bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of magnetostriction in amorphous delay lines
Authors: Hristoforou E. , Hauser H. , Ktena A.
Published: May 2003

A macroscopic model of the response of magnetostrictive delay lines (MDLs) under pulsed current excitation Ie is presented. The basic principles of the MDL operation are outlined and the relevant considerations of the control parameters are discussed. It is shown that the peak value of the voltage response is proportional to the derivative of the magnetostriction λ versus field H. The analytic expression derived for dλ/dH involves two identification parameters, c and A0. Parameter c has field dimensions and is a function of material parameters as described by the energetic model (EM) and proportional to the effective anisotropy field as predicted by EM, and A0 is a normalization constant, related to Ic and saturation magnetostriction λs. Preliminary results are presented comparing theoretical curves with experimental data on a Fe78Si7B15 amorphous ribbon sample with sufficient agreement.
Magnetic anisotropy in Ni–Mn–Ga martensites
Authors: Straka Ladislav , Heczko Oleg
Published: May 2003

We study the temperature dependence of the magnetic anisotropy of three different martensites known to exist in the Ni–Mn–Ga alloys. The anisotropy constants were determined from magnetization curves measured at different temperatures. The anisotropy of five-layered modulated tetragonal martensite is uniaxial with easy magnetization direction along short crystallographic axis. At room temperature K1(rt)=1.65×105 J m−3 and K2 is negligible. Seven-layered modulated orthorhombic martensite exhibits easy magnetization direction along the shortest crystallographic axis. K1(rt)=1.7×105 J m−3 and K2(rt)=0.9×105 J m−3 referring to hard and mid-hard magnetization axes. Nonmodulated tetragonal martensite possesses a uniaxial anisotropy with easy plane and hard magnetization direction along the long crystallographic axis with K1(rt)=−2.3×105 J m−3 and K2(rt)=0.55×105 J m−3. The temperature dependence of K1(T) of five-layered martensite follows magnetization power law with exponent n=3 suggesting a single ion origin of the magnetic anisotropy in Ni–Mn–Ga martensite.
Magnetotransport properties of CrO2 films down to single-grain sizes
Authors: Yuan L. , Ovchenkov Y. , Sokolov A. , Yang C.-S. , Doudin B. , Liou S. H.
Published: May 2003

The magnetotransport properties of CrO2 films down to single-grain sizes have been investigated. The magnetoresistance of highly grain oriented CrO2 (100) films, deposited onto TiO2 (100) substrates using a high pressure cell, has been measured in the temperature range of 1.5–300 K and in magnetic fields up to 6 T. The difference in the magnetoresistance at the low temperature of these samples is related to the value of the residual resistance ratio (RRR=R273 K/R4.2 K) of the samples. The negative magnetoresistance observed in some of these films is likely due to defects (disorder) in the CrO2 crystal. We observed a large positive longitudinal magnetoresistance in the CrO2 sample when it was measured with the current perpendicular to the c axis. The nature of the large value of the longitudinal magnetoresistance may be related to the complex configuration of the Fermi surface of CrO2.
Method for the synthesis of CrO2 at ambient pressure and temperature
Authors: Ye H. , Zhang Q. , Saito F. , Jeyadevan B. , Tohji K. , Tsunoda M.
Published: May 2003

In this article, we report the synthesis of metastable chromium dioxide at ambient temperature and pressure. The formulation of this method is based on the fact that the CrO2 particles used in magnetic recording tapes are chemically unstable and react with the organic binder, leading to a disproportionate reaction that produces nonmagnetic Cr(III)OOH and H2Cr(VI)O4 compounds. This suggests that it is also possible to synthesize CrO2 by reacting Cr(III) with Cr(VI) under appropriate conditions. Thus, metastable chromium dioxide (CrO2) powder was synthesized by mechanically grinding a mixture of CrO3 and Cr(OH)3 to induce a solid–solid reaction at atmospheric temperature and pressure. The characteristics of the product obtained after mechanical grinding of the above mixture depended on parameters such as the preparation conditions for the starting material Cr(OH)3, the grinding time, the rotational speed of the mill and the mole ratio of CrO3 and Cr(OH)3. An x-ray diffraction pattern of the product obtained under optimum conditions was in excellent agreement with the profile of CrO2. Furthermore, the powder was strongly ferromagnetic, with saturation magnetization of 63 emu/g in an applied field of 1 T.
Interaction between Cs and Fe
Authors: Zhang Y. S. , Zeng Zhi
Published: May 2003

Following the fact that giant magnetic moments of Fe observed as impurities either in bulk or thin film of cesium, the component as well as the distance dependence of Fe–Cs interactions were studied by density functional theory with local spin density approximation. Modeled ordered alloys of FexCs1−x (x=1.0, 0.75, 0.5, 0.25, 0) are studied by placing Fe and Cs at various lattice sites with different interatomic distances on a body-centered-cubic crystal structure. The total spin moments per Fe atom decrease as the Cs concentration increases in modeled FexCs1−x, which indicates that stronger hybridization between Fe and Cs exists when there are more Cs atoms in the system. The spin moments per Fe atom become larger with an increase in the Fe–Cs distance in Fe0.5Cs0.5. Polarization to the cesium atom is small even for shorter Fe–Cs distances. The interaction between Cs and Fe is analyzed by the change of Fe 3d states in these systems, and is significantly different from that in either single iron or bulk iron.
Magnetic property and electronic structure of itinerant PdxCoyO2 magnets
Authors: Okabe H. , Matoba M. , Kyomen T. , Itoh M.
Published: May 2003

The electronic structure of the delafossite-type compound PdCoO2 is calculated by the full potential linear muffin-tin orbital method and the magnetic property of the composition modified (defect injection introduced) compound Pd0.72Co0.68O2 was investigated. The calculation reveals that the density of states of PdCoO2 around the Fermi level is mainly composed of Pd 4d and Co 3d and O 2p orbitals, and the Fermi level is located just at the depression of the density of states. The magnetic nature of the composition modified Pd0.72Co0.68O2 compound is quite complex. The slow spin dynamics found below Tf can be interpreted by considering that the weak ferromagnetic moment in each CoO6 plane is itself disordered. This behavior is explained by the short-range character of the randomness which leads to a finite-size magnetized cluster, and suggests that the presence of these clusters would induce a spin-glass like feature for T<Tf.
Micromagnetic simulation of thermal ripple in thin films: “Roller-coaster” visualization
Authors: Feng Xuebing , Visscher P. B. , Apalkov D. M.
Published: May 2003

We present simulations of pulse-induced magnetization-rotation experiments in Permalloy. These lead to temporary domain formation (“thermal ripple”) and help to explain the time dependence of experimental results. To understand and visualize the motion, we find it very useful to exploit a mathematical isomorphism of this problem (in the limit Ms≫Hpulse) to the problem of a massive particle on a circular track (“roller coaster”). The height (gravitational potential energy) of this track is proportional to the Stoner–Wohlfarth energy. The fact that the resulting “precession” is really oscillation in a plane, and the fact that this oscillation overshoots the minimum-energy configuration (the inertia effect) are much more intuitive in the roller coaster picture than in the conventional “M precesses about the effective field” picture. Animated simulations of this behavior are available on the web (http://bama.ua.edu/∼visscher/mumag/).
Statistical coarse graining as an approach to multiscale problems in magnetism (invited)
Authors: Dobrovitski V. V. , Katsnelson M. I. , Harmon B. N.
Published: May 2003

Multiscale phenomena which include several processes occurring simultaneously at different length scales, and exchanging energy with each other, are widespread in magnetism (e.g., nucleation of magnetization reversal at an atomic-scale defect or grain boundary, a domain wall breaking through an atomic-scale defect, etc.). We demonstrate that in such multiscale processes, a role of short-wavelength excitations dynamically generated near the defects is crucial. As a result, standard micromagnetic theory is not always sufficient for description of these phenomena. We present a novel coarse-graining approach to length scales coupling in dynamical magnetic modeling, which allows accurate treatment of microscopic defects in nanomagnets. The results of simulations performed on simplified model systems, where an atomic-scale variation of anisotropy and exchange represent defects in real systems, show that the coarse-graining approach achieves practically the precision of exact atomic simulations, while describing the system with a much smaller number of degrees of freedom.
Multiscale modeling of hysteretic phenomena in magnets (invited)
Authors: Antropov V. P. , Belashchenko K. D.
Published: May 2003

Methodology of multiscale modeling of hysteretic phenomena in magnets is discussed. A practical combination of first-principles, micromagnetic, and microstructural calculations is constructed which allows one to study the hysteretic phenomena in hard magnets. Advantages and shortcomings of this approach are discussed. Multiscale nature of coercivity in CoPt type magnets is elucidated. Two sources of coercivity in polytwinned CoPt type magnets developing at different length scales, domain wall pinning at antiphase boundaries and splitting at twin boundaries, are illustrated for a realistic microstructure.
High-frequency permeability of thin NiFe/IrMn layers
Authors: Acher O. , Queste S. , Barholz K.-U. , Mattheis R.
Published: May 2003

Thin permalloy films without and with an 8 nm IrMn exchange layer have been grown by sputter deposition. Permalloy thickness ranges from 15 nm to 50 nm. The microwave permeability of these layers has been investigated in the 50 MHz to 3 GHz range using a permeameter based on a microstrip cell. A satisfactory measurement precision is observed on permalloy films as thin as 15 nm. The gyromagnetic resonance frequency increases from 670 MHz for the unbiased sample, to 2.7 GHz for the thinner biased sample. This increase in frequency is slightly larger than expected from the exchange-bias field measured by Kerr effect using a simple model for the gyromagnetic behavior. The properties of a two-period superlattice are also reported.
Effect of compressive stress on hysteresis loss and magnetostriction of grain oriented Si–Fe sheets
Authors: Yamamoto Ken-ichi , Yamashiro Yasumasa
Published: May 2003

Changes in hysteresis loss and magnetostriction λ due to compressive stress σ are discussed for grain oriented Si–Fe sheets. L and T samples are prepared for each kind of sheets, where the longitudinal direction of the L/T sample is parallel/transverse to the rolling direction of the sheets. Hysteresis loss of all samples increases owing to compressive stress σ, and the increased value is in proportion to λσ. The proportional ratio between them varies depending on the conditions of the sample and measuring maximum flux density.
Magnetic properties of FeCo laminates subjected to axial loading
Authors: Turgut Z. , Horwath J. C. , Huang M. Q. , Fingers R. T.
Published: May 2003

While manufacturing the stator/rotor assemblies of the aircraft power components, increased power losses are typically pronounced due to an imposed axial load onto the stack of magnetic laminates to increase the rigidity of the stack. An axial loader to enable core loss measurements to be made while the laminate stack is subjected to an axial load has been designed. The apparatus and multilayered slotted interface plates, allowing for the toroidal windings and simultaneous load application were used to evaluate the effect of compression up to 27.5 MPa on magnetic properties of commercially available Fe–Co based Hiperco® 50, Hiperco® 50 HS, and Hiperco® 27 alloys. For each composition, we tested two sets of samples: (1) completely insulated and (2) uninsulated. We report the increased losses due to an axial load and discuss the origin of these increased losses in terms of constant size anisotropy for completely insulated laminates.
Magnetoresistance and Hall effect in the ferromagnetic semiconductor Ga1−xMnxAs
Authors: Edmonds K. W. , Campion R. P. , Wang K. -Y. , Neumann A. C. , Gallagher B. L. , Foxon C. T. , Main P. C.
Published: May 2003

The resistivity, temperature, and magnetic field dependence of the anomalous Hall effect in a series of metallic Ga1−xMnxAs thin films with x=0.015 to 0.08 is presented. A quadratic dependence of the anomalous Hall resistance on the resistivity is observed, with a magnitude which is in agreement with Berry phase theories of the anomalous Hall effect in dilute magnetic semiconductors.
Anisotropic magnetoresistance in a-Fe90−xMnxZr10 alloys (x=0–12)
Authors: Perumal A. , Srinivas V. , Roy Aparna , Nigam A. K. , Chandra Girish , Dunlap R. A.
Published: May 2003

The anisotropy between longitudinal and transverse magnetoresistance has been measured in a number of amorphous Fe90−xMnxZr10 (x=0–12) alloys in fields up to 50 kOe at various temperatures between 4.2 and 300 K. The observed composition dependence of the spontaneous resistivity anisotropy can straightforwardly be explained in terms of the two-current-conduction model. The composition dependence of spin-up and spin-down residual resistivity has been determined, and provides conclusive evidence of weak-itinerant ferromagnetism in the entire range of compositions of the present investigation. These observations are further supported by high-field susceptibility data obtained from magnetization measurements.
Magnetic interaction between GaMnAs layers via spin-polarized quasi-two-dimensional hole gas: Monte Carlo simulation
Authors: Boselli M. A. , Loureiro da Silva L. , da Cunha Lima I. C. , Ghazali A.
Published: May 2003

The magnetic order resulting from an indirect exchange between magnetic moments in the metallic phase of a Ga1−xMnxAs bilayered structure is studied via Monte Carlo simulation. The coupling mechanism involves a perturbative calculation in second order of the interaction between the magnetic moments and carriers (holes). We take into account a possible polarization of the hole gas establishing, thus self-consistency between the magnetic order and the electronic structure. It leads to a ferromagnetic order even in the case of thin layers. This fact is analyzed in terms of the inter- and intralayer interactions.
Improvement of magnetic property of GaMnN by codoping of Mg
Authors: Kim K. H. , Lee K. J. , Kim D. J. , Kim C. S. , Lee H. C. , Kim C. G. , Yoo S. H. , Kim H. J. , Ihm Y. E.
Published: May 2003

GaMnN and Mg-codoped GaMnN films were grown in molecular beam epitaxy using a GaN single precursor. The structural, electrical, and magnetic properties were examined. The Mg-codoped GaMnN layers revealed room-temperature ferromagnetism. Codoping with Mg reduces the Mn incorporation, but increases the conductivity of the GaMnN films. At the same time, the saturation magnetization and coercivity have increased at room temperature. As this improvement was obtained with much reduced Mn concentration of ∼0.3%, the crucial role of the carriers for carrier-induced ferromagnetism in magnetic semiconductors is observed.
Magnetic semiconductors based on cobalt substituted ZnO
Authors: Rode K. , Anane A. , Mattana R. , Contour J.-P. , Durand O. , LeBourgeois R.
Published: May 2003

We have investigated the microstructure and the magnetic properties of cobalt substituted ZnO thin films deposited on sapphire (0001) substrates by pulsed laser deposition. We have optimized the growth condition using in situ monitoring by reflection high-energy electron diffraction. We found that ferromagnetic films need to be grown at low oxygen partial pressure (<10−6 Torr). Films with 25% of Co are ferromagnetic at room temperature with clear out-of-plane anisotropy. We have looked for spurious origins of the ferromagnetic signal and found none.
Effect of carrier for magnetic and magnetotransport properties of Si:Ce films
Authors: Yokota T. , Fujimura N. , Wada T. , Hamasaki S. , Ito T.
Published: May 2003

Epitaxial Si:Ce films with smooth surfaces were prepared by low temperature molecular beam epitaxy. Although as-deposited films showed positive magnetization due to the existence of Ce having been substituted in Si lattice, the conduction was n type. The conduction changed to p type as a result of hydrogen termination, indicating that the film had contained dangling bonds. The magnetization behavior of the as-deposited n-type sample is completely identical to that of the p-type sample. The temperature dependence of resistivity (ρ–T) for each sample with n-type or p-type conduction has a cusp at 150 K, which is related to the spin–glass transition. But the cusp observed in the ρ–T curve of the sample with n-type conduction is broader than that of the sample with p-type conduction. It seems to have originated from the spin dynamics in Si:Ce with different carrier types.
Room-temperature ferromagnetism in highly Cr-doped II–VI diluted magnetic semiconductor Zn1−xCrxTe
Authors: Saito H. , Zayets V. , Yamagata S. , Ando K.
Published: May 2003

Highly Cr-doped II–VI diluted magnetic semiconductor (DMS) Zn1−xCrxTe films with a ferromagnetic long-range order have been grown. A phase diagram of Zn1−xCrxTe in relation to the growth temperature and Cr concentration was determined. Magnetic circular dichroism measurements revealed that a magnetically single phase of DMS Zn1−xCrxTe is obtained in the films with Cr concentration up to x=0.20. Spontaneous magnetization of the film with x=0.20 disappears around 300 K, indicating that the Zn1−xCrxTe is a DMS with room-temperature ferromagnetism.
Anomalous magnetoresistance in high-temperature organic-based magnetic semiconducting V(TCNE)x films
Authors: Raju N. P. , Savrin T. , Prigodin V. N. , Pokhodnya K. I. , Miller Joel S. , Epstein A. J.
Published: May 2003

Anomalous positive magnetoresistance (MR) in high temperature organic-based magnet V(TCNE)x (TCNE=tetracynoethylene) thin films is reported. MR increases linearly with applied magnetic field and shows a maximum at the ferrimagnetic ordering temperature. The suggested roles of oppositely spin polarized π* electronic subbands and magnetization fluctuations due to the disordered nature of V(TCNE)x films are discussed.
Ferromagnetism of anatase Ti1−XCoXO2−δ films grown by ultraviolet-assisted pulsed laser deposition
Authors: Yang Hyuck Soo , Choi Jaeyoung , Craciun Valentin , Singh Rajiv K.
Published: May 2003

We report the effect of ultraviolet irradiation during pulsed laser deposition on the growth of cobalt-doped TiO2 films in terms of the crystalline and magnetic properties. Due to more active oxygen gas species created by photodissociation and enhanced atomic migration on the substrate surface, highly crystalline films could be obtained at a relatively low deposition temperature of 400 °C. The full width at half maximum of the (004) peak rocking curve was 0.46° for the UV-assisted film deposited at 400 °C on the LaAlO3(001) substrate. Fourfold symmetry was observed in phi scans of the Ti0.93Co0.07O2−δ film, which indicated that this film was grown in cube-on-cube epitaxial orientation with the [001] normal to the LaAlO3(001) substrate surface. The temperature dependence of the zero-field-cooled/field-cooled magnetization was examined and the hysteresis was clearly identified by the nonzero difference between them. The Curie temperature was estimated to be well over 350 K, and at room temperature the coercive field was ∼180 Oe and the remanence was ∼17% of the saturation magnetization. The in-depth distribution of the constituent elements was characterized by Auger depth profiling. No segregation was found at the surface of the film and the interdiffusion between the substrate and the film was significantly reduced at the low deposition temperature used.
Epitaxial growth and transport properties of Sr2CrWO6 thin films
Authors: Philipp J. B. , Reisinger D. , Schonecke M. , Opel M. , Marx A. , Erb A. , Alff L. , Gross R.
Published: May 2003

We report on the preparation and characterization of epitaxial thin films of the double-perovskite Sr2CrWO6 by pulsed laser deposition. On substrates with low lattice mismatch like SrTiO3, epitaxial Sr2CrWO6 films with high crystalline quality can be grown in a molecular layer-by-layer growth mode. Due to the similar ionic radii of Cr and W, these elements show no sublattice order. Nevertheless, the measured Curie temperature is well above 400 K. Due to the reducing growth atmosphere required for double perovskites, the SrTiO3 substrate surface undergoes an insulator–metal transition impeding the separation of thin film and substrate electric transport properties.
Magnetic properties of ZnO-based diluted magnetic semiconductors
Authors: Yoon S. W. , Cho S.-B. , We S. C. , Yoon S. , Suh B. J. , Song H. K. , Shin Y. J.
Published: May 2003

We report a study of the magnetic properties of transition-metal doped Zn1−xTMxO (TM=Mn, Co, Fe). Polycrystalline powder samples were synthesized by both solid-state and liquid-phase reactions. From the Curie–Weiss behavior of susceptibility at high temperatures, it was found that the TM–TM interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constants J=−90 to −30 K. The magnetization data measured at low temperature as a function field H are fit to a parameterized Brillouin function to obtain the effective concentration xeff of magnetically active TM2+ ions. As x increases, the fraction of magnetically active ions, xeff/x, decreases. This is ascribed to an increase in average AF interaction between doped magnetic spins as the average distance between them decreases with an increase in x.
Nucleation and evolution of hybrid spin spiral in soft/hard ferromagnetic bilayer
Authors: Kabanov Yu. P. , Gornakov V. S. , Nikitenko V. I. , Shapiro A. J. , Shull R. D.
Published: May 2003

The hybrid exchange spring behavior in a bilayer consisting of an epitaxial film of Fe (500 Å)/Sm2Co7 (350 Å) deposited onto a Cr (200 Å) buffered MgO(110) substrate was investigated using the magneto-optical indicator film technique. Two critical magnitudes of the remagnetizing magnetic field were found that dramatically change the bilayer remanent magnetization. One of them is determined by exchange spring penetration in the interface; another is determined by hybrid spin spiral transformation to domain walls in the hard ferromagnet layer. Unexpected dependence of the direction of average remanent magnetization on the external magnetic field direction and value is revealed.
Magneto-structural phase transition in Gd5(Si2Ge2) and MnFe(P1/3As2/3) systems
Authors: Samolyuk G. D. , Antropov V. P.
Published: May 2003

We study the electronic structure and magnetic properties of Gd5(Si2Ge2) and MnFe(P1/3As2/3) system of alloys using the density functional approach. The stability of their magnetic structures was investigated. The total energy as a function of structural deformation was calculated. The exchange coupling calculations show that the effective Heisenberg model parameters are decreased with structural deformation. The free energy as a function of temperature and deformation was calculated. The decrease of effective exchange coupling leads to first order magneto-structural phase transition, |∂H/∂T| and giant magneto-caloric effect.
Magnetic properties of SmCo5 and YCo5
Authors: Larson P. , Mazin I. I.
Published: May 2003

We have studied the magnetic moments and magnetic anisotropy energy (MAE) of YCo5 and SmCo5 using full-potential linear augmented plane wave (LAPW) electronic structure calculations. Most previous calculations of the MAE for YCo5, using local density approximation (LDA) for the exchange-correlation potential, have found values significantly smaller (∼0.6 meV/f.u.) than experiment (∼3.8 meV/f.u.). The rest of the MAE is attributed to many body corrections. Our LAPW calculations using the generalized gradient approximation (GGA) instead of LDA and including nonspherical corrections give values ∼1.5 meV/f.u. The Co magnetic moment of YCo5−xCux, unlike the prediction of the virtual crystal approximation, decreases slowly with impurity concentration until dropping suddenly to zero at a critical dopant concentration. Correlation effects were found to be crucial for the MAE in SmCo5. While GGA calculations give MAE for SmCo5 of the wrong sign, including the LDA+U, correction brings it to ∼21 meV/f.u., in good agreement with the experimental value of 13–16 meV/f.u.
Electronic susceptibility and Curie temperature of the double-exchange model within dynamical mean-field theory
Authors: Fishman R. S. , Jarrell M.
Published: May 2003

Due to its applications to the manganites, the double-exchange (DE) model has been intensively studied over the past ten years. An especially promising approach to investigate the DE model is dynamical mean-field theory (DMFT), which becomes exact in infinite dimension but accurately describes local quantum fluctuations in three dimensions. In this article, we use DMFT to solve the Bethe–Salpeter equation for the electronic susceptibility above TC. Assuming a semicircular density-of-states, we obtain an analytic relation for TC in the limit of large Hund’s coupling from the condition that the inverse electronic susceptibility vanishes. Our expression for TC agrees with an earlier result based on the local-moment susceptibility and corrects a mistake in a result based on the self-consistent equation for the magnetization below TC.
Local spin-density waves in Cr–V alloys: Dependence on temperature and applied magnetic field
Authors: de Oliveira L. M. , Ortiz W. A. , de Oliveira A. J. A.
Published: May 2003

The appearance of Curie–Weiss behavior in itinerant antiferromagnetic systems is an important subject of research in the case of Cr alloys. The nature of the phenomenon is associated to stabilization of a short-range order, which allows formation of local spin-density waves below a certain temperature, TLOC. In this work we present results of magnetic susceptibility of Cr–x at. %V alloys (x=0, 0.1, 0.2, and 0.4) where, TLOC is observed. Our results show that not only does a dependence of TLOC with V concentration exist, but it also presents a linear dependence on the applied magnetic field. The effective magnetic moment estimated from Curie–Weiss fittings also show a decreasing dependence with the applied magnetic field.
Entropy change at the martensitic transformation in ferromagnetic shape memory alloys Ni2+xMn1−xGa
Authors: Khovailo V. V. , Oikawa K. , Abe T. , Takagi T.
Published: May 2003

The entropy change ΔS between the high-temperature cubic phase and the low-temperature tetragonally based martensitic phase of Ni2+xMn1−xGa (x=0–0.20) alloys was studied. The experimental results obtained indicate that ΔS in the Ni2+xMn1−xGa alloys increases with the Ni excess x. The increase of ΔS is presumably accounted for by an increase of magnetic contribution to the entropy change. It is suggested that the change in modulation of the martensitic phase of Ni2+xMn1−xGa results in discontinuity of the composition dependence of ΔS.
Prediction and measurements of a current derivative sensor response for voltage pulses applied to induction machines
Authors: Wolbank Thomas M. , Machl Juergen L. , Hauser H.
Published: May 2003

Current derivative sensors (CDI’s) can advantageously be used in modern drives to determine the position of the rotor or flux in induction machines if no sensor at the shaft is available or to realize an effective condition monitoring of the drive. This is achieved by exploiting the transient current slope of the machine due to voltage pulses impressed by the switching of the inverter. To enable a CDI optimization with reference to the demands given by the drive application, a simulation model is presented which describes the nonlinear dependence on the history of the magnetic field and magnetization in ferromagnetic materials by statistical domain behavior using phenomenological parameters. In addition, measurements and simulations of the developed sensor output signals are presented which are used in a 15 kW induction machine drive. The optimized CDI sensitivity is S=0.35 μΩs and the bandwidth exceeds the characteristic of standard current sensors by five times.
Volume change due to the transition from the ferromagnetic to the antiferromagnetic state in La(Fe0.862Al0.138)13 compound
Authors: Irisawa K. , Fujita A. , Fukamichi K.
Published: May 2003

Linear thermal expansion, specific heat, and Mössbauer effect measurements have been performed for a La(Fe0.862Al0.138)13 compound which exhibits a temperature-induced first-order transition from the ferromagnetic (F) to the antiferromagnetic (AF) state. The volume decrease due to this transition is about 0.42%, and the magnetic contributions to the volume and the specific heat are still present even in the paramagnetic temperature range. The temperature dependence of the spontaneous magnetostriction ωs is compared with that of the average hyperfine field expected to be proportional to the order parameter M0 (MQ) in the F (AF) state. The difference in the temperature dependence between ωs and M0 (MQ) suggests significant local spin fluctuations.
Ferromagnetic semiconductors based upon AlGaP
Authors: Overberg M. E. , Thaler G. T. , Frazier R. M. , Abernathy C. R. , Pearton S. J. , Rairigh R. , Kelly J. , Theodoropoulou N. A. , Hebard A. F. , Wilson R. G. , Zavada J. M.
Published: May 2003

Ion implantation of Mn or Cr at concentrations of 1–5 at. % were performed in AlxGa1−xP (x=0.24,0.38) epilayers grown by gas source molecular beam epitaxy. Ferromagnetic-like ordering above 100 K for Cr and 300 K for Mn was observed in superconducting quantum interference device measurements. Structural characterization revealed no second phases that could influence the measured magnetic properties. As the AIP mole fraction in the AlxGa1−xP layers increased, the magnetic ordering temperatures were generally observed to increase, while the calculated magnetic moment decreased. Mn appears to be a more promising choice than Cr for high temperature ferromagnetism in AlGaP.
Fe
3
O
4
thin films sputter deposited from iron oxide targets
Authors: Peng Yingguo , Park Chandro , Laughlin David E.
Published: May 2003

Fe 3 O 4 thin films have been directly sputter deposited from a target consisting of a mixture of Fe3O4 and Fe2O3 onto Si and glass substrates. The magnetic properties and microstructures of the films have been characterized and correlated. The columnar growth of the Fe3O4 grains was found to be initialized from the substrate surface without any critical thickness. Substrate bias was found to be a very effective means of improving the crystal quality and magnetic properties of the thin films. The crystallographic defects revealed by high resolution transmission electron microscopy seem to be a characteristic of the films prepared by this method.
Magnetic and neutron diffraction study on Ni2In type (Mn1−xPdx)2Ga
Authors: Shiraishi Hiroshi , Hori Tomiei , Ohkubo Naoki , Ohoyama Kenji , Yamaguchi Yasuo
Published: May 2003

Pseudobinary intermetallic compounds (Mn1−xPdx)2Ga of Ni2In type structure are found in the range of 0.3≦x<0.6. This article describes the results of investigations of the crystal and magnetic properties of the phase with x-ray and neutron diffraction, as well as magneto metric methods. The neutron diffraction study for x=0.4 proved that in the Ni2In type crystal structure, the 2(a) site is occupied by Mn atoms, the 2(d) site by Pd and residual Mn atoms, and the 2(c) site is occupied by Ga atoms. (Mn1−xPdx)2Ga compounds exhibit a ferromagnetic nature characterized by TC, and in the temperature below TN, antiferromagnetic components originating from canted spin structure in 2(a) structures in the 2(a) site. It is similar to canted spin structures of CoMnSn. The existence of a small concave region in the magnetization–temperature dependence curve is discussed in terms of the canting of spins in the 2(a) site.
Magnetic properties of L1 FePt and FePt:Ag nanocluster films
Authors: Xu Yingfan , Sun Z. G. , Qiang Y. , Sellmyer D. J.
Published: May 2003

A sputtering gas-aggregation technique has been used to prepare FePt and FePt:Ag nanocluster films. The cluster size was controlled in a range from 3 to 6 nm. FePt cluster films were directly deposited onto Si substrate; FePt:Ag cluster films were fabricated by depositing a FePt cluster layer between a Ag underlayer and overlayer. Nanostructure and magnetic properties of the samples were characterized by x-ray diffraction, transmission electron microscopy, and magnetometry. The high magnetic anisotropy L10 fct phase was realized in the films annealed at a temperature of 550 °C and above. The orientation of clusters is random. The coercivity increases with an increase of annealing temperature; high in-plane and out-of-plane coercivities, exceeding 10 kOe, were achieved in both FePt and FePt:Ag cluster films after annealing. For FePt:Ag films, the coercivity increases with Ag underlayer thickness, tAg, and reaches about 17 kOe at room temperature for tAg=5 nm after annealing at 650 °C for 10 min. The high coercivity is closely correlated with the degree of L10 ordering and nanostructure of the films.
Highly oriented nonepitaxially grown L1 FePt films
Authors: Yan M. L. , Powers N. , Sellmyer D. J.
Published: May 2003

A method of preparing nonepitaxially grown, highly textured L10 FePt thin films is described. A nearly perfect (001) texture was obtained by direct deposition of FePt films on Corning 7059 glass substrates and subsequent rapid thermal annealing. The ordering and orientation of the L10-phase FePt grains were controlled by the initial as-deposited film structure, and also by the annealing process. Magnetic measurements reveal large perpendicular anisotropy for these (001) textured films. The substrates and processes used for nonepitaxial growth of L10 ordered FePt films are much more compatible with practical applications than those grown epitaxially.
Room temperature active regenerative magnetic refrigeration: Magnetic nanocomposites
Authors: Shir Farhad , Yanik Levent , Bennett Lawrence H. , Della Torre Edward , Shull Robert D.
Published: May 2003

Nanocomposites have several advantages as a refrigerant for 100–300 K applications compared to the other common methods of assembling a magnetic refrigeration bed, such as a layered thermal bed, or mixing of different magnetic materials. This article discusses the thermodynamics and heat transfer analysis of an ideal and real active magnetic regenerative refrigeration cycle. An algorithm for the choice of optimum parameters is derived.
Large magnetoresistance in (La1−xCaxMnO3)1−y:ZrO2 composite
Authors: Das D. , Saha A. , Russek S. E. , Raj R. , Bahadur D.
Published: May 2003

Colossal magnetoresistance (CMR) composite materials have been synthesized to explore the possibility of improving magneto-transport and structural properties in CMR systems. In this work we describe (La1−xCaxMnO3)1−y (LCMO) (ZrO2)y (x≈0.3 and 0.0⩽y⩽0.40 mole %) composites that have been synthesized using a modified (non Pechini type) sol–gel technique. Magnetoresistivity of the composites was evaluated at 5 T field and in the temperature range 5–300 K. The composites show higher magnitude of MR compared to pure LCMO. The MR rises from a base value 76%, for the case y=0, to a maximum value of 93.8%, obtained at y=0.05. dc susceptibility measurements show a distinct ferromagnetic to paramagnetic transition in all composites. The ferromagnetic transition temperature (TC) drops from 225 K in pure LCMO (y=0) to 121 K in y=0.05 and then slowly rises to 157 K as y increases. The plots of zero field cooled susceptibility χZFC (T) and field cooled susceptibility χFC (T) diverge clearly below TC, indicating magnetic irreversibility. The composite exhibits a clear metal–insulator transition (TMI) at or just above the magnetic transition. The peak resistivity ρMI at the metal–insulator transition also exhibits interesting changes. For pure LCMO polycrystals, ρMI=102 Ω cm, but it increases to 228 Ω cm for y=0.05 and then gradually decreases to 1.94 Ω cm for y⩾0.10. The phase evolution in the LCMO:ZrO2 composites was studied by x-ray powder diffraction and correlated to the magnetic and electrical properties.
Magnetic properties of Nd5SixSn4−x
Authors: Wang H. B. , Elouneg-Jamróz Miryam , Ryan D. H. , Altounian Z.
Published: May 2003

The structure and magnetic properties of Nd5SixSn4−x have been investigated using x-ray diffraction, Sn119 Mössbauer spectroscopy and bulk magnetic techniques. The crystal structure is orthorhombic for x⩽3.5, above which it is tetragonal. The ac-susceptibility shows that the magnetic ordering temperatures of Nd5SixSn4−x increase from 35 to 120 K as x increases. Magnetization data indicate that the Nd moments cannot be ordered ferromagnetically at 5 K and that the magnetic structure must be mixed ferro-antiferromagnetic form as seen in the Nd5SixGe4−x system. The Sn119 Mössbauer spectra of Nd5SixSn4−x at 9 K are dominated by two, sharp hyperfine sextets with fields between 10 and 16 T that increase with x.
Study of magnetic ordering states of CeGex with 1.66<x⩽2.0
Authors: Zan J. A. , Yuen Tan , Lin C. L. , Huang X.-Y. , Li Jing
Published: May 2003

The existence of an antiferromagnetic transition at TN=7 K is reported for CeGex with a small range of Ge concentration of 1.66<x<1.71. Measurements of x-ray diffraction and the temperature and magnetic field dependence of magnetization and specific heat have been carried out for CeGex with 1.66⩽x⩽2.0. The magnetic ground state and the crystal structure change abruptly near a Ge concentration of x=1.710. Within the range of Ge concentrations between x=1.71 and 2.0, CeGex crystallizes in the orthorhombic structure and exhibits two magnetic transitions: an antiferromagnetic transition at TN=7 K and a ferromagnetic transition at TC=4.3 K. Interestingly, when x is lowered to 1.66<x<1.71, CeGex crystallizes in a tetragonal phase, exhibits an antiferromagnetic transition at TN=7 K, and remains in the antiferromagnetic state down to a temperature of 0.5 K.
Pressure and magnetic field effects In CeNiGe2
Authors: Alsmadi A. M. , Nakotte H. , Sarrao J. L. , Jung M. H. , Lacerda A. H.
Published: May 2003

CeNiGe 2 crystallizes in the orthorhombic CeNiSi2-type layered structure and exhibits large magnetic and transport anisotropy. The compound undergoes two antiferromagnetic transitions at TN=4 K and TM=3 K. We report measurement of the electrical resistivity on a CeNiGe2 single crystal as a function of the temperature and magnetic field (i ∥ b axis, H ⊥ b axis) performed under hydrostatic pressure up to 9 kbar. Upon application of magnetic field, antiferromagnetic transitions merge into a single transition and this transition is completely suppressed in fields greater than 5 T. Application of pressure causes a reduction of the ordering temperatures. The results are compared with findings for other Ce compounds and discussed in terms of moment instability and the variation of exchange due to increased 4f-ligand hybridization.
Magnetic anisotropy and electronic transport properties in single-crystalline compounds of EuCu2Ge2 and EuCu2Si2
Authors: Rhyee Jong-Soo , Cho B. K. , Ri H. C.
Published: May 2003

We present the temperature dependent magnetization, M(T), and the resistivity, ρ(T), of EuCu2Ge2 and EuCu2Si2 in single crystalline form. The EuCu2Ge2, which is synthesized in single-crystalline form, shows two different magnetic transitions in M(T) at TMc=8.5 K and TMab=4 K depending upon the magnetic field orientation. On the other hand, the ρ(T) reveals only one transition at T=4 K as a sudden drop of the ρ(T) without any noticeable change at T=8.5 K. For the EuCu2Si2, M(T) shows a two step antiferromagnetic transition at TN1≈10 K and TN2≈4 K, which is smeared out under H⩾1 T, probably due to field induced spin orientation. The transition at TN2 is also manifested as a decrease in ρ(T) due to loss of magnetic scattering. Interestingly, another exotic transition is found at T=2 K in ρ(T) data. The ρ(T) values rapidly increase by two orders of magnitude from ρ=20 μΩ cm at T=2 K and reach ρ=5 mΩ cm at T=0.5 K, which is similar to a metal–insulator transition.
Magnetic phase behavior for Gd-rich Gd–Y
Authors: Collingwood J. F. , Edwards C. , Palmer S. B. , Santos J. A. M. , Sousa J. B.
Published: May 2003

The elastic constants and electrical transport properties of single-crystal Gd and Gd–Y are measured for Y⩽26 at. %. There are clear features in the elastic constant C33(T) and in the temperature derivative of the a-axis resistivity, dρa/dT, at the accepted ferromagnetic Curie temperature (TC), and at the accepted spin-reorientation at TSR. In the Gd-rich Gd–Y, there is a third feature at T* below TSR, which is consistent with evidence for a further second-order transition. A modified magnetic phase diagram for Gd–Y is proposed.
Low-temperature magnetic structure of UPdGe
Authors: El-Khatib S. , Chang S. , Nakotte H. , Brown D. , Brück E. , Schultz A. J. , Christianson A. , Lacerda A.
Published: May 2003

We report on the low-temperature magnetic structure of UPdGe, which crystallizes in the orthorhombic TiNiSi structure. Magnetic space group analysis was employed to determine potential moment configuration allowed by crystal symmetry. By fitting the allowed models to neutron diffraction results, we find that UPdGe orders ferromagnetically below TC=28 K with collinear uranium moments of about 1.5 μB/U atom pointing along the orthorhombic b axis at 5 K.
Magnetic Kondo scattering in the electrical resistivity of sputtered Ti–Co films
Authors: Arranz M. A. , Riveiro J. M.
Published: May 2003

We have investigated the influence of the granular microstructure in the electrical resistivity of RF-sputtered Ti100−xCox thin films. Their x-rays characterization has been correlated with dc resistivity measurements from 7 to 300 K. A minimum in the resistivity, ρ(T), centered around Tm∼50 K, was recorded for Ti93Co7 samples grown at 30 W. Contrarily, samples grown at 110 W and the same x value displayed no evidences of that low-temperature anomaly. This phenomenon correlated with the change of the majority Ti granular phase: hcp-Ti for the sample grown at 30 W and bcc-Ti for the 110 W sample. That minimum arises from the great difference in Co solubility into both Ti granular phases (0.8 at. % in hcp-Ti and 14.5 at. % in bcc-Ti). Hence, the very small quantity of Co diluted into the hcp-Ti grains allows us to consider Co particles as uncorrelated magnetic impurities which scatter the conduction electrons. This magnetic Kondo scattering would be the mechanism responsible for the observed minimum in the resistivity of Ti93Co7 samples grown at 30 W.
Suppression of superconductivity due to spin imbalance in Co/Al/Co single electron transistor
Authors: Johansson Jan , Urech Mattias , Haviland David , Korenivski V.
Published: May 2003

Transport properties of ferromagnetic/nonmagnetic/ferromagnetic single electron transistors are investigated as a function of external magnetic-field, temperature, bias, and gate voltage. By designing the magnetic electrodes to have different switching fields, a two-mode device is realized having two stable magnetization states, with the electrodes aligned in parallel and antiparallel. Magnetoresistance of approximately 100% is measured in Co/AlOX/Al/AlOX/Co double tunnel junction spin valves at low bias, with the Al spacer in the superconducting state. The effect is substantially reduced at high bias and temperatures above the TC of the Al. The experimental results are interpreted as due to spin imbalance of charge carriers resulting in suppression of the superconducting gap of the Al island.
Synthesis and properties of hole-doped Li1−xBC
Authors: Zhao L. , Klavins P. , Liu Kai
Published: May 2003

Stoichiometric LiBC has been synthesized using both arc melting and sealed tantalum ampoule methods. Hole-doped Li1−xBC (0<x⩽0.37) compounds have been realized through vacuum deintercalation from LiBC. The hexagonal crystal lattice has remained largely intact, with only slight decreases in lattice parameters upon hole-doping. The samples are intrinsically diamagnetic and are semiconducting in the 2–300 K range studied. Increased hole-doping leads to a color darkening as well as a decrease in resistivity.
Interplay of superconductivity and magnetism in the single crystals of Er1−xTbxNi2B2C
Authors: Cho B. K. , Kim C. A. , Ri H.- C.
Published: May 2003

The measurements of temperature-dependent resistivity ρ(T) and magnetization M(T) are carried out for the single crystals of Er1−xTbxNi2B2C (x=0.0, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.6, 0.7, 0.8, 0.9, and 1.0) in order to study the superconductivity, magnetism, and their interplay in the temperature range of 2 K⩽T⩽20 K. From the analysis of the ρ(T) and M(T) data, the superconducting (Tc), antiferromagnetic (TN), and weak ferromagnetic (TWF), transition temperatures are derived and represented in terms of De Gennes (DG) factor. It is found that the Tc is suppressed with increasing Tb concentration and a crossover for Tc<TN occurs at x=0.2. Tc variation follows well the DG scaling even when Tc<TN. The TN is almost independent of temperature at T≈6 K in the Er-rich side of the compounds (x⩽0.4) and follows DG scaling based on the Ruderman–Kittel–Kasuya–Yosida interaction in Tb-rich side (x⩾0.6). The TWF also shows distinct behaviors in a Er- and Tb-rich region. The observed superconductivity, AF-ordered state, and weak ferromagnetic state are found to be closely correlated and will be discussed based on the magnetic interactions between Er and Tb elements.
Periodic orbit resonance in (TMTSF)2ClO4
Authors: Kovalev A. E. , Hill S. , Takahashi S. , Dhakal T. N. , Takasaki S. , Yamada J. , Anzai H. , Brooks J. S.
Published: May 2003

We have studied the magnetoelectrodynamics of the layered molecular conductor (TMTSF)2ClO4 at millimeter wave frequencies. The high-quality data were obtained in a split-pair magnet using a cavity perturbation technique. We found clear periodic orbit resonance due to the quasi-one-dimensional Fermi surface in the studied compound. These phenomena were investigated in detail. Using our data, we extracted a value for the Fermi velocity which is about 1.8×105 m/s.
Magnetic tunnel-valve barriers with boron
Authors: Childress J. R. , Py J.-S. , Ho M. K. , Fontana R. E. , Gurney B. A.
Published: May 2003

The properties of magnetic tunnel junctions with aluminum oxide barriers alloyed with boron are presented. When aluminum is deposited by sputtering, 10 at. % B addition is sufficient to completely transform the deposited Al film from crystalline to amorphous. We have investigated if the deposition of amorphous AlB prior to oxidation results in a variation of tunnel-valve properties. It is found that ultrathin Al90B10 films can be readily oxidized using a procedure similar to that for Al films, resulting in tunnel valves with good magnetic properties. In the ultrathin regime (when the specific junction resistance is Rj<20 Ω μm2), the barriers with boron yield magnetoresistance values slightly above that of pure aluminum oxide, although the effect is not significant for Rj<5 Ω μm2.