Dorin Cimpoesu

Micromagnetic and Preisach analysis of the First Order Reversal Curves (FORC) diagram

The First Order Reversal Curve (FORC) diagrams of interacting single-domain ferromagnetic particle systems have been found experimentally to contain negative regions. In this paper, we use micromagnetic and phenomenological (Preisach-type) models to help explain the occurrence of these negative regions. In Preisach-type modeling, the position of the negative region is correlated with the sign of the mean-field interactions. In micromagnetic modeling, the position of the negative region is correlated with the spatial arrangement of the particles in the model.

Micromagnetic calculation of the transverse susceptibility of patterned media

In this paper, we present a generalized model of the transverse susceptibility (TS) for realistic fine particle systems that may have any type of anisotropy (uniaxial,cubic,or unidirectional), distribution of easy axes orientation, particles positions and volumes and different strength of magnetostatic interactions between particles. The field dependence of TS is obtained using a micromagnetic algorithm based on the Landau-Lifshitz-Gilbert equation. For the uniaxial particles, where the critical curve approach can also be used, we have compared the results of the micromagnetic algorithm and those obtained by integration of Aharonis formula over the orientation distribution. Tests were also made on the calculation of the TS for single particles with various orientation of the easy axis with respect to the AC/DC applied fields.

Effect of damping on the laser induced ultrafast switching in rare earth-transition metal alloys

In this paper, we present simulations of thermally induced magnetic switching in ferrimagnetic systems performed with a Landau-Lifshitz-Bloch (LLB) equation for damping constant in a wide range of values. We have systematically studied the GdFeCo ferrimagnet with various concentrations of Gd and compared for some values of parameters the LLB results with atomistic simulations. The agreement is remarkably good, which shows that the dynamics described by the ferrimagnetic LLB is a reasonable approximation of this complex physical phenomenon. As an important element, we show that the LLB is able to also describe the intermediate formation of a ferromagnetic state which seems to be essential to understand laser induced ultrafast switching. The study reveals the fundamental role of damping during the switching process.

The state dependence of the dynamic interactions in nanostructured particulate systems

This paper is dedicated to the micromagnetic study of the temperature dependence of interactions in samples increasingly structured, with the standard deviations of the volume and orientation distributions becoming smaller and smaller. We have used the stochastic Landau–Lifshitz–Gilbert equation, observed the evolution of the interaction field distribution as a function of temperature during a number of magnetization processes and analyzed separately the contributions of the superparamagnetic and blocked particles.

Kinetic effects observed in dynamic first-order reversal curves of magnetic wires: Experiment and theoretical description

This study is focused on the possibility to extend the use of the first-order reversal curve (FORC) diagram method to rate-dependent hysteresis observed experimentally on soft magnetic wires. The FORC system of an amorphous magnetic wire was measured with an inductometric experimental setup in which the field-rate was maintained constant. The FORC experiment was performed for four different field-rates. We have developed and implemented a model based on the hypothesis that the magnetization processes in the samples are mainly due to the movement of a domain wall between the central domains of the wire. The differential equation of the domain wall movement is able to give a remarkably accurate description of the experimental FORC diagrams for all the field rates used in the experiment. The experimental FORCs, the FORC susceptibility diagram, and the classical FORC diagram show, however, a number of details that the model is not able to describe. In each such case, one discusses the possible physical cause ...

Generalized reversible susceptibility tensor

A theory of reversible susceptibility tensor based on magnetization vector dynamics, as described by the Landau–Lifshitz equation of motion, is given. It is shown that the reversible transverse susceptibility (RTS) is in fact the zero frequency limit of the ferromagnetic resonance (FMR). Thus, the methods which have been developed previously for the theoretical description of FMR may be applied to predict the RTS behavior. The importance of these results resides in the generality of the approach which allows one to find the reversible susceptibility tensor for virtually any magnetic system if an expression for the magnetic free energy density is known.

Measurements on Real and Imaginary Parts of Transverse Susceptibility of Particulate System

In this paper, the influence of ac field frequency on transverse susceptibility (TS) curves was investigated. Two different experimental setups were used to reach the excitation frequency of 6 GHz. At low frequencies, the real and imaginary parts of the susceptibility were directly obtained by measuring the impedance of a coil incorporating the sample using a RLC bridge. In the microwave frequency domain, the susceptibility was correlated with the scattering parameters of a transmission line containing the sample. The experimental data show that for a Ni-Zn ferrite polycrystalline sample Ni0.65Zn0.35Fe2O4 and for ac field frequencies up to 100 MHz, the TS peeks do not present an important shift in field positions. The measured real and imaginary parts of the TS have the same peek structure, but the amplitude and line width differ. At higher ac frequencies, the peeks field are frequency dependent and a supplementary peek was observed.

Temperature Dependence of FORC Diagrams in Nanostructured Materials

The paper is dedicated to the first-order reversal curves (FORC)-based study of the interactions in nanostructured both longitudinal and perpendicular media. Two types of algorithms were used to simulate the FORC diagrams at different temperatures: Monte-Carlo-Metropolis and stochastic Landau-Lifshitz-Gilbert models. The results are compared and the discussed