bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase diagram of the two-dimensional quantum antiferromagnet in a magnetic field
Authors: Cuccoli Alessandro , Gori Giacomo , Vaia Ruggero , Verrucchi Paola
Published: June 2006

The two-dimensional Heisenberg antiferromagnet experiences an effective easy-plane anisotropy when a magnetic field is applied, giving rise to Berezinskii-Kosterlitz-Thouless (BKT) critical behavior. Remarkably, the strength of the effective anisotropy, and consequently the critical BKT temperature, can be tuned by varying the field. By means of the pure-quantum self-consistent harmonic approximation, that reduces the quantum model to an effective classical system, the phase diagram is constructed from its known classical counterpart. By classical Monte Carlo simulations of the effective system the thermodynamic quantities can be calculated. Comparison is made with the behavior observed experimentally in the S=5∕2 compound Mn(HCOO)22H2O.
Large magnetocaloric effect in Pr1−xPbxMnO3 (0.1⩽x⩽0.5) perovskites
Authors: Phan Manh-Huong , Peng Hua-Xin , Yu Seong-Cho , Tho Nguyen Duc , Hanh Duong Thi , Chau Nguyen
Published: June 2006

This research reports the findings of large low-field magnetocaloric effect in polycrystalline Pr1−xPbxMnO3 (0.1⩽x⩽0.5) perovskites. It is found that, upon an applied field of 13.5kOe, the magnetic entropy change (ΔSM) reached values of 3.91, 3.68, and 3.34J∕kgK for x=0.1, 0.4, and 0.5 compositions, respectively. These values are larger than that of Gd (3.32J∕kgK) and were attained by a low applied magnetic field that can be generated by permanent magnets. These superior magnetocaloric features together with a relatively low material cost make the Pr1−xPbxMnO3 perovskites attractive candidate materials for magnetic refrigerators in a temperature range of 150–270K.
Multiple magnetic resonances in permeability spectra of thick CoTaZr films
Authors: Lee D. W. , Wang S. X.
Published: June 2006

Multiple resonance peaks were observed below the ferromagnetic resonance in the permeability spectra of CoTaZr films with thickness up to 3μm. Analysis shows the power-law-like dependence of peak frequencies on the film thickness and the peak order, and the observed behavior was identical for both blanket and patterned CoTaZr samples. Experimental findings suggest that the observed modes corresponding to the multiple resonance peaks may not be related to the in-plane domain structure but rather are likely associated with a mechanism in the out-of-plane direction. Multiple resonance peaks disappeared when the lamination structures were used.
Microanalytical characterization of multi-rare-earth nanocrystalline magnets by transmission electron microscopy and atom probe tomography
Authors: Wu Y. Q. , Tang W. , Miller M. K. , Anderson I. E. , McCallum R. W. , Dennis K. W. , Kramer M. J.
Published: June 2006

The partitioning behavior of various rare-earth (RE) elements during solidification and their segregation behavior at the grain boundaries were investigated in nanocrystalline (Y0.5Dy0.5)2.2Fe14B and (Nd0.5Y0.25Dy0.25)1.8Zr0.4Co1.5Fe12.5B alloys by transmission electron microscopy and atom probe tomography. The best hard magnetic properties obtained are Hcj=22kOe, Br=5.10kG, and (BH)max=5.97MGOe for the Y–Dy-based alloy and Hcj=10.6kOe, Br=6.64kG, and (BH)max=9.56MGOe for the Y–Nd–Dy based alloy. The grain size of the Y–Dy based alloy was ∼50nm. The Y–Nd–Dy based alloy had an overall finer, bimodal grain size. An intergranular (Y0.36Dy0.64)6Fe23 phase was detected in the Y–Dy based alloy. A uniform distribution of RE elements was found within the 2-14-1 grains in both alloys. The Y:(Dy+Nd) ratio in the Y–Nd–Dy alloy was lower than its nominal composition, indicating that the Y is segregating to grain boundaries or forming a second phase.
Enhancement of the magnetic anisotropy of Co clusters by Au capping
Authors: Luis F. , Bartolomé J. , Bartolomé F. , Martínez M. J. , García L. M. , Petroff F. , Deranlot C. , Wilhelm F. , Rogalev A.
Published: June 2006

We study the magnetic properties of Co nanoparticles, prepared by sputtering, with diameters ranging from 1to3.5nm. The effective anisotropy, which determines the activation energy for the magnetization reversal, increases with decreasing particle’s size, showing the dominant role played by surface atoms. We find that the superparamagnetic blocking temperature and the coercive field are significatively enhanced when the clusters are capped by a thin Au layer. This enhancement is probably caused by the bonding or hybridization of Co and Au atoms at the interface between the two metals. It provides thus a method to modify the magnetic anisotropy, which can be of interest for the applications of magnetic nanoparticles.
Mössbauer and magnetic studies of multiferroic Mg0.95Mn0.05Fe2−2xTi2xO4 system
Authors: Kumar Shalendra , Alimuddin  , Kumar Ravi , Dogra Anjana , Reddy V. R. , Banerjee A.
Published: June 2006

A series of polycrystalline multiferroic system Mg0.95Mn0.05Fe2−2xTi2xO4 is prepared by a solid-state reaction route. The effect of Ti+4 substitution in Mg0.95Mn0.05Fe2−2xTi2xO4 has been studied using x-ray diffraction (XRD), Mössbauer spectroscopy, isothermal magnetization hysteresis, temperature dependent dc susceptibility, and temperature dependent dielectric measurements. Structural transformation from cubic spinel to tetragonal has been revealed from the XRD analysis. From the analysis of Mössbauer spectra, the decrease in hyperfine field at both tetrahedral and octahedral sites has been observed as a function of the concentration of Ti+4 ions, which has been explained on the basis of a supertransferred hyperfine field. The appearance of the paramagnetic doublet within the sextet is due to the interaction of the electric field gradient (EFG) with the quadrupole moment of Fe57 nucleus and the decrease in magnetic interaction between Fe ions with Ti dilution. The variation in the quadrupole splitting indicates that the EFG changes with the increase in the substitution of Ti+4 ions. Isothermal hysteresis loop shows the ferromagnetic behavior of Mg0.95Mn0.05Fe2−2xTi2xO4 and the saturation magnetization is decreased with the increase in the substitution of Ti+4, which is explained by the weakening of the exchange interaction in the system. The dielectric measurements of these samples exhibit ferroelectric transition depending upon the concentration of the Ti+4.
The magnetoelectric effect in pulsed magnetic fields
Authors: Bueno-Baques Dario , Grossinger Roland , Schonhart Markus , Doung Giap V. , Sato Reiko , Corral-Flores Veronica , Matutes-Aquino Jose
Published: June 2006

A measurement method for studying the magnetoelectric effect in pulsed magnetic fields was developed. Besides magnetization, the system allows the measurement of magnetostriction (using strain gauges) and the magnetoelectric effect. For measuring the magnetoelectric effect a special sample holder with fixed contacts and compensation coils was developed to ensure the reproducibility of the measurements. The magnetoelectric effect can be measured by two different methods: (i) During the application of a magnetic field pulse when the voltage appearing on the sample is directly recorded and (ii) As a function of a high frequency (f=10kHz) ac field superimposed to the main field pulse. In this way an ac voltage appears in the sample which is proportional to the charge change ΔQ=Q0sin(wt+Φ). This voltage is measured using a fast analog lock-in amplifier. A detailed description of the total system is provided. The magnetoelectric effect on well known composites of cobalt ferrite and barium titanate was measured at room temperature. A maximum magnetoelectric coefficient of 1.5mV∕cmOe was obtained for a composite with a composition of 50wt% ferrite sintered at 1200°C. A model to explain the results obtained by this measurement method is discussed.
Configurational anisotropy in hexagonal arrays of submicron Co elements
Authors: Weekes S. M. , Ogrin F. Y. , Keatley P. S.
Published: June 2006

The angular dependence of the in-plane magnetization reversal in hexagonally ordered Co arrays has been investigated. A sixfold anisotropy has been observed which mirrors the structural symmetry of the array. The arrays consist of polycrystalline, circular elements with constant center-to-center separation and diameters of 360nm and below. It has been observed that the anisotropy is strongly dependent on element diameter and disappears for densely packed elements. This anisotropic coupling cannot be explained by a simple model of interacting point dipoles situated at regular lattice sites, as this predicts an isotropic interaction field for a hexagonal array. Instead, it is believed that the anisotropy arises from interactions between unsaturated regions of the elements, particularly at adjacent edges.
Improvement of energy product in exchange coupled Fe49−xCoxPt51 (x=0.0,0.7,1.3,2.2) thin films
Authors: Yuan F. T. , Hsiao S. N. , Liao W. M. , Chen S. K. , Yao Y. D.
Published: June 2006

Magnetic properties and crystal structure of the Fe49−xCoxPt51 (x=0.0,0.7,1.3,2.2) thin films deposited on a quartz substrate heated at 500°C were investigated. A significantly enhanced energy product of 18.4MGOe (87% larger than that of the binary film) was obtained in the x=1.3 sample. This improvement can be attributed to the well exchange coupling between the Ll0 and the residual disordered FePt regions. Unlike the common exchange spring (coupled) magnets or films showing a well defined two-phase structure, the chemical ordering of Fe–Co–Pt films changes continuously from the ordered regions to the disordered regions. We consider that this continuous change of chemical ordering enhances exchange coupling. Furthermore, the addition of cobalt also accompanied the decrease of crystal domain (subgrain) size in FePt grains. The fine-grain dispersion also can be a reason for the high energy product.
Complex permittivity and permeability of strontium ferrites at millimeter waves
Authors: Korolev Konstantin A. , Subramanian Lakshmi , Afsar Mohammed N.
Published: June 2006

Magneto-optical approach for the measurements of complex dielectric permittivity and magnetic permeability of solid and powdered strontium ferrite materials has been performed in the Q-, V-, and W-band frequency ranges. Free-space quasioptical millimeter-wave spectrometer equipped with a backward-wave oscillator as a tunable source of coherent radiation provides the transmittance spectra in transverse magnetic field up to 7.5kOe. Frequency dependences of dielectric and magnetic parameters of strontium ferrites have been calculated by matching theoretical curves to the experimental transmittance spectra. Shifting of the ferromagnetic resonance to higher frequencies and broadening of the zone of strong absorption in magnetic field for solid ferrite materials have been observed.
Amorphous-FeCoCrZrB ferromagnets for use as high-temperature magnetic refrigerants
Authors: Johnson F. , Shull R. D.
Published: June 2006

Magnetic metallic glasses, having large magnetic moments and high Curie temperatures (TC), have not been widely studied as magnetic refrigerants. In this study, we report on the magnetocaloric effects of five FeCoCrZrB amorphous alloys [(FexCoyCrz)91Zr7B2, with Fe:Co:Cr ratios of 100:0:0, 90:15:5, 85:5:10, and 75:15:10] chosen for efficiently surveying the high TC metallic glass composition surface. Magnetic isotherms were measured at 25K intervals from 25to525K. The entropy change (ΔS) due to the magnetocaloric effect was computed by integrating the magnetic isotherms. The refrigeration capacity (RC) was computed by Wood-Potter method. At 3979kA∕m (50kOe) the FeCoCrZrB alloys have RC values of 240–320J∕kg; at 796kA∕m (10kOe) the alloys have values of 32–54J∕kg. The RC values favorably compare with values reported for other refrigerant materials. The peak ΔS temperature depends strongly on composition, varying from 200to450K. The wide flexibility in choosing amorphous alloy compositions allows them to be fine tuned for multistage high temperature refrigerators. The saturation magnetization at 25 and 300K and Curie temperatures are also reported for the five alloys.
Effective method to measure back emfs and their harmonics of permanent magnet ac motors
Authors: Jiang Q. , Bi C. , Lin S.
Published: June 2006

As the HDD spindle motors become smaller and smaller, the back electromotive forces (emfs) measurement faces the new challenges due to their low inertias and small sizes. This article proposes a novel method to measure the back emfs and their harmonic components of PM ac motors only through a freewheeling procedure. To eliminate the influence of the freewheeling deceleration, the phase flux linkages are employed to obtain the back emf amplitudes and phases of the fundamental and harmonic components by using finite Fourier series analysis. The proposed method makes the freewheeling measurement of the back emfs and their harmonics accurate and fast. It is especially useful for the low inertia PM ac motors, such as spindle motors for small form factor HDDs.
Field controlled magnetic thermal hysteresis in Co∕Tb multilayers
Authors: Hossu M. R. , Koymen A. R.
Published: June 2006

Experimental results proving that Co∕Tb multilayers show thermal hysteresis in the total magnetic moment are presented. [Co(30Å)∕Tb(45Å)]8 was synthesized using dc magnetron sputtering. Due to antiferromagnetic coupling between the Co and Tb layers this system behaves like an artificial ferrimagnet. The measurement of magnetic moment (M) as a function of temperature (T) shows that magnetic phase transition occurs at different temperatures during heating (superheating) and cooling (supercooling) cycles resulting in a characteristic bow-tie shaped thermal hysteresis curve for M(T). The width of the thermal hysteresis was measured to be around 90 K in an external magnetic field of 2000 Oe. Increasing the field to 4000 Oe reduces the thermal hysteresis width to about 40 K and above 6000 Oe the width disappears. Co∕Tb multilayers with thicker layers such as [Co(40Å)∕Tb(60Å)]8 show the same trend. However, the width of the thermal hysteresis is generally smaller at a given magnetic field.
Electron magnetic resonance study on the surface states of La0.7Sr0.3MnO3 nanoparticles
Authors: Wu Jia-Hung , Lin Jauyn Grace
Published: June 2006

The technique of electron magnetic resonance (EMR) with a broad temperature regime (80–400 K) was employed for studying the surface states of La0.7Sr0.3MnO3 nanoparticles, which were synthesized by sol-gel routes with three different gelification agents (grain size≈17–24nm). Our EMR results reveal that all La0.7Sr0.3MnO3 nanoparticles have two magnetic states below the Curie temperatures TC. One is attributed to the core state and the other to surface state. Furthermore, the absorption field and the linewidth of surface state have a significant temperature dependence in the sample synthesized by citric acid, which may be associated with the existence of spin-glass state on the surface of nanoparticles.
Combined optical and force microscopy of patterned magnetic films
Authors: Sievers S. , Albrecht M. , Siegner U. , Dreyer S. , Jooss Ch.
Published: June 2006

Combined magneto-optic indicator film (MOIF) and magnetic force microscopy (MFM) is introduced as an advanced tool for the characterization of patterned ferromagnetic films. The MOIF technique combines quantitative stray field imaging of individual micron-sized magnetic elements with a large-area overview of the patterned film while MFM provides nanometer spatial resolution. In order to demonstrate the potential of this approach, lithographically patterned CoPt films with out-of-plane anisotropy were characterized. The large-scale MOIF images reveal variations of the magnetization state and the magnetization reversal among individual magnetic elements. Corresponding high-resolution MFM images relate these results to the micromagnetic configuration of the elements.
Microfluxgate sensor with amorphous cobalt (Co–Nb–Zr) soft magnetic core for electronic compass
Authors: Na Kyoung-Won , Yuan Jingli , Ji Joon-Ho , Choi Sang-On
Published: June 2006

A silicon based microfluxgate sensor with a cobalt based amorphous soft magnetic core for electronic compass is presented in this paper. A sputtered Co85Nb12Zr3 magnetic core having a rectangular ring shape is combined with microcopper solenoid coils for excitation and pickup, which were wound alternately around the core to increase the number of coil turns. The Co85Nb12Zr3 as a core material is adopted for improving properties of the magnetic core and easy integration with micromachining processes to achieve a small size of the sensor. The sputtered Co85Nb12Zr3 showed dc effective permeability of ∼10000 and an extremely low coercivity of ∼0.03Oe with the thickness of 1μm. The Co85Nb12Zr3 as a thin film core with high permeability and low coercivity was easily saturated by a low excitation magnetic field, enhancing the sensitivity and linearity of the microfluxgate sensor. Finally, the sensor showed excellent linearity response over the range of −300 to 300μT with sensitivity of 60V∕T at the excitation condition of 3.0Vp-p and 5.0 MHz square wave form. The sensor size excluding pad region is about 0.55×1.4mm2.
Magnetic properties of self-assembled ferritin-core arrays
Authors: Yuan Zhen , Atanassov Plamen , Alsmadi Abdel M. , te Velthuis S. G. E. , Welp Ulrich , Hammetter Chris I. , Hjelm Rex , Nakotte Heinz
Published: June 2006

We studied the magnetic properties of a ferritin-core array, deposited on a Si wafer and encapsulated in silica. The array was formed by a simple evaporation-induced self-assembly mechanism of ferritin proteins, and it consists of 6 nm ferritin cores crystallized in a two-dimensional hexagonal lattice with a core-core distance of about 10.5 nm. Magnetization studies indicate that the array undergoes a magnetic transition around 210 K. Our small-angle neutron-scattering studies provide evidence that huge net magnetic moments are stabilized in each core and that long-range magnetic order between the core moments occurs in the array.
Monte Carlo simulations of NiFe2O4 nanoparticles
Authors: Zhou Chenggang , Schulthess T. C. , Landau D. P.
Published: June 2006

We use Monte Carlo simulations to study NiFe2O4 nanoparticles. Finite size and surface effects differentiate them from their bulk counterparts. A continuous version of the Wang-Landau algorithm is used to calculate the joint density of states g(Mz,E) efficiently. From g(Mz,E), we obtain the Bragg-Williams free energy of the particle, and other physical quantities. The hysteresis is observed when the nanoparticles have both surface disorder and surface anisotropy. We found that the finite coercivity is the result of interplay between surface disorder and surface anisotropy. If the surface disorder is absent or the surface anisotropy is relatively weak, the nanoparticles often exhibit superparamagnetism.
Transport properties of multilayered MgB2∕Mg2Si superconducting thin film
Authors: Zhao Y. , Qin M. J. , Dou S. X. , Ionescu M. , Munroe P.
Published: June 2006

Electronic transport measurements have been carried out on superconducting MgB2∕Mg2Si multilayer film, using a standard four-probe method in perpendicular and parallel applied fields. The film, which was prepared by pulsed-laser deposition, has a layered structure with each MgB2 layer being 40–50 nm thick and the Mg2Si interlayers about 5 nm thick. The flux flow activation energy is deduced from the resistivity-temperature curves using an Arrhenius fit. The results show a clearly enhanced anisotropy of the vortex activation energy in the multilayered film. The irreversibility field and the vortex activation energy are significantly increased in parallel fields.
Influence of ferromagnetic current screen layer on current perpendicular to the plane-giant magnetoresistance
Authors: Hoshino Katsumi , Hoshiya Hiroyuki
Published: June 2006

We investigated the influence of the ferromagnetic current screen layer on a current perpendicular to the plane-giant magnetoresistance (CPP-GMR). We prepared a bottom-type synthetic-pinned spin-valve film with a ferromagnetic current screen layer of naturally oxidized CoFe, which was separated magnetically from the free and pinned layers. When the CoFe layer thickness increased to above 1.0nm before oxidation, we observed a magnetic moment corresponding to the unoxidized CoFe. We also found that the MR ratio abruptly increased from 1.6% to 7.2% with the increase in the CoFe layer thickness from 1.0to3.7nm before oxidation, which was the same tendency observed in the magnetic moment. The MR ratio seemed to be strongly dependent on the magnetic moment of the current screen layer. The MR curve with the high MR ratio had two peaks near the zero magnetic fields. Based on the analysis of the magnetization configuration of the free, pinned, and current screen layers, we found that the ferromagnetic properties of the current screen layer contributed to the high MR ratio. From transmission electron microscopy observation and magnetic properties of the current screen layer, we estimated that the metal CoFe bridge exists in the current screen layer. We believe that the metal bridge made from the ferromagnetic metal in the current screen layer is important for enhancing the MR ratio of CPP-GMR for both current screening and spin dependent scattering.
Large-scale and highly accurate magnetic field analysis of magnetic shield
Authors: Takahashi Y. , Wakao S. , Kameari A.
Published: June 2006

This paper describes a large-scale and highly accurate magnetostatic field analysis targeting a magnetic shield. The hybrid finite element–boundary element method and the magnetic moment method do not require mesh division for the free space and can easily treat the nonlinearity of magnetic property. Therefore, these methods are considered very effective for the analysis of a magnetic shield which has a high aspect ratio of the size scale to the thickness. However, large memory and computational time have been required due to the dense matrices generated by those integral-based formulations. To overcome the difficulties, we introduce the fast multipole method into the hybrid method and the magnetic moment method. Furthermore, to achieve more reduction of CPU time, we propose an effective preconditioning technique suitable for the hybrid method. Some numerical results that demonstrate the effectiveness of these approaches are also presented.
Simple theory of hot electron dynamics observed by femtosecond ellipsometry
Authors: Kruglyak V. V. , Hicken R. J.
Published: June 2006

The dynamics of the linear and angular momenta of hot electrons in metals are of key importance for the design and operation of hot electron devices such as spin and tunnel valve transistors. The corresponding relaxation times are expected to lie in the subpicosecond range and must be studied with experimental techniques of adequate (femtosecond) temporal resolution. Here we report a simple theory of the ultrafast ellipsometric response of metals after excitation with femtosecond optical pulses. Although developed in the relaxation time approximation, the theory allows electron linear and angular momentum relaxation times to be extracted.
Effect of thermal agitation on the switching field distributions of CoPtCr–SiO2 perpendicular recording media
Authors: Shimatsu T. , Kondo T. , Mitsuzuka K. , Watanabe S. , Aoi H. , Muraoka H. , Nakamura Y.
Published: June 2006

The effect of thermal agitations on the switching field distribution (SFD) of CoPtCr–SiO2 perpendicular media was investigated. Dc demagnetizing (DCD) magnetization curves and minor dc demagnetizing (M-DCD) magnetization curves were measured at applied field sweep rates of ∼10 and ∼108Oe∕s. We estimated the SFD from the difference between the DCD and M-DCD curves, and defined them as ΔHr∕Hr (at ∼10Oe∕s) and ΔHrP∕HrP (at ∼108Oe∕s). The values of ΔHrP∕HrP were found to be 60%–70% of ΔHr∕Hr. The difference between ΔHrP∕HrP and ΔHr∕Hr should be caused by thermal agitation of the magnetization. The influence of the distribution of the thermal agitation on SFD was calculated using the distribution of the grain volumes. It is concluded that the SFD measured at vibrating-sample magnetometer (VSM) time scales is significantly influenced by thermal agitation of the magnetization, and reduction of the grain size distribution is the most effective way to reduce the SFD at VSM time scales.
Field assisted spin switching in magnetic random access memory
Authors: Jeong W. C. , Park J. H. , Oh J. H. , Koh G. H. , Jeong G. T. , Jeong H. S. , Kim Kinam
Published: June 2006

A switching method called by field assisted spin switching has been investigated. A field assisted spin switching consists of a metal line induced magnetic field and a spin switching through a magnetic tunnel junction. It is a variation of a current induced switching and assisted by the magnetic field induced by the current-carrying metal line. Various current paths have been tested to investigate how and how much the spin switching contributes to the overall switching and the results will be explained. A computer simulation has been complemented to measure the degree of the thermal effect in the switching.
Hybrid nanograin rare earth magnets with improved thermal stability
Authors: Shen Y. , He Y. , Huang M. Q. , Lee D. , Bauser S. , Higgins A. , Chen C. , Liu S.
Published: June 2006

Attempts to fill a wide gap in thermal stability between Nd–Fe–B and Sm–Co based magnets by making a conventional hybrid Nd15Fe79B6∕Sm(Co,Fe,Cu,Zr)7.4 type of magnets encountered two technical difficulties: incompatible processes and interdiffusion between two materials. Because a fundamental change in coercivity mechanism occurs when reducing the grain size in these magnets to nanometer range, high coercivity can be obtained in both nanograin Sm2(Co,Fe)17 and Nd2Fe14B after a short anneal at ∼700°C. Thus, the processes of making two different nanograin magnets are completely compatible. Further, very short time of elevated temperature processing can minimize the interdiffusion. Applying this approach, hybrid nanograin Nd2Fe14B∕Sm2(Co,Fe)17 magnets with (BH)max=27.4MGOe and HcM=14kOe were made using rapid inductive hot compaction and hot deformation. In addition, better magnetic performance of (BH)max=34.5MGOe and HcM=16.9kOe was obtained in a hybrid nanograin Pr2(Fe,Co)14B∕Pr(Co,Fe)5 (80wt%∕20wt%) magnet.
Drag force measurement: A means for determining hysteresis loss
Authors: Garshelis Ivan J. , Tollens Stijn P. L. , Kari Ryan J. , Vandenbossche Lode P. , Dupré Luc R.
Published: June 2006

A method for determining hysteresis losses in thin strips of soft magnetic materials is described. It is based on the measurement of a drag force which arises with the movement of the sample through the strong field existing in the space near a permanent magnet. Not associated with macro eddy currents, the force is shown to originate from the magnetic hysteresis of the material, having, in fact, an amplitude equal to the product of hysteresis loss and the area of the sample cross section. Correlation within 18% with the measurements made by conventional methods is shown for a wide range of experimental materials.
Interplay between magnetic interactions in spin-valve structures
Authors: Rodríguez-Suárez R. L. , Oliveira A. B. , Rezende S. M. , Azevedo A.
Published: June 2006

In this work we report on the experimental and theoretical investigations of the effects of various magnetic couplings existing in spin-valve structures. Magnetic interactions such as the exchange bias that occurs at the ferromagnetic/antiferromagnetic interface and the indirect coupling between ferromagnetic (FM) layers mediated by a nonmagnetic metal spacer as well as the interchange between both kinds of interactions were investigated. The unidirectional anisotropy that pins the magnetization of the pinned layer transmits to the free layer throughout the nonmagnetic spacer. Also, the resonance fields of the FM layers show an upward (downward) shift for an antiferromagnetic (ferromagnetic) bilinear coupling.
Magnetic properties of monodisperse iron oxide nanoparticles
Authors: Lin Chun-Rong , Chiang Ray-Kuang , Wang Jiun-Shen , Sung Ti-Wen
Published: June 2006

We have synthesized a set of monodisperse iron oxide nanoparticles ranging from 7.8to17.9nm by thermal decomposition methods. Based on the evidence of high-resolution transmission electron microscopy, the iron oxide nanoparticles appear as spherical dots with size standard deviations of less than 5%. Blocking temperatures of the set of nanoparticles were measured by the zero-field-cooled magnetization measurements. The anisotropy energy constants are estimated from the measured blocking temperatures. The contribution from the surface anisotropy is the dominant factor of the higher anisotropy energy found. The saturation magnetization and coercive force HC (77K) are functions of the particle size and increase with the particle size.
Optimal detection for perpendicular recording channels with transition noise
Authors: Zhang Xun , Negi Rohit
Published: June 2006

This paper proposes a jitter-sensitive-sequence-detection scheme based on a realistic transition noise model for high-density perpendicular magnetic recording channels. In this scheme, the transition jitter is assumed to have one of the Q discrete values, and the discretized jitter sequence is estimated jointly with the recorded bit sequence using maximum a posteriori criterion. The detection performance is compared with that of the pattern dependent noise predictive scheme based on an autoregressive Gaussian noise model. Simulation results show that the scheme provides a significant gain when the transition jitter is large and the resulting transition noise dominates over the electronic noise.
Anisotropy transition of Co in IrMn∕Co∕FeOx∕Co by field cooling
Authors: Shen Chih-Ta , Lai Chih-Huang , Huang Po-Hsiang , Hsu S. Y. , Chung Ting Yi
Published: June 2006

The temperature dependence of Co anisotropy on a nano-FeOx layer was studied in the structure of IrMn∕Co(FM1)∕FeOx∕Co(FM2). An anisotropy transition of the FM2 was observed from a combination of uniaxial and unidirectional anisotropies at room temperature (RT) to unidirectional anisotropy at temperature below 80K through field cooling process. Various ferromagnetic (FM) and antiferromagnetic (AFM) components existing in the FeOx layer were attributable to the observed anisotropy of FM2. AFM domains with TN higher than room temperature were responsible for the observed uniaxial anisotropy at RT and AFM domains with TN of 80K were accountable for the anisotropy transition, below which the unidirectional anisotropy became dominant. In addition, the direction of the shifted loop could be determined by the cooling field direction.
Magnetic properties of Nd–Fe–Ti–C–B nanocomposite magnets produced by spark plasma sintering method
Authors: Saito Tetsuji
Published: June 2006

Amorphous melt-spun ribbons of a Nd9Fe73Ti4C1B13 alloy were consolidated into bulk materials by the spark plasma sintering method. The resultant bulk material was amorphous when the ribbons were consolidated at 773K. The amorphous bulk material showed a low coercivity, as would be expected for amorphous Nd–Fe–B materials. On the other hand, the bulk materials produced at 873K and above consisted of α-Fe and Nd2Fe14B phases. The resultant nanocomposite magnets showed high coercivity values exceeding 10kOe. The bulk material produced at 973K showed a high remanence of 8.4kG with a maximum energy product of 13.8MGOe.
Magneto-optical properties of group-IV ferromagnetic semiconductor Ge1−xFex grown by low-temperature molecular beam epitaxy
Authors: Shuto Yusuke , Tanaka Masaaki , Sugahara Satoshi
Published: June 2006

Group-IV ferromagnetic semiconductor Ge1−xFex was grown by low-temperature molecular beam epitaxy without precipitation of ferromagnetic Ge–Fe intermetallic compounds. The ferromagnetism of Ge1−xFex films was investigated by magnetic circular dichroism (MCD). In particular, the influence of the Fe content (x=2.0%–17.5%) and growth temperature (100 and 200°C) on the ferromagnetism was carefully studied. The MCD measurements revealed that the overall spectral features reflecting the band structure of the Ge1−xFex films were identical with those in bulk Ge, and that the large spin splitting of the band structure was induced by the incorporation of Fe atoms into the Ge matrix, indicating the existence of s,p-d exchange interactions. The Ge1−xFex films showed ferromagnetic behavior and the ferromagnetic transition temperature linearly increased with increasing the Fe concentration. These results indicate that the epitaxially grown Ge1−xFex is an intrinsic ferromagnetic semiconductor.
Spin screening of magnetic moments and inverse proximity effect in ferromagnet/superconductor nanostructures
Authors: Fazleev N. G. , Khusainov M. M. , Proshin Yu. N. , Khusainov M. G.
Published: June 2006

It is shown that the short-range oscillating spin polarization of conduction electrons around a magnetic moment embedded in a superconducting film is screened by a long-range antiferromagnetic term, which has its origin in Cooper pairing. Using these results, an exchange model of the proximity effect in ferromagnet/superconductor (F/S) nanostructures is proposed. The mutual accommodation of inhomogeneous superconducting and magnetic order parameters in the F/S nanostructures is studied within the framework of this model. It is shown that F/S nanostructures of the first type allow only the presence of homogeneous ferromagnetic ordering in the F layers that coexists with superconductivity in the S layers if the exchange fields h are lower than a critical field hc. It is found that in the F/S nanostructures of the second type for exchange fields h between hc1 and hc2, where hc1(2) is the lower (upper) critical exchange field, a nonuniform cryptoferromagnetic modulation is induced in the spin structure of the F films. The conditions for the coexistence of inhomogeneous magnetism and superconductivity in the F/S nanostructures EuO∕Al, EuO∕V, EuS∕Al, and LaCaMnO∕YBaCuO are explored. The theory is used to interpret the experimental data.
Structure and magnetic properties of pure and Gd-doped HfO2 thin films
Authors: Wang Wendong , Hong Yuanjia , Yu Minghui , Rout Bibhudutta , Glass Gary A. , Tang Jinke
Published: June 2006

Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates (silicon, R-Al2O3, and LaAlO3) by pulsed laser deposition. X-ray diffraction (XRD) patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD pattern except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with postdeposition annealing of the HfO2 films. In addition to the films, HfO2 powders were annealed in pure hydrogen flow, and a ferromagnetic signal was not observed.
Heating ability of magnetite nanobeads with various sizes for magnetic hyperthermia at 120kHz, a noninvasive frequency
Authors: Okawa K. , Sekine M. , Maeda M. , Tada M. , Abe M. , Matsushita N. , Nishio K. , Handa H.
Published: June 2006

We synthesized four kinds of magnetite particles having average sizes of 7, 18, 40, and 80nm and investigated their heating ability when they were dispersed in an agar gel and exposed to an ac magnetic field at 120kHz, a noninvasive frequency for anticancer hyperthermia. The particles 18nm in average diameter gave the highest heating ability, though they exhibited narrow hysteresis loops as compared to the particles having average diameters of 40 and 80nm. This indicates that hysteresis loss does not contribute much to the heat rise by the 120kHz ac field, and Néel relaxation is dominantly contributing to the heat rise by the 18nm sized particles. A calculation based on Néel relaxation loss gave a plausible explanation.
Order-disorder phase transition in two-dimensional Ising model with exchange and dipole interactions
Authors: Rastelli E. , Regina S. , Tassi A.
Published: June 2006

The Ising model with nearest neighbor ferromagnetic (J) exchange and dipole (g) interaction has been extensively investigated both analytically and numerically. A general agreement was found about the main features of the phase diagram characterized by striped configurations of spins up and down of increasing width h at increasing J∕g. However, the nature of the phase transition between the striped and the paramagnetic phase needs to be clarified. Here we present results of extensive Monte Carlo simulations to throw light on the phase transition between the striped configurations with h=1, 2, and 4 and the paramagnetic phase.
Anisotropic polymer bonded hard-magnetic films for microelectromechanical system applications
Authors: Romero J. J. , Cuadrado R. , Pina E. , de Hoyos A. , Pigazo F. , Palomares F. J. , Hernando A. , Sastre R. , Gonzalez J. M.
Published: June 2006

Thick hard-magnetic films have been prepared from NdFeB microparticles dispersed in polymethylmethacrylate solutions. The preparation process included the application of different magnetic fields during the drying of the hard particles dispersion, which resulted on different degrees of out-of-plane macroscopic anisotropy. From the analysis of the temperature dependence of the coercivity of the films it was concluded that the induced macroscopic anisotropy was linked to the occurrence of columnar particle aggregates.
Observation of azimuthal ferromagnetic resonance modes in 1.0μm diameter Permalloy rings
Authors: Zhu Xiaobin , Liu Zhigang , Metlushko Vitali , Giovannini L. , Montoncello F. , Nizzoli F. , Freeman Mark R.
Published: June 2006

The spin dynamics of thin film Permalloy individual rings in the circulating state is studied in the presence of an in-plane bias field and in response to a transient field applied perpendicular to the ring plane. Only one dominant precessional mode is observed in the remanence state, which corresponds to an angularly uniform excitation. In contrast, when an external bias field is applied, multiple precessional modes are observed, with the number of modes increasing as the field increases. The static magnetic field distorts the circular symmetry of the initial magnetization state. As a result, the precessional torque produced by the pulsed field is inhomogeneous, which promotes the excitation of higher order azimuthal modes.
Evaluation of nanocrystalline materials, amorphous metal alloys, and ferrites for magnetic pulse compression applications
Authors: Burdt Russell , Curry Randy D. , McDonald Kenneth F. , Melcher Paul , Ness Richard , Huang Chaofeng
Published: June 2006

A magnetic pulse compressor test stand was developed to evaluate the switching and loss properties of magnetic core materials that included ferrite and several alloys of nickel-iron, Metglas, and the nanocrystalline material Vitroperm. The test stand generated 1−cos(ωt) voltage pulses across the core under evaluation to simulate the magnetic excitation encountered in pulse compressors. Pulse amplitude and repetition rate were user controlled in order to vary the initial time to saturation while keeping magnetic core temperature constant. Switching losses were calculated directly as ∫(νi)dt, and an advanced figure of merit was utilized to compare test results. The test stand, data acquisition and analysis, and test results are discussed.
Measurement of magnetization dynamics in single-molecule magnets induced by pulsed millimeter-wave radiation
Authors: Bal M. , Friedman Jonathan R. , Tuominen M. T. , Rumberger E. M. , Hendrickson D. N.
Published: June 2006

We describe an experiment aimed at measuring the spin dynamics of the Fe8 single-molecule magnet in the presence of pulsed microwave radiation. In earlier work, heating was observed after a 0.2ms pulse of intense radiation, indicating that the spin system and the lattice were out of thermal equilibrium at millisecond time scales [Bal et al., Europhys. Lett. 71, 110 (2005)]. In the current work, an inductive pickup loop is used to probe the photon-induced magnetization dynamics between only two levels of the spin system at much shorter time scales (from nanoseconds to microseconds). The relaxation time for the magnetization, induced by a pulse of radiation, is found to be on the order of 10μs.
Disproportionation and critical interaction strength in NaxCoO2: Concentration dependence
Authors: Lee K.-W. , Pickett W. E.
Published: June 2006

We present results of studies of charge disproportionation (CD) and spin differentiation in NaxCoO2 using the correlated band theory approach [local density approximation (LDA)+Hubbard U:LDA+U]. The simultaneous CD and gap opening can be followed through a first-order charge disproportionation transition. By comparison with experiments, we propose a value of the Coulomb repulsion strength U(x) that has significant dependence on the carrier concentration x, for which we obtain a phase diagram. The connection between the Co–O bond length and effects of on-site correlation is also reported.
Interfacial characteristics of a Fe3O4∕Nb(0.5%):SrTiO3 oxide junction
Authors: Kundaliya Darshan C. , Ogale S. B. , Fu L. F. , Welz S. J. , Higgins J. S. , Langham G. , Dhar S. , Browning N. D. , Venkatesan T.
Published: June 2006

The temperature dependent Schottky diode characteristics of epitaxial junctions between Nb:SrTiO3 (Nb concentrations: 0.5%) and Fe3O4 are studied. Epitaxial thin films of Fe3O4 were grown on Nb:SrTiO3 substrates by pulsed laser deposition technique. The films and heterointerfaces were characterized by x-ray diffraction, Z-contrast transmission electron microscopy, magnetic susceptibility, four-probe in-plane resistivity, and the temperature dependent junction current-voltage (I-V) characteristics. The nonlinear nature of the characteristics is analyzed within the framework of thermionic emission theory. Junction parameters such as the Schottky barrier height (ϕB) and ideality factor (η) are extracted. The temperature evolution of these parameters shows interesting and systematic trends, with remarkable changes near the Verwey transition (TV=120K). The magnetic field dependence of I-V characteristic data is also recorded and a spin polarization of ∼80% is estimated for the magnetite electrode.
Micromagnetic energy barriers
Authors: Skomski R. , Zhou J. , Kirby R. D. , Sellmyer D. J.
Published: June 2006

The structure of micromagnetic energy barriers responsible for slow magnetization processes is investigated. Thermally activated slow magnetization processes proceed over energy barriers whose structure is determined by the micromagnetic free energy. This restricts the range of physically meaningful energy barriers. An analysis of the underlying micromagnetic free energy yields power-law dependences with exponents of 3∕2 or 2 for physically reasonable models. This must be contrasted to other power laws, such as linear laws, and to 1∕H-type dependences. In the limit of small energy barriers, corrections to the Arrhenius law become important. In this regime, there is no simple expression for the relaxation behavior, but two requirements help to judge models and approximations. First, at low temperatures, the Arrhenius-type power laws must be reproduced. Second, as in the Arrhenius limit, the approaches must correspond to well-defined energy landscapes.
Angular dependence of the giant magnetoresistance in multilayer rings with different contact configurations
Authors: Morecroft D. , Castaño F. J. , Jung W. , Ross C. A.
Published: June 2006

We report on the effects of electrical contact configuration on the giant magnetoresistance (GMR) response of NiFe∕Cu∕Co pseudo-spin-valve elliptical rings. Room temperature GMR measurements show different approaches to remanence, depending on the position of the voltage leads with respect to the curvature of the rings. Computational analysis of micromagnetic simulations reproduces these experimental trends, which result from changes in the relative alignment between the NiFe and Co layers in different regions of the rings.
Highly coercive rapidly solidified Sm–Co alloys
Authors: Aich S. , Ravindran V. K. , Shield J. E.
Published: June 2006

Highly coercive (Hc up to 37kOe at 300K), high remanent permanent magnets have been achieved by rapid solidification of binary Sm–Co alloys and Sm–Co alloys modified with Nb and C. Rapidly solidified SmCox alloys with x ranging from 5 to 11.5 formed predominantly a solid solution TbCu7-type SmCo7 phase, although hcp Co was observed for x>7.3. A coercivity value of 10kOe was observed for x<6.1, even though the microstructural scale was on the order of 1μm. The coercivity decreased significantly with the presence of the hcp Co phase, which formed initially as ∼80nm grains and, at higher x, as primary dendrites. Additions of 3at.% Nb or 3 and 5at.% C profoundly affected the coercivity values. Transmission electron microscopy (TEM) investigations revealed the origin of the improved coercivity. The addition of Nb resulted in a significant reduction in microstructural scale. The SmCo7 grain size decreased systematically with Nb content, reaching 150–200nm at 3at.% Nb. The addition of C also significantly enhanced the coercivity, which systematically increased with C content and reached 37kOe at 5at.% C. The effect of C, however, resulted in morphological changes as TEM revealed the formation of an intergranular phase that effectively isolated the hard magnetic SmCo7 grains from one another, reducing magnetic interactions. Excellent isotropic energy products of 6–8MGOe were also achieved.
Structure and magnetic properties of melt-spun (Nd0.625Ni0.375)85Al15 ribbons
Authors: Xu Feng , Wu Xiaoshan , Du Youwei , Cui Chong , Chen Guang
Published: June 2006

A metallic glass (Nd0.625Ni0.375)85Al15 with ferromagnetism at low temperatures is presented in the following paper. The structure, thermal stability, and magnetic properties have been investigated. The result demonstrates that the amorphous alloy has a certain glass-forming ability and forms a kind of spin glass below its Curie temperature.
Irradiation-controlled giant magnetoresistance of PtMn-based spin valve
Authors: Huang Sheng-Huang , Lai Chih-Huang , Chiang C. C. , Yang Cheng-Han
Published: June 2006

He + -ion irradiation resulted in the direct ordering of PtMn without postannealing. Samples were irradiated with 2MeV He+ ions and a beam current of 1.08μA∕cm2 such that the corresponding surface temperature was 190°C. The exchange bias direction was set in situ during irradiation in a field of 900Oe. A high giant magnetoresistance (GMR) ratio of 11% was obtained in PtMn-based spin valves after He+ irradiation. The GMR is completely eliminated after it is irradiated with oxygen ions at 42keV. Combining He+ with oxygen-ion irradiation can provide magnetic patterning for GMR sensors.
Detection of wide band signal by a high frequency carrier-type magnetic probe
Authors: Tan Kenji , Yamakawa Kiyoshi , Komakine Takashi , Yamaguchi Masahiro , Kayano Yoshiki , Inoue Hiroshi
Published: June 2006

A high frequency carrier-type magnetic field sensor [gaint magnetoimpedance (GMI) sensor] probe with a narrow magnetic strip was fabricated. High frequency field detection was performed by this probe. For specific dc bias fields, high sensitivity could be achieved in a wide range of frequency from 5kHzto1GHz. The sensitivity with a high dc bias field was lower than that with a low dc bias field. The outputs of the GMI probe were proportional to input power which generated the ac magnetic field.