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In this paper noise enhanced stability in magnetic systems is studied by both an Ising-type model and a Preisach–Arrhenius model as well as a dynamic Preisach model. It is shown that in one nonequilibrium Ising system noise enhanced stability occurs and that dynamic Preisach model has the capability to predict the occurrence of noise enhanced stability in magnetic systems. On the contrary, in a Preisach–Arrhenius model of a single quadrant magnetic material, noise enhanced stability is not detected. [ABSTRACT FROM AUTHOR]

Complementary nature of coercivity enhancement and exchange bias in a ferro-antiferromagnet (F-AF) exchange coupled system with arbitrary configuration refers to the relation Hsw0-=Ms/χin,max, with Ms being the saturation magnetization of the F part, Hsw0- being the field at which the F magnetization starts to be unstable, applied opposite to the stable direction of the F magnetization in the absence of external field, and χin,max being the maximal initial susceptibility, for which the field is applied perpendicular to that stable direction. This relation is a special case of a more general result for a single domain particle, proved in this paper. In a F-AF system, when the F magnetization behaves coherently, the effect of F-AF coupling can be absorbed into the anisotropy of the F part and the system can be treated as a single domain in an external field. [ABSTRACT FROM AUTHOR]

Attractive features of soft magnetic composites (SMCs) are high resistivity, isotropic three-dimensional flux behavior, and easy compression into the complicated shapes required in electromagnetic devices. Comprehensive understanding of the materials will help optimize design of electromagnetic devices. This paper presents the magnetization processes in a SMC sample in micron scale by means of the magneto-optical imaging technique. The sample was magnetized by magnetic fields tangential or perpendicular to the observation surface. It is observed that the flux density is higher at the particle region but lower at the interparticle space. When a tangential field is applied, the stray fields change polarization at the particle boundaries. Both results suggest that the magnetized sample behaves as a collection of individual magnetized particles rather than as a uniform and continuous magnetic substance such as soft iron although there are some interactions between neighboring particles. [ABSTRACT FROM AUTHOR]

In this paper one presents a method for statistical characterization of the magnetization processes in two-dimensional networks of single-domain ferromagnetic particles. These systems are similar with the structured materials that are seen as future high-density recording media. The method allows very interesting developments in the characterization of the stability of the magnetic states which is an important factor for a recording medium. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper represents the force performance of a brushless dc motor with a continuous ring-type permanent magnet (PM), considering its magnetization patterns: trapezoidal, trapezoidal with dead zone, and unbalanced trapezoidal magnetization with dead zone. The radial force density in PM motor causes vibration, because vibration is induced the traveling force from the rotating PM acting on the stator. Magnetization distribution of the PM as well as the shape of the teeth determines the distribution of force density. In particular, the distribution has a three-dimensional (3-D) pattern because of overhang, that is, it is not uniform in axial direction. Thus, the analysis of radial force density required dynamic analysis considering the 3-D shape of the teeth and overhang. The results show that the force density as a source of vibration varies considerably depending on the overhang and magnetization distribution patterns. In addition, the validity of the developed method, coupled 3-D equivalent magnetic circuit network method, with driving circuit and motion equation, is confirmed by comparison of conventional method using 3D finite element method. [ABSTRACT FROM AUTHOR]

The Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation is widely used to describe phase transformation kinetics. This description, however, is not valid in finite size domains, in particular, thin films. A new computational model incorporating the level-set method is employed to study phase evolution in thin film systems. For both homogeneous (bulk) and heterogeneous (surface) nucleation, nucleation density and film thickness were systematically adjusted to study finitethickness effects on the Avrami exponent during the transformation process. Only site-saturated nucleation with isotropic interface-kinetics controlled growth is considered in this paper. We show that the observed Avrami exponent is not constant throughout the phase transformation process in thin films with a value that is not consistent with the dimensionality of the transformation. Finitethickness effects are shown to result in reduced time-dependent Avrami exponents when bulk nucleation is present, but not necessarily when surface nucleation is present. [ABSTRACT FROM AUTHOR]

In this paper, phase locking dynamics of vortices induced by spin transfer torque in bridge-coupled nanodisks are studied by micromagnetic simulations. In the presence of the bridge coupling, the required time for the phase locking is dramatically reduced, and the phase difference between the two vortices keeps at a nonzero value after the phase locking. Moreover, the phase difference is affected significantly by bridge coupling, Oersted field distribution, nanodisk size, as well as inplane bias magnetic field. In addition, the coupled gyrotropic frequency of vortices depends linearly on the perpendicular magnetic field. This systematic study of phase locking parameters, especially the phase difference, is important for the applications of vortex-based spin-torque nano-oscillators. [ABSTRACT FROM AUTHOR]

Smart control is an attracting and important function for modern electromagnetic wave absorber. This paper presents the design, fabrication, and measurement of a frequency and bandwidth tunable metamaterial absorber (MA) in X-band. The unit cell of the MA consists of a microstrip resonator loaded with the varactors. Simulation and measurement results show that by tuning the bias voltage on the varactors, the peak absorption frequency can be tuned by 0.44 GHz with the peak absorption greater than 95%. Field and circuit model analysis is conducted to reveal the working mode and predict the absorbing frequency. After that, by specially designing the bias circuit so as to adjust the bias voltage on neighboring unit cells separately, dual resonance and absorption peaks occur, and the overall absorption bandwidth can thus be tuned conveniently by controlling the difference of the two resonance frequencies. The center absorbing frequency can also be tuned. Simulation and experiment results show that the 75% absorption (-6 dB reflection) bandwidth can be tuned from 0.40 GHz to 0.74 GHz, which is a two-fold tuning range. This work is believed to improve the state-of-the-art smart metamaterial absorber. [ABSTRACT FROM AUTHOR]

Magnetostriction (MS)-caused strain in single-phase three-legged cores with different core cutting forms, which suffer from induced magnetic loss and noise, was studied. It is found that adopting each different core form types induces magnetostriction ε variation in a transformer core operating with a high-frequency AC signal. The results are compared with finite element analysis simulations. It is also indicated that magnetostriction ε variations are significant in the rolling direction and along limbs and yokes. In this paper, it is proposed that core corner sides and T-joint parts without cutting structure, the core exhibits lower core loss and lower heat dissipation due to the fact that the magnetic flux that passes through corner sides shows lower magnetostriction variation. The magnetic properties resulting from magnetostriction variation in core loss and heat dissipation phenomena are significantly different from other core forms because of stronger contributions from magnetostatic forces. The main contribution for reducing core loss and noise, making them much less in corner numbers and cutting-fabricated forms, can be expected to come from lower magnetic flux and magnetostriction variation. [ABSTRACT FROM AUTHOR]

This paper micromagnetically studies the magnetization dynamics driven by the spin-Hall effect in a Platinum/Permalloy bi-layer. For a certain field and current range, the excitation of a uniform mode, characterized by a power with a spatial distribution in the whole ferromagnetic cross section, is observed. We suggest to use the ferromagnet of the bi-layer as basis for the realization of an array of spin-torque oscillators (STOs): the Permalloy ferromagnet will act as shared free layer, whereas the spacers and the polarizers are built on top of it. Following this strategy, the frequency of the uniform mode will be the same for the whole device, creating an intrinsic synchronization. The synchronization of an array of parallely connected STOs will allow to increase the output power, as necessary for technological applications. [ABSTRACT FROM AUTHOR]

In this paper, it is theoretically demonstrated that the electromagnetically induced transparency (EIT) and absorption (EIA) can be achieved in a magnetic metamaterial. Unit cell of metamaterial consists of a split ring resonator and an "I" shaped cut-wire pair, which serves as a bright resonator and a dark resonator, respectively. It is found that the EIT effect in metamaterial results from the magnetic interaction between bright and dark resonators. A classical model is also introduced to describe the EIT behavior in magnetic metamaterial, and its analytical results are in good agreement with numerical results. Significantly, by controlling the space separation between resonators, we can obtain the destructive and constructive interferences, and thus observe the transition between EIT and EIA. These results may achieve potential applications on enhancing the nonlinear interaction. [ABSTRACT FROM AUTHOR]

Heat-Assisted Magnetic Recording (HAMR) is a likely successor of Perpendicular Magnetic Recording (PMR) in the Hard disk drive industry. In PMR, recording performance is strongly affected by the following distributions in magnetic granular media: magnetic anisotropy field (HK), volume/ grain size, and interaction field from neighboring grains. Since HAMR writing occurs in a narrow temperature region below Curie point (Tc), additional grain-to-grain Tc variation would strongly affect HAMR recording performance. Thus, Tc distribution should be examined for successful HAMR media development. In this paper, we demonstrate a new approach of extracting HK and Tc distributions (σHK and σTc) from thermo-remanence measurements. During the measurement process, a thin film is magnetically saturated, laser heated to specific peak temperature (for a time typically of 5 μs), then cooled to room temperature and magnetic thermo-remanence is measured. Analytical fit to the experimental curves enables independent evaluation of both σTc (±0.5% absolute) and σHK (±2% absolute). Parameters of the analytical statistical model include: temperature dependencies Ms(T), HK(T); mean field effective demagnetization factor N; grain size, HK; and Tc distributions. Thermal fluctuations are taken into account using Arrhenius-Neel formalism. Here, we report experimental σTc values as a function of grain volume. Increase of σTc with grain size reduction might be a limiting factor for HAMR extendibility. [ABSTRACT FROM AUTHOR]

In order to understand the magnetocaloric response of materials, it is important to analyze the interactions between the different phases present in them. Recent models have analyzed the influence of these interactions on the magnetocaloric response of composites, providing an estimate value of the interaction field that is consistent with experimental results. This paper analyzes to which extent magnetization first-order reversal curve (FORC) method can be used to calculate these interactions. It is shown that the different field ranges that are explored using these techniques (inside the hysteretic region for FORC; close to magnetic saturation for magnetocaloric effect) produce interaction field values that differ in order of magnitude, with FORC being sensitive to the lower values of the interaction field and magnetocaloric analysis accounting for the larger interactions. [ABSTRACT FROM AUTHOR]

Memory machines with both high-power-density and wide-speed-range are becoming very attractive most recently. The purpose of this paper is to propose a novel type of memory machine, namely, claw pole memory machine. It engages an axially magnetized AlNiCo PM ring on the claw pole rotor to build the main magnetic flux in air-gap which is responsible for the electromechanical energy conversion. A magnetizing coil is equipped to online regulate the magnetization level of the permanent magnet ring, so as to achieve wide-speed-range operation. The operating principle is analyzed. The Preisach hysteresis model is combined with 3D finite element method to conduct performance assessment of the proposed claw pole memory machine. Calculation results demonstrate that the air-gap flux density can be readily adjusted by injecting DC pulse into the magnetizing coil, and the speed-range of the proposed machine can be extended as wide as six times of its base speed. [ABSTRACT FROM AUTHOR]

This paper deals with the effect of a biocompatible surface coating layer on the magnetic properties of ultrasmall iron oxide nanoparticles. Particles were synthesized by laser pyrolysis and fully oxidized to maghemite by acid treatment. The surface of the magnetic nanoparticles was systematically coated with either phosphonate (phosphonoacetic acid or pamidronic acid) or carboxylate-based (carboxymethyl dextran) molecules and the binding to the nanoparticle surface was analyzed. Magnetic properties at low temperature show a decrease in coercivity and an increase in magnetization after the coating process. Hysteresis loop displacement after field cooling is significantly reduced by the coating, in particular, for particles coated with pamidronic acid, which show a 10% reduction of the displacement of the loop. We conclude that the chemical coordination of carboxylates and phosphonates reduces the surface disorder and enhances the magnetic properties of ultrasmall maghemite nanoparticles. [ABSTRACT FROM AUTHOR]

This paper presents two-dimensional vector measurements of major hysteresis loops of magnetic particle tape measured with a vibrating sample magnetometer. The measurements, presented in magnet and sample coordinates, may be used to evaluate how well the computations of a vector model fit the corresponding measured data. The measurement process is described, and the characteristics of the measurements are discussed. [ABSTRACT FROM AUTHOR]

Recently, interest in alnico magnetic alloys has been rekindled due to their potential to substitute for rare-earth based permanent magnets provided modest improvements in their coercivity can be achieved without loss of saturation magnetization. Recent experimental studies have indicated that atomic and magnetic structure of the two phases (one AlNi-based, the other FeCo-based) that comprise these spinodally decomposed alloy is not as simple as previously thought. A key issue that arises is the distribution of Fe, Co, and Ti within the AlNi-based matrix phase. In this paper, we report the results of first-principles calculations of the site preference of ternary alloying additions in DO3 Fe3Al, Co3Al, and Ni3Al alloys, as models for the aluminide phase. For compound compositions that are Al rich, which correspond to experimental situation, Ti and Fe are found to occupy the a sites, while Co and Ni prefer the c sites of the DO3 lattice. An important finding is that the magnetic moments of transition metals in Fe3Al and Co3Al are ordered ferromagnetically, whereas the Ni3Al were found to be nonmagnetic unless the Fe or Co is added as a ternary element. [ABSTRACT FROM AUTHOR]

In this paper, the physical properties of half-doped manganite La0.5Ca0.5MnO3 with crystallite sizes ranging from 15 to 40 nm are investigated. As expected, ferromagnetic order strengthens at expense of antiferromagnetic one as crystallite size is reduced to 15 nm. However, contrary to previously reported works, an enhancement of saturation magnetization is observed as crystallite size increases from 15 to 22 nm. This unexpected behavior is accompanied by an unusual cell volume variation that seems to induce ferromagnetic-like behavior at expense of antiferromagnetic one. Besides, field cooled hysteresis loops show exchange bias field and coercivity enhancement for increasing cooling fields, which suggest a kind of core-shell structure with AFM-FM coupling for crystallite sizes as small as 15 nm. It is expected that inner core orders antiferromagnetically, whereas uncompensated surface spins behave as spin glass with ferromagnetic-like ordering. [ABSTRACT FROM AUTHOR]

Waves in magnetic systems often show nonreciprocal propagation, where the properties of a wave depends on whether it is travelling in a forward or reversed direction. This fundamental property is used in devices such as isolators, which often use a transverse electric (TE) polarized wave. In this paper, we study propagation of electromagnetic waves in small microstrip waveguides where the wave propagates at an arbitrary angle with respect to the magnetization direction. The angle allows mixtures of TE and transverse magnetic (TM) modes. This has distinct benefits, allowing the nonreciprocity typically found in TE modes, but also allowing propagation of low frequency TM modes, which do not have a cut-off frequency. Our calculations, using Barium hexaferrite as the magnetic material, show significant nonreciprocity and isolation are possible. Differences in attenuation above 75 dB/cm are found for these mixed-polarization waves for propagation angles of about 40°. We study nonreciprocal propagation as a function of propagation angle and other material parameters for both primarily TM and primarily TE modes. [ABSTRACT FROM AUTHOR]

In magnetic recording media, the grains or clusters reverse their magnetization over a range of reversal field, resulting in a switching field distribution. In order to achieve high areal densities, it is desirable to understand and minimize such a distribution. Clusters of grains which contain stacking faults (SF) or fcc phase have lower anisotropy, an order lower than those without them. It is believed that such low anisotropy regions reverse their magnetization at a much lower reversal field than the rest of the material with a larger anisotropy. Such clusters/grains cause recording performance deterioration, such as adjacent track erasure and dc noise. Therefore, the observation of clusters that reverse at very low reversal fields (nucleation sites, NS) could give information on the noise and the adjacent track erasure. Potentially, the observed clusters could also provide information on the SF. In this paper, we study the reversal of nucleation sites in granular perpendicular media based on a magnetic force microscope (MFM) methodology and validate the observations with high resolution cross-section transmission electron microscopy (HRTEM) measurements. Samples, wherein a high anisotropy CoPt layer was introduced to control the NS or SF in a systematic way, were evaluated by MFM, TEM, and magnetometry. The magnetic properties indicated that the thickness of the CoPt layer results in an increase of nucleation sites. TEM measurements indicated a correlation between the thickness of CoPt layer and the stacking fault density. A clear correlation was also observed between the MFM results, TEM observations, and the coercivity and nucleation field of the samples, validating the effectiveness of the proposed method in evaluating the nucleation sites which potentially arise from stacking faults. [ABSTRACT FROM AUTHOR]

In a sequel to our previous paper (J. Appl. Phys. 113, 036908 (2013)), where we reported comprehensive analysis of inter-grain connectivity (AF), pinning, percolation threshold (Pc), and anisotropy (γ) in a series of ex-situ processed MgB2, we address the same issues in in-situ processed samples. MgB2 samples with stoichiometric composition, excess Mg (5wt. %) and further 3wt. % addition of various non-carbon based additives like nano-Ag, nano-Ni, and YBCO are synthesised by the in-situ route. Detailed investigations of X-ray diffraction, magnetization (M), and resistivity (ρ) as a function of temperature (T) and field (B) in the range 5–300K and 0–8 T are carried out in all the samples. The resistive superconducting onset Tcon~38.66 ± 0.3K and offset (where ρ goes to zero) Tc0 ~ 38.1 ± 0.3K of the samples stay nearly unchanged. The inter-grain connectivity (AF) of the samples varies between 11%–20%. All the additives result in a critical current density (Jc) higher than the stoichiometric MgB2 sample, where the highest values (e.g., Jc(1 T, 5K)~ 1.2 x 109 A/m2) are observed for the sample with 5 wt. % excess Mg. The major findings based on quantitative analysis of ρ (T, B) and Jc (B, T) data in all the samples are: (1) along with previously studied ex-situ samples, the Jc(AF) shows a significant increase at AF ~ 7%; (2) the irreversibility lines lie lower than the characteristic Tc0(B) lines in the B-T phase diagram; (3) a universal core pinning (δ1- and/or δTc- type) mechanism is revealed in the entire T range 5–30 K; and (4) typical values of Pc~0.57 ± 0.04 is indicative of weak link networks. [ABSTRACT FROM AUTHOR]

In this paper, we report the effects of carbon content on crystal structure, magnetic and electrical/thermal transport properties in antiperovskite compounds GaCxFe3 (0.15 ≤ x ≤ 0.3). Our experimental results indicate that all the physical properties we measured are sensitive to the carbon concentration for GaCxFe3. With decreasing the carbon content x, the Curie temperature (TC), saturated magnetization, and coercive force (HC) increase while the lattice constant, Debye temperature, and resistivity decrease gradually. Further analysis suggests that the serial GaCxFe3 can be a new kind of promising high-temperature soft magnetic material considering their high-TC (∼788 K) and low-HC (∼22 Oe). Interestingly, the low-temperature resistivity exhibits an abnormal semiconducting behavior below a characteristic temperature for each of GaCxFe3 (0.15 ≤ x ≤ 0.3). This abnormality of resistivity may be attributed to a combination of the renormalized electron-electron interaction and the weak-localization. Furthermore, the studies of the thermal transport and the Hall measurement suggest that the type of dominant carriers in GaCxFe3 changes gradually from hole- to electron-type with decreasing x. [ABSTRACT FROM AUTHOR]

We report a comprehensive investigation of the magnetic properties of LuFe2O4 (LFO) samples with different oxygen stoichiometries. Samples with excess oxygen exhibit spin glass behavior without long-range magnetic order, while three-dimensional ferrimagnetic order exists in a stoichiometric sample. Dissimilar experimental observations reported in several papers can be understood consistently when oxygen stoichiometry is taken into account. The stoichiometric sample orders magnetically below TN = 243 K, and a monoclinic lattice distortion sets in below TL = 175 K. This structural change is sensitive to the applied magnetic field, indicating strong spin-lattice coupling in this material. Unusual low-field thermal magnetization behavior was observed near TL, and its origin is discussed. [ABSTRACT FROM AUTHOR]

In this paper, different definitions of the distribution functions that can be used to account for the magnetization of dispersed nano-particle systems are discussed. The volume and number weighted distributions are both found to be equally valid for the representation of distribution functions in nanoparticle magnetic systems. This study also shows that the room temperature magnetization curve M(H) of a superparamagnetic system is sensitive to the particle size distribution parameters and, that for a non-interacting system, these parameters can be unambiguously determined. Furthermore, the temperature variation of the initial susceptibility χ(T) for a dispersion of particle sizes is also examined using both the exact and the critical approaches. The critical approach is found to be a reasonable and valid approximation, since the discrepancy in the calculated χ(T) curves between the exact and the approximated form of the function exp(-tm/τ) was found to be very small. For a dispersion of anisotropy energies, both approaches adequately describe the unblocking process of the particle magnetic moments within the system when the temperature is increased. In this study, the distribution of the critical transition temperatures that can be obtained from the temperature variation of the initial susceptibility is also examined. For different definitions of the distribution functions, the retrieved distributions from the experimental data are bound to be different. Furthermore, the calculated temperature variation of the initial susceptibility is found to be sensitive to the constant value of the frequency factor f0 used in the calculations. The discrepancy in calculating the χ(T) curve using an improper value of f0 is larger than that discrepancy arising from the step-like approximation of exp(-tm/τ). Thus, the f0 value has to be calculated using the physical parameters of the system and not just taken as a constant value between 109 and 1011 Hz. [ABSTRACT FROM AUTHOR]

In this paper structural, electric, magnetic, and Mössbauer spectroscopy studies were conducted in (x)BiFeO3-(1-x)BaTiO3, 0.9 ≥ x ≥ 0.3, solid solutions. X-ray diffraction and Rietveld refinement studies indicated the formation of single-phased materials crystallized in a distorted perovskite structure with the coexistence of rhombohedral and monoclinic symmetries. Room temperature ferroelectric hysteresis loops showed that the electric polarization increases with the increase of the BaTiO3 content due to the singular structural evolution of the studied solid solutions. All samples presented weak ferromagnetic ordering, which indicates that the BaTiO3 substitution in the BiFeO3 matrix released the latent magnetization. Mössbauer studies revealed a magnetic spectral signature corresponding to ordered Fe3+ ions, and a decrease of the magnetic hyperfine magnetic fields with the increase of the BaTiO3 content. The composition 0.3BiFeO3-0.7BaTiO3 presented a spectral signature corresponding to a paramagnetic behavior, which strongly suggests that the observed magnetization in this sample is due to the Ti3+ ions. [ABSTRACT FROM AUTHOR]

During the last years, cylindrical structures like dots, wires, and tubes have been intensively investigated. However, stadium-shaped particles have been scarcely investigated because of the complexity associated to the control of its geometry. In this paper, we used a scaled atomistic representation with Monte Carlo simulations to investigate systematically the magnetization reversal process in stadium-shaped magnetic structures as a function of the size of the central volume. Different reversal mechanisms have been identified and its existence has been explained. The reversal process for short, almost circular, stadium structures occurs via the formation of a single vortex, whereas for longer stadium structures the reversal process involves either single vortices or vortex pairs. For the longest structures investigated, the reversal always involves two vortices that generate a central diamond region or a central antivortex, depending on the region of the nucleation of the vortices. Our results provide guidelines for the use of stadium-shaped particles in the control of vortex motion when a magnetic field is applied. [ABSTRACT FROM AUTHOR]

The present paper presents the effects of a nanostructured, ferromagnetic La1-xSrxMnO3(LSMO) thin film on the pinning characteristics of an epitaxial YBa2Cu3O7(YBCO) thin film deposited on top. Ordered arrays of LSMO grains were obtained upon growing the film on a terraced (001) SrTiO3 substrate. The analyses of magnetic measurements revealed the presence of a complex vortex pinning mechanism within the YBCO film. With respect to a reference single YBCO layer, an additional pinning potential was observed. Its temperature evolution suggests that a magnetic pinning mechanism is responsible for improved pinning characteristics in the high temperature and field region. Based on the morphology of the underlying LSMO buffer, a strong pinning force is expected to arise due to the anti-dot formations, where a high magnetization gradient exists. [ABSTRACT FROM AUTHOR]

This paper presents a comparative study on the tetragonal magnetostriction constant, λγ,2, [ = (3/2)λ100] and magnetoelastic coupling, b1, of binary Fe100-xZx (0 < x < 35, Z = Al, Ga, Ge, and Si) and ternary Fe-Ga-Al and Fe-Ga-Ge alloys. The quantities are corrected for magnetostrains due to sample geometry (the magnetostrictive form effect). Recently published elastic constant data along with magnetization measurements at both room temperature and 77 K make these corrections possible. The form effect correction lowers the magnetostriction by ∼10 ppm for high-modulus alloys and by as much as 30 ppm for low-modulus alloys. The elastic constants are also used to determine the values of the magnetoelastic coupling constant, b1. With the new magnetostriction data on the Fe-Al-Ga alloy, it is possible to show how the double peak magnetostriction feature of the binary Fe-Ga alloy flows into the single peak binary Fe-Al alloy. The corrected magnetostriction and magnetoelastic coupling data for the various alloys are also compared using the electron-per-atom ratio, e/a, as the common variable. The Hume-Rothery rules link the e/a ratio to the regions of phase stability, which appear to be intimately related to the magnetostriction versus the solute concentration curve in these alloys. Using e/a as the abscissa tends to align the peaks in the magnetostriction and magnetoelastic coupling for the Fe-Ga, Fe-Ge, Fe-Al, Fe-Ga-Al, and Fe-Ga-Ge alloys, but not for the Fe-Si alloys for which the larger atomic size difference may play a greater role in phase stabilization. Corrections for the form effect are also presented for the rhombohedral magnetostriction, λ&eh;,2, and the magnetoelastic coupling, b2, of Fe100-xGax (0 < x < 35) alloys. [ABSTRACT FROM AUTHOR]

This paper describes a full micromagnetic characterization of the magnetization dynamics driven by spin-polarized current in anisotropic spin-torque oscillators (STOs). For field angles approaching the hard in-plane axis, the excited mode is uniform and a super-critical Hopf-bifurcation takes place at the critical current density JC. For field angles close to the easy axis of the free layer, the excited mode is localized (non-uniform) and a sub-critical Hopf-bifurcation occurs at JC. In this latter region, a hysteretic behaviour is, therefore, found. We demonstrate numerically that the non-linearities of the STO are strongly reduced when the oscillation frequency at the critical current is near the ferromagnetic resonance (FMR) frequency computed at zero bias current, and in particular, this condition corresponds to the field orientation at which a minimum in the FMR-frequency is achieved. [ABSTRACT FROM AUTHOR]

Experimental and analytical studies are carried out on features of damages-induced magnetization (natural magnetization) of the 304 austenitic stainless steel. Various tensile plastic deformations are introduced into test pieces with different initial discontinuities. The surface strain distributions and natural magnetic fields are measured during and after the tensile testings. The magnetization distributions inside the material are reconstructed from the measured natural magnetic field signals through inverse analysis. From the experimental and numerical results, it is clarified that at first, the magnetization direction of the material depends on the load direction and damage distribution and the distribution pattern of magnetization reflects the existence of concentrative damages. Second, the relationship between the intensity of damage-induced magnetization and the maximum plastic strain follows a simple formula proposed in this paper. Third, the magnetization intensity is correlated with the volume fraction of the martensitic phase. Finally, the theory of minimum energy is applicable to qualitatively explain the mechanism of the damage-induced magnetization for the 304 austenitic stainless steel. [ABSTRACT FROM AUTHOR]

Axial magnetostriction in giant magnetostrictive materials with specific crystal orientation is highly anisotropic when changing magnetization direction. Based on a single-{111}-twin crystal growth model, this paper presents a domain rotation simulation of anisotropic magnetostrictions in <110> oriented crystals Tb0.36Dy0.64(Fe0.85Co0.15)2 by considering the demagnetization effect. Simulation results show that the axial magnetostrictions under different external magnetic field directions, which correspond to specific magnetic moment 'jump' and 'rotation' processes, are in good agreement with experimental data. The demagnetization effect, arising from non-coaxial magnetization, hinders moment alignments especially at low external fields and reduces the initial magnetostriction. Change of the saturation magnetostriction is also presented by multi-domain calculations through revealing the detailed magnetization processes. Such domain rotation simulations might provide a better understanding of the magnetostriction dependence on magnetization direction, i.e., the anisotropic magnetostrictive responses. [ABSTRACT FROM AUTHOR]

In this paper we report high magnetic field (- 140 ≤ H ≤ 140 kOe) magnetization data for cobalt ferrites (crystallites size ∼ 42 nm) for temperatures (T) varying from 5 to 340 K. The T-dependence for the cubic magnetocrystalline anisotropy constant K1 was determined by using the 'law of approach' (LA) to saturation. The values of K1 were found to be substantially different from previously reported values obtained using the same procedure but with H up to 50 kOe. By properly choosing the quantum parameters we found a very good agreement between the values calculated by using the model proposed by Tachiki [Prog. Theor. Phys. 23, 1055 (1960)] and the K1 versus T data. For instance, the values of K1 measured (calculated) for 5 K and 340 K were 28.3 × 106 erg/cm3 (27.7 × 106 erg/cm3) and 7.4 × 106 erg/cm3 (6.8 × 106 erg/cm3), respectively. The values of the parameters used to fit the data in both magnetic field regimes were chosen based on cation distribution over the A and B-sites on the spinel structure of the nanoparticles. [ABSTRACT FROM AUTHOR]

In this paper, we present an analytical method to study the magnetization processes of two magnetic particles with ferromagnetic or antiferromagnetic coupling by using a modified expression for the anisotropy free energy. By considering a system consisting of two uniaxial particles, coupled by a simple exchange interaction, we describe analytically: the particles orientations, switching points and hysteresis loops for different ferromagnetic or antiferromagnetic couplings and for different orientations of the applied field. [ABSTRACT FROM AUTHOR]

It is well-known that the optimum condition to obtain the highest permeability and lowest coercive force for a soft magnetic material is that both K and λs are very small. In this paper, it is suggested that this optimum condition is sufficient only for the single phase soft magnetic material. For the material containing two phases, in order to obtain the best magnetic properties, a new condition must be added; the difference in the magnitude of the saturation magnetization of the two phases, ΔMS, should be very small. Only when the K, λs, and ΔMS are all very small can the best soft magnetic properties be obtained. A model describing the hindrance of the domain well motion by the ΔMS effect is suggested. Some examples in the nanocrystalline soft magnetic alloys are used to demonstrate the existence of the ΔMS effect. [ABSTRACT FROM AUTHOR]

In this paper, we show that when nanoparticles of Fe3O4 are coated with gold there is a distinct enhancement of magnetization by a factor of six. This increase of magnetization has been attributed to large orbital magnetic moment formation at the magnetic particle/Au (core/shell) interface. Our theoretical analysis indicates that the enhanced magnetism observed in Fe3O4-Au (core-shell) nanoparticles is an interfacial effect. The origin of magnetism in Au as an interfacial phenomenon is supported by the observation of positive magnetization in citrate coated gold nanoparticles. In citrate coated gold nanoparticles, we observe a crossover from positive magnetization value to negative magnetization value upon increasing magnetic field indicating cancellation of interfacial magnetization by the diamagnetic contribution from the bulk. We propose a theoretical formalism which semi-quantitatively explains our experimental results and supports the origin of magnetization in Au as an interfacial effect. [ABSTRACT FROM AUTHOR]

The magnetization orientation of the polarizing reference layer would fluctuate between the parallel and antiparallel states when the reference layer loses stability. In this paper, a combined analytical and simulation study is presented to predict the magnetic dynamics of the spin valve element with single-domain magnets as the free and reference layers. We acquire a complete phase diagram that includes the normal spin-torque switching and random magnetization fluctuations by tuning the spin torque strength between the free and reference layers. The phase dynamics strongly depends on the magnetization state of the reference layer, showing that the instability of the reference layer could be responsible for the random fluctuation events. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the influence of the dipolar magnetic interaction on the current-driven vortex dynamics in a two-dimensional array of nanodisks. A general formula is established for the prediction of the correlation between the restoring force of the target nanodisk and the polarities and circulations of the neighboring vortices. Large numbers of possible magnetization configuration combinations among the target and the surrounding neighbors are reduced into four groups under an extreme condition. The maximum and minimum restoring forces are analytically derived, which can be used to predict the critical switching current dependence on the magnetic configuration combinations of vortices. [ABSTRACT FROM AUTHOR]

Traditional magnetic/stress coupling models are based on the magnetoelastic theory, which cannot explain magnetic phenomena when ferromagnetic materials are subjected to a plastic deformation. To promote the development of magnetomechanical theories, a new physical-based magnetic/stress coupling model was proposed in this paper, where the effects of the applied magnetic field, elastic and plastic deformations on the magnetization are, respectively, considered in terms of different acting mechanisms. Moreover, the influences of variants on the strength in different magnetization stages are also calculated. [ABSTRACT FROM AUTHOR]

This paper reports effects of inter-film interactions on static and dynamic magnetization behavior at film edges in magnetic trilayer stripe arrays under transverse applied fields. The trilayers consist of two magnetic films of Ni80Fe20, 10 nm and 20 nm thick that interact via dipolar fields across thin, nonmagnetic spacers from 1 nm to 20 nm thick. These trilayer films are patterned into stripe arrays by optical interference lithography. Ferromagnetic resonance spectra exhibit separate edge modes that are identified as excitation of edge modes primarily in one magnetic film or the other. When interlayer exchange is negligible, we observe several effects of dipolar edge-edge coupling including shifts of the edge saturation fields and changes in the mode intensity ratio relative to single-film edges. These interaction effects are reproduced by micromagnetic modeling and explained by simple models. [ABSTRACT FROM AUTHOR]

Finding the relations between the microstructural material parameters and the macroscopic hysteresis behavior is indispensable in the design of ferromagnetic materials with minimal (hysteresis) losses. Micromagnetic hysteresis simulations enable a rigorous and structured investigation of these relations since in the numerical model each material parameter can be altered independently. This paper describes a procedure to extract the Preisach distribution function, quantifying the macroscopic hysteresis properties, from micromagnetic simulations incorporating the materials' microstructure. Furthermore, the instantaneously added, stored and dissipated energy while running through the hysteresis loop as described in the macroscopic Preisach model and in the micromagnetic hysteresis model are compared, evidencing a very good agreement. Moreover, using the micromagnetic model, the energy rearrangements between the different micromagnetic interaction terms is studied at each time point of the hysteresis loop. It is concluded that the micromagnetic hysteresis model is a valuable tool in the study of hysteresis properties and loss mechanisms in ferromagnetic materials. [ABSTRACT FROM AUTHOR]

This paper presents properties and nanocrystallization characteristics of HITPERM based Fe68.8Co17.2Hf7Cu1B6 and Fe69.6Co17.4Hf7B6 alloys. Both alloys differing only in copper content were subjected to series of characterizations in order to study the effect of copper. Evolution of magnetic properties as a function time has been investigated by isothermal annealing experiments and optimum annealing conditions of amorphous precursors have been reported. Resulting magnetic properties (ac and dc) measured at room temperature are presented. Nanocrystallization kinetics studied by using time dependant magnetization measurements was discussed in the framework of Johnson-Mehl-Avrami model. Results of thermal stability measurements conducted in air at 500 K for up to 1000 h are reported. [ABSTRACT FROM AUTHOR]

In this paper we report results of structural, spectroscopic, and magnetic investigations of MgFe2O4 nanoparticles prepared by soft mechanochemical synthesis. MgFe2O4 nanoparticles crystallize in Fd3m space group with mixed cation distribution and reduced percentage of Fe3+ at tetrahedral (8a) sites. Discrepancy in the cation distribution compared to that in the bulk Mg–ferrite is one of the highest known. X-ray line broadening analysis reveals crystallite size and strain anisotropy. The saturation magnetization, Msat=62 emu/g measured at 5 K is twice higher than that found in the bulk counterparts. Such high value of Msat is attributed to the low value of cation inversion parameter (δ=0.69), to the core/shell structure of the nanoparticles and to the surface/volume ratio. Mössbauer spectrum collected at room temperature reveals ferrimagnetic ordering between Fe3+ ions in 8a and 16d sites, while zero-field-cooled (ZFC) and field-cooled (FC) M(T) measurements were shown superparamagnetic state above 350 K. [ABSTRACT FROM AUTHOR]

A study of magnetic aggregation in the context of magnetic resonance imaging (MRI) based actuated targeting is proposed. MRI systems can induce displacement forces on magnetized particles as they flow through the blood vessels. Magnetic aggregation of the particles happens when they are placed within the magnetic field of the MRI system and can greatly influence the MRI steering dynamics of magnetic particles. In this paper, a review of the different parameters that can be used to tailor the size, geometry, stiffness, and density of magnetic aggregates is proposed. Then, magnetic aggregation experiments on a suspension of Fe3O4 microparticles ranging from 0.1 to 100 μm in diameter are described. The effects of particle concentration, flow rate, and magnetic field amplitude were evaluated. Field amplitudes of 1.5 mT, 0.4 T, and 1.5 T fields were applied without any magnetic steering gradients and caused aggregates that could sometimes exceed 1 mm in length. Since magnetic aggregates can reach higher magnetophoretic velocities than individual particles, large aggregates could be exploited in larger arteries with important blood flows. A few strategies are discussed to assist in the design of MRI steering experiments by enhancing the positive effects of magnetic aggregation over its negative effects. [ABSTRACT FROM AUTHOR]

Magnetic annealing can reorient the domain moments, improving dramatically the magnetostriction of Tb–Dy–Fe oriented polycrystals without applying any prestress. In this paper, totally different stress dependences of magnetization and magnetostriction were observed in <110> oriented Tb0.36Dy0.64(Fe0.85Co0.15)2 polycrystals after perpendicular magnetic annealing. Under compressive prestress, no obvious increase in magnetostriction occurred but the linear magnetostriction performance was improved. The magnetization initially decreased and approached to saturation at relatively higher magnetic fields with increasing the stress magnitude. Such variation can be contributed mainly by the redistribution of initial magnetic moments and enlargement of 71° wall energy. [ABSTRACT FROM AUTHOR]

Phenomena occurring at the interface may significantly modify the properties of magnetic thin films. In this paper, the effect of temperature on the magnetic properties of Co(001)/MgO/Co trilayers is studied. The trilayer structure was modified by partial coverage of Co layers by Au at the Co/MgO interface. It is found that when the quantity of deposited gold is small, formation of islands at the Co/MgO interfaces takes place, which is proved by the topological surface analysis and by the x-ray reflectivity. In the investigated trilayer structure, gradual changes of the easy magnetization axis from the in-plane to the out-of-plane direction are observed during lowering the temperature from 300 to 5 K. The analysis of the ferromagnetic resonance spectra within the multiple-magnetic-layer model system indicates that the surface anisotropy provides a significant contribution to the magnetic anisotropy of complete Co(001)/MgO/Co heterostructure. [ABSTRACT FROM AUTHOR]

In this paper we numerically conduct micromagnetic modeling with an expended micromagnetic model that includes the spin torque term and an impedance model to investigate methods to replace external field sources with internal ones and to investigate its tuneability on nanopillar geometries. We present results for three methods: interlayer coupling, large perpendicular anisotropy, and magnetostatic coupling. The internal field sources are evaluated as function of frequency shift with current, its dependency on temperature, and are tested against analytical predictions. [ABSTRACT FROM AUTHOR]

We present a three-dimensional study of the torque transmitted between tile permanent magnets uniformly magnetized. All this study is based on the Coulombian model. The torque is calculated semianalytically by considering all the surface densities that appear on the tiles. In addition, no simplifying assumptions are done in the expressions given in this paper. Consequently, the evaluation of the torque is very accurate and allows us to show the drawbacks of using tile permanent magnets uniformly magnetized instead of using tile permanent magnets radially magnetized. Such an approach is useful because it allows us to realize easily parametric studies. [ABSTRACT FROM AUTHOR]

We compared the magnetoresistance (MR) ratios of current-confined-path (CCP) current-perpendicular-to-plane (CPP) giant MR spin valves (SVs) calculated using the two-current-model. The simplified two-current model, which places the parasitic resistance external to the two-current network, has been inaccurate in determining the overall resistance of a SV in parallel magnetization configuration. The characteristics of the error in MR depend on the parasitic resistance value, the spin asymmetry coefficient, and the spacer layer resistivity. In comparison to the complete two-current model, we found that although the simplified model could generally predict the trend of various MR behaviors, it also overestimates MR for different CCP densities. A more comprehensive calculation on the CCP SVs was then performed using the spin-drift-diffusion equations based on the Valet–Fert model. It was found that the MR behavior upon considering spin relaxation in the ferromagnetic layers has a closer match with results predicted by the simplified model due to the strong reduction in spin accumulation in the parasitic layers. Spin relaxation in the spacer layer, however, shows a competing effect that tends to reduce the MR ratios. Our results in this paper are useful for accurate evaluation of MR performance in a CPP SV sensor with current-confined paths. [ABSTRACT FROM AUTHOR]

Nanopillars containing a perpendicular polarizer and a vortex free layer have been considered as promising structures for the selective switching of the vortex core by changing the spin polarized current direction. In this paper, two distinct switching routes of the vortex cores are found to be classified according to the lateral dimension of samples. For a relatively large element, the core polarity is switched through a creation and annihilation process of vortex-antivortex pair and the vortex circulation remains unchanged; however, for a small element, the core switching is mediated by a transient C-state, and moreover, the circulation is dominated by the current-generated Oersted field. [ABSTRACT FROM AUTHOR]

We have studied the dynamic switching triggered by spin angular momentum transfer in a pulsed current of a spin-valve-type trilayer structure, and its dependence on thermal effects. In order to determine the current pulse parameters, where fast and stable switching can be achieved, we have studied the magnetization’s dynamics properties as a function of applied current pulse amplitude and shape, waiting time, and initial orientation, and also as a function of the Gilbert damping constant. The magnetic layer is assumed to be single domain, ellipsoid shaped. In this paper also we present the thermal fluctuation effects on the switching behavior. The model is based on the Landau–Lifshitz–Gilbert equation and the stochastic Landau–Lifshitz–Gilbert equation with a spin-transfer term included, which are numerically integrated. [ABSTRACT FROM AUTHOR]