position. Both probes are numerically aligned in the second step in order that the gradient reading is zero in homogenous magnetic field. The third step consists of periodic drift calibration during measurement. The results and detailed description of each calibration step are presented and discussed in the paper. The gradiometer is finally verified during the detection of the metal object in the measuring grid.
Effect of electrode and EuO thickness on EuO-electrode interface in tunneling spin filter
Authors: Negusse Ezana , Holroyd J. , Liberati M. , Dvorak J. , Idzerda Y. U. , Santos T. S. , Moodera J. S. , Arenholz E.
Published: June 2006

The effect of electrode material and EuO film thickness on the interface between the two was studied. Of the electrodes examined, yttrium was found to decrease the formation of the nonmagnetic oxide Eu2O3. By decomposing the x-ray absorption spectroscopy (XAS) spectra of the samples with different electrodes against the reference EuO and Eu2O3 spectra, the relative fraction of these two species was quantified. Multilayers with silver electrodes had the highest amount of Eu2O3, about 41%, whereas aluminum had 28% compared to the less than 5% observed for the multilayers with yttrium electrodes. A slight decrease in the fraction of Eu2O3 with increasing EuO thickness was found. Angle dependent XAS measurements, done on 80Å EuO film, indicated the presence of Eu2O3 at high grazing angles which then decreased drastically with decreasing grazing angle. This indicated that the Eu2O3 was localized at the EuO-electrode interface.