Search

Abstract Layered 3d‐orbital ferromagnet is an ideal research platform to experimentally achieve intrinsic 2D ferromagnetism and theoretically study the quantum nature of magnetic exchange interactions therein. A variety of magnetic phases can emerge from the strongly correlated feature of 3d‐orbital electrons, in which their exchange interactions can be effectively modulated by various kinds of external stimuli. Therefore, controlling the emergent magnetic phenomena of layered 3d‐orbital ferromagnets is significant in both fundamental science and practical applications. Considering the roles of magnetic exchange interactions, this review summarizes recent progress in controlling the emergent magnetic properties of layered 3d‐orbital ferromagnets by systematically introducing modulation methods, underlying mechanisms, and device applications. The existing challenges and future prospects for this research field are also outlined, shedding light on finding optimized magnetic materials, exploring powerful modulation techniques, and designing multifunctional new concept devices.

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]

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.

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]

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.

In this study, size controlled FeCo colloidal magnetic nanoalloys in the range of 11.5–37.2 nm were synthesized by surfactant assistant ball milling method. Magnetic separation technique was performed subsequent to synthesis process so as to obtain magnetic nanoalloy fluid with narrow size distribution. Particle distribution was determined by transmission electron microscope (TEM) while X-ray diffraction (XRD) measurements verified FeCo alloy formation as BCC structure. Vibrating sample magnetometer (VSM) method was used to investigate magnetic properties of nanoalloys. Maximum saturation magnetization and maximum coercivity were obtained as 172 Am 2 /kg for nanoparticles with the mean size of 37.2 nm and 19.4 mT for nanoparticles with the mean size of 13.3 nm, respectively. The heating ability of FeCo magnetic nanoalloys was determined through calorimetrical measurements for magnetic fluid hyperthermia (MFH) applications. Heat generation mechanisms were investigated by using linear response theory and Stoner-Wohlfarth (S-W) model. Specific absorption rate (SAR) values were obtained in the range of 2–15 W/g for magnetic field frequency of 171 kHz and magnetic field strength in between 6 and 14 mT. [ABSTRACT FROM AUTHOR]

In this work we present a faster modified protocol for first order reversal curve (FORC) measurements. The main idea of this procedure is to use the information of the ascending and descending branches constructed through successive sweeps of magnetic field. The new method reduces the number of field sweeps to almost one half as compared to the traditional method. The length of each branch is reduced faster than in the usual FORC protocol. The new method implies not only a new measurement protocol but also a new recipe for the previous treatment of the data. After of these pre-processing, the FORC diagram can be obtained by the conventional methods. In the present work we show that the new FORC procedure leads to results identical to the conventional method if the system under study follows the Stoner-Wohlfarth model with interactions that do not depend of the magnetic state (up or down) of the entities, as in the Preisach model. More specifically, if the coercive and interactions fields are not correlated, and the hysteresis loops have a square shape. Some numerical examples show the comparison between the usual FORC procedure and the propose one. We also discuss that it is possible to find some differences in the case of real systems, due to the magnetic interactions. There is no reason to prefer one FORC method over the other from the point of view of the information to be obtained. On the contrary, the use of both methods could open doors for a more accurate and deep analysis. [ABSTRACT FROM AUTHOR]

A model for the off-axis vector magnetization of a distribution of uniaxial particles is presented. Recent work by the authors decomposed the magnetization into two components and modeled the total vector magnetization as their vector sum. In this paper, to account for anisotropy, the direction of the reversible magnetization component is specified by the vector sum of the applied field and an effective anisotropy field. The formulation of the new anisotropy field (AF) model is derived and its results are discussed considering (i) oscillation and rotational modes, (ii) lag angle, and (iii) unitary magnetization. The advantages of the AF model are outlined by comparing its results to the results of the classical Stoner–Wohlfarth model. [ABSTRACT FROM AUTHOR]

Top panel: Inhomogeneity of magnetic field during the sample deposition (a) leads to random fluctuations of the angle between the easy axis and magnetic field during the coercivity measurements (b). Bottom panel: A Stoner-Wohlfarth based approach is applied to estimate the angle and the low-temperature coercivity from the reference values at RT (c). This diminishes the error in measured coercivity, caused by the easy axis direction fluctuations (d). [Display omitted] • Sputtering magnetic field inhomogeneity tilts anisotropy axes in a thin film. • Tilting the axes in respect to field affects coercivity in exchange biased films. • Inclination in 2–3° causes 50% error in measured coercivity H C. • A two-step diagram method is proposed to assess the easy axis H C. • The proposed recalculation diminishes the experimental error in H C. Exact measurement of coercivity in thin magnetic films is of high importance for sensor industry. In exchange biased multilayers, incorporated in magnetoresistive sensors, coercivity of the coupled ferromagnetic layers strongly depends on the orientation of easy axis in respect to field. We show, that in NiFe/IrMn films with high H B /H C ratio, field misalignment ∼2° results in ∼50 % change in coercivity. Inhomogeneous magnetic field in a film deposition device as well as manual installation of the sample in magnetometers result in large error in coercivity. In this work, we propose measurement and analysis protocol to avoid this uncertainty and to evaluate easy axis coercivity values from tilted angle measurements in a wide temperature range using just one reference value of easy axis coercive field, measured at room temperature. [ABSTRACT FROM AUTHOR]

In the present study, the magnetization reversal of 2-D rectangular array of soft magnetic permalloy thin truncated conical disks is simulated by using micromagnetic modeling. The base radius (R) of the conical disk is varied between 20 – 100 nm and upper radius (r) varied between 10 nm to R − 10 nm such that the ratio σ = r / R lies between 0 to 1. The analysis of angular dependence (θ = 0 ° to 90°) of hysteresis loops for the case of R = 100 nm is done in particular and its comparison with Stoner–Wohlfarth (SW) model reveals the presence of a bi-axial anisotropy which is attributed to the dipolar interaction between the nanodisks in the array. When the nanodisks are close to regular cylindrical shape (for σ ∼ 0. 9), the orthogonal axes of the facet of the array acts as magnetic easy axes. As the nanodisks are tapered to σ ∼ 0. 5 and 0.1, the same set of axes turns as hard axis of the bi-axial anisotropic system. Corresponding bi-axial anisotropy constant is estimated from the coercivity of the hysteresis loop along the easy axis as K e f f = 13. 508 kJ / m 3 , − 5. 403 kJ / m 3 and − 4. 322 kJ / m 3 respectively for σ = 0. 9 , 0. 5 and 0.1 with R = 100 nm. In addition to this, an analytical expression is also formulated to calculate the total demagnetization energy of the 2-D array as a function of the magnetization orientation. This demagnetization energy is fitted with the effective bi-axial anisotropy energy expression considering an angular dispersion of the bi-axial anisotropy. The fitting suggests that the extent of angular dispersion of bi-axial anisotropy reduces as the conical disks are tapered (σ decreased). Irrespective of the magnetic field orientation, the remanent state exhibits a s-state of spatial magnetization profile when σ is large whereas a buckled magnetization state symmetric to body diagonal is exhibited for lower σ. • Micromagnetic simulation of hysteresis loop in 2D array of conical nanodisks. • Effective biaxial anisotropy arising due to dipolar interaction in the array. • Spin rotation is the dominant contribution to magnetization reversal. • Fitting of demagnetization energy with biaxial anisotropy model is done. • Fitting indicates angular dispersion of biaxial anisotropy axes. [ABSTRACT FROM AUTHOR]

The Stoner–Wohlfarth (SW) model provides an efficient analytical model to describe the behavior of magnetic layers within magnetoresistive sensors. Combined with a proper description of magneto-resistivity an efficient device model can be derived, which is necessary for an optimal electric circuit design. Parameters of the model are determined by global optimization of an application specific cost function which contains measured resistances for different applied fields. Several application cases are examined and used for validation of the device model. [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]

Room-temperature magnetic properties of planar nanowire arrays of Fe have been studied using comprehensive analysis of FMR and magnetometry data. It has been shown that the Fe NWs are ferromagnetic at room temperature and their magnetic properties are mainly governed by shape anisotropy. Combining parameters derived from the FMR study with experimental data of magnetometry, simulations of hysteresis loops have been performed based on Stoner–Wohlfarth approach. Calculations show that magnetisation reversal of Fe NWs has a coherent-rotation type of localised character. As NW thickness decreases, localisation of the magnetisation reversal mode is found to enhance due to increased inhomogeneity of thinner NWs. [ABSTRACT FROM AUTHOR]

Abstract: In this work we present a comparative study of the classical Stoner Wohlfarth model and the analytical vector hysteron model. We analyze the hysteretic behavior description when the thermal fluctuations are included. We found a different behavior in the evolution of the coercive field as function of temperature for the vector hysteron model when the anisotropy easy axis is non-parallel to the magnetic field orientation. At low temperatures a plateau is observed as consequence of the switching field behavior. At high temperature both models give identical description of the magnetization behavior, which merges with the superparamagnetic approximation. [Copyright &y& Elsevier]

Abstract: The microstructure and magnetic properties of 2:17 type isotropic magnets were investigated. The slow cooling heat treatment (cooling at 1°C/min from 820 to 400°C, and isothermal treatment during 24h) was interrupted after the temperatures of 820, 700, 600 and 500°C and their hysteresis were measured with fields up to 9T. The fully heat treated sample presented coercivity (μ0H) of 3.32T, after 24h at 400°C. The microstructure was investigated with SEM–FEG (Scanning Electron Microscope with Field Emission Gun) and X-ray Diffraction Rietveld analysis. The application of the Stoner–Wohlfarth–Callen–Liu–Cullen (SW–CLC) model points out exchange coupling between ferromagnetic Sm2(CoFe)17 nanocells and ferromagnetic Sm(CoCu)5 present at the cell boundary phase. The results are interpreted with the double shell model: first-a cobalt-rich ferromagnetic Sm(CoCu)5 shell originates exchange coupling and second-a copper-rich paramagnetic Sm(CuCo)5 shell produces magnetic decoupling. This double shell helps to maximize coercivity and remanence. The anisotropy field of the Sm2(CoFe)17 cell phase was estimated in 7T with the SW–CLC model. [Copyright &y& Elsevier]

Abstract: The magnetic response of nanocomposites formed by non-interacting well dispersed γ-Fe2O3 nanoparticles in a polymer matrix is presented. Various low loading fraction of particles in polymer leads to an observation of similar values of blocking temperatures and coercive fields. ac response confirms that particles are non-interacting and follow Neel–Brown model. Effect of particle size distribution on hysteresis behavior and saturation magnetization as a function of temperature is discussed. Since particles have a size distribution, the experimental results of magnetic response are compared with simulations based on Stoner–Wohlfarth model of single size particles. We have devised a measurement method in which a constant magnetic field was applied while the thermal energy is varied by sequentially heating and cooling the sample below the blocking temperature. Nanoparticle–polymer composites show reversible magnetization behavior for sequential heating/cooling cycles. However, simulation based on single size particle system shows irreversible magnetization behavior during the heating and cooling cycles. These observations are qualitatively explained in terms of different behavior of magnetization as a function of temperature for smaller superparamagnetic particles and larger blocked particles below overall blocking temperature of the composite. [Copyright &y& Elsevier]

Abstract: A comparative analysis between linear and non-linear energy barriers used for modeling statistical thermally-excited ferromagnetic systems is presented. The linear energy barrier is obtained by new symmetry considerations about the anisotropy energy and the link with the non-linear energy barrier is also presented. For a relevant analysis we compare the effects of linear and non-linear energy barriers implemented in two different models: Preisach–Néel and Ising–Metropolis. The differences between energy barriers which are reflected in different coercive field dependence of the temperature are also presented. [Copyright &y& Elsevier]

Abstract: Magnetic hysteresis curves of bulk amorphous ferromagnet alloys of composition Nd60Fe30Al10, Nd60Fe20Co10Al10 and Pr58Fe24Al18 have been measured in applied magnetic fields up to 9T at temperatures in the range 10–350K. The behaviour of the demagnetisation curve in the first quadrant is interpreted using a mean field interaction model as proposed by Callen et al. [Phys. Rev. B 16 (1977) 263], which extends the Stoner–Wohlfarth model [Philos. Trans. Roy. Soc. A 240 (1948) 599] for a random distribution of non-interacting uniaxial grains. Application of the mean field interaction model enables the determination of the saturation magnetisation M s, anisotropy field H a, and interaction parameter d, and from these other magnetic parameters, such as the anisotropy constant, K, are deduced. For the three alloys, the temperature dependent behaviour of M s, H a, d and K over the range 20–350K are found to be qualitatively similar, though there are quantitative differences. In all cases M s increases with decreasing temperature, both H a and K increase with decreasing temperature, reaching a peak in the range 75–120K, and then decreasing, and d decreases approximately linearly as the temperature decreases. The physical mechanisms responsible for coercivity in these materials are discussed in the context of random anisotropy and a strong pinning model of domain walls. [Copyright &y& Elsevier]

Abstract: The analytical way to describe uniaxial single-domain particles, using new symmetry considerations on the magnetic anisotropy, as a vector hysteresis unit (vector hysteron), is presented. The main characteristics of the vector hysteron such as the longitudinal hysteresis loops, transversal hysteresis loops and rotational hysteresis loops are presented. An extension of the vector hysteron and a vector hysteron that can be applied to the ferroelectric hysteresis are also presented. [ABSTRACT FROM AUTHOR]

Abstract: Combination of magneto-optical (MO) vector magnetometry and magneto-optical Kerr microscopy is used to investigate the surface magnetic properties of amorphous CoFeCrSiB ribbons. Strongly inhomogeneous magnetic behavior of ribbons in as-quenched state is improved by field-annealing and stress-annealing processes that induce weak uniaxial longitudinal and transverse anisotropy. It was shown that values of coercive and anisotropy field increase with increasing annealing temperature. Inclination of easy axis from the ribbon axis is estimated by comparing the measured surface hysteresis loops with the Stoner–Wohlfarth model, and is supported also by the Kerr microscopy. Method with the current flowing through the ribbon is proposed for magnetic domains observations. [Copyright &y& Elsevier]

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]

Abstract: Small-angle neutron scattering (SANS) experiments on nanostructured Fe reveal grain-size-dependent magnetic correlations with a minimal correlation length for grain sizes on the order of the bulk domain-wall width. To investigate the evolution of these correlations during the magnetization process, we performed SANS experiments in external fields of various strengths. In intermediate fields, we find anisotropic scattering profiles with an unusual intensity enhancement for scattering vectors parallel to the field direction. These observations are compared with a modeled granular microstructure containing magnetic domains of arbitrary size and orientation, demonstrating that magnetic domains extend over several grains and have a magnetization that is tilted considerably away from the external field direction. Since the domain size does not change significantly with the magnitude of the external field, we conclude that the magnetization process does not proceed via domain-wall motion. Rather, our SANS data suggests that the magnetization process proceeds by simultaneous reversal of a few adjacent domains, presumably in the form of small avalanches. The latter supposition is supported by theoretical arguments showing the existence of marginally stable domains within the random-anisotropy model. [Copyright &y& Elsevier]

Reversible and irreversible magnetization processes of the single-domain elliptical permalloy dot are studied by simulation. The magnetization curve of the reversible process obtained by applying the field along the hard axis of the elliptical dot is almost linear, as is the case in the single-domain ellipsoidal particle. Due to the occurrence of the non uniform reversal, the switching field is reduced significantly compared to that predicted by the Stoner–Wohlfarth model. The nucleation fields of elliptical dots are calculated as a function of ellipse''s aspect ratio, and the function reveals a step-like behavior. [Copyright &y& Elsevier]

We review some recent results beyond the now established theory of magnetization switching of a nanoparticle within the single-spin approximation. The first extension is that of the Stoner–Wohlfarth model for magnetization static switching under applied magnetic field including the effect of temperature at long-time scales. The second concerns a generalization of the Néel–Brown model for thermoactivated dynamic magnetization switching to include the effect of exchange interaction in the framework of Langer''s theory in the intermediate-to-high damping limit. We finally argue why the single-spin approximation is not appropriate for very small nanoparticles. [Copyright &y& Elsevier]

A modified version of the Stoner–Wohlfarth model, which takes into account thermal effects, has been used to study the influence of measuring temperature on the magnetic anisotropy derived from demagnetization curves. To be able to reproduce the thermal dependence of the magnetization curve, and not only of coercivity, a Neel relaxation approach is considered, avoiding the two-state approximation. Anisotropy results are compared with calculations for field and temperature dependent torque magnetometry, indicating that the most reliable parameter in the apparent distribution of anisotropy is the mean value of the anisotropy field. [Copyright &y& Elsevier]

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]

The magnetisation reversal of the free layer of two different spin-valve (SV) films with crossed anisotropy was studied using Lorentz microscopy in a modified transmission electron microscope. Both SVs were of similar construction, the principal difference being the replacement of Co by CoFe in the layers surrounding the non-magnetic spacer. Marked differences in the reversal mode were apparent. For CoFe SVs reversal was usually by rotation and experimental results were in good agreement with those predicted by a modified Stoner–Wohlfarth model. By contrast, reversal in Co SVs tended to involve complex domain processes with a high degree of irreversibility. The differences are attributed to the much higher magnetostriction constant of Co which gives rise to substantial local property variation. [Copyright &y& Elsevier]

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]

• Nd-Fe-B hot-deformed magnets were produced by low pressure spark plasma sintering (SPS). • Low pressure SPS gave comparable magnetic properties than the conventional process. • Cone-like shape of the magnet was observed in the initial stages for the first time. • The experiments were perfectly interpreted in the frame of Stoner-Wohlfarth model. We have produced hot-deformed Nd-Fe-B magnets from commercial (MQU-F) Nd-Fe-B ribbons. The spark-plasma-sintering technique was used to deform the samples under low pressures of 40 MPa. The initial stages of the hot-deformation process were investigated in terms of microstructures and magnetic properties. Hot-deformed magnets with different deformation ratios were produced and the dependence of the remanence on the deformation ratio was determined. In the initial stages of the hot-deformation process, a cone-like shape of the magnet was observed for the first time. The experimental data can be qualitatively interpreted by applying the basic Stoner–Wohlfarth model. [ABSTRACT FROM AUTHOR]

Abstract: We report on a method to orient the easy axes of ferromagnetic nanoclusters during their deposition, with the use of a magnetic field supplied by a set of permanent magnets. This field is applied close to the substrate where the preformed clusters are deposited. The efficiency is demonstrated on an assembly of 4nm Co clusters produced by a sputtering and gas-condensation technique. The magnetization of the deposited clusters measured at 6K exhibits an overall in-plane alignment of the easy axes in the direction of the field applied during deposition, H D . Using a Stoner–Wohlfarth model, it is estimated that half of the clusters easy axes lie within from H D . [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]

Abstract: Experimental data of bicrystal grain-boundary magnetoresistance hysteresis shows that the resistance at remanence after saturation depends on whether the magnetic field was applied parallel or perpendicular to the grain boundary. In the latter case the behavior can be described by the Stoner–Wohlfarth magnetization reversal model for coherent rotation of the magnetization, which leads to an alignment along the easy magnetization directions of the bicrystal in zero field. In the parallel configuration the magnetization directions of the two grains remain parallel in zero field. We have used this information to deduce an effective spin polarization of % of the grain boundary in a bicrystal of . [Copyright &y& Elsevier]

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]

The magnetoresistance (MR) curves during the rotation of magnetic field in NiO spin-valve are well described by taking into account the involved magnetization process of free and pinned layers according to rotating field strength. In particular, hysteretic MR characteristics pronounced in a field strength of 1.5 times the exchange field are ascribed for by the viscosity effect on magnetization rotation of pinned layer. These analyses of MR curves provide a basis decomposing the MR components from each magnetization process of free and pinned layers. [Copyright &y& Elsevier]