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In the present work is present an alternative to calculate, by analytical considerations, the angular dependence of coercivity for an extensive (infinite) array of ellipsoid chains. Ellipsoids of Ni with semi-axis a = 15 nm and c = εa, were the basis of the present study with ε = 1, 2, 4, 8, 16, and 32. The center to center ellipsoid's distance (into the chain) was r = 2c, and the coercivity was calculated for different values of ε and d as a function of the angle of application of external field, measured from the chain axis. The energetic contributions, for calculations, were the dipolar and Zeeman. Combinations of the chain's packing factor with different ellipsoid's aspect ratios were used for the calculation of anisotropy field, HA. A direct calculation of inter-ellipsoid interaction was performed assuming uniform reversion by using the Stoner-Wohlfarth model. The dipolar contribution corresponding to the inter-chain interaction was estimated according to experimental results obtained for Ni nanowires arrays with hexagonal spatial distribution. In order to evaluate the inter-chain interaction the distance (d) was set at 55, 60, and 65 nm. The combination of these geometric parameters results in great modifications of coercivity values. With some limitations, the work here is important for comparative studies with experimental results. The model results reproduce those obtained by micromagnetic simulation reported in previous works. [ABSTRACT FROM AUTHOR]

This paper reviews my research on the role of domain structures and domain wall motion, especially, in anisotropic sintered permanent magnets. The existence of domain wall motion in Nd–Fe–B sintered magnets during the intermediate stages of the demagnetization process has been revealed experimentally using the step method, susceptibility measurements of magnetically demagnetized samples, and direct Hall probe sensor measurements of flux densities in holes drilled in magnets. This evidence of multi-domain regions during demagnetization has shown that the magnetically reversed regions should be larger than several micrometers. To explain these experimental results, the critical size of the magnetic reversal center, which is a nanometer-sized nucleus, and the explosive growth of the nucleus to micrometer-sized reversed regions with volumes 105 –106 times larger were discussed based on classical equations. Accordingly, the Stoner–Wohlfarth model, Kronmüller’s equation modified by considering the surface domain wall of the nucleus, and Becker’s model of growth of the reversal center were discussed. The phenomenological Ginzburg–Landau free energy curves were also determined for each case of magnetic reversal in the single-domain grain and in the saturable multi-domain grains in conjunction with the energy equations. [ABSTRACT FROM AUTHOR]

The variation in the magnetoresistance of a spin-valve sensor during its linear movement in an inhomogeneous field of a magnetic label at different orientations of the magnetic moment of the spin-valve structure is considered. It is shown that the shape of the sensor signal and its value critically depend on the initial orientation of the magnetic moment and the trajectory of the sensor in the field of the magnetic label. [ABSTRACT FROM AUTHOR]

A new model to describe magnetization of a magnetoactive elastomer (MAE) filled with magnetically hard powder is proposed. The magnetization process is treated as a result of a joint contribution of an assembly of single‐domain particles each of which dwells inside its own elastomer cavity. Magnetization of such a particle is a complex magnetomechanical process that combines intrinsic magnetization reversal and particle mechanical rotation. Possible slippage of the particles, in the course of which they disconnect from the matrix and, on their move, rub against the cavity wall, is explicitly taken into account. The influence of the slippage effect on the macroscopic magnetization of the composite is analyzed by means of computer simulation. Occurrence of asymmetrical magnetization loops of single‐domain particles located in an elastic medium is demonstrated. It is shown that by allowing for the slippage, it becomes possible to explain the experimentally observed anomalously low but non‐zero half‐width (coercivity) of the hysteresis loops of the MAE samples. [ABSTRACT FROM AUTHOR]

Periodic systems of magnetic nanoparticles are now of interest as they support GHz spin waves. Their equilibrium configurations, switchable with the external magnetic field, are crucial for such applications. We study infinite and finite chains of particles of two shapes (i) ellipsoidal and (ii) rectangular stripes with long axes perpendicular to the chain axis. A variable magnetic field is applied parallel to the long axes. Micromagnetic simulations are compared with the corresponding discrete spin models (Stoner-Wohlfarth model, S-W). An antiferromagnetic configuration is the ground state for all the systems at vanishing field but a ferromagnetic configuration occurs when the field is strong enough. The switching of the infinite chains to the reversed ferromagnetic configuration proceeds directly for the ellipsoids and by an intermediate configuration, in which the magnetization within the particle is non-uniform, in the case of the stripes. The non-uniform configurations are well represented by tilted states in S-W model. Important differences are found in the finite analogs: the switching of ellipsoids becomes multistage and starts from the innermost particles relatively well reproduced with S-W model, whereas the reversal of the stripes, starts from the outermost particles and has no analog in S-W model. Practical consequences of the findings are discussed. [ABSTRACT FROM AUTHOR]

A Stoner–Wohlfarth-type model is used to demonstrate the effect of the buildup of magnetic charges near the grain boundaries of low anisotropy polycrystalline materials, revealed by measuring the magnetization during positive-field warming after negative-field cooling. The remnant magnetization after negative-field cooling has two different contributions. The temperature-dependent component is modeled as an assembly of particles with thermal relaxation. The temperature-independent component is modeled as an assembly of particles overcoming variable phenomenological energy barriers corresponding to the change in susceptibility when the anisotropy constant changes its sign. The model is applicable to soft-magnetic materials where the buildup of the magnetic charges near the grain boundaries creates demagnetizing fields opposing, and comparable in magnitude to, the anisotropy field. The results of the model are in qualitative agreement with published data revealing the magneto-thermal characteristics of polycrystalline gadolinium. [ABSTRACT FROM AUTHOR]

The barium and strontium hexaferrites (BaM and SrM) were successfully prepared by the standard solid-state reaction method. To inhibit crystal growth, 1 wt % BO was added into the initial mixture. The structural and magnetic properties of pure hexaferrites were compared with the samples prepared with boron addition. Powders were annealed at temperatures between 800 and 1200 C. The crystal structure, morphology, and magnetic properties of hexaferrites were investigated with X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). BaM was synthesized at temperatures as low as 800 C with boron addition having saturation magnetization of ∼50 emu/g and coercivity of ∼3 kOe. At the same temperature, pure sample has very low magnetization about 1.3 emu/g. Moreover, at higher sintering temperatures, boron-containing samples have higher magnetization values compared to pure BaM. The optimal Fe/Ba ratio was determined as 10.5 for boron-containing samples, while it is 11.5 for the pure one. The coercivity started to decrease at 1100 C indicating that single to multi-domain transition occurs. The saturation magnetization values are nearly equal in pure- and boron-containing SrM samples in the whole temperature range. However, maximal coercivities occurred at different temperatures for boron-containing and the pure samples as 1000 and 1100 C, respectively. Magnetic interactions deduced from the Stoner-Wohlfarth model using the isothermal magnetization and demagnetization measurements showed that boron addition suppresses the destructive (demagnetizing-like) interactions among domains in both SrM and BaM samples, and thus, stabilizes the remanence. [ABSTRACT FROM AUTHOR]

In this paper, a new method to calculate the critical points (CPs) for a symmetric synthetic antiferromagnet, using the Stoner-Wohlfarth model for each layer is proposed. For given initial conditions, the system has multiple equilibrium states, and consequently, there are multiple energy barriers (EBs), so it is rather difficult to estimate the actual EBs for the real switches. Using the developed method, one can determine all the minimum energy points as well as the saddle points (SPs) based on the solutions of polynomial equations associated to the critical points conditions. These are necessary conditions to estimate the EB. Defining the EB as the difference between the lowest SP and the highest energy minimum for an applied field, one obtains for the lowest values of the EB the well-known critical curves. The plot of the EBs in the field planes is compared with the graphs of the critical curves and the results are discussed. [ABSTRACT FROM AUTHOR]

Magnetic properties of two FeC samples with different amounts of carbon have been studied. In both cases, the amount of carbon was well above the mass sufficient to transform nanocrystalline iron into iron carbide (cementite). Through the dc magnetic and transmission electron microscopy (TEM) measurements it was shown that cementite nanoparticles formed agglomerates; the size distribution of these nanoparticles was very wide, and superparamagnetic-like behaviour was not observed even at room temperature. [ABSTRACT FROM AUTHOR]

Based on a qualitative study of the Stoner-Wohlfarth model, we point out that driving a magnetic tunnel junction (MTJ) with an alternative two-dimensional magnetic field allows to measure simultaneously two components of an external magnetic field. Only one single MTJ without a pinning layer is needed to measure both components of a magnetic field parallel to the junction plane. The response of the magnetometer does not depend on the resistance of the junction or the amplitude of its variations. A prototype has been manufactured and encouraging experimental results are presented. Sensitivities higher than 500 V/T and a noise level of 2 μT/√Hz are reported. [ABSTRACT FROM PUBLISHER]

Magnetic field sensors based on the giant magnetoresistance (GMR) effect have a number of practical current and future applications. We report on a modeling of the magnetoresistive response of moving spin-valve (SV) GMR sensors combined in certain cluster networks to an inhomogeneous magnetic field of a label. We predicted a large variety of sensor responses dependent on the number of sensors in the cluster, their types of interconnections, the orientation of the cluster, and the trajectory of sensor motion relative to the label. The model included a specific shape of the label, producing an inhomogeneous magnetic field. The results can be used for the optimal design of positioning devices. [ABSTRACT FROM AUTHOR]

Angular dependence of Hex and Hc, and the critical angle behavior are measured in CoFe/MnIr bilayers annealed at 200°C and 340°C. The interfacial exchange coupling anisotropy and the antiferromagnet anisotropy at tAF < tAFcare estimated from the best fitting of angular dependence of Hex and Hc using Stoner-Wohlfarth (S-W) model. These results confirm existence of interfacial exchange coupling anisotropy between F and AF layers for tAF < tAFc. The measured critical angles for tAF > tAFc as well as for tAF < tAFc are well explained using S-W model. [ABSTRACT FROM AUTHOR]

We use the micromagnetic simulation to study the reversible and irreversible magnetization processes of the single- domain elliptical permalloy particles. The magnetization curve of the reversible process, which has no hysteresis, obtained by applying the field along the hard axis of the elliptical particle is almost linear, as is the case in the single-domain ellipsoidal particle. On the other hand, due to the occurrence of the nonuniform reversal, the switching field of the irreversible process, which has hysteresis, is reduced significantly compared to that predicted by the Stoner-Wohlfarth model. The nucleation fields of elliptical particles are calculated as a function of ellipse's aspect ratio, and the function reveals a step-like behavior. [ABSTRACT FROM AUTHOR]