bluuebunny/crossref_metadata_2025 huggingface dataset:
Current detection of vortex motion in patterned S-shape wires with constrictions
Authors: Chen Yi-Chun , Yao Yeong-Der , Lee Shang-Fan , Lin Yu-An , Chen Dong-Cheng , Liou Yung
Published: June 2006

The current-driven effect on the vortex domain wall in a series-connected permalloy S-shape patterns was investigated. When a domain wall is initially formed in the wire section, in contrast to the ac result, applying a low dc current (J∼105A∕cm2) will break the degeneracy of the switching energy in different connected sections. Moreover, three kinds of effects on vortex states are observed by applying a high dc current (J∼107A∕cm2). The current applied slightly below the original switching field will drive vortices to constrictions while the current applied far below the original switching field will only perturb the vortex to another stable state. At the fields slightly over the original switching field, the domain walls are initially trapped at constrictions, injecting current will cause both the depinning and deformation of vortices.
Planar Hall effect in magnetite (100) films
Authors: Jin Xuesong , Ramos Rafael , Zhou Y. , McEvoy C. , Shvets I. V.
Published: June 2006

Giant planar Hall effect (GPHE) has been observed in epitaxial magnetite (100) films grown on MgO substrates. The effect is manifested as jumps in the transverse resistivity when the film is subjected to a swept, in-plane magnetic field. The jumps are two orders of magnitude higher than previously observed in metallic ferromagnets. Recently, the same effect has been reported for other materials, but unlike our results, they present GPHE at low temperature only. The magnitude of the GPHE observed at room temperature has potential applications such as magnetic sensors and nonvolatile memory elements.
Hard magnetic FePt nanoparticles by salt-matrix annealing
Authors: Li Daren , Poudyal Narayan , Nandwana Vikas , Jin Zhiqiang , Elkins Kevin , Liu J. Ping
Published: June 2006

To transfer face-centered-cubic (fcc) FePt nanoparticles to the face-centered-tetragonal (fct) phase with high magnetic anisotropy, heat treatments are necessary. The heat treatments lead to agglomeration and sintering of the nanoparticles. To prevent the particles from sintering, salts as the separating media (matrix) have been used for annealing the nanoparticles in our experiments. The fcc nanoparticles produced by chemical synthesis were mixed with NaCl powders. The mixture was then annealed in forming gas (93%H2+7%Ar) in different conditions to complete the fcc to fct phase transition. After the annealing, the salt was washed out by water and monodisperse fct FePt nanoparticles were obtained. Detailed studies on the effect of the NaCl-to-FePt weight ratios (from 1:1 to 400:1) have been performed. It was found that a suitable NaCl-to-FePt ratio is the key to obtain monodisperse fct FePt nanoparticles. A higher NaCl-to-FePt ratio is needed for larger particles when the annealing conditions are the same. Increased annealing temperature and time should be accompanied by a higher NaCl-to-FePt ratio. Magnetic measurements show very high coercivity (up to 30 kOe) of the monodispersed fct nanoparticles by the salt-matrix annealing.
Nature of magnetism in Co- and Mn-doped ZnO prepared by sol-gel technique
Authors: Manivannan A. , Dutta P. , Glaspell G. , Seehra M. S.
Published: June 2006

Magnetic properties of sol-gel-prepared bulk samples of Co0.05Zn0.95O and Mn0.05Zn0.95O are reported before and after annealing in 5%H2∕95%Ar at 573 K for 6 h. The as-prepared samples are paramagnetic with the magnetic susceptibility χ following the Curie-Weiss law: χ=χ0+C∕(T−θ). The magnitudes of C are consistent with the magnetic moments expected for the Co2+ and Mn2+ states. After hydrogenation, the magnetism of Mn∕ZnO is unchanged but Co∕ZnO acquires room-temperature ferromagnetism (RTFM) with a magnetic moment of 0.35μB∕Co site and hysteresis loop with coercivity Hc≃600Oe, remanence Mr≃0.45emu∕g, and saturation magnetization Ms≃1.2emu∕g. Electron magnetic-resonance spectroscopy at 9.28 GHz gives signals corresponding to the Co2+ and Mn2+ states for the paramagnetic states and a broad FM signal for the hydrogenated Co∕ZnO. This difference under hydrogenation between Co∕ZnO and Mn∕ZnO suggests that n-type doping leads to stabilizing of RTFM in Co∕ZnO but not in Mn∕ZnO, the latter perhaps requiring p-type doping.
Fabrication and magnetic properties of nanopatterned FePt media
Authors: Tang Y. J. , AuBuchon J. F. , Chen L. H. , Jin S. , Kim J. W. , Kim Y. H. , Yoon C. S.
Published: June 2006

A method for fabricating nanopatterned media has been investigated. A monolayer of regularly distributed Ni nanoparticles of uniform diameter (∼5nm) fabricated on the surface of a polyimide-coated Si substrate using metal film-polyamic acid interactions was utilized as a protruding template to deposit an array of FePt phase. A FePt film (5 nm thick) was sputtered onto this template with or without an underlayer of either 5-nm-thick MgO or CoO and analyzed before and after annealing. It was observed that while the FePt films sputtered on flat Si substrates with underlayers exhibited low Hc of only about 1 kOe after the L10 conversion, the FePt samples deposited onto protruding Ni nanoparticle templates showed a significant increase in Hc to ∼8.1kOe. The FePt sample with CoO underlayer exhibited a possibly strong perpendicular magnetic anisotropy. The intergranular exchange coupling of the samples was also investigated by using ΔM method.
Magnetic properties of one-dimensional quasiperiodic Co∕Pt multilayers
Authors: Zhu L. Y. , Cheng X. M. , Chien C. L.
Published: June 2006

We have fabricated Fibonacci [Co∕Pt] multilayers with two constituent Co∕Pt bilayers, as well as periodic multilayers of the same Co∕Pt bilayers, all of which exhibit perpendicular magnetic anisotropy. Magnetic properties of the quasiperiodic and periodic [Co∕Pt] multilayers have been studied macroscopically by vibrating sample magnetometry and microscopically by magnetic force microscopy. The Fibonacci [Co∕Pt] multilayers show enhanced squareness in magnetic hysteresis loops compared to the corresponding periodic [Co∕Pt]n multilayers, due to a smaller density of isolated nucleation sites in the saturation state.
Magnetic properties of hybrid polymer bonded Nd–Fe–B/ferrite magnets
Authors: Wang X. F. , Lee D. , Jiang Z. L.
Published: June 2006

The influences of constituents on magnetic properties, thermal stabilities, and magnetizing abilities have been investigated for hybrid bonded magnets containing Nd–Fe–B/ferrite or two distinctly different Nd–Fe–B materials. The experiments indicated that the Br of the hybrid Nd–Fe–B/ferrite magnets increases nonlinearly with increasing volumetric portion of Nd–Fe–B. The values of Br are higher than the volumetric averages of the constituents in the range of Nd–Fe–B content below 50 vol %. For the 40 wt % Nd–Fe–B/ferrite magnet, the magnetic properties of Br=3.8kG, Hcm=4.6kOe, and (BH)max=3.2MGOe were attained. The temperature coefficient of Hcm in the temperature range from 25 to 150 °C is −17%∕°C and obviously improved over the corresponding straight Nd–Fe–B magnet. The aging losses of open circuit induction at 100 and 125 °C decreased with increasing ferrite portion in hybrid Nd–Fe–B/ferrite magnets. By mixing 40 wt % of Nd14B5.8Fe80.2 and 60 wt % of Nd10.4B5.6Co2Fe82, the Br,Hcm, and (BH)max of the magnet reached to 7 kG, 9 kOe, and 9.7 MG Oe, respectively, which are similar to the magnetic properties of commercial MQ1-B magnet. However, the total rare-earth content is lower, and the magnetizing ability at low magnetizing field is better.
Unusual magnetization reversal in [Co∕Pt]4 multilayers with perpendicular anisotropy
Authors: Cheng X. M. , Nikitenko V. I. , Shapiro A. J. , Shull R. D. , Chien C. L.
Published: June 2006

Unusual magnetization reversal of [Co(4Å)∕Pt(10Å)]4 multilayers with perpendicular magnetic anisotropy has been revealed macroscopically by magnetometry measurements and microscopically by magneto-optical Kerr effect microscopy and magnetic force microscopy (MFM) imaging. During the first-order reversal process, the magnetization first decreases, then reaches a plateau, and finally rises back to saturation, corresponding to expanding bubble domains, stationary domains, and fading contrast but unchanged boundary domains, respectively. MFM imaging reveals the existence of many submicron-scaled unreversed channels within the boundary of the “bubble” domains. The magnetization reversal behavior can be accounted for by the evolution of the unusual domain structures in different field regimes.
Influence of ferromagnetic thickness on structural and magnetic properties of exchange-biased manganite superlattices
Authors: Campillo G. , Gomez M. E. , Berger A. , Hoffmann A. , Escudero R. , Prieto P.
Published: June 2006

The structural and magnetic properties of superlattice structures of alternating ferromagnetic La2∕3Ca1∕3MnO3 (F-LCMO) and antiferromagnetic La1∕3Ca2∕3MnO3 (AF-LCMO) layers were systematically studied as functions of F-LCMO layer thickness, tF. Samples were grown via a high-oxygen pressure sputtering process. Magnetic hysteresis measurements after field cooling revealed an exchange bias, Hex, at low temperatures in such superlattices. We found a correlation of the structural and magnetic properties with tF. In particular, we observed diminished resistance, increased metal-insulator transition temperature, TMI, as well as increased Curie temperature with increasing tF. Additionally, we found that the temperature dependence of Hex*tF for superlattices with the same antiferromagnetic layer thickness, tAF, is a unique function and independent of tF. We also find that the low-temperature saturation magnetization, MS, follows a power-law dependence with temperature, according to M0(1−BTα) with an exponent of α=2.5±0.2.
Investigation of Cr substitution in Co ferrite (CoCrxFe2−xO4) using Mossbauer spectroscopy
Authors: Krieble K. , Lo C. C. H. , Melikhov Y. , Snyder J. E.
Published: June 2006

Substitution of other metals for Fe in cobalt ferrite has been proposed as a method to tailor the magnetic and magnetoelastic properties for sensor and actuator applications [H. Zheng et al., Science 303, 661 (2004)]. However, to understand the effect of Cr substitution, one needs atomic-level information on the local environments and interactions of the transition-metal ions. In this study, Mossbauer spectroscopy was used to investigate the local environments of the Fe atoms in these materials. A series of five powder samples with compositions CoCrxFe2−xO4 (x=0.0 to 0.8) was investigated using transmission geometry. Results show two distinct six-line hyperfine patterns, indicating Fe in A and B spinel sites. Increasing Cr concentration is seen to decrease the hyperfine field strength for both A and B sites, as well as increasing the width of those distributions. Results for Cr substitution show generally similar behavior to a prior study using Mn; however, Cr substitution has more pronounced effects: the hyperfine fields decrease and distribution widths increase at greater rates for Cr substitution, and the differences between A and B site behavior are more pronounced. Results are consistent with a model in which Cr has an even stronger B-site preference than Mn, and displaces more of the Co from the B to the A sites.
Electron doping in the double perovskite LaxA2−xCrWO6 with A=Sr and Ca
Authors: Geprägs S. , Majewski P. , Gross R. , Ritter C. , Alff L.
Published: June 2006

The ferrimagnetic double perovskites can have high Curie temperatures TC above 600K. In the system Sr2FeMoO6 (TC≈420K) electron doping by substituting, for example, La3+ for Sr2+ ions leads to a considerable increase in TC. Here we report the same effect for LaxCa2−xCrWO6. This demonstrates that electron doping generically increases TC in the ferrimagnetic double perovskites. However, in the system Sr2CrWO6 (TC≈450K) the preferential formation of LaCrO3 prevents electron doping by La substitution as supported by neutron scattering and x-ray diffraction.
Lorentz microscopy of elliptical magnetic rings
Authors: Bromwich T. J. , Petford-Long A. K. , Castaño F. J. , Ross C. A.
Published: June 2006

A study of the magnetization reversal process of NiFe elliptical rings has been made as a function of ring size and ring orientation with respect to the applied field. The long axis of the rings ranged from 600nm to 6μm and the ring width and thickness were fixed at ∼220 and 20nm, respectively. The magnetization reversal mechanism was characterized using Lorentz transmission electron microscopy as a function of the angle between the long axis of the ellipse and the applied field. A decrease in the range of stability of the intermediate vortex state with increasing angle between the long axis and the applied field was observed. The structure of the remanent domain walls was dependent on the size of the ring, and a phase diagram of domain-wall type as a function of ring geometry calculated from Landau-Lifshitz-Gilbert simulations is presented for comparison with the experimental data.
Magnetic properties, microstructure and phase evolution of (Ce1−xPrx)9.5Febal.CoyTi2B10 (x=0–1 and y=0, 2.5) nanocomposites
Authors: Chen C. H. , Chiu C. H. , Chang H. W. , Chang C. W. , Chang W. C.
Published: June 2006

Magnetic properties, microstructure, and phase evolution of (Ce1−xPrx)9.5Febal.CoyTi2B10 (x=0–1 and y=0, 2.5) nanocomposites have been investigated. A progressive substitution of Pr for Ce enhances the remanence (Br), intrinsic coercivity (Hci), and maximum energy product [(BH)max] of the ribbons effectively. In addition, the Curie temperature of 2:14:1 phase also increases with increasing Pr concentration. For Co-free (Ce1−xPrx)9.5Febal.Ti2B10 ribbons, magnetically hard 2:14:1 phase and two soft phases, α-Fe and Fe3B, are found in the ribbons with x=0, 0.3, 0.4, and 0.7, while Fe3B phase disappears in the ribbons with x=1. On the other hand, the value of Br, (BH)max, temperature coefficient α, and Curie temperature of 2:14:1 and α-Fe are improved with a slight substitution of 2.5 at. % Co for Fe. The magnetic properties of Br=8.5kG, Hci=7.5kOe, (BH)max=13.5MGOe, α=−0.185%∕°C, and β=−0.54%∕°C can be achieved in (Ce0.5Pr0.5)9.5Febal.Co2.5Ti2B10 nanocomposites.
Magnetic soft x-ray microscopy at 15 nm resolution probing nanoscale local magnetic hysteresis (invited)
Authors: Kim Dong-Hyun , Fischer Peter , Chao Weilun , Anderson Erik , Im Mi-Young , Shin Sung-Chul , Choe Sug-Bong
Published: June 2006

Recent progress in x-ray optics has pushed the lateral resolution of soft x-ray magnetic microscopy to below 15 nm. We have measured local magnetic hysteresis on a nanometer scale at the full-field x-ray microscope XM-1 at the Advanced Light Source in Berkeley, approaching fundamental length scales such as exchange lengths, Barkhausen lengths, and grain diameters. We have studied the evolution of magnetic domain patterns in a nanogranular CoCrPt film with a pronounced perpendicular magnetic anisotropy and revealed nanoscopic details associated with the granular film structure. From a quantitative analysis of the field-dependent magnetic domain patterns, we are able to generate local magnetic hysteresis map on a nanometer scale. Our findings indicate a significant variation of local coercive fields corresponding to the nanoscopic behavior of magnetic domains.
Effect of varying sputtering parameters on the observation of magnetic domains using high-resolution Bitter microscopy technique
Authors: Shah Prasanna , Gavrin A.
Published: June 2006

High-pressure-sputtered iron nanoparticles have been synthesized in a gun designed for this purpose. We observe a gradual increase in the particle size with increasing gas pressure inside the gun. X-ray-diffraction studies indicate that the major phase is bcc iron and the super conducting quantum interference device measurements clearly show that the particles are ferromagnetic at room temperature. We have used these particles to image the domains on magnetic recording media using high-resolution Bitter microscopy and have studied the quality of the images thus produced over a range of sputtering conditions.
Tunnel magnetoresistance in epitaxial magnetic tunnel junctions using full-Heusler alloy Co2MnGe thin film and MgO tunnel barrier
Authors: Marukame Takao , Ishikawa Takayuki , Matsuda Ken-ichi , Uemura Tetsuya , Yamamoto Masafumi
Published: June 2006

We fabricated fully epitaxial magnetic tunnel junctions (MTJs) using a Co-based full-Heusler alloy Co2MnGe (CMG) thin film as a lower electrode, an MgO tunnel barrier, and a Co50Fe50 upper electrode and investigated their tunnel magnetoresistance (TMR) characteristics. The microfabricated MTJs showed strongly temperature-dependent TMR characteristics with typical TMR ratios of 70% at 7 K and 14% at room temperature (RT). Furthermore, the TMR characteristics exhibited the following notable features in the bias voltage (V) dependence: (1) a cusplike V dependence within a range of ∼±200mV around V=0 at 7 K, which was smeared out at RT, and (2) a V dependence with pronounced asymmetry regarding the polarity, showing negative TMR ratios in a certain negative bias voltage range around −400mV at both 7 K and RT (V was defined with respect to the lower CMG electrode). A possible transport mechanism leading to the notably asymmetric V dependence along with the negative TMR for a certain bias voltage region is direct tunneling that reflects the spin-dependent density of states of the CMG electrode.
Monte Carlo analysis of magnetic aftereffect phenomena
Authors: Andrei Petru , Stancu Alexandru
Published: June 2006

Magnetic aftereffect phenomena are analyzed by using the Monte Carlo technique. This technique has the advantage that it can be applied to any model of hysteresis. It is shown that a logt-type dependence of the magnetization can be qualitatively predicted even in the framework of hysteresis models with local history, such as the Jiles-Atherton model. These models are computationally much more efficient than the models with global history such as the Preisach model. Numerical results related to the decay of the magnetization as of function of time, as well as to the viscosity coefficient, are presented.
Micromagnetic simulation study on exchange-coupled composite media
Authors: Lee S. C. , Lee T. D.
Published: June 2006

The magnetic dynamics of the total recording system with a single pole head, recording layer (RL), and semihard layer (SHL) or soft underlayer were examined by a three-dimensional micromagnetic model to understand the recording characteristics of the composite perpendicular media. The effects of the exchange strength and the SHL properties on read/write properties and the possibility of the composite media for high recording density were investigated. By comparing the three different cases: (i) single layer, (ii) conventional double-layered perpendicular media, and (iii) exchange-coupled composite media, the improvement of writability and signal-to-noise ratio (SNR) was studied. The effect of interlayer exchange coupling (Jex) between the RL and the SHL on SNR in the exchange-coupled composite media will be discussed. The effect of nonmagnetic grain-boundary region in the RL and the SHL on read/write properties will be also discussed.
Optically pumped spin-polarized carrier transport across Fewire∕GaAs interfaces
Authors: Taniyama T. , Wada E. , Yamazaki Y.
Published: June 2006

Spin-selective transport of optically pumped spin-polarized electrons across 5-nm-thick Fewires∕GaAs and 5-nm-thick Fewires∕AlOx(1.5nm)∕GaAs interfaces is studied as functions of magnetic field and temperature. We find characteristic temperature dependence of the spin selectivity at the Fe∕AlOx∕p-GaAs interface, where the selectivity shows a maximum at 200 K, which we attribute to the tunneling effect of spin-polarized electrons across the AlOx layer in the Fe∕AlOx∕p-GaAs structure.
Depinning fields of a magnetic domain wall from asymmetric notches
Authors: Himeno A. , Kasai S. , Ono T.
Published: June 2006

Depinning fields of a magnetic domain wall (DW) from asymmetric notches artificially introduced into submicron magnetic wires were investigated by utilizing the giant magnetoresistance effect. It was found that the depinning field from the asymmetric notch depends on the propagation direction of the DW. The asymmetric notch works as an asymmetric potential barrier against the DW propagation. The depinning field increases with increasing slope of the asymmetric notch.
Magnetic anisotropy and crystal structure of Co–P films synthesized by electrodeposition from alkaline electrolytes
Authors: Xu X. , Zangari G.
Published: June 2006

Co–P and Co films were electrodeposited on polycrystalline Au from a Co-sulphamate-based alkaline solution with ammonium citrate as complexing agent, with or without Na hypophosphite as the P source. Pure cobalt films exhibit a hexagonal-close-packed (hcp) structure. Scanning electron microscopy images show an acicular microstructure, with shorter features at larger current densities. In the presence of the P source, the films show diffraction peaks only at very small deposition current densities (up to 2mA∕cm2), when more packed and slightly elongated grains are formed and x-ray-diffraction (XRD) patterns exhibit hcp (10.0) and (11.0) peaks. With increasing current density, P content in the films increases, the apparent grain size decreases, and the films lose crystallinity, as detected by XRD. Co films show in-plane magnetization and coercivity less than 100 Oe. The codeposition of P results in an increased coercivity of crystalline films, up to about 900 Oe at 100 nm thickness. Amorphous Co–P films obtained at high current density become very soft and for a thickness larger than 400 nm exhibit a stripe domain structure.
Magnetic behavior of rapidly solidified Pr–Co alloys with the TbCu7-type structure
Authors: Kostogorova J. , Shield J. E.
Published: June 2006

Rapid solidification has been utilized to produce a series of Pr–Co alloys between the Pr2Co17 and PrCo5 stoichiometries. In this system, PrCo5 has easy axis magnetization while Pr2Co17 has easy-plane magnetization. Alloys of the form (PrxCo1−x)94Ti3C3 with x ranging from 0.105 corresponding to the Pr2Co17 compound to x=0.167 corresponding to the PrCo5 compound were produced by melt spinning at a tangential wheel speed of 40m∕s. The rapid solidification and alloying additions were found to suppress the formation of the Pr2Co17 ordered phase, leading to the formation of the disordered TbCu7-type structure over a range of Pr∕Co ratios. Hysteresis loops were characterized by a smooth demagnetization curve reflective of single-phase demagnetization. Heat treatment at 800 °C led to the formation of the Pr2Co17 and PrCo5 phases, and the presence of the soft magnetic Pr2Co17 phase drastically decreased the coercivity. The soft magnetic behavior was consistent with in-plane magnetization of the Pr2Co17 structure that formed during heat treatment. However, the relatively high coercivity observed in the as-solidified alloys with the disordered TbCu7-type structure suggests that dumbbell disorder may create easy axis magnetization, and changes in saturation magnetization also imply that dumbbell configuration is important to the magnetic properties.
Large uniaxial magnetic anisotropy by lattice deformation in CoPt∕Ru perpendicular films
Authors: Shimatsu T. , Sato H. , Okazaki Y. , Aoi H. , Muraoka H. , Nakamura Y. , Okamoto S. , Kitakami O.
Published: June 2006

A large uniaxial magnetic anisotropy Ku of about 2×107erg∕cm3 was obtained in Co–(20–30) at. % Pt perpendicular films deposited on Ru seed layers. The films showed good c-axis orientation perpendicular to the film plane, and no evidence of a Co3Pt-type-ordered structure was found in the X-ray-diffraction patterns. The value of Ku (the sum of the first- and second-order terms of uniaxial anisotropy, Ku1 and Ku2) increased as the thickness of the CoPt layer decreased, and reached about 2×107erg∕cm3 at 2 nm (Co–20 at. % Pt films) or 5 nm (Co–(25–30) at. % Pt films). The ratio of Ku2 to Ku1 was 0.16 in 40 nm films, but reduced to less than 0.03 for thicknesses below 5 nm. The experimental results suggest that, besides the surface anisotropy, the increase in Ku on reducing the thickness is mostly related to the reduction of the hcp-CoPt lattice c∕a ratio, probably due to epitaxial growth of CoPt on the Ru seed layer.
Effect of length and annealing conditions on magnetoimpedance of Co68Fe5Si12B15 amorphous ribbons
Authors: Murthy V. Satya Narayana , Venkatesh S. , Markandeyulu G.
Published: June 2006

The effect of conventional annealing and Joule annealing on magnetoimpedance (MI) of Co68Fe5Si12B15 ribbons was studied. The ribbons used for the MI measurements were 1 and 5 cm long. The as cast 5-cm-long ribbons exhibit a larger MI compared with the 1-cm-long ribbons, at all frequencies. The maximum MI of the as cast ribbons of 5 cm length is 17% at 600 kHz whereas for the 1-cm-long ribbons it is 3% at 500 kHz. Conventional annealing carried out at 200 and 300 °C for 1 h results in the decrease of the MI in both the 5 and 1-cm-long ribbons, respectively. The Joule annealing of the 5-cm-long ribbons employing a current of 500 mA for 5 min. causes the increase of the MI to 26% at 800 kHz. For the 1 cm Joule-annealed ribbon, employing a current of 500 mA for 5 min., the maximum MI observed is 6% at 500 kHz.
The high-speed operation of single phase switched reluctance motor considering magnetic saturation
Authors: Ahn JoonSeon , Won Sung Hong , Lee Ju
Published: June 2006

In the high-speed operation of SRM, the conventional pulse width modulation (PWM) drive method is not available because of the limitation of switching speed; therefore the single-pulse drive method is commonly employed. On the contrary, the use of the single-pulse drive method cannot avoid the overcurrent in the low-speed operation because of the insufficient back emf and the difficulty of duty control. With these reasons, the switching method is commonly changed from PWM at the low-speed operation to the single-pulse method at the high-speed operation. In the fan application, the required load torque increases as a square of the fan speed; it requires more current for the torque generation. Therefore at the mode transition between PWM and single-pulse drive, it is unavoidable that the phase current rapidly increases if the nonlinearity of inductance to the current is not considered. In this paper, by using finite element method (FEM) which is considered with the nonlinearity of the inductance with respect to the current, the speed of mode transition is calculated (18 000 rpm) and verified by the experiment.
Electrical spin injection and accumulation in ferromagnetic/Au/ferromagnetic lateral spin valves
Authors: Ku J. , Chang J. , Han S. , Ha J. , Eom J.
Published: June 2006

Spin injection and accumulation have been investigated for Ni81Fe19∕Au∕Ni81Fe19 lateral spin valves with various separations between spin injector and detector. The spin valve has been studied by using two distinct measurement configurations: the local and nonlocal measurement configurations. For both configurations, a spin valve signal has been found in the wide temperature range from 15to300K, and the magnitude of the spin valve signal decreases as the temperature increases. Based on the nonlocal spin valve signals, we estimate that the spin-diffusion length of a Au film is 85nm and that the spin-injection efficiency is 73% at T=15K.
Design and dynamic analysis of permanent magnet linear synchronous machine for servoapplication
Authors: Jang Seok-Myeong , You Dae-Joon , Jang Won-Bum , Han Sang-Chul
Published: June 2006

In servosystem demanding precision dynamic characteristics, application of the permanent magnet linear synchronous machines (PMLSMs) has an advantage of analysis convenience by simple geometry and thrust ripple reduction from the sinusoidal back electromotive force and excited stator. Therefore, this paper presents the design of surface-mounted PMLSM with a slotless iron-cored stator according to coil turns to satisfy the rate thrust. Also, from the dynamic analysis for servoapplication of manufactured motor with heavy mass, we offer an accurate range of the dc link voltage and acceleration in rate speed. This is applied to speed reference profile considering the system characteristics in total length of moving position.
Characterization of magnetic properties at edges by edge-mode dynamics
Authors: Maranville B. B. , McMichael R. D. , Kim S. A. , Johnson W. L. , Ross C. A. , Cheng Joy Y.
Published: June 2006

We have used “trapped spin wave” or edge modes of magnetic precession to probe the magnetic environment near magnetic film edges magnetized perpendicular to the edge. Micromagnetic models of dynamics in stripes reveal that the edge mode frequency-field relationship depends on whether the edge surface is vertical or tapered, while the “bulk” modes are nearly unaffected. The models show the edge-mode frequency going to zero at the edge saturation field. This critical field becomes much less distinct for applied fields misaligned from the edge normal by as little as 1°. Ferromagnetic-resonance and Brillouin light-scattering measurements of the edge modes in an array of 480-nm-wide×12-nm-thick Ni80Fe20 stripes have a lower edge saturation field than the vertical edge models, but agree well with the model of 45°-tapered edges.
The influence of shape and structure on the Curie temperature of Fe and Co nanoparticles
Authors: Evans R. , Nowak U. , Dorfbauer F. , Shrefl T. , Mryasov O. , Chantrell R. W. , Grochola G.
Published: June 2006

We have investigated the effect of lattice fluctuations on the magnetic properties of nanoparticles of Fe and Co. Atomic structures were simulated using a molecular-dynamic approach, with the system slowly cooled into the ordered phase. The magnetic properties were then simulated using an atomistic approach using a classical spin Hamiltonian taking into account the long-range nature and atomic separation dependence of the exchange. The magnetic properties are found to be affected by both the particle shape and the lattice fluctuations. For a perfectly ordered lattice we find that a spherical particle has a larger magnetization for a given temperature than a cube containing the same number of atoms. We have also studied the effect of lattice fluctuations. This involves a comparison of M(T) for two cases, firstly, a nanoparticle with a fixed lattice corresponding to the low-temperature annealed state (T=20K), and secondly a nanoparticle with a lattice structure equilibrated at the temperature T, the latter case being subject to fluctuations in the lattice spacing, and the nanoparticle shape. The dynamic structure gives rise to a reduction in the magnetization and Tc, which is a finite size effect to be considered beyond others such as the reduction in coordination at the nanoparticle surface.
Manipulating polar ferromagnetism in transition-metal-doped ZnO: Why manganese is different from cobalt (invited)
Authors: Kittilstved Kevin R. , Gamelin Daniel R.
Published: June 2006

High-temperature magnetic ordering in Mn2+- and Co2+-doped ZnO diluted magnetic semiconductors has been predicted theoretically and confirmed experimentally to have different charge-carrier requirements. This paper summarizes some of these experimental and theoretical results and relates the different carrier polarity requirements for 300K ferromagnetism in Mn2+:ZnO and Co2+:ZnO to differences in the charge-transfer electronic structures of these two materials.
Determination of spin polarization of Gd and Dy by point-contact Andreev reflection
Authors: Valentine J. M. , Chien C. L.
Published: June 2006

We report measurements of spin polarization P of c-axis Gd and Dy films, epitaxially grown on (112¯0) sapphire substrates with a W(110) seed layer. Magnetometry measurements show that the Gd layer is ferromagnetic with TC≈293K, whereas Dy is antiferromagnetic below TN≈180K followed by a transition to ferromagnetic ordering at a lower temperature. We have obtained P=52% and 50% for c-axis Gd and Dy films, respectively, using the point-contact Andreev reflection (PCAR) technique with quantitative analyses. These values are in good agreement with previous PCAR results of 45% (Gd), but differ substantially from results measured using superconducting tunnel junctions of 14% (Gd) and 6.8% (Dy).
Manipulation of nonmagnetic nanobeads in dilute ferrofluid
Authors: Halverson D. , Kalghatgi S. , Yellen B. , Friedman G.
Published: June 2006

Patterns of submicron Co islands in conjunction with a uniform, static, or rotating magnetic field are used to demonstrate the possibility of assembling 100–300nm nonmagnetic latex beads in designated locations and manipulating their movements on surfaces.
Surface energy and adhesion of perfluoropolyether nanofilms on carbon overcoat: The end group and backbone chain effect
Authors: Chen Haigang , Li Lei , Merzlikine Alexei G. , Hsia Yiao-Tee , Jhon Myung S.
Published: June 2006

In this paper, we have investigated the surface energy and adhesion of one functional PFPE (Zdol) and two series of nonfunctional PFPEs (Z and D) on carbon-overcoated disk surfaces. The effects of end group functionality, backbone chain flexibility, molecular weight, and film thickness were systematically examined. Our results indicated that nonfunctional PFPEs have weak attraction with carbon overcoat. However, due to backbone chain effect, Z has slightly stronger attraction than D. Based on the surface energy analyses and bonded thickness results, schematic bonding models were proposed, which indicate strong hydrogen bonding∕ordered packing structure∕low mobility for functional PFPE films and weak attraction∕less-ordered packing structure∕high mobility for nonfunctional PFPE films.
Study of the magnetic behavior of single-crystalline Nd0.5Sr0.5MnO3
Authors: Venkatesh R. , Nirmala R. , Rangarajan G. , Malik S. K. , Sankaranarayanan V.
Published: June 2006

Zero-field-cooled and field-cooled magnetization data in single-crystalline Nd0.5Sr0.5MnO3 revealed the existence of thermodynamic irreversibility below Curie temperature (TC), indicating the presence of frustration of spins. The imaginary part of ac susceptibility (χ″) indicates a prominent cusp below TC, which shifts to higher temperatures with increasing frequency. Magnetization, ac susceptibility, resistivity, and specific-heat measurements were used to find three critical exponents α=0.12(1), β=0.5(0), and γ=1.02(2). The values of the critical exponents are all between mean-field values and three-dimensional Heisenberg model values.
Midpoint numerical technique for stochastic Landau-Lifshitz-Gilbert dynamics
Authors: d’Aquino M. , Serpico C. , Coppola G. , Mayergoyz I. D. , Bertotti G.
Published: June 2006

The implicit midpoint time-integration technique is applied to the stochastic Landau-Lifshitz-Gilbert (LLG) equation. The numerical scheme converges to the Stratonovich solution in the limit of vanishing time step. It preserves the magnetization magnitude and the main energy balance properties of the LLG equation independently of the time step. The numerical technique is then applied to the study of superparamagnetic state in a small spheroidal particle, and the numerical results are compared with the theory.
Magnetic states in wide annular structures
Authors: Kläui M. , Rüdiger U. , Vaz C. A. F. , Bland J. A. C. , Cherifi S. , Locatelli A. , Heun S. , Pavlovska A. , Bauer E. , Heyderman L. J.
Published: June 2006

The results of nonintrusive high-resolution imaging of the magnetic states present in wide Co rings with a small inner hole (1.7μm outer diameter, 300nm inner diameter, and 700nm width) are presented. Rather than the reproducible onion state commonly found in rings, a prevalent triangle state is observed, with other magnetic states characteristic of disks also occurring. The uniformity and the magnetic states in these wide rings are found to be more typical of disks than of rings. Analyzing the energetics of the formation of the magnetic states, this is attributed to the reduced shape anisotropy in wide rings as compared to narrow rings.
High saturation magnetization and soft magnetic properties of nanocrystalline (Fe,Co)90Zr7B3 alloys annealed under a rotating magnetic field
Authors: Suzuki K. , Ito N. , Garitaonandia J. S. , Cashion J. D.
Published: June 2006

The effects of magnetic-field annealing on the soft magnetic properties of nanocrystalline (Fe1−xCox)90Zr7B3 (x=0, 0.1, 0.2, and 0.3) have been investigated. The annealing-induced uniaxial anisotropy (Ku) in these alloys was found to follow an x2 dependence and Ku appears to reflect the population density of Co–Co atomic pairs. The annealing-induced anisotropy is suppressed by applying a rotating field during annealing and the coercivity is reduced by up to 60%. Nanocrystalline (Fe0.8Co0.2)90Zr7B3 annealed under a rotating field of 640kA∕m exhibits a low coercivity of 9.1A∕m and high permeability values (under 0.4A∕m) of 4500 at 1 kHz and 3100 at 100 kHz along with a high saturation magnetization of 1.74 T. These permeability values are comparable to those of Fe-based amorphous alloys while the saturation magnetization is clearly higher than those of amorphous or nanocrystalline soft magnetic alloy ribbons developed to date.
Effects of substrate bias on CoCrPt-SiO2 magnetic recording media
Authors: Lee Hwan-Soo , Bain James A. , Laughlin David E.
Published: June 2006

CoCrPt and CoCrPt-SiO2 magnetic layers were investigated as a function of substrate bias. It was found that the use of substrate bias greatly modified the CoCrPt-SiO2 film microstructure. This was due to a large change in the SiO2 content during bias sputtering. Biasing had an effect of reducing the amount of oxide in the growing films and promoting the growth of larger grains, but it was also found to be very effective in decoupling the grains as determined by ΔM curves. In addition, in the biased films, a large dependence of the coercivity on film thickness was observed. The in-plane coercivity nearly doubled its value from 2400 to 4300 Oe as the thickness increased from 5 to 15 nm.
Fabrication of ultrathin Ni–Zn ferrite films using electron cyclotron resonance sputtering method
Authors: Tanaka Terumitsu , Kurisu Hiroki , Matsuura Mitsuru , Shimosato Yoshihiro , Okada Shigenobu , Oshiro Kazunori , Fujimori Hirotaka , Yamamoto Setsuo
Published: June 2006

Well-crystallized Ni–Zn ferrite (Ni0.4Zn0.6Fe2O4) highly oriented ultrathin films were obtained at a substrate temperature of 200 °C by a reactive sputtering method utilizing electron cyclotron resonance microwave plasma, which is very effective to crystallize oxide or nitride materials without heat treatment. Thin films of Ni–Zn ferrite deposited on a MgO (100) underlayer showed an intense X-ray-diffraction peak of (400) from the Ni–Zn ferrite as compared to similar films deposited directly onto thermally oxidized Si substrates. A 1.5-nm-thick Ni–Zn ferrite film, which corresponds to twice the lattice constant for bulk Ni–Zn ferrite, crystallized on a MgO (100) underlayer.
A survey of anisotropy measurement techniques and study of thickness effect on interfacial and volume anisotropies in Co∕Pt multilayer media
Authors: Guo Vickie W. , Lu Bin , Wu Xiaowei , Ju Ganping , Valcu Bogdan , Weller Dieter
Published: June 2006

A survey of various measurement techniques, including the rotation method, the hard-axis hysteresis loop, and the torque measurement, is presented using Co∕Pt multilayer recording media. It is found that for highly exchange-coupled samples, the results from the rotational method are consistent with those from the hard-axis hysteresis loop and torque measurements. Such anisotropy measurements are applied to determine the interfacial anisotropy in Co∕Pt multilayers (MLs) by studying a series of Co∕Pt ML samples with Co thickness ranging from 0.1 to 2 nm. The Pt thickness is fixed at either 1 or 0.5 nm. It is found that the multilayer with 1 nm Pt layer shows a larger interfacial anisotropy constant (KS∼0.8erg∕cm2) than the media with 0.5 nm Pt layer (KS∼0.6erg∕cm2), corresponding to about 80% and 60% of the interfacial anisotropy of the ML made by molecular-beam epitaxy, respectively [D. Weller et al., Mater. Res. Soc. Symp. Proc. 313, 791 (1993)]. Such thickness dependence is supported by the x-ray-diffraction superlattice peak measurements that indicate the discontinuous growth for the case with thinner Pt.
Structure and magnetic properties of FePt and FePt–Ag nanostructured magnets by cyclic cold rolling
Authors: Cui B. Z. , Han K. , Garmestani H. , Schneider-Muntau H. J.
Published: June 2006

This paper reports phase components, nanostructure, coercivity mechanism, and hard magnetic properties of FePt and FePt–Ag bulk nanostructured magnets prepared by cyclic cold rolling and subsequent annealing. Annealing temperature of 500 °C or higher and annealing time of 120 min or longer are necessary for the as-rolled nanolaminate foils to complete the diffusion and ordering phase transformation. The addition of a small amount of Ag to the as-rolled Fe∕Pt foils results in an increase of both the ordering temperature and relative volume fractions of Fe3Pt and FePt3 phases. The diffusion and phase formation mechanism linked to the specific nanostructures of the cold-rolled foils are discussed. The coercivity mechanism of FePt and FePt–Ag nanostructured magnets is analyzed by Gaunt’s domain-wall pinning model.
Multiscale analysis of magnetization dynamics driven by external fields
Authors: Dimian M. , Mayergoyz I. D. , Bertotti G. , Serpico C.
Published: June 2006

The multiscale approach to the magnetization dynamics driven by external magnetic fields is presented. In this approach, the Landau-Lifshitz equations are represented in an equivalent form, where the magnetic free energy is used as one of the state variables. This form clearly separates fast and slow dynamics and, consequently, it can be used for the development of perturbation and multiple scale techniques. Several applications of this approach to the precessional magnetization switching are discussed.
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f
noise in magnetic tunnel junctions with MgO tunnel barriers
Authors: Gokce Aisha , Nowak E. R. , Yang See Hun , Parkin S. S. P.
Published: June 2006

Electrical noise measurements are reported for magnetic tunnel junctions having magnesium oxide tunnel barriers. These junctions have resistance-area products (RAPs) of order 10–100MΩμm2 and exhibit zero-bias tunneling magnetoresistance ratios (TMRs) as high as 120% at room temperature. The TMR is bias dependent and decreases to half its maximum value for biases near 300 mV. The dominant low-frequency electrical noise is due to resistance fluctuations having a 1∕f-like power spectral dependence and a nonmagnetic origin. The normalized 1∕f noise parameter, α, is found to be of order 10−7 to 10−6 which compares favorably to magnetic tunnel junctions consisting of an aluminum oxide barrier with comparable RAPs but lower TMR. At high biases, α is found to decrease which we attribute to defect-assisted tunneling mechanisms.
Domain wall resistance in FePt wire with perpendicular magnetic anisotropy
Authors: Tanigawa H. , Yamaguchi A. , Kasai S. , Ono T. , Seki T. , Shima T. , Takanashi K.
Published: June 2006

The domain wall resistance was measured in an FePt wire with perpendicular magnetic anisotropy. By the local heating with pulsed current through the wire, the magnetization in a notch of an FePt wire can be selectively reversed. As a result, two domain walls can be created in the vicinity of the notch. The domain wall resistance in this system is positive. The measured interface resistance per domain wall is 2.6×10−16Ωm2, and is almost independent of temperature.
Micromagnetic recording model of writer geometry effects at skew
Authors: Plumer M. L. , Bozeman S. , van Ek J. , Michel R. P.
Published: June 2006

The effects of the pole-tip geometry at the air-bearing surface on perpendicular recording at a skew angle are examined through modeling and spin-stand test data. Head fields generated by the finite element method were used to record transitions within our previously described micromagnetic recording model. Write-field contours for a variety of square, rectangular, and trapezoidal pole shapes were evaluated to determine the impact of geometry on field contours. Comparing results for recorded track width, transition width, and media signal to noise ratio at 0° and 15° skew demonstrate the benefits of trapezoidal and reduced aspect-ratio pole shapes. Consistency between these modeled results and test data is demonstrated.
Giant magnetoimpedance sensor integrated in an oscillator system
Authors: Giouroudi I. , Hauser H. , Musiejovsky L. , Steurer J.
Published: June 2006

This article presents a magnetic field measurement system using giant magnetoimpedance (GMI) sensor as a phase shift element. Investigations were carried out using CoFeB trilayer thin films with 2kA∕m anisotropy field structured as strips and meanders of 300μm length presented in previous studies. The GMI sensor is integrated into an oscillator circuit. The measurement system detects the phase change caused by the magnetic field to be measured. A 6.85% impedance decrease at an applied field of 1.4mT was observed. The phase difference in such cases was Δφ=1.12°. The transmission constant was μa∕Δφ=1mV∕deg.
Spin valve biosensors: Signal dependence on nanoparticle position
Authors: Li Guanxiong , Sun Shouheng , Wang Shan X.
Published: June 2006

Experimental and theoretical studies have been carried out on the spin valve sensor signal dependence on the spatial locations of magnetic nanoparticles as potential biomolecular labels in the magnetic biodetection technology. Superparamagnetic 16 nm magnetite (Fe3O4) nanoparticles were site specifically deposited at different positions relative to a submicron-wide spin valve sensor. The spin valve sensor signal showed both polarity and magnitude differences with the particles at different positions. A theoretical model including magnetic sensor-particle interaction confirms the experimental results and provides a design guide to the sensing area. Moreover, the theoretical calculations reveal a nonmonotonic signal dependence on the vertical particle-to-sensor distance due to the sensor-particle interaction, and show that an optimum distance exists for signal strength and quantification.
Cu doping effect on FePt grains prepared by rapid thermal annealing on SiO2 substrate and wall structure in TbFeCo∕FePt CGC-like film
Authors: Itoh Akiyoshi , Itoh Yujii , Nanba Kensuke , Adachi Yoshiharu , Motohashi Masataka , Tsukamoto Arata
Published: June 2006

We report that Cu substitution is effective in raising the degree of (001) crystal orientation of FePt grains prepared by rapid thermal annealing on SiO2. We fabricated coupled granular and continuous (CGC) like films such as TbFeCo on FePt grains. The wall coercivity was about twice that of the TbFeCo single layer film. From three-dimensional micromagnetic simulations with Landau-Lifshitz-Gilbert (LLG) equation, it was confirmed that wall coercivity was enhanced and domain shapes made smooth in CGC film. When the thickness of the TbFeCo layer was thin, wall width was thinner than the theoretical value.