bluuebunny/crossref_metadata_2025 huggingface dataset:
Double-barrier magnetic tunnel junctions with GeSbTe thermal barriers for improved thermally assisted magnetoresistive random access memory cells
Authors: Cardoso S. , Ferreira R. , Silva F. , Freitas P. P. , Melo L. V. , Sousa R. C. , Redon O. , MacKenzie M. , Chapman J. N.
Published: June 2006

Double-barrier magnetic tunnel junction (MTJ) cells incorporating one thermal barrier (GeSbTe) were fabricated for improved thermally assisted magnetic switching. The MTJ has two Al2O3 barriers with a common weakly pinned structure (storage layer) and two pinned layers (reference). The structural quality of the double junction stack and the roughness at the (buffer/thermal barrier) level were investigated and optimized. To minimize the required heating during writing, the blocking temperature (TB) of the storage layer is reduced to 110 °C by thinning the MnIr layer to 80 Å, while a strong exchange coupling and TB∼300°C are obtained at the reference layers with a synthetic antiferromagnetically coupled CoFeB∕Ru∕CoFeB structure pinned to 250-Å-thick MnIr. For the write experiments, the current flowing through the MTJ (patterned down to 2μm2) increases the temperature above the storage layer TB, under an external field of +∕−80Oe. Current densities <1mA∕μm2 were enough to write in the MTJs with a thermal barrier (almost half the values needed without thermal barriers, which also showed a stronger dependence of the write power on the junction area). Write power values of the order of 0.3–1.8mW∕μm2 were achieved.
Enhanced Mr and (BH)max in anisotropic R2Fe14B∕α-Fe composite magnets via intergranular magnetostatic coupling
Authors: Gabay A. M. , Marinescu M. , Hadjipanayis G. C.
Published: June 2006

Composite magnets were prepared by hot pressing followed by hot deformation of blends composed of Nd14Fe79.5Ga0.5B6 or (Nd0.75Dy0.25)14Fe79.5Ga0.5B6 ribbon powders as a high-coercivity component and Fe powder as a high-magnetization component. The addition of 15 wt % α-Fe to (Nd0.75Dy0.25)14Fe79.5Ga0.5B6 increases the remanent magnetization of the hot-deformed magnets from 10.6 to 12.04 kG, while the maximum energy product is also increased from 27.3 to 29.5 MG Oe for hot-deformed magnets with 10 wt % α-Fe addition. Microstructure investigations of the composite magnets revealed the size of the Fe particles in the micrometer range, exceeding by far the size for effective exchange interactions. Despite a less refined microstructure, the particular layered configuration of the composite magnets gives rise to a positive magnetostatic coupling of the grains and therefore a unitary magnetic behavior with enhanced magnetic properties. The cooperative demagnetization process, together with the magnetic coupling of the grains, was pointed out through a smooth demagnetization curve and a sharp single peak of the irreversible susceptibility.
Electrodeposited p-type magnetic metal-base transistor
Authors: Delatorre Rafael Gallina , Munford Maximiliano Luis , Stenger Vagner , Pasa André Avelino , Schwarzacher Walther , Meruvia Michelle S. , Hümmelgen Ivo A.
Published: June 2006

In this work the development of a magnetic metal-base transistor that operates by hole transport is reported. The transistor is constructed using p-type silicon as the collector, Co as the base, and Cu2O as the emitter. Both base and emitter are deposited using electrochemical procedures. The transistor shows a magnetic-field-dependent current gain and a magnetocurrent of ∼40% observed for a low emitter current value of 2 mA.
Nd
2
Fe
14
B
-based nanocomposite magnets with transition metal and carbon additions
Authors: Harrison N. J. , Davies H. A. , Todd I.
Published: June 2006

Nanocomposite intermediate rare-earth∕high-boron content hard magnetic alloys, having compositions Nd9Fe73B12.6C1.4M4 (M=Ti, V, Cr, Zr, or Nb), have been produced as melt spun ribbon. As for the Ti addition, all the other solutes were effective in modifying the crystallization behavior so that the Nd2Fe14B phase formed rather than the cubic Nd2Fe23B3 phase. The magnetic properties of the V-, Cr-, Zr-, and Nb-containing alloys in the as-spun state and following annealing are compared with those of the Ti-containing alloy. Good combinations of intrinsic coercivity (1020kAm−1) and of maximum energy product (97kJm−3) were obtained for the V-containing alloy, closely comparable with those for the M=Ti alloy, and the Zr addition also yielded a good property combination.
Effect of mutual coupling on torque production in switched reluctance motors
Authors: Walker Jill A. , Dorrell David G. , Cossar Calum
Published: June 2006

In many cases, the normal operation of switched reluctance machines requires excitation of two or more phases simultaneously. When multiple phases are conducting simultaneously, the flux paths from each phase will overlap, which may lead to localized saturation. In such cases, the flux linkage must be considered a function not just of the current in the test winding but of all excited windings. The degree of mutual coupling between phases influences the per-phase magnetization curves and torque characteristics. In machines with even phase numbers, the degree of mutual coupling between phases varies due to discontinuities in the phase polarity arrangement. From nonlinear finite element simulations, it is possible to compare the i-ψ loop diagrams under single-phase and multiphase excitations, and hence the torque produced. The mutual flux linkage from each phase can be calculated separately for each rotor position using the frozen permeability method, to further analyze the mutual coupling effects. For a given excitation current profile, the torque can be maximized by careful arrangement of the phase polarities.
Microwave and magnetic properties of self-biased barium hexaferrite screen printed thick films
Authors: Chen Yajie , Geiler Anton L. , Sakai Tomokazu , Yoon Soack D. , Vittoria Carmine , Harris Vincent G.
Published: June 2006

The interest in barium hexaferrite thick films, particularly with high remanent magnetization, is driven by the development of small planar ferrite microwave devices. We report here processing and microwave characterization of BaFe12O19 ferrite thick films (100–400μm). The films were deposited on silicon and alumina substrates by screen printing, oriented under a magnetic field of 8kOe, then annealed at 250°C and sintered at temperatures ranging from 850to1300°C. Scanning electron microscopy and x-ray diffraction exhibited strong crystallographic alignment of c-axis crystals perpendicular to the film plane. The magnetization measurement indicated that a typical dense film with 270μm thickness yielded a high squareness (Mr∕Ms) of 0.93. Ferrimagnetic resonance (FMR) measurements were performed in the frequency range of 40–55GHz. From the linear dependence of FMR frequency on the external field, a g factor of 2.03±0.08 was deduced, while the smallest linewidth was obtained to be 1.2kOe at 40GHz. The broadening of the FMR linewidth arises predominantly from magnetic inhomogeneities in the polycrystalline films. The thick films have great potential for use in future microwave and millimeter wave monolithic integrated circuits.
CoPt patterned media in anodized aluminum oxide templates
Authors: Gapin A. I. , Ye X. R. , Aubuchon J. F. , Chen L. H. , Tang Y. J. , Jin S.
Published: June 2006

Patterned recording media consisting of a vertically aligned array of L10 phase CoPt nanowires embedded in a thin anodized aluminum oxide (AAO) template on silicon has been prepared. A sputter deposited thin film of aluminum on silicon was anodized and a CoPt magnetic alloy was electrodeposited into the pores of the AAO. The vertically aligned arrays of CoPt nanowires were about ∼100nm tall with ∼20nm average diameter. Since the CoPt nanowire array is laterally constrained by the surrounding AAO, the nanowire diameter is maintained without coarsening during the L10 phase conversion heat treatment at 700°C. After annealing and conversion to the L10 phase, the ∼20nm CoPt nanowires exhibit a large coercivity of ∼8kOe measured in the in-plane and perpendicular directions.
Structural and magnetic properties of a chemically ordered face-centered-cubic (111) Mn alloy film
Authors: Zhou Zhuowei , Li Qiangyong , Venus D.
Published: June 2006

A 4 ML Ni∕W(110) substrate is used to establish a (111) face-centered-cubic (fcc) template upon which 3 ML of Fe is deposited and annealed to 580K to form a substrate with very good short and long range fcc (111) order, that is Fe rich at the surface. Mn alloy films are formed by annealing a subsequent Mn deposit of 0.3–1.6 ML. Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and directional AES show that an ordered alloy is not formed until an annealing temperature of 580K, upon which a multilayer alloy with a P(23×23)R30° LEED pattern is created. The alloy films formed from 0.3 to 0.5 ML of Mn have magnetic properties similar to the FeNi substrate. Hysteresis loops and ac-susceptibility curves measured using the Kerr effect give square loops with a ferromagnetic moment along the in-plane fcc [-211] direction and a Curie temperature TC of about 460K. There is an increase in coercive field likely due to the inhomogeneities introduced by the Mn. Alloy films formed from 0.8 to 1.6 ML of Mn show a marked increase in the width of the susceptibility peak, and a decrease in the peak temperature. The hysteresis loop becomes slanted with a reduced coercive field. The measurements are consistent with a paramagnetic or antiferromagnetic Mn alloy forming an uneven interface within the FeNi film, so that the remaining FeNi film has a wide distribution in TC.
Magnetic properties of Co–Al melt-spun ribbons
Authors: Saito Tetsuji
Published: June 2006

Co – Al alloy ribbons were produced by melt spinning followed by heat treatment. Although the Co–Al melt-spun ribbons had low coercivities, annealing of the ribbons resulted in an increase in the coercivity. The optimally annealed Co75Al25 melt-spun ribbon exhibited a high coercivity of 1.6kOe with a high saturation magnetization of 45emu∕g. The annealed Co75Al25 melt-spun ribbon consisted of the hcp-Co and CoAl phases together with a small amount of the fcc-Co phase. It was found that the application of a magnetic field of 20kOe during annealing promotes the formation of the fcc-Co phase and results in the increase in the remanence value.
Magnetic properties of Sm–Fe–N bulk magnets produced by compression shearing method
Authors: Saito Tetsuji , Sato Hisanobu , Fukui Masato , Takeishi Hiroyuku
Published: June 2006

Sm-Fe-N powder was successfully consolidated into bulk materials at room temperature by the compression shearing method. Regardless of the applied shear stress, the resultant bulk materials retained the original Sm2Fe17N3 phase structure without any appreciable decomposition of the Sm2Fe17N3 phase. The density of the resultant bulk materials and the remanence of the Sm-Fe-N magnets increased with increasing the applied shear stress. A magnetically anisotropic Sm-Fe-N bulk magnet was obtained using the premagnetized Sm-Fe-N powder. The resultant Sm-Fe-N bulk magnet exhibited a remanence of 6.8kG with a coercivity of 10.8kOe.
In situ resonant photoemission characterization of La0.6Sr0.4MnO3 layers buried in insulating perovskite oxides
Authors: Kumigashira H. , Hashimoto R. , Chikamatsu A. , Oshima M. , Ohnishi T. , Lippmaa M. , Wadati H. , Fujimori A. , Ono K. , Kawasaki M. , Koinuma H.
Published: June 2006

We have investigated the Mn 3d partial density of states in SrTiO3(STO)∕La0.6Sr0.4MnO3(LSMO) heterointerfaces as well as of LSMO films using Mn 2p-3d resonant photoemission spectroscopy. The strong enhancement of the Mn 3d spectra at the Mn 2p-3d threshold enables us to extract the Mn 3d spectra of LSMO layers in the vicinity of the interface with the STO overlayers. We have found that the spectral intensity of eg↑ states near the Fermi level is drastically reduced when the LSMO film is capped with STO overlayers. The close similarity in reduction of the intensity of the eg↑ states between the STO/LSMO interface and hole-doped LSMO films suggests that the hole doping into the LSMO layer close to the STO layer originates from the chemical carrier-concentration modulation at the valence-mismatched interface composed of the stacking sequence -TiO2-SrO-MnO2-La0.6Sr0.4O-, which is inevitable for multilayers based on perovskite oxides.
Growth and magnetic properties of IV-VI diluted magnetic semiconductor Ge1−xCrxTe
Authors: Fukuma Y. , Taya T. , Miyawaki S. , Irisa T. , Asada H. , Koyanagi T.
Published: June 2006

IV-VI diluted magnetic semiconductor Ge1−xCrxTe films were grown on BaF2 substrates by molecular-beam epitaxy. The Ge1−xCrxTe film up to x=0.103 is single phase as determined by reflection high-energy electron diffraction and x-ray diffraction measurements. The optical band gap decreases with increasing Cr composition. Ferromagnetic order of the Ge1−xCrxTe films is characterized by direct magnetization and anomalous Hall effect measurements.
Nanostructural M-type barium hexaferrite synthesized by spark plasma sintering method
Authors: Zhao Wen-Yu , Zhang Qing-Jie , Tang Xin-Feng , Cheng Hai-Bin , Zhai Peng-Cheng
Published: June 2006

This investigation shows that the spark plasma sintering (SPS) method may not only absolutely prevent the formation of intermediate phase α-Fe2O3 but also significantly lower the crystallization temperature of M-type barium hexaferrite. A kind of textured sample composed of major phase M-type barium hexaferrite and trace amount of BaFe0.24Fe0.76O2.88 has been successfully synthesized in one step by SPS process at 800°C for 10min. Scanning electron microscopy, energy dispersive spectroscopy, x-ray-diffraction, transimission electron microscopy, and vibrating-sample magnetometer techniques are used to study the microstructure, composition, phase formation, crystal orientation, and magnetism of the textured sample, respectively. It is confirmed that nanostructural M-type barium hexaferrites in the holes of the textured sample are in nanobelt and nanorod microstructures; these nanobelts and nanorods are nonstoichiometric and completely random and are characterized by barium or iron surplus, and that their composition may be described as BaFe12+xO19+1.5x (−4.49⩽x⩽6.60). The prolongation orientation of a nanobelt is parallel to the zone axis [101¯0]. The values of σs and Hc of the textured sample are 65.52emu∕g and 1412.17Oe, respectively. The Hc value decreases to one-fourth of that of randomly packed platelike M-type barium hexaferrite with same composition synthesized by sol-gel process at 900°C for 4h, while the σs value was independent of the synthesis conditions. This indicates that M-type barium hexaferrite magnets synthesized by SPS process may be applied to electromagnetic wave absorption and/or magnetic recording.
Precessional and thermal relaxation dynamics of magnetic nanoparticles: A time-quantified Monte Carlo approach
Authors: Cheng X. Z. , Jalil M. B. A. , Lee H. K. , Okabe Y.
Published: June 2006

A hybrid Monte Carlo (MC) method is proposed to study the full magnetization dynamics of a magnetic nanoparticle, comprising damping, thermal fluctuations, and precessional motion involved in a magnetization reversal process. The precessional motion is an athermal process, and has been neglected in previous MC schemes used to model magnetization dynamics. We introduce it into our hybrid method by adding a precessional step of the appropriate size to the random walk of the heat-bath Metropolis MC method. This hybrid MC method is applied for the study of the role of precession in the magnetization switching process of a magnetic nanoparticle under the influence of an oblique field. We numerically predict two distinct behaviors in the switching process corresponding to the high and low damping limits.
Concentration effects on the grafting of magnetic nanoparticles by Monte Carlo simulations
Authors: Castro L. L. , Miotto R. , Bakuzis A. F.
Published: June 2006

In this work, we report a systematic comparison between three-dimensional Monte Carlo simulations and magnetic resonance measurements of polydisperse Fe3O4 based nanoparticles. Magnetization curves clearly show the validity of the simple model employed by us. Our theoretical results are then compared to real ferrofluids samples that have been experimentally characterized by transmission electron microscopy and magnetic resonance measurements. Our data indicates that the number of surfactant molecules and its physical distribution (grafting) decisively affects the surface-surface particle distance within an agglomerate. Two different grafting regimes are clearly identified from our theoretical data.
Effect of rare-earth Ho ion substitution on magnetic properties of Fe3O4 magnetic fluids
Authors: Upadhyay R. V. , Gupta Amita , Sudakar C. , Rao K. V. , Parekh Kinnari , Desai Rucha , Mehta R V
Published: June 2006

Rare-earth atoms play an important role in determining the magnetocrystalline anisotropy in 4f-3d intermetallic compounds. Recently we reported on the synthesis and magnetic properties of Gd-substituted Mn-Zn ferrite nanoparticles as potentially suitable for magnetic fluid hyperthermia (MFH). In MFH the specific power absorption rate is related to the lossy magnetocrystalline anisotropic properties of the magnetic fluids. In this paper we report the role of Ho substitution in magnetite nanoparticles, which is found to enhance the KV product arising from the large anisotropy of Ho3+ moments. The zero-field-cooled magnetization data is then simulated by assuming noninteracting magnetic particles with uniaxial anisotropy and lognormal particle size distribution. The fit parameters give the values of particle diameter (D) 9 nm, standard deviation 0.3 in ln(D), and the anisotropy constant K to be 3.5×104J∕m3. The value of K thus obtained is an order of magnitude larger than the value known for the undoped for magnetite (104J∕m3).
Magnetic properties of Co–Zn–P alloy powders produced by chemical reduction
Authors: Saito Tetsuji
Published: June 2006

A hard magnetic powder of Co–Zn–P alloy was produced by chemical reduction. The powder had a spherical shape with an average diameter of about 1μm. The Co–Zn–P alloy powder mainly consisted of the hcp-Co and Co2P phases. The saturation magnetization of the Co–Zn–P powder decreased as its Zn content increased. On the other hand, the addition of Zn to Co–P alloy powder resulted in an increase in the coercivity. Co–Zn–P alloy powder with a Zn content of 4wt% showed a remanence of 45.3emu∕g with a coercivity of 1.64kOe.
Soft magnetic powder-core composites of Fe90Zr7B3 and Fe49Co21Al5Ga2P9.65C5.75B4.6Si3 alloys
Authors: Turgut Zafer , Attenweiler Thomas , Huang Meiqing , Horwath John C. , Fingers Richard T.
Published: June 2006

Amorphous and nanocrystalline alloys in ribbon form exhibit excellent soft magnetic properties, but their forms are limited to tape wound cores. Complex shapes require the implementation of a powder metallurgical approach resulting in reduced permeabilities. The present study investigates Fe-based Fe90Zr7B3 (C1) and Fe49Co21Al5Ga2P9.65C5.75B4.6Si3 (C2) melt-spun ribbons as precursors for compacted powder cores. Single-roller melt spinning of C1 produced partially crystallized structures while C2 resulted in amorphous ribbons. Annealing studies were carried out based on the crystallization temperatures of various phases extracted from M(T) measurements. In ribbon form and under optimum annealing conditions, C1 revealed a 1.88T saturation flux density (Bs) and 44A∕m coercivity (Hc), while C2 exhibited a Bs of 0.78T and Hc of 2.4A∕m.
Nanostructure and magnetic properties of highly (001) oriented L1 (Fe49Pt51)1−xCux films
Authors: Yan M. L. , Xu Y. F. , Sellmyer D. J.
Published: June 2006

We report on nonepitaxially grown L10 Cu-alloyed FePt thin films with strong (001) texture. The FePt films with different Cu contents were deposited directly on Si wafers with a Fe49Pt51∕Cu multilayer structure. The Cu content was varied from 0to13at.%. All films were annealed at 600°C for 5min. X-ray-diffraction characterization showed that only one set of L10 diffraction peaks appeared and no elemental Cu diffraction peaks were visible. This result, along with a varying c∕a lattice-parameter ratio, suggests that Cu substitutes Fe or Pt in the L10 lattice and ternary FePtCu alloy films are formed. (001) texture was enhanced with the increase of Cu content. Transmission electron microscope images showed that the grain size of FePtCu was about 10nm. For FePt film with 11at.% Cu substitution, coercivity was about 5kOe, which is suitable for writing in a practical perpendicular-recording film.
Effect of very thin Cr films on the magnetic behavior of epitaxial Co
Authors: Reith Timothy , Shaw J. M. , Falco C. M.
Published: June 2006

A single monolayer (ML) of Cr was found to have a significant effect on the magnetic properties of subsequently deposited ultra-thin epitaxial Co layers. Brillouin light scattering (BLS) indicated that the Cr interlayer reversed the sign of the interface contribution to the total perpendicular magnetic anisotropy (PMA) of the Co. The negative value of the interfacial anisotropy term implied in-plane behavior. As a result, the samples with Cr became in-plane at about 11 ML of Co (1 ML of Co is about 0.2 nm), but the structures without Cr, while behaving qualitatively similar, exhibited higher total PMA and became in-plane at about 13 ML of Co. This behavior was also confirmed using perpendicular magneto-optic Kerr effect measurements. These results suggested that the in-plane nature of the interface anisotropy term for those samples with Cr was significant in determining their greater sensitivity to increased Co thickness. The impact of interdiffusion at the Cr/Co interface should not be neglected; by itself Cr is antiferromagnetic, and as an additive to Co and with no diffusion to grain boundaries (these films are single crystal), a greater effect from it is expected than from simple diluents such as Cu or Au. A monotonic decrease in magnetization with decreasing Co thickness was observed, but only for the samples with Cr, further suggesting the importance of interdiffusion.
Space mapping method for the design of passive shields
Authors: Sergeant Peter , Dupré Luc , Melkebeek Jan
Published: June 2006

The aim of the paper is to find the optimal geometry of a passive shield for the reduction of the magnetic stray field of an axisymmetric induction heater. For the optimization, a space mapping algorithm is used that requires two models. The first is an accurate model with a high computational effort as it contains finite element models. The second is less accurate, but it has a low computational effort as it uses an analytical model: the shield is replaced by a number of mutually coupled coils. The currents in the shield are found by solving an electrical circuit. Space mapping combines both models to obtain the optimal passive shield fast and accurately. The presented optimization technique is compared with gradient, simplex, and genetic algorithms.
Three-dimensional analysis of tubular permanent magnet machines
Authors: Chai J. , Wang J. , Howe D.
Published: June 2006

This paper presents results from a three-dimensional finite element analysis of a tubular permanent magnet machine, and quantifies the influence of the laminated modules from which the stator core is assembled on the flux linkage and thrust force capability as well as on the self- and mutual inductances. The three-dimensional finite element (FE) model accounts for the nonlinear, anisotropic magnetization characteristic of the laminated stator structure, and for the voids which exist between the laminated modules. Predicted results are compared with those deduced from an axisymmetric FE model. It is shown that the emf and thrust force deduced from the three-dimensional model are significantly lower than those which are predicted from an axisymmetric field analysis, primarily as a consequence of the teeth and yoke being more highly saturated due to the presence of the voids in the laminated stator core.
Effect of TiC addition on microstructure and magnetic properties for MRE2(Fe,Co)14B melt-spun ribbons (MRE=Nd+Y+Dy)
Authors: Tang W. , Wu Y. Q. , Dennis K. W. , Kramer M. J. , Anderson I. E. , McCallum R. W.
Published: June 2006

Effects of a TiC addition on microstructure and magnetic properties in [MRE2.2Fe14B](100−2x)∕17.2+TixCx(MRE=Nd+Y+Dy,x=1–5) ribbons, melt spun at a wheel speed of 16m∕s, were systematically studied. X-ray diffraction and differential thermal analysis data revealed that the addition of TiC improves the glass formability in the mixed rare earth alloys without Co, resulting in partially amorphous alloys. TEM observations showed that the average grain size in the as spun samples decreases from 200 to 20 nm with increasing x from 1 to 5, confirming that the addition of TiC can significantly improve microstructure. For an optimized [MRE2(Fe,Co)14B](100−2x)∕17.2+TixCx sample with x=2, spun at 25m∕s and annealed at 750 °C for 15 min, the room-temperature magnetic properties of Hcj=11.8kOe, Mr=7.2kGs, and (BH)max=11.3MGOe were obtained. Temperature coefficients for Mr and Hcj of −0.06 and −0.37%∕°C, respectively, also were measured in the temperature range of 27–100 °C. The new magnet alloy exhibits more uniform magnetic properties and a usable energy product to nearly 300 °C.
In situ transport in alumina-based magnetic tunnel junctions during high-vacuum annealing
Authors: Hindmarch A. T. , Anderson G. I. R. , Marrows C. H. , Hickey B. J.
Published: June 2006

We have performed in situ transport measurements on CoFeB∕AlO∕Co spin-valve magnetic tunnel junctions during annealing up to 200 °C. For optimally oxidized junctions we observe a marked decrease in the parallel state junction resistance, which occurs at around 150 °C and continues throughout the anneal process until the temperature is reduced back below 150 °C. Spectroscopic measurements reveal a stronger bias dependence due to enhanced magnon-assisted tunneling, and show that features relating to weak fcc texture in the cobalt upper electrode fade after annealing at 200 °C. We attribute this to a possible amorphization of the cobalt electrode in close proximity to the barrier interface.
Magnetic carriers of iron nanoparticles coated with a functional polymer for high throughput bioscreening
Authors: Maeda M. , Kuroda C. S. , Shimura T. , Tada M. , Abe M. , Yamamuro S. , Sumiyama K. , Handa H.
Published: June 2006

We synthesized iron nanoparticles, 15–20 nm in size, having saturation magnetization, Ms, of 170emu∕g. They were embedded in copolymer beads of styrene (St) and glycidyl methacrylate (GMA), which were coated with poly-GMA by seed polymerization. The resultant Fe∕St-GMA/GMA beads had diameters of 100–200 nm and Ms of 60emu∕g. By coating with poly-GMA, the zeta potential of the beads changed from −93.7 to −54.8mV, as measured by an electrophoresis method. This facilitated (as revealed by gel electrophoresis method) nonspecific protein adsorption suppression, a requisite for nanoparticles to be applied to carriers for bioscreening.
Time-dependent solution of the N-state Stoner-Wohlfarth model
Authors: Gilmore Keith , Idzerda Yves U.
Published: June 2006

A set of N coupled differential equations has been developed for calculating the history-dependent magnetization of an ensemble of single domain magnetic particles. These equations, which are a generalization of a previously reported two-state model, have been solved for the case of hysteresis loops for various numbers of states, N, and several field sweep rates. It has been found that the two-state model significantly misestimates the coercivity and that satisfactory convergence may be achieved by N=16. Coercivity was observed to increase logarithmically with field sweep rate.
Application of the Villari effect to electric power harvesting
Authors: Zhao X. , Lord D. G.
Published: June 2006

Transducers utilizing the Villari effect (inverse-magnetostrictive effect) consist of a coil wound on a core of magnetostrictive material. In this paper, a linear magnetomechanical coupling model is developed to analytically calculate the potential electrical power such transducers can generate when subjected to applied harmonic mechanical vibration. Two vibration modes, force driven and displacement driven, are distinguished. The eddy current effect in the magnetostrictive core material and the leakage inductance of the coil are taken into account. Comparisons of output electrical power are presented for Terfenol-D and Galfenol magnetostrictive cores.
Experimental verification of the performance of the self-tunable loop-current array shielding module
Authors: Goleman K. , Yamato T. , Sasada I.
Published: June 2006

An active shielding module consisting of a self-tunable loop-current array is verified experimentally. Loop currents in the array are independently controlled by the magnetometer dedicated to each of them. Since magnetic field decays as 1∕z3 along the distance z from the loop, the magnetic noise should be overcompensated at the location of the loop-current array to provide a good cancellation in the shielded area. In order to enable overcompensation in a stable feedback system, a small additional loop is combined to the magnetometer that reduces the magnetic direct coupling from the main loop current to the magnetometer. A feedback electronic circuit with a proportional-integral (PI) controller is combined with each of the array element to establish a stable canceling operation. An orthogonal fluxgate magnetometer is used to monitor incident magnetic noise. Shielding performance of an array consisting of nine loop currents arranged in a 3×3 grid was evaluated numerically and confirmed by experiments. Evaluation and measurements are done over the 1.5×1.5m2 area 2m in front of the array. The magnetic dipole disturbance source is placed 2m further beyond the array. Attenuation of the magnetic field is calculated as 1∕15. Experimental data show 1∕10 attenuation.
Reduction of switching field distributions by edge oxidization of submicron magnetoresistive tunneling junction cells for high-density magnetoresistive random access memories
Authors: Yoshikawa M. , Kitagawa E. , Takahashi S. , Kai T. , Amano M. , Shimomura N. , Kishi T. , Ikegawa S. , Asao Y. , Yoda H. , Nagahara K. , Numata H. , Ishiwata N. , Hada H. , Tahara S.
Published: June 2006

An edge oxidization effect on magnetization reversals is investigated for submicron-patterned magnetoresistive tunneling junctions (MTJs). By the MTJ edge oxidization which causes the MTJ edge saturation magnetization (Ms) reduction, the switching field distributions (SFDs) for 0.24×0.48μm2 MTJs are reduced to less than 10%. The offset fields and the kinks in resistance-magnetic-field curves are reduced. Micromagnetic simulation results predict that the edge magnetization reversals are suppressed by the MTJ edge Ms reduction and the edge domain size at the remanent states becomes small. Consequently, the edge domain motion suppression by the edge oxidization is effective for decreasing the SFDs.
Magnetic property characterization of magnetite (Fe3O4) nanorod cores for integrated solenoid rf inductors
Authors: Kim Jinsook , Ni Weiping , Lee Chungho , Kan Edwin C. , Hosein Ian D. , Song Yanning , Liddell Chekesha
Published: June 2006

The on-chip magnetic solenoid inductors with Fe3O4 magnetite nanorod (MN) cores are fabricated and characterized up to 40GHz. By vibrating-sample magnetometer measurements, the magnetic property of MN as a magnetic core for a solenoid inductor is investigated. In addition, high-frequency characterization with scattering parameter measurements is performed to estimate the high-frequency performance of the solenoid inductors with the MN cores.
Enhanced magnetoelectric effect in core-shell particulate composites
Authors: Corral-Flores V. , Bueno-Baques D. , Carrillo-Flores D. , Matutes-Aquino J. A.
Published: June 2006

A method for the preparation of magnetostrictive-piezoelectric particulate composites with enhanced magnetoelectric effect was developed. The composites were synthesized in situ forming a shell of barium titanate around nanoparticles of cobalt ferrite, varying the composition of the cobalt ferrite magnetostrictive phase from 20to60wt.%. Cobalt ferrite nanoparticles were obtained by coprecipitation and then added to the precursor gel of barium titanate, allowing the in situ formation of the composite and thereby restricting the contact of the ferrite particles during sintering. The samples were sintered at a temperature ranging from 1100to1250°C for 12h, followed by a plating step to be electrically poled. Additional samples were prepared by conventional mechanical milling for comparison, starting from cobalt ferrite prepared either by coprecipitation or the sol-gel technique and commercial barium titanate. Samples of same compositions prepared by different methods and sintered under the same conditions showed different behavior. For example, the in situ synthesized sample showed a piezoelectric d33 constant approximately six times larger and a magnetoelectric voltage coefficient approximately three times larger than the corresponding mechanically milled samples. The piezoelectric d33 constant decreased with the content of ferrite, achieving the maximum value of 44.6pC∕N for the in situ prepared sample with 20wt.% of ferrite sintered at 1200°C. The highest magnetoelectric effect was present in the composition of 50wt.% ferrite sintered at 1200°C, with a magnetoelectric coefficient of 1.48mV∕cmOe at room temperature.
Diode/magnetic tunnel junction cell for fully scalable matrix-based biochip
Authors: Cardoso F. A. , Ferreira H. A. , Conde J. P. , Chu V. , Freitas P. P. , Vidal D. , Germano J. , Sousa L. , Piedade M. S. , Costa B. A. , Lemos J. M.
Published: June 2006

Magnetoresistive biochips have been recently introduced for the detection of biomolecular recognition. In this work, the detection site incorporates a thin-film diode in series with a magnetic tunnel junction (MTJ), leading to a matrix-based biochip that can be easily scaled up to screen large numbers of different target analytes. The fabricated 16×16 cell matrix integrates hydrogenated amorphous silicon (a-Si:H) diodes with aluminum oxide barrier MTJ. Each detection site also includes a U-shaped current line for magnetically assisted target concentration at probe sites. The biochip is being integrated in a portable, credit card size electronics control platform. Detection of 250nm diameter magnetic nanoparticles by one of the matrix cells is demonstrated.
Analysis of random Landau-Lifshitz dynamics by using stochastic processes on graphs
Authors: Bertotti G. , Mayergoyz I. D. , Serpico C.
Published: June 2006

A randomly perturbed Landau-Lifshitz-Slonczewski equation is transformed into a stochastic differential equation for free energy. The stochastic differential equation is defined in graphs that reflect the energy landscape of magnetic system. The stochastic differential equation for energy contains additional thermally generated drift terms that may appreciably affect slow time-scale magnetization dynamics. The effect of these thermal drift terms on energy distribution near equilibria and limit cycles (self-oscillations) is discussed.
Thickness dependence of the microstructure and magnetic anisotropy of sputtered Fe50Ni50 films
Authors: Zeng Q. , Baker I. , Sun Y. , Cui J. B. , Daghlian C. P.
Published: June 2006

Fe 50 Ni 50 thin films were prepared by dc magnetron sputtering with a field of ∼40mT applied parallel to the plane of the film. The microstructure and magnetic anisotropy of the films have been studied as a function of film thickness, which was varied from 13–1100nm. Vibrating sample magnetometer and magnetic force microscope measurements show that only the very thinnest films (13nm) showed an out-of-plane magnetic component. The in-plane anisotropy strongly depended on the thickness of the film: films with thickness of ⩽20nm were magnetically isotropic with perfectly square B-H loops; when the thickness increased to 30nm, the film began to show in-plane anisotropy with the magnetic easy axis parallel to the direction of the magnetic field applied during sputtering. The easy axis direction was confirmed by both magneto-optic Kerr effect and B-H loop tracer measurements.
Competing magnetic interactions in perpendicular exchange-biased [Co∕Pt]y∕FeMn multilayers
Authors: Ji Xiaosong , Krishnan Kannan M.
Published: June 2006

Perpendicular exchange bias in multilayers arises from a complex interplay between unidirectional anisotropy at the terminating ferromagnet(FM)/antiferromagnet(AFM) interface, the perpendicular anisotropy of the FM/nonmagnet multilayer stack, and the overall magnetostatic energy of the structure. Exchange bias field (Heb) and coercivity (Hc) of [Co∕Pt]y∕FeMn with perpendicular anisotropy have been investigated by varying the thickness of a top Co layer in direct contact with the FeMn or number of Co∕Pt bilayers. An unusual dependence of Heb and Hc on these parameters has been observed. As the top Co layer thickness of [Co∕Pt]y∕FeMn multilayer varies, both Heb and coercivity Hc show a peak in values and decrease when the top Co is too thin or too thick. Heb of [Co∕Pt]y∕FeMn is inversely proportional to the number of Co∕Pt bilayers, y for 2⩽y⩽5, while Hc increases. For y>5, Heb increases and Hc decreases with y until both of them reach constant values. These observations have been attributed to the role of the effective perpendicular anisotropy of the FM multilayer, especially the FM layer adjacent to the AFM layer, in maintaining the perpendicular exchange bias.
Supergiant magnetoimpedance effect in a glass-coated microwire LC resonator
Authors: Kim Yong-Seok , Yu Seong-Cho , Le Anh-Tuan , Kim Chong-Oh , Rhee Jang-Roh , Vázquez Manuel , Hwang Myung-Joo , Lee Heebok
Published: June 2006

Supergiant magnetoimpedance (SGMI) effect was found to be present in a LC resonator consisting of a glass-coated amorphous Co83.2B3.3Si5.9Mn7.6 microwire. It is found that the form of the impedance curves as a function of applied field is highly sensitive with increasing frequency. The phase angle was also strongly found to be dependent on this field. As one approached the resonance frequency, extremely large increments of magnetoimpedance are observed. SGMI values of 400 000% are found by precise tuning of the frequency around 518.51MHz. Accordingly, the sudden change of the phase angle, as large as 180 °C, evidenced the occurrence of the resonance at a given intensity of the external dc magnetic field. These phenomena can be explained in terms of a ferromagnetic resonance in an ultrasoft magnetic microwire.
Transverse susceptibility of nanoparticle systems: The effect of interaction, dispersion, and texture
Authors: Matarranz R. , Contreras M. C. , Pan G. , Presa B. , Corrales J. A. , Calleja J. F.
Published: June 2006

The effect of texture, dispersion, and interaction on transverse susceptibility has been studied. We have developed a model based on the well-known Stoner-Wohlfarth model, by taking into account the texture, the anisotropy field value distribution, and the intergranular magnetic interactions. This model shows a good agreement with experimental measurements on granular FeCoV thin films and allows us to determine the mean value of the local magnetic anisotropy, intergrain interaction, and texture of these alloys.
Optimum design criteria for a synchronous reluctance motor with concentrated winding using response surface methodology
Authors: Lee Jung-Ho , Park Seong-June , Jeon Su-Jin
Published: June 2006

This paper presents an optimization procedure using response surface methodology (RSM) to determine design parameters for reducing torque ripple. The RSM has been achieved to use the experimental design method in combination with finite element method and well adapted to make analytical model for a complex problem considering a lot of interaction of design variables.
Mesoscopic thin-film magnetic rings (invited)
Authors: Ross C. A. , Castaño F. J. , Morecroft D. , Jung W. , Smith Henry I. , Moore T. A. , Hayward T. J. , Bland J. A. C. , Bromwich T. J. , Petford-Long A. K.
Published: June 2006

The magnetic properties and magnetoresistance of thin-film circular and elliptical magnetic rings made from Co, NiFe, NiFe∕FeMn, and Co∕Cu∕NiFe have been explored. Single-layer rings show stable onion and vortex states and metastable twisted states containing a 360° wall. For NiFe rings, four-point magnetotransport results can be explained quantitatively by anisotropic magnetoresistance. NiFe∕FeMn exchange-biased rings show offset hysteresis loops, and the easy axis is determined by a combination of the ring ellipticity and the exchange coupling. In Co∕Cu∕NiFe multilayer rings the behavior is dominated by the magnetostatic coupling between the domain walls in the Co and NiFe. In the major loop the giant magnetoresistance varies between three distinct levels corresponding to combinations of onion and vortex states in the NiFe and Co layers.
Electromagnetic analysis and control parameter estimation of moving-coil linear oscillatory actuator
Authors: Jang Seok-Myeong , Choi Jang-Young , Jeong Sang-Sub
Published: June 2006

This paper deals with an electromagnetic analysis and control parameter estimation of a moving-coil linear oscillatory actuator (MCLOA). Analytical solutions for electromagnetic characteristics of the MCLOA are obtained from transfer relations derived in terms of a magnetic vector potential and two-dimensional (2D) rectangular coordinate systems. And then, on the basis of 2D analytical solutions, control parameters such as the thrust constant, the back-emf constant, and winding inductances are estimated. Finally, analytical results for both electromagnetic characteristics and control parameters of the MCLOA are validated extensively by finite element analyses. In particular, test results such as static thrust, resistance, and inductance measurements are given to confirm the analyses.
Temperature dependence of magnetic domain structures of single-crystal Co thin films and particles
Authors: Kageyama Yasuyuki , Suzuki Takao
Published: June 2006

Submicron-sized Co particles were fabricated by patterning single-crystal Co thin films, and their magnetic domain structures were observed by magnetic force microscopy (MFM) with temperature up to 200 °C. The Co films (19–220 nm in thickness with their c axis parallel to the surface normal) were epitaxially deposited onto Al2O3 (001) single-crystal substrates by electron-beam evaporation. Prior to dot fabrication, magnetic properties of these films were measured by a vibrating-sample magnetometer and a torque meter with changing temperature from room temperature 4 to 200 °C to confirm the film’s quality, and then, from these films circular dots with diameters of 0.5, 1, and 2.5μm were patterned by electron-beam lithography and Ar-ion-beam etching. For the thick (220 nm) film case, maze patterns in domain structure were clearly observed in all types of dots. MFM images taken at elevated temperatures did not show drastic change in the domain pattern; whereas for the 56-nm-thick film’s case, thin stripe domains accompanied with large (up to 10μm) leaflike domain changes its distribution inside dots after fabrication from the plane film. Landau-Lifshitz-Gilbert simulation for domain pattern visualization was also conducted, and the results showed good agreement with experimental results with thick dot, though for the thin dot case the agreement was not so satisfactory.
Effect of interparticle interaction on the magnetic relaxation in NiO nanorods
Authors: Shim H. , Manivannan A. , Seehra M. S. , Reddy K. M. , Punnoose A.
Published: June 2006

Temperature variation (5 K–300 K) of the AC magnetic susceptibilities (χ′ and χ″) at frequency f=0.1, 1, 50, 100, 500, 1000, 2000, 5000, and 10, 000 Hz are reported in 5 nm diameter nanorods of NiO, with and without oleic acid (OA) coating. Using the peak in χ′ as the blocking temperature TB, it is observed that TB increases with increasing f. The data for the two samples fit the Vogel-Fulcher law: f=f0exp[−Ea∕k(TB−T0)], with f0=9.2×1011Hz, Ea∕k=1085 K, and T0=165 K (0 K) for the uncoated (coated) particles. This shows that T0 provides a good measure of the effects of interparticle interactions on magnetic relaxation and that these interactions are essentially eliminated with the OA coating.
Sintering behavior of spin-coated FePt and FePtAu nanoparticles
Authors: Kang Shishou , Jia Zhiyong , Zoto I. , Reed D. , Nikles David E. , Harrell J. W. , Thompson Gregory , Mankey Gary , Krishnamurthy Vemuru V. , Porcar L.
Published: June 2006

FePt and [FePt]95Au5 nanoparticles with an average size of about 4nm were chemically synthesized and spin coated onto silicon substrates. Samples were subsequently thermally annealed at temperatures ranging from 250to500°C for 30min. Three-dimensional structural characterization was carried out with small-angle neutron scattering (SANS) and small-angle x-ray diffraction (SAXRD) measurements. For both FePt and [FePt]95Au5 particles before annealing, SANS measurements gave an in-plane coherence length parameter a=7.3nm, while SAXRD measurements gave a perpendicular coherence length parameter c=12.0nm. The ratio of c∕a is about 1.64, indicating the as-made particle array has a hexagonal close-packed superstructure. For both FePt and FePtAu nanoparticles, the diffraction peaks shifted to higher angles and broadened with increasing annealing temperature. This effect corresponds to a shrinking of the nanoparticle array, followed by agglomeration and sintering of the nanoparticles, resulting in the eventual loss of positional order with increasing annealing temperature. The effect is more pronounced for FePtAu than for FePt. Dynamic coercivity measurements show that the FePtAu nanoparticles have both higher intrinsic coercivity and higher switching volume at the same annealing temperature. These results are consistent with previous studies that show that additive Au both lowers the chemical ordering temperature and promotes sintering.
Controlled grain-boundary defect formation and its role in the high-Tc ferromagnetism of Ni2+:SnO2
Authors: Archer Paul I. , Gamelin Daniel R.
Published: June 2006

Understanding the roles of defects in the ferromagnetism of oxide diluted magnetic semiconductors is a central challenge in the field of magnetism. In this paper, we report a systematic study of the activation and deactivation of high-Tc (⪢300K) ferromagnetism in Ni2+:SnO2 by gentle annealing at 100°C. We attribute this activation and deactivation to the generation and passivation of nonstoichiometric grain-boundary defects, respectively.
The mechanism of magnetic properties improvement and microstructure refinement of Zr in Nd2Fe14B
Authors: Xu Youwen , Kramer M. J. , Wu Y. Q. , Dennis K. W. , McCallum R. W.
Published: June 2006

In order to understand the role of Zr addition in Nd2Fe14B (2-14-1), a systematic study of Zr in Nd2Fe14B was done on a series of stoichiometric samples (Nd1−xZrx)2Fe14B with 0⩽x⩽0.16. As-cast ingots formed significant amounts of Nd2Fe17 starting at x=0.06, while in the melt-spun ribbons, no Nd2Fe17 was observed for the range studied. A monotonic decrease in lattice parameters for the 2-14-1 phase indicate Zr solubility in the melt-spun ribbons, but this is not observed in the ingots. Magnetic and thermal measurements on the ribbons showed that the saturated magnetization Ms (around 16kG), the spin reorientation temperature (125K), and the Curie temperature (583K) do not depend on the Zr content, while the anisotropy field Ha slightly decreased as the Zr content increased. The energy product (BHmax) and the coercive force Hc are highly dependent on the solidification and annealing process. Scanning electron microscopy and transmission electron microscopy on the as-melt spun alloys are consistent with x-ray diffraction showing Zr solubility in the 2-14-1 grains. After annealing, Zr diffuses to the grain boundaries to form Zr enriched compounds. Thus, while Zr may exhibit a metastable solubility in Nd2Fe14B, the enhanced magnetic behaviors observed in melt-spun ribbons are due to the refinement of the microstructure and enhanced grain-boundary pinning.