bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical and magnetic properties induced by structural confinement of ternary chalcogenide in SBA-15 nanotube
Authors: Jung J.-S. , Choi K.-H. , Kim Y.-R. , Chae W.-S. , Lee S.-H. , Malkinski L. , Golub V. , O’Connor C. J.
Published: May 2003

The amorphous ternary material, Ni3(SbTe3)2 was prepared using a rapid precipitation reaction between the Zintl phase K3SbTe3 and NiBr2 in solution. This material induced by the incorporation of ternary chalcogenide into long range ordered mesoporous SBA-15 was transformed into ternary nanoparticles. Structural confinement of the amorphous ternary chalcogenide in the mesoporous silica tube causes the suppression of phonon coupling to photoexcited carriers in the confined ternary chalcogenide, resulting in narrower photoluminescence (PL) band than those of the bulk ternary chalcogenide. It also gives rise to ferromagnetic behavior of the nanoparticles made from the material, which is a spin glass with the freezing temperature of 4.0 K in a bulk form. The physical and chemical properties of obtained ternary bulk and nanoparticles have been investigated by x-ray diffraction, superconducting quantum interference device, transmission electron microscopy, Brunauer–Emmett–Teller, and PL (steady-state photoluminescence spectroscopy). The role played by the modified SBA-15 porous silica in controlling ternary chalcogenide size and quantities has been discussed.
Formation of FePt nanoparticles in annealed FePt/C multilayers
Authors: Zhang Y. , Wan J. , Bonder M. J. , Hadjipanayis G. C. , Weller D.
Published: May 2003

The formation of FePt nanoparticles by annealing FePt/C multilayer in the range of 600–800 °C was studied. During annealing at 700 °C, the coercivity of FePt (5 Å)/C (5 Å) sample increases from 0.2 to 6 kOe after 2 min of annealing, reaching 14.5 kOe after 1 h, while the coercivity of FePt (5 Å)/C (20 Å) sample shows a much slower change with annealing, exhibiting a value of 3.5 kOe after 1 h of annealing. Transmission electron microscopy results did not show a layer structure in as-deposited 5 Å/5 Å samples, and a clear superlattice reflection was observed in 5 Å/5 Å samples subjected to 2 min of annealing, indicative of the L10 fct phase. The degree of atomic ordering of the fct structure increases with further annealing. After 1 h of annealing, the 5 Å/5 Å samples show aggregates of particles with a well-ordered structure and a wide particle size distribution. In the as-deposited 5 Å/20 Å sample, a layer structure was observed with a uniform particle size distribution. After 5 min annealing at 700 °C, the layers are completely broken with the appearance of weak superlattice reflections. Both particle size and degree of atomic ordering increase slowly with further annealing in the 5 Å/20 Å sample. After 1 h annealing, isolated particles were observed with an average particle size of 6 nm. Depositing thicker carbon layers appear to restrict the growth of particles and delay the onset of atomic ordering with annealing.
Magnetic nanoparticles of Fe2O3 synthesized by the pulsed wire evaporation method
Authors: Uhm Y. R. , Kim W. W. , Kim S. J. , Kim C. S. , Rhee C. K.
Published: May 2003

Nanoparticles of Fe2O3 with a mean particle size of 4–50 nm have been prepared by the pulsed wire evaporation method, and its structural and magnetic properties were studied. From the main peak intensity of x-ray diffraction the amount of v̥-Fe2O3 and ɑ-Fe2O3 in sample is composed about 70% and 30%, respectively. The coercivity (53 Oe) and the saturation magnetization (14 emu/g) are about 20% of those of the bulk v̥-Fe2O3. A quadrupole line on the center of Mössbauer spectrum represents the superparamagnetic phase of v̥-Fe2O3 with a mean particle size of 7 nm or below.
Tailoring magnetic nanocrystal superlattices by chemical engineering
Authors: Sudfeld D. , Wojczykowski K. , Hachmann W. , Jutzi P. , Reiss G. , Hütten A.
Published: May 2003

Recognizing the technological potential of magnetic nanocrystals for the next generation of high density data storage [S. Sun and C. B. Murray, J. Appl. Phys. 85, 4325 (1999)] and as markers in biological systems for detection and manipulation of attached molecules, [L. Lagae, R. Wirix-Speetjens, J. Das, D. Graham, H. Ferreira, P. P. F. Freitas, G. Borghs, and J. De Boeck, J. Appl. Phys. 91, 7445 (2002)] ligand stabilized nanoparticles prepared via high–temperature solution–phase synthesis [C. B. Murray, S. Sun, W. Gaschler, H. Doyle, T. A. Betley, and C. R. Kagan, IBM J. Res. Dev. 45, 47 (2001); V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115 (2001)] rapidly gain interest in research and development. Following a chemical preparation route recently proposed [V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, Science 291, 2115 (2001)] Co and FeCo alloyed nanocrystals have been synthesized with different nanocrystal size distributions employing different growth rates during preparation. The resulting magnetic properties are a function of the particle size distribution and can be tailored for superparamagnetic or ferromagnetic behavior at room temperature. In addition, the possibility of synthesizing FeCo alloyed nanoparticles is demonstrated and discussed.
Thermally induced magnetization reversal in antiferromagnetically coupled media
Authors: Dittrich Rok , Schrefl Thomas , Suess Dieter , Scholz Werner , Forster Hermann , Fidler Josef
Published: May 2003

The calculation of energy barriers between stable equilibrium states in strongly interacting systems requires the calculation of saddle points in high dimensional energy landscapes. We combine the nudged elastic band method with finite element micromagnetics to calculate optimal paths, giving a global view of thermal switching in antiferromagnetically coupled media. Energy barriers and transition rates can be estimated from the saddle points and the energy minima along this path. We investigate the dependence of the energy barrier on the strength of the antiferromagnetic coupling and study the reversal in the data bit transition region. Below 1.1 mJ/m2 a two step reversal occurs passing a metastable state. Above 1.1 mJ/m2 this metastable state disappears and the energy barrier is constant for a coupling strength greater than 1.5 mJ/m2.
Development and process control of magnetic tunnel junctions for magnetic random access memory devices
Authors: Kula Witold , Wolfman Jerome , Ounadjela Kamel , Chen Eugene , Koutny William
Published: May 2003

We report on the development and process control of magnetic tunnel junctions (MTJs) for magnetic random access memory (MRAM) devices. It is demonstrated that MTJs with high magnetoresistance ∼40% at 300 mV, resistance–area product (RA) ∼1–3 kΩ μm2, low intrinsic interlayer coupling (Hin) ∼2–3 Oe, and excellent bit switching characteristics can be developed and fully integrated with complementary metal–oxide–semiconductor circuitry into MRAM devices. MTJ uniformity and repeatability level suitable for mass production has been demonstrated with the advanced processing and monitoring techniques.
Magnetic field effects on strain and resistivity during the martensitic transformation in Ni–Mn–Ga single crystals
Authors: Jin X. , Bono D. , O’Handley R. C. , Allen S. M. , Hsu T. Y.
Published: May 2003

Effects of 1 T external magnetic field on the transformation strains and resistivity changes associated with martensitic transformation in Ni2MnGa single crystals have been investigated by simultaneously monitoring the variations in strain and electrical resistivity with temperature. The results show that the strains on transforming from austenite to martensite are larger than that for the reverse transformation when the transformation takes place under 1 T external magnetic field. The reason for larger transformation strain mainly arises from the growth of favorable martensitic twin variants under external magnetic field rather than the magnetic field effect on the shift of martensitic transformation temperatures.
Reversible magnetization processes and energy density product in Sm–CoFe and Sm–Co/Co bilayers
Authors: Schrefl T. , Forster H. , Dittrich R. , Suess D. , Scholz W. , Fidler J.
Published: May 2003

The hysteresis properties of epitaxial SmCo/Co and SmCo/Fe bilayers are calculated by the solution of the Landau–Lifshitz Gilbert equation. The thin film grain structure is taken into account using appropriate finite element techniques. The J(H) curve shows the typical exchange spring behavior for the bilayer if the soft magnetic layer thickness exceeds 10 nm. However, the reversible rotations of the magnetization for low external field deteriorate the maximum energy density product. Straight B(H) curves are obtained only for a Fe layer thickness of 5 nm. Magnetization reversal starts with the reversible rotation of the soft layer magnetization. Initially, the magnetization rotates in opposite directions in different regions of the film. The reversible rotations penetrate substantially into the hard layer.
Fabrication of Sm–Co/Co (Fe) composites by electroless Co and Co–Fe plating
Authors: Zeng Q. , Zhang Y. , Bonder M. J. , Hadjipanayis G. C.
Published: May 2003

Sm–Co/Co and Sm–Co/Fe–Co nanocomposites have been fabricated by electroless plating of Sm–Co powders with Co and Co–Fe, respectively. The influence of electroless plating conditions on the magnetic properties and structure of the coatings were studied. The saturation magnetization (Ms) and morphology of the deposited material were found to be dependent on the concentration of hypophosphite, the temperature of the solution, and plating time. By plating at an elevated temperature and decreasing the NaH2PO2⋅H2O concentration of the bath, the phosphorus content decreases, and it is possible to form Co particles with a Ms of up to 142 emu/g as compared to 160 emu/g for bulk Co. For 50 μm Sm(Co,Cu,Fe,Zr)7.5 particles, coating with Co has had little effect on the coercivity, maintaining a value of 7 kOe, while there is a substantial increase in the value of Ms that varies with Co plating time. However, SmCo5 particles with a size of a few microns, plating with the bcc Co–Fe alloy increases Ms significantly, at the expense of the coercivity which decreases drastically.
High-resolution giant magnetoresistance on-chip arrays for magnetic imaging
Authors: Smith C. H. , Schneider R. W. , Pohm A. V.
Published: May 2003

Arrays of micrometer-sized giant magnetoresistance (GMR) magnetic sensors with corresponding micrometer-sized spacing on a single chip are being used to detect very small magnetic fields with high spatial resolution. These very small magnetic fields, or changes in magnetic fields, are associated with magnetic biosensors, nondestructive evaluation, precision position sensing, document validation including currency and magnetic tape, and other magnetic imaging. By using a silicon substrate, the signal conditioning and logic capability of integrated circuits can be incorporated to optimize system performance. This integrated technique reduces the effect of noise and simplifies the sensor/signal-processing interface. GMR sensor arrays with sensor spacing as small as 5 micrometers as well as single-chip arrays with 128 sensors covering a 4 mm width have been constructed. The GMR sensors are made of multilayer material consisting of FeCo/Cu layers with 8% to 20% GMR and maximum sensitivities of 1.0 mV/V/G (100 nV/nT @10 V). The sensor elements are photolithographically patterned with 1.5 to 2 μm wide stripes for high resistance and directional sensitivity. These on-chip magnetic arrays are being applied to various forms of magnetic media as well as work with biosensors and nondestructive test.
Enhanced coercivity in REFeAl-based bulk amorphous alloys
Authors: Betancourt I. , Valenzuela R.
Published: May 2003

A systematic study of die-cast REFeAl (RE=Nd+Pr) bulk amorphous alloys with small B, Cu, and Dy additions is presented. Differential scanning calorimetry and x-ray diffraction data are consistent with an amorphous microstructure and a small amount of hcp Nd-type crystallites embedded in the amorphous matrix for B, Cu+B-, and Dy-containing alloys. Coercivity, Hc, exhibits a progressive enhancement upon B, B+Cu, and Dy addition (from 284 kA/m for Nd30Fe60Al10 up to 388 kA/m for Dy substituted alloy). This Hc improvement is associated not only with domain wall pinning on the crystallites, but also with the increase of nucleation sites and thus, with the number of RE and RE–Fe–Al clusters. In agreement with the latter, a superparamagnetic behavior, which is characteristic of a clustered structure, was observed above 451 K for all the alloy compositions.
Temperature dependence of magnetic resonance in NiO nanoparticles
Authors: Pishko V. V. , Gnatchenko S. L. , Tsapenko V. V. , Kodama R. H. , Makhlouf Salah A.
Published: May 2003

Magnetic resonance measurements of different size NiO nanoparticles were performed at frequencies below 33 cm−1 and in the temperature range 300–500 K. The samples were earlier investigated by superconducting quantum interference device magnetometry and electron spin resonance spectrometry. The spectra were scanned by changing the sample temperature at fixed frequencies. At room temperature, resonance frequency for bulk NiO is 36.5 cm−1. At higher temperatures, the magnetic resonance frequency becomes lower, and at the Néel temperature goes to zero. For the 435 Å nanoparticles, we detected only one resonance peak. Extrapolation of the dependence to zero frequency gives a Néel temperature of 492 K. For 57 Å NiO, we observed two different peaks. One of them was at the same place as for 435 Å NiO, and another one was at higher temperatures. There exists several mechanisms which determine the magnetic structure of NiO nanoparticles, and, respectively, its resonance spectra. We believe that the magnetic resonance in 435 Å NiO corresponds to a bulklike structure, and the detection of two separate peaks in 57 Å NiO by a “size effect,” which is consistent with a many-sublattice magnetic structure and corresponding additional exchange modes of magnetic resonance.
Submicron Co(TaC) line array produced by electron-beam direct writing
Authors: Zhao Y. , Zhou T. J. , Wang J. P. , Thong J. T. L. , Yao X. F. , Chong T. C.
Published: May 2003

( Co 60 C 40 ) 97 Ta 3 and Co60C40 films with thickness 30 nm were prepared by cosputtering Co, Ta, and C onto C-buffered glass substrates. The as-deposited (Co60C40)97Ta3 and Co60C40 films were amorphous and nonferromagnetic. These films became magnetic upon annealing and the magnetic performance of annealed (Co60C40)97Ta3 films are better than that of annealed Co60C40 films at the same annealing condition. Magnetic patterning (line array) of the as-deposited (Co60C40)97Ta3 films was realized by subjecting it to electron-beam radiation using a focused 30 keV beam with a current of 7.1 nA and a dwell time per line of 0.75 s and longer. By increasing the dwell time, the whole region where an electron beam was scanned became magnetic with clear domain structures because of thermally activated diffusion. The required dwell time of magnetically patterning nonmagnetic (Co60C40)97Ta3 thin films (0.75 s) is much shorter than that of Co60C40 films (3.8 s). The magnetic measurements show that the lines [(Co60C40)97Ta3] and dots (Co60C40) are magnetically soft. The present method of magnetically patterning a nonmagnetic film has potential application for nanoscale solid magnetic devices.
Transition of magnetocrystalline anisotropy and domain structure in epitaxial Fe(001) nanomagnets
Authors: Pulwey R. , Zölfl M. , Bayreuther G. , Weiss D.
Published: May 2003

The magnetocrystalline anisotropy of epitaxially grown bcc-Fe(001) films on GaAs(001) shows a transition from a fourfold intrinsic anisotropy in thick films to an uniaxial one in ultrathin films (<3 nm) and hence can be tuned by varying the film thickness. Here we investigate the consequence of such an anisotropy tuning for the magnetization configurations of nanomagnets. The thickness was varied between 2.5 and 30 nm in steps of 2.5 nm. Disks with diameters between 200 nm and 2 μm were patterned with electron beam lithography and ion beam etching. The remanent and ac-demagnetized states as well as the switching behavior were examined by magnetic force microscopy. In addition, we employed micromagnetic simulations to compare with the measured results.
High-frequency measurements of spin-valve films and devices (invited)
Authors: Kaka Shehzaad , Nibarger John P. , Russek Stephen E. , Stutzke N. A. , Burkett S. L.
Published: May 2003

High-frequency measurements of spin-valve films and devices, made using several different measurement techniques, are presented and compared. Pulsed inductive measurements were made on sheet films and provide insight into the intrinsic dynamical properties of the component films and multilayer stacks. The damping parameter, in the completed spin-valve stack, is larger than in the constituent films. Direct time and frequency domain measurements of the dynamical response of micrometer-size spin-valve devices, made using high-bandwidth magnetoresistance techniques, showed damping parameters comparable to these measured on spin-valve sheet films. The small-angle magnetization response was also determined by high-frequency magnetic noise measurements. The damping parameters were smaller than those obtained by direct susceptibility measurements. The device-level measurements show a different dependence of the damping parameter on the easy-axis field as compared to sheet-level measurements. In addition to the uniform rotation mode, other peaks can be observed in the noise spectra that correspond to fluctuation modes arising from the micromagnetic structure. Electrical device measurements have much greater sensitivity than other high-frequency magnetic measurement techniques, which allow the direct observation of magnetization motion in submicrometer elements without averaging. This technique is used to directly examine thermally activated events and nonrepetitive dynamical motions.
Polyol-process-derived CoPt nanoparticles: Structural and magnetic properties
Authors: Chinnasamy C. N. , Jeyadevan B. , Shinoda K. , Tohji K.
Published: May 2003

We report the synthesis and magnetic properties of CoPt nanoparticles by using the polyol process. Since the reduction potential of Pt is more positive than Co, Pt is easily reduced compared to Co. Hence, CoPt nanoparticles were realized by coreducing cobalt and platinum acetylacetonate in the presence of an appropriate amount of OH ions in trimethylene glycol. X-ray diffraction and transmission electron microscopy studies showed that the as-synthesized CoPt nanoparticles had fcc structure and about 5 nm in diameter. Composition analysis reveals that the as-synthesized particles are almost equiatomic Co50Pt50. Magnetic characterization revealed that these nanoparticles are ferromagnetic at room temperature, and that the magnetization and coercivity values were 8 emu/g and 380 Oe, respectively. Differential scanning calorimetry studies showed that the ordering temperature of the as-synthesized particles was only 550 °C (peak temperature), against 825 °C of the bulk. Annealing the CoPt nanoparticles above 550 °C induced ordering with enhanced magnetic properties.
Structure and magnetic properties of in-plane oriented FePt–Ag nanocomposites
Authors: Zhou Y. Z. , Chen J. S. , Chow G. M. , Wang J. P.
Published: May 2003

By cosputtering Ag with FePt on a Cr90Ru10 underlayer, the film texture and magnetic easy axis were successfully changed from the perpendicular to the longitudinal direction. The crystallographic structure and magnetic properties of Ag-doped FePt thin films were investigated. Doping of Ag in FePt films led to greater in-plane coercivity and smaller magnetic grain size. The in-plane coercivity of the nanocomposite films varied from 0.8 to 6.5 kOe, depending on the Ag content. With an increase in Ag content, the FePt grain size became smaller, and the magnetic reversal mechanism approached the nucleation mode. These results suggest a practical way by which to fabricate L10 FePt thin films with longitudinal anisotropy using a nanocomposite approach.
Effects of grain size and morphology on the coercivity of Sm2(Co1−xFex)17 based powders and spin cast ribbons
Authors: Chen Christina H. , Kodat Steve , Walmer Michael H. , Cheng Shu-Fan , Willard Matthew A. , Harris Vincent G.
Published: May 2003

The effects of grain/particle size and morphology on the coercivity of Sm2(Co1−xFex)17 based powders and spin cast ribbons are investigated in an attempt to make pure Sm–Co 2:17 magnets with a higher energy product than that currently available. It is observed that the coercive force is not only affected by the composition, but also significantly by the grain/particle size. The coercivity of the as-cast SmCo6 powder was increased from 0.5 to ∼12 kOe when the grain/particle size was decreased from 14 to 1.5 μm. A twofold increase in coercivity was also observed for some Sm(CO0.85Fe0.15)7.6 spin cast ribbons when the wheel speed was increased from 6 to 25 m/s and the grain size of the ribbons was decreased from 2–15 to 0.5–5 μm. The initial magnetization curves show a high degree of texture that exists in the free side of ribbons, with the c axis parallel to the longitudinal direction of the ribbons. The presence of a trace of boron in the ribbons changes the grain morphology and eliminates the texture. Some results show promise of making a high coercivity higher energy product Sm2(Co1−xFex)17 based magnets without adding nonmagnetic elements such as Cu and Zr.
Magnetism of Ir in Fe2IrSi from Ir L2,3 edge x-ray magnetic circular dichroism spectroscopy
Authors: Krishnamurthy V. V. , Weston J. L. , Mankey G. J. , Suzuki M. , Kawamura N. , Ishikawa T.
Published: May 2003

The formation of an induced 5d magnetic moment on Ir in a new ferromagnetic compound Fe2IrSi [Curie temperature 662(15) K], which forms in the L21 structure, has been investigated by x-ray magnetic circular dichroism and x-ray absorption spectra measurements at Ir L2,3 edges at 297 K. Using the sum rules which relate the integrals of these spectra with the expectation ground state value of the orbital angular momentum 〈LZ〉 of the probe atom, the orbital moment of Ir could be determined as 0.0036(2)μB. The orbital moment to spin moment ratio is found to be 0.0240(15). Assuming ferromagnetic coupling between Ir and Fe and that the total magnetic moment of Fe2IrSi follows Slater–Pauling behavior, we estimate the magnetic moment of Fe at 0 K as 1.4μB/atom.
Quantum transport properties in ferromagnetic nanorings at low temperature
Authors: Kasai S. , Saitoh E. , Miyajima H.
Published: May 2003

The low-temperature magnetoresistance in ferromagnetic Ni and Fe19Ni81 nanorings were studied. Although the aperiodic fluctuation and the periodic oscillation were observed in Fe19Ni81 nanoring, the periodic oscillation disappears in Ni nanoring. The estimated phase coherence length in Ni nanoring is about 80 nm, which is much smaller than that in Fe19Ni81 nanoring (∼500 nm). These results imply that there exists a mechanism, such as coupling between conduction electron and local magnetic anisotropy.
Magnetic hysteresis of mechanically alloyed Sm–Co nanocrystalline powders
Authors: Zhou J. , Skomski R. , Sellmyer D. J.
Published: May 2003

Mechanically alloyed Sm–Co powders and Sm12(Co,Cu,Ti)88 powders are investigated. X-ray diffraction patterns show that after annealing, structures of 2:7, 1:5, and 1:7 phases form. A room-temperature coercivity of 41 kOe was obtained in Sm2Co7 powders. Magnetic hysteresis was investigated by the method of δm curves. Positive value δm curves were obtained in Sm2Co7 and SmCo5, while negative values were found in SmCo7.3 and SmCo8 indicating different magnetization reversal mechanisms. Nanocrystalline powders of Sm12(Co,Cu,Ti)88 with a mixture of 1:5 and 2:17 phases form after long-time heat treatment. The intrinsic coercivity of the powders increases with an increasing amount of Cu. Short annealing time produces 1:7 phase with higher crystalline anisotropy, which results in larger coercivity.
Magnetization patterns of permalloy square frames
Authors: Lai Mei-Feng , Wei Zung-Hang , Chang Ching-Ray , Wu J. C. , Hsieh W. Z. , Usov Nickolai A. , Lai Jun-Yang , Yao Y. D.
Published: May 2003

Four different magnetization configurations of micron- and submicron-sized permalloy square frames are investigated by numerical simulations and experiments. Beside the pure conventional 90° Neel type wall with zero net magnetic pole, we also obtain numerically another high energy domain wall with positive or negative net magnetic poles in the corner. These three kinds of domain walls constitute four different patterns in square frames. We compare the magnetic pole density distributions derived from the spin configurations of simulation results with the images taken by magnetic force microscopy, and find reasonable agreement between them.
Microscale resin-bonded permanent magnets for magnetic micro-electro-mechanical systems applications
Authors: Cho Hyoung J. , Ahn Chong H.
Published: May 2003

A micromachining technique has been developed for the fabrication of microscale resin-bonded permanent magnets. Magnetic paste has been prepared from Sr-ferrite powder and an epoxy resin, filled into lithographically defined molds, and formed into resin-bonded magnets after room temperature curing. Coercivity of 356 kA/m (4480 Oe), retentivity of 33 mT (330 G), and energy density of 2.7 kJ/m3 have been achieved in 65-μm-thick disk arrays with lateral dimensions ranging from 50 to 200 μm. Based on the developed magnet, a magnetic MEMS actuator has been designed, fabricated, and characterized. Actuation current up to ±60 mA operated the actuator up to 70 μm in attractive and repulsive motion. This work can be used for producing thick-film type permanent magnets, which can be scaled from a few tens of micrometers to millimeters on various substrates.
Effect of additions on the crystallization process, structure, and magnetic properties of (NdPr)–Fe–B amorphous ribbons
Authors: Wang Z. C. , Davies H. A.
Published: May 2003

The influence of Zr and Mo additions on the crystallization process, microstructure, and magnetic properties of (Nd0.75Pr0.25)12Fe82B6 amorphous alloy ribbons has been studied. Results show that the addition of 3 at. % of either Zr or Mo refines the grains for the optimally annealed alloys, which enhances the intergrain exchange coupling between the RE2Fe14B and the hard magnetic properties. The effect of Zr is, however, superior to that of Mo. Analysis of the crystallization kinetics for the three alloys, investigated by DTA, shows that the activation energy for crystallization Ec(x) for the basic (Nd0.75Pr0.25)12Fe82B6 alloy remains almost constant with increasing crystal fraction x, whereas for both the Mo- and Zr-containing alloys Ec(x) increases gradually with x giving a reduced crystal growth rate consistent with the observed grain refinement. The increase in Ec(x) with x, is larger for the Zr-doped alloy than for the Mo counterpart, which is in accord with the more refined grains and the larger enhancement of Br and (BH)max for the former alloy.
Magnetic two-dimensional short-range order in hexagonal manganites
Authors: Lonkai Th. , Tomuta D. G. , Hoffmann J.-U. , Schneider R. , Hohlwein D. , Ihringer J.
Published: May 2003

Hexagonal manganites REMnO3 consist of layers of two-dimensional connected distorted MnO5 bipyramids separated by distorted layers of RE, leading to frustrated Mn3+–O2−–Mn3+ in-plane exchange paths (superexchange) and Mn3+–O2−–O2−–Mn3+ interplane exchange paths (super-superexchange). We report single-crystal diffraction experiments (RE=Ho,Er,Y) indicating magnetic 2D short-range order up to temperatures of T=150 K (TN∼74 K), and SQUID magnetization data (RE=Y) indicating short-range order up to at least 250 K. Fits of a model based on an extended mean-field theory to the single-crystal data show in-plane exchange about 3 orders of magnitude stronger than the interplane exchange, explaining the unusual strong deviation from the Curie–Weiss law of the SQUID magnetization data.
Static torque profiles of a hybrid stepper motor having relative eccentricity between stator and rotor axes
Authors: Rajagopal K. R. , Singh Bhim , Singh B. P.
Published: May 2003

A smooth and controlled driving load for a hybrid stepper motor requires detailed knowledge of the motor’s static and detent torque profiles. Uncertainties to measured torque profiles may be due to rotor/stator concentric misalignment as one possible factor. This article presents the results of a detailed experimental analysis on a stepper motor’s relative eccentricity between the rotor and stator axes. The nonuniform airgap not only introduces a position-dependent fundamental detent torque along with the conventional fourth harmonic detent torque, but also changes the static torque profile. The sum effect on static torque is a changed torque equilibrium point, dependent both on rotor position and motor phase. Detent torque harmonic component analysis of a set of 0.5° hybrid stepper motors, with rotor-to-stator eccentricity as the control variable, showed how increased eccentricity exceeding a given limit will cause a loss of stable equilibrium of the detent torque at each step position.
Proton nuclear magnetic resonance investigation of the spin dynamics in cobalt based one-dimensional magnetic molecular chains
Authors: Lascialfari A. , Micotti E. , Aldrovandi S. , Caneschi A. , Gatteschi D.
Published: May 2003

H 1 nuclear magnetic resonance was used to investigate the spin dynamics of the molecular chain Co(hfac)2NITPhOMe. This helical chain is composed of alternating Co(II) ions and radical moieties with dominant nearest-neighbor antiferromagnetic interaction (J∼220 K). Below 50 K, the chain behaves as a 1/2–1/2 Ising chain, with the electronic spins freezing before a transition to three-dimensional order. Previous results stated that the relaxation time τM of the magnetization M follows a thermally activated mechanism τM=τ0 exp(Δ/T) with τ0≈3(±1)×10−11 s and Δ≈152(±2) K. Measurements of the H1 NMR spin-lattice relaxation rate T1−1 were performed as a function of the temperature for two different external magnetic fields (0.35 and 1.7 T). Below 50 K, where the 1/2–1/2 spin chain approximation is valid, explanation of the experimental data was found by assuming the presence of two thermally activated relaxation mechanisms. One of these corresponds to the correlation time detected by M while a second mechanism, undetected in the magnetization measurements, becomes effective in the low temperature range. The possibility of the two nuclear relaxation mechanisms being related to different spin dynamics of the Co(II) and the radical spins, respectively, is discussed.
Magnetization reversal and aftereffect in orthoferrites
Authors: Didosyan Y. S. , Hauser H. , Reider G. A. , Glatz R. , Wolfmayr H.
Published: May 2003

A magnetic aftereffect of several μs duration has been discovered in 1.2 mm thick samples of yttrium orthoferrite crystals cut perpendicularly to the optical axis at 1.3 μm wavelength. The aftereffect is associated with nucleation and expansion of oppositely magnetized domains that occur in a monodomain sample after removal of an external magnetic field. Magnetization reversal processes are visualized in a Faraday setup and studied by measuring the temporal dependence of the state of polarization of a transmitted light beam.
Magnet shape optimization for high performance single-phase line start synchronous motor
Authors: Kim Sol , Choi Jae-Hak , Lee Ju
Published: May 2003

This article presents an optimal design for a single-phase line start synchronous motor according to permanent magnet and flux barrier geometries. To explore the improved torque and efficiency over that of conventional single-phase induction motors for compressors, three types of magnet shape and flux barrier are introduced. A time-stepped two-dimensional finite-element method that considers the external running capacitor is used for analysis of the steady state characteristics. Counter electromotive force at the rated speed, Ld and Lq inductances, torque, and efficiency are calculated to optimize the shape of the magnet and the flux barrier. From the torque and efficiency characteristics taking variation of the load angle into consideration in the steady state, we can determine an optimal design for the permanent magnet and the flux barrier.
Ferromagnetic quantum dots formed by external laser irradiation
Authors: Yang J. Y. , Yoon K. S. , Do Y. H. , Kim C. O. , Hong J. P. , Rho Y. H. , Kim H. J.
Published: May 2003

We present an efficient method for the formation of cobalt ferromagnetic quantum dots (FMQDs) by exposing a Nd: YAG laser (wavelength=355 nm) on ferromagnetic thin films grown on Si substrates. Primary emphasis is placed on the simple and direct fabrication of the FMQDs without performing any micro- or nanopattering process. Scanned laser irradiation of low power causes localized segregation of ultrathin as-deposited films by the laser-induced heat, resulting in the fabrication of FMQDs. Observations by scanning electron microscope and atomic force microscope clearly confirm highly uniform controllable nanoscale FMQDs in our case. The average size and density of the FMQDs were about 10 nm and 2×1011/cm2, respectively. Magnetic characteristics of the FMQDs were also measured by a superconducting quantum interference device at 5 K.
Effect of maximum torque according to the permanent magnet configuration of a brushless dc motor with concentrated winding
Authors: Lee Kab-Jae , Kim Sol , Lee Ju , Oh Jae-Eung
Published: May 2003

A brushless dc (BLDC) motor, which has a permanent magnet (PM) component, is a potential candidate for hybrid or electric vehicle applications. Minimizing the BLDC motor size is an important requirement for application. This requirement is usually satisfied by adopting a high performance permanent magnet or improved winding methods. The PM configuration is also a critical point in design. This article presents the effect of the PM configuration on motor performance, especially the maximum torque. Four representative BLDC motor types are analytically investigated under the condition that the volume of the PM and magnetic material is constant. An embedded interior permanent magnet motor has the best torque performance the maximum torque of which is more than 1.5 times larger than that of the surface mounted permanent magnet motor. The performance of back electromotive force, instantaneous torques is also investigated.
Structure and magnetic properties of NiFe/SiO2 and Co/SiO2 nanocomposites consolidated by detonation compaction
Authors: Zhang Y. D. , Ma X. Q. , Hui S. , Wu Mingzhong , Ge S. H. , Hines W. A. , Budnick J. I. , Cetegen B. M. , Semenov S. Y.
Published: May 2003

In this article, the structural and magnetic properties of NiFe/SiO2 and Co/SiO2 nanocomposites fabricated via powder processing are presented. The NiFe/SiO2 and Co/SiO2 nanoparticles were both synthesized by a wet chemistry approach. The as-synthesized nanoparticles were characterized by scanning electron microscopy, x-ray diffraction, and superconducting quantum interference device magnetometer, yielding detailed information concerning the structure and size of NiFe or Co particles, coating, and composition purification. The nanoparticles were then consolidated into solid components via detonation compaction. Depending on the powder morphology and detonation conditions, the density of the consolidated sample can reach over 91% of the theoretical density of the conventional materials. X-ray diffraction experiments on the samples both before and after consolidation indicate that the crystal structure of the nanocomposites remains unchanged during detonation consolidations; however, a rather large increase in particle size for the magnetic constituent was observed. Static magnetic property studies carried out on the samples both before and after detonation showed no change in the saturation magnetization after consolidation, indicating that the detonation consolidation does not cause oxidation of the nanopowders. Initial complex permeability measurements on the consolidated samples showed an essentially flat μ′ versus frequency response over a wide frequency range between 5 kHz and 13 MHz, implying a high resistivity of the consolidated sample. These experiments show that detonation compaction is a promising approach for consolidating magnetic nanoparticles, and NiFe/SiO2 and Co/SiO2 are good candidates for high frequency soft magnetic materials.
Magnetic, magneto-optical, and transport properties of ferromagnetic shape-memory Ni2MnGa alloy
Authors: Lee S. J. , Lee Y. P. , Hyun Y. H. , Kudryavtsev Y. V.
Published: May 2003

The magnetic, transport, and magneto-optical properties of Ni2MnGa alloy in the martensitic and austenitic states were investigated. The temperature dependence of resistivity shows kinks at the structural and ferroparamagnetic transitions. The temperature dependence of magnetization was also investigated for both the bulk and the film. It was shown that the lack of crystallinity in the amorphous Ni2MnGa alloy films leads to the loss of ferromagnetic order above 4 K. The nature of the interband absorption peaks in the optical-conductivity spectra and in the absorptive part of off-diagonal components of the dielectric function for the authentic state of Ni2MnGa alloy was elucidated.
Resonant magnetic reflectivity from U/Fe multilayers
Authors: Brown S. D. , Beesley A. , Herring A. , Mannix D. , Thomas M. F. , Thompson P. , Bouchenoire L. , Langridge S. , Lander G. H. , Stirling W. G. , Mirone A. , Ward R. C. C. , Wells M. R. , Zochowski S. W.
Published: May 2003

U/Fe multilayers have been produced through dc sputtering. The high quality of these multilayers has been confirmed through x-ray reflectivity measurements and fitting to the resultant spectra. These samples are currently the subject of investigations using techniques including polarized neutron reflectivity, conversion electron Mössbauer spectroscopy, and magnetotransport and magneto-optic measurements. In this article, we report on results obtained from a [U 30 Å/Fe 40 Å]×30 multilayer through resonant magnetic scattering performed on the XMaS beamline at the European Synchrotron Radiation Facility. The asymmetry ratio is defined as (I+−I−)/(I++I−), where I+(I−) is the normalized intensity obtained with circular polarization and the applied field parallel (antiparallel) to the beam direction. Asymmetry data are presented as functions of both energy and momentum transfer. We have identified a Uranium moment with the use of resonant magnetic reflectivity at 12 K. Reversal of the polarity of the asymmetry ratio between the first and second superlattice peaks indicates a nonuniform moment distribution through the uranium layers. Temperature dependence measurements confirm that the moment also exists at room temperature.
Magnetic structures of CrPt3 by first-principles calculations
Authors: Kwon Youngsoo , Rho T. H. , Lee Sangsoo , Hong Soon C.
Published: May 2003

In order to investigate systematically the magnetic structures of CrPt3, four different magnetic structures of ferrimagnetism (FIM), A-, C-, and G-type antiferromagnetism (AFM) for L12 structure and two different ones of FIM and A-type AFM for D022 structure were calculated. The full-potential linearized augmented plane wave (FLAPW) method was employed to solve Kohn–Sham equation self-consistently, using general gradient approximation for exchange-correlation potential. We found that L12 FIM is most stable among all of the aforementioned states. For D022, AFM is more stable than FIM by a significant energy difference (102 meV/Cr–atom). For D022 AFM the coupling between Cr and Pt atoms is ferromagnetic unlike FIM. The calculated magnetic moments are calculated to 2.78, 2.81, 2.81, and 2.88 μB for FIM, A-, C-, and G-type AFM in L12, and 2.36, 2.59 μB for FIM and AFM in D022, respectively. The magnetism is proven to be very sensitive to a crystal structure.
Ferromagnetic properties in Cr, Fe-doped Ge single crystals
Authors: Choi Sungyoul , Hong Soon Cheol , Cho Sunglae , Kim Yunki , Ketterson John B. , Jung Chi-Un , Rhie K. , Kim Bong-Jun , Kim Y. C.
Published: May 2003

We have fabricated Cr and Fe-doped bulk Ge single crystal using the vertical gradient solidification method. The Cr-doped Ge single crystal showed ferromagnetic ordering at 126 K, as determined from temperature dependent magnetization and resistance measurements. The measured magnetic moment per Cr was 0.83 μB at 5 K. On the other hand, Fe0.05Ge0.95 showed ferromagnetic ordering at 233 K. The coersive field was 80 Oe at 5 K.
Formation of two-dimensional electron gas and the magnetotransport behavior of ZnMnO/ZnO heterostructure
Authors: Edahiro T. , Fujimura N. , Ito T.
Published: May 2003

ZnO based heterostructure, which contains magnetic impurities in the barrier layer, is grown on c-cut sapphire substrate by pulsed laser deposition. The temperature dependence of the mobility of the Zn0.9Mn0.1O/ZnO heterostructure exhibits the suppression of ionized impurity scattering below 100 K. The carrier concentration and the mobility measured at 1.85 K are 4.0×1012 l/cm2 and 360 cm2/V s. Shubnikov–de Haas oscillations are clearly observed in longitudinal magnetoresistance above 3.7 T at the same temperature. These transport properties reveal that two-dimensional electron gas is successfully obtained at the Zn0.9Mn0.1O/ZnO interface.
Atomic, electronic, and magnetic properties of magnetic tunnel junctions
Authors: Oleynik I. I. , Tsymbal E. Yu.
Published: May 2003

We report results of first-principles density-functional studies of the atomic and electronic structure of Co/Al2O3/Co and Co/SrTiO3/Co magnetic tunnel junctions (MTJs). The atomic structure has been studied for different interface terminations and the interfacial energetics was quantified based on the work of separation. Based on energetics of cohesion, the O-terminated and TiO2-terminated interfaces were identified as the most stable structures for Co/Al2O3/Co and Co/SrTiO3/Co MTJs, respectively. The electronic structure was analyzed using local densities of states. We found that the electronic structure of the O-terminated Co/Al2O3/Co tunnel junction exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 Å. Calculated electronic structure of the TiO2-terminated Co/SrTiO3/Co MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen. This coupling induces a magnetic moment of 0.25 μB on the interface Ti atom, which is aligned antiparallel to the magnetic moment of the Co layer. This is a possible cause of inversion of the spin polarization of tunneling across the SrTiO3 barrier that was found in recent experiments.
Verification of magnetic field gradient effects on medium convection and cell adhesion
Authors: Iwasaka M. , Yamamoto K. , Ando J. , Ueno S.
Published: May 2003

We investigated the effects of strong static magnetic fields on the convectional flow in a cell culture medium and their consequences on cell adhesion patterns. Gradient magnetic fields of 6 T with 60 T/m affected the convection with floating cell aggregations in a cell culture flask, and reversibly changed the direction of convectional flow. HeLa cells exhibited streamlike-cell distribution patterns in the direction of the applied magnetic field gradient. The study also revealed a promotion of cell orientation by a thermal convectional flow in the medium after magnetic field exposure of MC3T3 cells at 8 T for one day. An application of gradient magnetic fields for the treatment of cell adhesion was proposed using a magnetically modulated convectional flow.
Fe
3
O
4
and its magnetic tunneling junctions grown by ion beam deposition
Authors: Aoshima Ken-ichi , Wang Shan X.
Published: May 2003

Magnetic properties of Fe3O4 and magnetic tunnel junctions with Fe3O4 bottom electrode have been investigated. Highly conductive V/Ru layers were used as an underlayer of the Fe3O4 films. The V/Ru/Fe3O4 on [110] out-of-plane oriented MgO single crystal substrate show an anisotropy and high squareness along [11̄0] direction, while the Fe3O4 films with an underlayer of just Ru show isotropic behavior and low squareness. X-ray diffraction shows tensile stress on Fe3O4 for V/Ru/Fe3O4 samples. The anisotropy was shown to be induced by the stress. Finally, magnetic tunnel junction stacks of MgO/V/Ru/Fe3O4/AlO/CoFe/NiFe/Ru were deposited and the magnetic tunnel junctions with a junction size ranging from 2×2 μm2 to 9×9 μm2 were fabricated by optical lithography. The junctions show magnetoresistance ratios of ∼14% and no geometrical effect due to the junction size.
Spontaneous magnetoencephalography alpha rhythm measurement in a cylindrical magnetic shield employing magnetic shaking
Authors: Tashiro K. , Nagashima K. , Sumida A. , Fukunaga T. , Sasada I.
Published: May 2003

This article shows a demonstration of magnetoencephalography measurement in a cylindrical magnetic shield made of cobalt-based amorphous tape with magnetic shaking. The noise levels of the first-order superconducting quantum interference device gradiometers that operated in the shield were reduced to as low as 40 fT/√Hz at 10 Hz by surrounding it with a simple rf shield made of conductive cloth. We observed spontaneous alpha rhythms from a human brain in this shielding system. Alpha rhythms and their suppression caused by opening the eyes were clearly found, which was also confirmed by electroencephalography measurement from the same volunteer under similar conditions.
Magnetocrystalline volume and interface anisotropies in epitaxial films: Universal relation and Néel’s model (invited)
Authors: Bayreuther Günther , Dumm Martin , Uhl Brigitte , Meier Roland , Kipferl Wolfgang
Published: May 2003

The thickness dependence of the fourfold in-plane magnetic anisotropy was first observed in epitaxial Fe(001) films and described by the volume anisotropy of bcc Fe with a positive anisotropy constant (K1vol>0) superimposed by a negative interface term (K1int<0). This results in a spin reorientation of the easy axis from [100] in thick films to [110] for thicknesses tFe<tcrit=6 ML. To clarify the origin of the interface term Fe and bcc Fe1−xCox alloy films (x<0.8) epitaxially grown on different substrates—Ag(001), Au(001), GaAs(001)—were studied. As a result, magnetoelastic interactions can be ruled out because exactly the same anisotropies are found for tensile strain [Fe on Ag(001) and Au(001)] and for compressive strain [Fe on GaAs(001)]. Also, K1int is not affected by the particular interface material or overlayer, e.g., vacuum or Au. A universal correlation was found between the volume and interface anisotropy constants: −K1int/K1vol=tcrit=6 ML. It is shown that this is indeed expected within Néel's phenomenological theory of magnetic anisotropy if contributions from nearest and next-nearest neighbors are taken into account. Electronic hybridization effects in this case seem to play a minor role compared to the local symmetry. It is suggested that observed effects of overlayers on perpendicular interface anisotropies might be related to surface relaxations which have not been sufficiently studied so far and which, on the other hand, would not affect the fourth-order in-plane anisotropy according to Néel's model in agreement with present experimental results.