Al. Stancu

Study of the microstructure and of the permeability spectra of Ni–Zn–Cu ferrites

Abstract Microstructure and magnetic properties of Ni–Zn ferrite are highly sensitive to the preparation methodology, sintering conditions and to the amount of constituent metal oxides, impurities or doping levels. This paper focuses its discussion on dependence of permeability spectra on the frequency for samples of Ni–Zn–Cu ferrite sintered at different temperatures. The experimental and calculated complex permeability curves are compared.

The initial susceptibility in the FC and ZFC magnetisation processes

Abstract A new ‘blocking temperature’ model for the field-cooled and zero-field-cooled magnetisation processes for fine ferromagnetic particle systems is presented. On the basis of Stoner-Wohlfarth model and the discrete two-level model for the thermal relaxation, both blocked and superparamagnetic contributions to the sample magnetic moment are calculated. The effects of volume and easy-axis orientation distributions and also of the temperature rate are taken into account; a Lorentz mean field term is also considered. The simulation results are discussed.

Model of ferromagnetic resonance in granular magnetic solids

Micromagnetic simulations of ferromagnetic fine particle systems have been used to study the effect of various parameters on the hysteresis loops and on the ferromagnetic resonance (FMR) curves. Systematic calculations show that the effect of the dipolar interactions on such systems significantly differs from that introduced by the exchange interactions. It is important to note that the anisotropy and damping parameter also contribute to the FMR response.

Differential Preisach model for the description of dynamic magnetization processes

A differential dynamic Preisach model developed by us and introduced as an idea suggested by Bertotti is generalized. The starting point of this model is that the instantaneous excess field due to the moving and dynamic processes can be approximated as a function of M and it’s variation rate dM/dt: F(M,dM/dt). Thus, we determine a differential equation for the magnetic susceptibility. The model’s parameters are found by an original identification method. As an application we simulate a simple electrical circuit and compare the results with the experimental data. A very good agreement between the measured and simulated data is observed.

Effect of the second-order anistropy constant on the transverse susceptibility of uniaxial ferromagnets

A generalized theoretical approach of the transverse susceptibility, χT, in the case of uniaxial ferromagnets is presented. This advances the classical model of transverse susceptibility where only the first-order anisotropy constant, K1, is taken into account and makes possible the study of the influence of the second-order anisotropy constant, K2 on the χT curves. It is shown that additional Barkhausen jumps driven by higher-order anisotropy constants emerge more evidently in the field dependence of χT making their detection much easier than from the hysteresis loop. Based on the obtained results, we propose a simple method to determine both, K1 and K2 independently from χT experiments.

Static and light induced hysteresis in spin-crossover compounds: experimental data and application of Preisach-type models

In this paper we focus on the different types of hysteresis exhibited by spin transition compounds: thermal hysteresis, pressure hysteresis and light induced thermal hysteresis. We analyse the manner in which different Preisach-type models can be applied to reproduce major and minor hysteresis loops.

Simulation of magnetization curves with Preisach-Néel models

Abstract The master equation was solved in each point of the Preisach plane for the particle system associated to it in order to check the expression of the critical curves and to reveal the dynamics of the critical curves when the applied field depends on time. Using this approach, the deltaM plots as a function of the waiting time and temperature have been calculated.

Time and temperature-dependent Preisach models

Abstract The critical curves in the Preisach–Neel model are investigated using the numerical solution of master equation with time-dependent magnetic field and temperature. The movement of the thermal boundaries in the Preisach plane is investigated in different magnetization processes. It is shown that the rate variation of the applied field and temperature, which are explicit parameters in this model, can have a strong influence on the configuration of the thermal boundaries. The results are compared with those obtained with critical curve- type Preisach–Neel models using linear and nonlinear energy barrier expression.

Interaction evaluation in particulate media using the integral generalized ΔM plot

This paper presents a method of estimating the parameters of the generalized moving Preisach model by simple dc measurements. Two points on the integral generalized ΔM (IGDM) plot are used, which are very sensitive to choice of the parameters describing the interactions. The technique allows a refinement of parameters once that the coercive field distribution is approximated by using the magnetic hysteresis loop (MHL), dc magnetization curve (DCD), and the isothermal remanent magnetization curve starting from the dc demagnetized state of the system (IRMdc). The method may also be used to test the parameters provided by different identification techniques by considering the two points on the IGDM curve as check points for the simulations.

Micromagnetic study of reversible transverse susceptibility

Abstract A generalized theoretical approach of the transverse susceptibility in the case of uniaxial ferromagnets is presented. This advances the classical model of transverse susceptibility where only the first-order anisotropy constant is taken into account, and makes possible the study of the influence of the second-order anisotropy constant on the transverse susceptibility curves. The complicated switching behavior occurring in certain cases was analyzed and discussed from the dynamical point of view, comparing the results with those obtained with a Landau–Lifshitz–Gilbert approach.