Measurement of magnetic properties of cobalt-rare earth permanent magnets

Co-rare earth permanent magnets generally display a high intrinsic coercive force. In order to make magnetic measurements with such materials fully saturated, we have found it necessary to use magnetizing fields in excess of 50 kOe. The superconducting solenoid is ideally suited for generating such fields. In this study, we have used a 100 kOe Nb3-Sn superconducting solenoid for magnetizing samples and for measuring saturation magnetization. Demagnetization properties of long cylindrical samples have been measured in the superconducting solenoid, and also with a conventional hysteresigraph after the samples have been saturated with the superconducting solenoid or in some cases with a pulsed field solenoid. Short disk samples or ring magnets such as are used in traveling wave tube (TWT) designs are difficult to measure by these techniques. We have found torque magnetometry to be useful for such shapes. Open circuit magnetization is measured in this case. Results for ring magnets will be compared with peak axial field measurements for a periodic TWT structure. Reversible and irreversible changes in magnetization with temperature are also of interest. Techniques for measuring these changes are discussed.     

Implementation of the Preisach-Stoner-Wohlfarth Classical Vector Model

A useful vector magnetic model must accurately simulate both the magnitude and direction of the magnetization when a magnetic medium is subjected to a linear or rotating applied field. Such a model has been recently presented as the Preisach-Stoner-Wohlfarth (PSW) model. The Preisach model computes the magnitude of the magnetization; the Stoner-Wohlfarth model computes the direction of the magnetization. These two models are combined into the PSW model. The PSW model is now initially implemented in a two-dimensional vector classical Preisach model. The model is computationally efficient since the magnetization angle is accessed for all applied fields from a single lookup table, which is generated by a one-time Stoner-Wohlfarth computation.     

Effective magnetization and forces due to eddy currents

A simple method for evaluating the effective magnetization due to eddy currents excited by AC magnetic fields in finite conductive objects is presented. The values of magnetization enable the estimation of the forces exerted on the object by the effect of eddy currents. The method relies on assuming a similarity between eddy current magnetization and the magnetization due to diamagnetic effects, which is easier to evaluate. Its validity is checked by comparing results from the eddy current forces with data obtained from conventional but much more complicated methods or from experimental data. The approach may be useful for evaluating the eddy current losses in finite conductive objects excited by AC magnetic fields or the influence of small conductive objects on AC excitation coils. The method combines techniques related to the conventional evaluation of eddy currents in certain 1-D geometries with techniques that approximate behavior of the AC magnetic field in finite objects in a way similar to that followed when the magnetic forces acting on a diamagnetic object are calculated     

Substantial reduction of critical current for magnetization switching in an exchange-biased spin valve

Great interest in current-induced magnetic excitation and switching in a magnetic nanopillar has been caused by the theoretical predictions of these phenomena. The concept of using a spin-polarized current to switch the magnetization orientation of a magnetic layer provides a possible way to realize future 'current-driven' devices: in such devices, direct switching of the magnetic memory bits would be produced by a local current application, instead of by a magnetic field generated by attached wires. Until now, all the reported work on current-induced magnetization switching has been concentrated on a simple ferromagnet/Cu/ferromagnet trilayer. Here we report the observation of current-induced magnetization switching in exchange-biased spin valves (ESPVs) at room temperature. The ESPVs clearly show current-induced magnetization switching behaviour under a sweeping direct current with a very high density. We show that insertion of a ruthenium layer between an ESPV nanopillar and the top electrode effectively decreases the critical current density from about 10(8) to 10(7) A cm(-2). In a well-designed 'antisymmetric' ESPV structure, this critical current density can be further reduced to 2 x 10(6) A cm(-2). We believe that the substantial reduction of critical current could make it possible for current-induced magnetization switching to be directly applied in spintronic devices, such as magnetic random-access memory.     

Co-Fe metal/native-oxide multilayers: a new direction in soft magnetic thin film design I. Quasi-static properties and dynamic response

The progression toward gigahertz data rates in magnetic recording has introduced considerable challenges to soft magnetic materials design. The difficulties lie in satisfying two sets of conflicting demands: 1) simultaneously achieving soft magnetic properties, high saturation magnetization, and a high resistivity, with the latter required to limit eddy-current losses and 2) balancing the inherent tradeoff between bandwidth and permeability imposed by the direct and inverse dependences, respectively, of these two parameters on the anisotropy field. This paper describes a new soft magnetic composite system that meets these requirements: a metal/native-oxide multilayer (MNOM) film consisting of nanogranular high-moment Co/sub x/Fe/sub 100-x/ layers separated by ultrathin magnetic native oxide layers. The high-resistivity magnetic oxide layers isolate the metallic layers electrically, while coupling them magnetically and minimizing the decrease in volume-averaged saturation magnetization that exists in traditional metal/nonmagnetic oxide composites. In addition, the "exchange-averaged" soft magnetic properties of the MNOM composite include an ideal low-dispersion in-plane uniaxial anisotropy whose magnitude varies linearly with the fraction x of Co in the alloy. The resulting anisotropy control, together with the large saturation magnetization, permits the permeability and resonance frequency to be tuned over a wide range to meet specific application requirements.     

A Nonlinear Magneto-Mechanical Coupled Constitutive Model for the Magnetostrictive Material Galfenol

In order to predict the performance of magnetostrictive smart material and push its applications in engineering, it is necessary to build the constitutive relations for the magnetostrictive material Galfenol. For Galfenol rods under the action of the pre-stress and magnetic field along the axial direction, the one-dimensional nonlinear magneto-mechanical coupling constitutive model is proposed based on the elastic Gibbs free energy, where the Taylor expansion of the elastic Gibbs free energy is made to obtain the polynomial forms. And then the constitutive relations are derived by replacing the polynomial forms with the proper transcendental functions based on the microscopic magneto-mechanical coupling mechanism. From the perspective of microscopic mechanism, the nonlinear strain related to magnetic domain rotation results in magnetostrictive strain changing with the pre-stress among the elastic strains induced by the pre-stress. By comparison, the predicted stress-strain, magnetostrictive strain, magnetic induction and magnetization curves agreed well with experimental results under the different pre-stresses. The proposed model can describe not only the influences of pre-stress on magnetostrictive strain and magnetization curves, but also nonlinear magneto-mechanical coupling effect of magnetostrictive material systematically, such as the Young’s modulus varying with stress and magnetic field. In the proposed constitutive model, the key material constants are not chosen to obtain a good fit with the experimental data, but are measured directly by experiments, such as the saturation magnetization, saturation magnetostrictive coefficient, saturation Young’s modulus, linear magnetic susceptibility and so on. In addition, the forms of the new constitutive relations are simpler than the existing constitutive models. Therefore, this model could be applied conveniently in the engineering applications.     

Influence of different conducting substrates on magnetic properties of electrodeposited Ni–Fe thin films

Electrodeposition of Ni–Fe soft magnetic alloy on copper and stainless steel substrates was performed in chloride bath. The deposition parameters such as current density, pH, temperature and deposition time have been investigated. From the investigation the optimized deposition parameters were current density 3.5 mA/cm², pH 3, temperature 30 °C and deposition time 15 min. The Ni–Fe magnetic alloys deposited on copper and stainless steel substrates under optimized deposition parameters are subjected to various characterizations. The structural and surface morphology of the Ni–Fe films were detected by using X-ray diffractogram (XRD) and scanning electron microscope (SEM) respectively. The constituents in the films were determined by energy dispersive X-ray spectroscopy (EDAX) technique. The magnetic properties such as the coercivity (H_c) and saturation magnetization of the films were studied with the help of vibrating sample magnetometer (VSM). From the magnetic studies it is concluded that the grain size are create a considerable impact on magnetic behavior of the films on both the substrates. The films prepared on stainless steel substrate of 0.1 M concentration at optimized deposition parameters exhibits higher coercivity (5010 Oe) which seems to be ideal for magnetic sensor applications.     

Small-Angle Neutron Scattering by the Magnetic Microstructure of Nanocrystalline Ferromagnets Near Saturation

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the magnetization to the magnetocrystalline and magnetoelastic anisotropy fields is derived using the theory of micromagnetics. In the limit where the scattering volume is a single magnetic domain, and the magnetization is nearly aligned with the direction of the magnetic field, closed form solutions are given for the differential scattering cross-section as a function of the scattering vector and of the magnetic field. These expressions involve an anisotropy field scattering function, that depends only on the Fourier components of the anisotropy field microstructure, not on the applied field, and a micromagnetic response function for SANS, that can be computed from tabulated values of the materials parameters saturation magnetization and exchange stiffness constant or spin wave stiffness constant. Based on these results, it is suggested that the anisotropy field scattering function S_H can be extracted from experimental SANS data. A sum rule for S_H suggests measurement of the volumetric mean square anisotropy field. When magnetocrystalline anisotropy is dominant, then a mean grain size or the grain size distribution may be determined by analysis of S_H.     

Distortion of inductive read head signal due to side-writtenmagnetization

We apply a Fourier series model to calculate distortion in an inductive read head signal due to side-written magnetization patterns in thin-film magnetic recording media. This model predicts that a magnetization pattern consisting of in-track longitudinal and transverse track edge magnetizations will produce a signal distorted by the flux from the side magnetization. By using parameters obtained from a direct magnetic image of the patterns, this analysis predicts the morphology and magnitude of signal distortion as a function of pattern dimensions. The implication of this distortion for higher bit densities suggested for the future was found to be significant. The proposed method will offer reliable means to quantitatively assess the effect of side writing at such densities     

Magnetic and optical properties of amorphous NdFeCo thin films by pulsed laser deposition technique

Rare earth and transition metal doped (NdFeCo) thin films were fabricated on Si (100) substrate by pulsed laser deposition technique keeping the substrate at constant temperature of 300 °C. A KrF Excimer laser (248 nm, 20 ns) was used as an energy source for the deposition. Thin films were deposited without and under the influence of transverse magnetic field applied across the plume. The applied magnetic field was varied from 3 to 6 kOe. The deposited films were characterized by XRD, FESEM, VSM and SE (Spectroscopic Ellipsometry). The deposited films were amorphous in nature. All the films regardless of the applied magnetic field exhibit perpendicular magnetic anisotropy. The thickness of the thin films was found to increase monotonically from 166 to 266 nm with the increase in the applied external magnetic field. The saturation magnetization has a maximum value of 1682 emu/cc for the film deposited under 4.5 kOe magnetic field. The value of optical band gap energy for the same film is found to have a maximum value of 3.1 eV. The values of both the saturation magnetization and the band gap energy were decreased with the increase in the applied magnetic field.     

Synthesis and characterization of CoFe(2)O(4) ferrite nanoparticles obtained by an electrochemical method

Uniform size cobalt ferrite nanoparticles have been synthesized in one step using an electrochemical technique. Synthesis parameters such as the current density, temperature and stirring were optimized to produce pure cobalt ferrite. The nanoparticles have been investigated by means of magnetic measurements, M?ssbauer spectroscopy, x-ray powder diffraction and transmission electron microscopy. The average size of the electrosynthesized samples was controlled by the synthesis parameters and this showed a rather narrow size distribution. The x-ray analysis shows that the CoFe(2)O(4) obtained presents a totally inverse spinel structure. The magnetic properties of the stoichiometric nanoparticles show ferromagnetic behavior at room temperature with a coercivity up to 6386?Oe and a saturation magnetization of 85?emu?g(-1).     

Colossal magnetic anisotropy of monatomic free and deposited platinum nanowires

Whenever a nanosystem such as an adatom, a cluster or a nanowire spontaneously magnetizes, a crucial parameter is its magnetic anisotropy, the intrinsic preference of magnetization to lie along an easy axis. Anisotropy is important in nanosystems because it helps reduce the magnitude of thermal (superparamagnetic) fluctuations, it can modify the flow of current, and it can induce new phenomena, such as the quantum tunnelling of magnetization. We discuss here, on the basis of density functional calculations, the novel and unconventional feature of colossal magnetic anisotropy--the strict impossibility of magnetization to rotate from the parallel to the orthogonal direction--which, owing to a quantum mechanical selection rule, the recently predicted Pt nanowire magnetism should exhibit. Model calculations suggest that the colossal magnetic anisotropy of a Pt chain should persist after weak adsorption on an inert substrate or surface step.     

Monte Carlo Study of the Magnetic Properties of ErRh Layers

The magnetic properties of antiferromagnetic ErRh layers have been studied using Monte Carlo simulations within the framework of the Ising model. The Hamiltonian considered includes both nearest-neighbor interactions and external magnetic field. Magnetizations and magnetic susceptibility versus temperature are computed for a fixed size. In addition, blocking temperature versus both size and applied external magnetic field is estimated. Hysteresis cycle versus temperature, saturation magnetization, coercive field, and the remanent magnetization are reported as well.     

Current-induced domain nucleation in ferromagnet

A key mechanism of the current-induced magnetization dynamics is the spin torque from a spin polarized current (spin current), which couples to spatial gradient of magnetization. Recently, it was pointed out that a large spin current applied to a uniform ferromagnet leads to a spin-wave instability. In this paper, we show that such instability is absent in a state containing a domain wall. This may indicate that nucleation of magnetic domains occurs above a certain critical spin current. This scenario is supported by an explicit energy comparison between the uniformly magnetized state and the domain-wall state under spin current.     

Optimization of magnetic calorimeters

The properties and performance of magnetic calorimeters based on Au containing a few hundred ppm of Er can be fully described by equilibrium thermodynamics. As a consequence, the magnitude of the change of magnetization of the sensor, resulting from the absorption of a particle, can be calculated with confidence. The magnetization change depends upon a number of parameters such as the external magnetic field, the temperature and the concentration of the Er ions. This theoretical understanding of the calorimeter also allows us to calculate the flux signal detected by a SQUID and how that signal depends on the detector geometry and other relevant parameters. To date we have measured only cylindrically shaped sensors, which are located directly in a circular SQUID loop. However, a sensor having the shape of a thin strip, possibly in form of a meander pattern, enclosed by a loop of the same geometry, has the potential of providing enhanced flux coupling to the SQUID. We discuss the relation of sensor geometry to other parameters such as the dimensions and heat capacity of an attached particle absorber. The values of the adjustable parameters that optimize the performance of a magnetic calorimeter are investigated under a number of different experimental constraints.     

Models for I and T pattern permalloy bars based upon bitter pattern observation of domain wall movements

The magnetostatic fields of the I and T pattern Permalloy overlay bars are analyzed by proposing a model based on the Bitter pattern observation of the domain wall structure in Permalloy bars. The magnetic charges that appear on the 90° domain walls are assumed to be the sources of the magnetic fields of the bars. The model has a two-dimensional reaction to an applied rotating in-plane field due to its two-dimensional domain wall movement and the consequent two-dimensional change of magnetic domain pattern inside the bar. The magnetization of the bar is equal to Msthe saturation magnetization of the bar at every section of the bar except on the domain walls. The magnetization curve and the magnetic field well Bz(bubble drive field) under the overlay bars are calculated and compared to that of the previous models. A qualitative explanation of the rotation of the bubble around the bars is given by the three-dimensional plots of the field well obtained for different orientations of the in-plane field.     

Statistical study of effective anisotropy field in ordered ferromagnetic nanowire arrays

Soft ferromagnetic nanowire arrays were obtained by electrodeposition of Co-Fe-P alloy into the pores of high quality home-made anodized aluminum oxide (AAO) templates. Bath acidity and current density were the two parameters used in order to tailor the orientation of local anisotropy axes in individual nanowires. In order to quantify the influence of the induced anisotropies on the magnetization processes in individual nanowires, the in-plane magnetization loops of the arrays are modeled as log-normal distributions of Stoner-Wohlfarth transverse magnetization processes. Using the lognormal mean parameter as an approximation for the saturation applied field of the array, we compute the effective anisotropy of the nanowires, which is found to increase with the pH of the electrodeposition bath.     

An empirical correlation between print through values and magnetic characteristics of chromium dioxide powders

While the measurement of the magnetic pro perties of the powders could be accomplished routinely, using gram-amount of materials, the determination of the print through requires the preparation of a magnetic tape in standard conditions. This latter fact introduces severe constraints for what concerns the amount of the material used and the reproducibility of the preparation method also in that critical laboratory phase which is the development of new materials or the improvement of older ones. It has been therefore thoroughly investigated the possibility of correlating magnetic properties of the CrO2powders, such as the specific saturation magnetization (σs), the coer civity factor (Hr-Hc)/Hcpercent and the magne tization at low fields with the print through. It has been shown that a good correlation is observed when those magnetic parameters are displayed in a function of the type:P.T. = A [sigma_{s}/C.F. times M.R.] + Band even better with a function of the type:P.T. = A [(sigma_{s})^{a}/(C.F.)^{b} times (M.R.)^{c}] + Bwhere P.T. is the print through value of the magnetic tape, C.F. is the coercivity factor defined above, M.R. = magnetization at 40 Oe/ saturation magnetization and a, b and c are numerical constants which could be obtained with a computer optimization procedure.     

Phenomenological model of the magnetocaloric effect and its correlation with critical behavior near room temperature in La<Subscript>0.7</Subscript>Ca<Subscript>0.2</Subscript>Sr<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> manganite

In this paper, we investigate the field dependence of the magnetocaloric properties of La_0.7Ca_0.2Sr_0.1MnO₃ powder sample using a phenomenological model. Our compound was elaborated by the conventional solid state reaction. The model parameters were determined from the magnetization data and were used to give better fits to magnetic transition and to calculate the magnetocaloric quantities. The magnetocaloric parameters such as the maximum of the magnetic entropy change \(\Delta S_M^{max}\) and the relative cooling power RCP, have been determined from the calculation of the magnetization as a function of temperature under several magnetic applied field. Thus, from the magnetocaloric results, such as RCP?≈?b(μ₀H)^1+1/δ and T_peak???T_C ≈ b (µ₀H)^1/Δ, the critical exponents values related to the magnetic transition have been determined. The estimated results are close to those expected by the tricritical mean-field model. Furthermore, the values of the ferromagnetic transition temperature T_C, as well as the critical exponents β, γ and δ obtained by the theoretical model, are compared with those obtained by other various techniques (such as the modified Arrott plots, the Kouvel–Fisher method and the critical isotherm analysis). A good agreement has been found in the vicinity of the Curie temperature.