[en] Multidimensional energy barriers have been evaluated for periodic hexagonal array and non-regular realistic arrangement of FePt and Co 4 nm particles. We have shown that the magnetostatic interactions broaden the energy barrier distribution and displace its center to larger values. The FePt particles have been shown to possess an extremely narrow energy barrier distribution comparatively to that of the Co particles. The validity of the widely used Pfeiffer one-particle approximation is discussed

[en] We present here the properties of the generalized and the integral generalized ΔM plots which are used to build an identification algorithm for the Generalized Moving Preisach Model. We also simulated systems of interacting particles with magnetic moments dynamics described by the Landau-Lifshitz-Gilbert equation. In each equilibrium state one studies the statistics of the interactions and other relevant parameters. These parameters are compared with the ones provided by the identification algorithm

[en] In this paper, the behaviour of the fluctuation of the fluctuation field (H_f) and its relationship with the activation energy for reversal in magnetic systems is examined. At a constant level of magnetisation, the link between thermal energy and field induced changes in the magnetisation of the system using different forms of the equation of state are found to give the same value of H_f. Accordingly, the former S/χ_irr and the ∂H/∂ln(M_irr)|_Mirr techniques for determining H_f are found to be consistent so long as the analysis of H_f is made at the same constant level of irreversible magnetisation (M_irr). Both techniques show that the fluctuation field can be determined using H_f=ΔH/Δln(t)|_Mirr which is more accessible to experiment. Hence, one can define H_f as the change in field required to maintain the magnetisation of the system unchanged during a time interval of Δln(t)=1. The behaviour of the fluctuation field in systems that contain a distribution of activation energies (f(ΔE)) is found to be governed by the nature of f(ΔE). For the case where this distribution arises from distributions of both particle volumes and anisotropy fields, the variation of H_f with M_irr is predicted to be approximately constant when both distributions have comparable standard deviations. Also H_f is predicted to be constant with M_irr is the case where both distributions become extremely narrow (i.e. the case of a single activation energy). Hence H_f remaining constant with M_irr cannot be used to show that the activation energy in the system is constant. In addition if the system contains a single activation energy, a constant value of H_f cannot be used to distinguish between reversal mechanisms such as coherent rotation or weak domain wall pinning

[en] Numerical studies of magnetisation fluctuations in a linear chain of spherical particles are carried out using the Langevin dynamic (LD) approach, and compared directly to the predictions of spin-wave theory. The results enable the identification of the magnetisation fluctuations in terms of exchange and magnetostatic spin-wave modes. Excellent quantitative agreement between the numerical and analytical results is obtained, suggesting that the LD approach correctly excites the normal modes (spin waves) of the chain

[en] Measurements are presented of the frequency-dependent (10 MHz-18 GHz) complex magnetic susceptibility of colloidal suspensions of different concentrations of nano-particles of magnetite in a hydrocarbon. Studies of three colloids, fluids, 1, 2 and 3, with packing fractions of magnetite particles of 0.22, 0.1 and 0.05, were made at different magnetic polarising fields in the range 0-100 kA m^-1. It is shown that the fluid with the highest packing fraction exhibits the highest resonant frequency. Using a local field approximation it has been shown that this observation can be explained by simply considering magnetic interactions between individual particles in the fluid. It is not necessary to invoke the presence of clustering as a function of increased packing fraction

[en] Magnetisation reversal in an exchange biased ferromagnetic/antiferromagnetic bilayer is extremely complex due to the coupling between the spins at the interface. Particular importance should be paid to the issue of thermally activated and hence time dependent reversal of the antiferromagnet during the reversal of the adjacent ferromagnet. This reversal of the antiferromagnet is one mechanism that can lead to an increased coercivity of the ferromagnetic layer. Waiting time experiments on two different systems, IrMn(X A)/NiFeCo(100 A) and a NiFe(100 A)/NiMn(250 A) bilayer, have shown that the degree of reversal in the antiferromagnet is highly dependent upon the particular antiferromagnet used. Measurements of bilayers with different thicknesses of the antiferromagnet show that the reversal rate is dependent upon the thickness. It is difficult to deduce the particular mechanism for reversal as the data is consistent both with a reversal by coherent rotation of grains and also by the formation of helical domains. The reversal is best viewed as a thermally activated process over a distribution of energy barriers, the particulars of which are determined by the morphology of the interface and the mechanism(s) for reversal. Further waiting time experiments show that this thermally activated reversal of the AFM is not the only mechanism determining the coercivity and that other mechanisms such as spin-flop coupling at a partially compensated interface may also be important

[en] A computational approach is presented which allows the simulation of time-dependent magnetisation processes over long time scales. The method is based on Monte Carlo methods and allows continuous magnetization changes, thereby allowing cooperative reversal mechanisms, whilst retaining a quantified time step. The method is applied to studies of the decay of information in written bits

[en] The question of how to introduce thermal fluctuations in the equation of motion of a magnetic system is addressed. Using the approach of the fluctuation-dissipation theorem we calculate the properties of the noise for both, the fluctuating field and the additive fluctuating torque (force) representation. In contrast to earlier calculations we consider the general case of a system of interacting magnetic moments. We show that the interactions do not result in any correlations of thermal fluctuations in the field representation and that the same widely used formula can be used in the most general case. We further prove that close to the equilibrium where the fluctuation-dissipation theorem is valid, both, field and additive torque (force) representations coincide, being different far away from it. We also show that the uncorrelated character of the noise is due to the form of the Landau-Lifshitz (or Gilbert) damping and under different damping formalisms, the normal mode analysis is proper

[en] Magnetic viscosity and irreversible susceptibility of a sample of anisotropic sintered NdFeB have been measured using a vibrating sample magnetometer. The fluctuation field as a function of local field is found to decrease with increasing demagnetizing field with a dip at the coercivity. This behavior is compared with an existing computer model based on a non-interacting system of fine, uniaxial, randomly oriented particles

[en] The coercivity H_c and fluctuation field H_f are intrinsic properties of any magnetic system exhibiting hysteresis. They are measures of the energy barriers resisting magnetization reversal and may be used to probe the nature of the magnetization processes. The variation of coercivity as a function of temperature and of the fluctuation field as a function of field and temperature have been studied for sintered and melt-spun NdFeB. The fluctuation field has been found to vary with field in the sintered material indicating that interactions play an important role in the nucleation of reverse domains while it remained constant in the melt-spun sample. The relationship between the coercivity and fluctuation field has been studied and found to be consistent with the Barbier plot though some deviation from the expected curve was found