[en] Cooperative and noncooperative magnetization processes in magnetic nanostructures are investigated. Using model calculations it is shown that the Preisach model and related approaches, such as Henkel, ΔM, and ΔH plots, describe magnetism on a mean-field level and cannot account for intra- and inter-granular cooperative effects. For example, the ΔM plot of a nucleation-controlled two-domain particle gives the false impression of a positive intergranular interaction. A simple but nontrivial cooperative model, consisting of two interacting but nonequivalent particles, is used to show that cooperative effects are most pronounced for narrow switching-field distributions, i.e., for nearly rectangular loops. This is unfavorable from the point of magnetic recording, where one aims at combining narrow loops with a noncooperative local switching behavior. A general rule is that the neglect of cooperative effects leads to an overestimation of the coercivity. [copyright] 2001 American Institute of Physics

[en] Macroscopic electromagnetic stationary solutions, in terms of equivalent charges of polarization and equivalent currents of magnetization, can be easily extended to points inside matter, avoiding the utilization of long mathematical developments. The concept of effective densities together with the use of the boundary conditions of electromagnetic fields, in an appropriate way, will allow us to generalize the solution obtained for external points to points inside matter, as will be proved in the present paper. (author)

[en] A numerical framework is presented for the solution of the Landau - Lifshitz - Gilbert equation of magnetization motion using a semi-implicit Crank - Nicholson integration scheme. Along with the details of both space and time domain discretizations, we report on the development of a physically based self-consistency criterion that allows for a quantitative measurement of error in dynamic micromagnetic simulations. In essence, this criterion relies in recalculating from actual magnetization motion the imposed phenomenological damping constant. Test calculations were performed with special attention paid to the determination of suitable integration time steps. [copyright] 2001 American Institute of Physics

No abstract available

No abstract available

[en] A dynamic generalization of the Stoner - Wohlfarth (SW) model is presented, in which the applied field component h_az parallel to the particle anisotropy axis is slowly varied, whereas the one in the perpendicular plane, h_a#perpendicular#, is rotated at the angular frequency ω. The Landau - Lifshitz - Gilbert equation is solved to calculate the system response under the constraint of spatially uniform magnetization at all times. Switching under slowly varying h_az is discussed. Various switching modes can occur in correspondence of different types of bifurcation present in the dynamics. In addition, it is shown that the system response can become quasiperiodic, with spontaneous magnetization oscillations at a frequency definitely lower than ω. [copyright] 2001 American Institute of Physics

[en] The magnetic relaxation of a superconducting slab in the flux creep regime has been investigated numerically in the framework of an approximate power-law dependence of the electric field E on the current density J (i.e. E^α/J/σJ). It has been shown numerically that the magnetization relaxation could be described in time as (t+t₀)^-1/^σ for a time in the range of t₀. This description was found to be valid for a short time, compared to t₀, after the start of relaxation. t₀ is a constant, which has also been analysed. Our result is useful in applications when transitory regimes are studied, and it has been extended to the case when the slab is in the flux flow regime. (author)

[en] A simple one-dimensional micromagnetic model is used to find the nucleation field and the energy barrier for the magnetization reversal at the end of a semi-infinite thin ferromagnetic cylinder. Time dependence of the thermal switching field is calculated. The intrinsic coercivity and thermal stability factor predicted by the model are in a good agreement with recent experiments on CrO₂ tapes and on individual Ni long particles. [copyright] 2001 American Institute of Physics

[en] We report the results of numerical simulations of nonisothermal dendritic flux penetration in type-II superconductors. We propose a generic mechanism of dynamic branching of a propagating hot spot of a flux flow/normal state triggered by a local heat pulse. The branching occurs when the flux hot spot reflects from inhomogeneities or the boundary on which magnetization currents either vanish, or change direction. The hot spot then undergoes a cascade of successive splittings, giving rise to a dissipative dendritic-type flux structure. This dynamic state eventually cools down, turning into a frozen multifilamentary pattern of magnetization currents

[en] We have carried out a systematic magnetic relaxation study, measured after applying and switching off a 5 T magnetic field to polycrystalline samples of La_0.5Ca_0.5MnO₃ and Nd_0.5Sr_0.5MnO₃. The long time logarithmic relaxation rate (LTLRR), decreased from 10 to 150 K and increased from 150 to 195 K in La_0.5Ca_0.5MnO₃. This change in behavior was found to be related to the complete suppression of the antiferromagnetic phase above 150 K and in the presence of a 5 T magnetic field. At 195 K, the magnetization first decreased, and after a few minutes increased slowly as a function of time. Moreover, between 200 and 245 K, the magnetization increased throughout the measured time span. The change in the slope of the curves, from negative to positive at about 200 K was found to be related to the suppression of antiferromagnetic fluctuations in small magnetic fields. A similar temperature dependence of the LTLRR was found for the Nd_0.5Sr_0.5MnO₃ sample. However, the temperature where the LTLRR reached the minimum in Nd_0.5Sr_0.5MnO₃ was lower than that of La_0.5Ca_0.5MnO₃. This result agrees with the stronger ferromagnetic interactions that exist in Nd_0.5Sr_0.5MnO₃ in comparison to La_0.5Ca_0.5MnO₃. The above measurements suggested that the general temperature dependence of the LTLRR and the underlying physics were mainly independent of the particular charge ordering system considered. All relaxation curves could be fitted using a logarithmic law at long times. This slow relaxation was attributed to the coexistence of ferromagnetic and antiferromagnetic interactions between Mn ions, which produced a distribution of energy barriers