D. Ledue

Monte Carlo investigation of transition and compensation temperatures of Fe/Tb multilayers

A Monte Carlo investigation of the A thickness (A≡Fe) dependence of the transition and compensation temperatures of a simple cubic Heisenberg A/B bilayer is carried out. Our model, which includes a few mixed planes of the A1−xBx type that represent the disordered interfaces, basically consists of several coupled magnetic parts with different bulk transition temperatures. Numerical results are compared with the experimental data for amorphous Fe/Tb multilayers. The simulated Fe thickness (tFe) dependence of TC, which reproduces the decrease of TC as tFe increases, is consistent with the experimental one. Since our simulations indicate that the magnetic ordering is driven by the disordered interfaces, the decrease of TC is explained by the fact that the interfaces are more and more decoupled as tFe increases. The experimental thermal behavior of each sublattice magnetization can be reproduced with a Tb-magnetic moment of 6μB which seems to be reasonable for amorphous layers, whereas a value of 9μB is too la...

Monte Carlo investigation of the magnetic anisotropy in Fe/Dy multilayers

By Monte Carlo simulations in the canonical ensemble, we have studied the magnetic anisotropy in Fe/Dy amorphous multilayers. This work has been motivated by experimental results which show a clear correlation between the magnetic perpendicular anisotropy and the substrate temperature during elaboration of the samples. Our aim is to relate macroscopic magnetic properties of the multilayers to their structure, more precisely their concentration profile. Our model is based on concentration dependent exchange interactions and spin values, on random magnetic anisotropy and on the existence of locally ordered clusters that leads to a perpendicular magnetisation. Our results evidence that a compensation point occurs in the case of an abrupt concentration profile. Moreover, an increase of the non-collinearity of the atomic moments has been evidenced when the Dy anisotropy constant value grows. We have also shown the existence of inhomogeneous magnetisation profiles along the samples which are related to the concentration profiles.

Monte Carlo simulations of magnetic properties in multilayers

Abstract A Monte Carlo method has been used to simulate Heisenberg multilayer systems ( L × L × 4 P ) consisting of alternating P ferromagnetic layers A and B with antiferromagnetic interface coupling J AB . Finite-size effects on the specific heat and magnetisation thermal variation for two kinds of boundary conditions at the top and bottom planes are investigated. In particular, our Monte Carlo data evidence that the specific heat exhibits two peaks and a single phase transition occurs at the temperature which corresponds to the location of the high temperature peak (as L → ∞).

Magnetoelectric properties of multiferroic CuCrO2 studied by means of ab initio calculations and Monte Carlo simulations

Motivated by the discovery of multiferroicity in the geometrically frustrated triangular antiferromagnet CuCrO2 below its Neel temperature T-N, we investigate its magnetic and ferroelectric propert ...

Magnetization reversal in amorphous Fe/Dy multilayers: A Monte Carlo study

The Monte Carlo method in the canonical ensemble is used to investigate magnetization reversal in multilayers of amorphous transition metal and rare earth elements. Our study is based on a model containing diluted clusters which exhibit an effective uniaxial anisotropy in competition with random magnetic anisotropy in the matrix. We simulate hysteresis loops for an abrupt profile and a diffuse one obtained from atom probe tomography analyses. Our results give evidence that the atom probe tomography profile favors perpendicular magnetic anisotropy in agreement with magnetic measurements. Moreover, the hysteresis loops calculated at several temperatures qualitatively agree with the experimental ones.

Magnetisation switching in a ferromagnetic Heisenberg nanoparticle with uniaxial anisotropy: a Monte Carlo investigation

Abstract We investigate the thermal-activated magnetisation reversal in a single ferromagnetic nanoparticle with uniaxial anisotropy using Monte Carlo simulations. The aim of this work is to reproduce the reversal magnetisation by uniform rotation at very low temperature in the high-energy barrier hypothesis, that is to realize the Neel–Brown model. For this purpose we have considered a simple cubic nanoparticle where each site is occupied by a classical Heisenberg spin. The Hamiltonian is the sum of an exchange interaction term, a single-ion anisotropy term and a Zeeman interaction term. Our numerical data of the thermal variation of the switching field are compared to an approximated expression and previous experimental results on Co nanoparticles.

Phase transitions in a ferrimagnetic Heisenberg multilayer: a Monte Carlo study

Abstract Using a Monte Carlo method, the magnetic properties of a Heisenberg multilayer system consisting of four alternating ferromagnetic layers (A and B) with different spins and interaction constants are examined. The effects of sampling size, antiferromagnetic interlayer coupling ( J AB ) and disordered interfaces on the specific heat and magnetisation thermal variation are investigated. For multilayers with steep interfaces, it is shown that the thermal variation of the specific heat exhibits two peaks, but only a single-phase transition occurs. The transition temperature is independent on J AB in the investigated range. On the other hand, this temperature depends on the kind of the interfaces.

Temperature effect in polycrystalline exchange-biased bilayers: A Monte Carlo study

A model of polycrystalline F/AF bilayers based on the Voronoi construction is considered to investigate the microstructure effect on exchange bias properties. This study is carried out by means of Monte Carlo simulations. The thermal variation of the exchange and coercive fields are in good qualitative agreement with previous experimental results on NiFe/NiMn bilayers. The blocking temperature distribution is determined by reproducing the experimental procedure based on an annealing. Then, it is evidenced that, by taking into account a decrease of the magnetization at low-temperature, at the F/AF interface in some of the AF grains, leads to the observation of the unusual low-T peak in the distribution.

Monte Carlo investigation of the eight-state Potts model on quasiperiodic tilings

A Monte Carlo investigation of the eight-state Potts model on the two-dimensional (2D) quasiperiodic octagonal tiling with free boundary conditions is performed in order to determine the nature of a temperature-driven transition. It is shown that numerical data suffer from drastic free boundary effects that strongly disturb the probability distributions of the internal energy and, consequently, the scaling behavior of the specific heat. An alternative way consisting in analysing the core of the tilings is applied to pass over free boundary effects. This analysis combined with the Lee–Kosterlitz method allows one to evidence that the system undergoes a first-order transition as in 2D periodic lattices. The first-order type of scaling is observed for the maximum in the susceptibility of the core of the tilings but not for the maximum in the specific heat.

Magnetic properties of Fe/Dy multilayers: A Monte Carlo investigation

Abstract We investigate the magnetic properties of a Heisenberg ferrimagnetic multilayer by using Monte Carlo simulations. The aim of this work is to study the local structural anisotropy model which is a possible origin of the perpendicular magnetic anisotropy in transition metal/rare earth amorphous multilayers. We have considered a face centered cubic lattice where each site is occupied by a classical Heisenberg spin. We have introduced in our model of amorphous multilayers a small fraction of crystallised Fe–Dy nanoclusters with a mean anisotropy axis along the deposition direction. We show that a competition in the energy terms takes place between the mean uniaxial anisotropy of the Dy atoms in the nanoclusters and the random anisotropy of the Dy atoms in the matrix.