Magnetoelectric properties of the multiferroic CuCrO2_2 studied by means of ab initio calculations and Monte Carlo simulations

Motivated by the discovery of multiferroicity in the geometrically frustrated triangular antiferromagnet CuCrO$_2$ below its N\'eel temperature $T_N$, we investigate its magnetic and ferroelectric properties using ab initio calculations and Monte Carlo simulations. Exchange interactions up to the third nearest neighbors in the $ab$ plane, inter-layer interaction and single ion anisotropy constants in CuCrO$_2$ are estimated by series of density functional theory calculations. In particular, our results evidence a hard axis along the [110] direction due to the lattice distortion that takes place along this direction below $T_N$. Our Monte Carlo simulations indicate that the system possesses a N\'eel temperature $T_N\approx27$ K very close to the ones reported experimentally ($T_N = 24-26$ K). Also we show that the ground state is a proper-screw magnetic configuration with an incommensurate propagation vector pointing along the [110] direction. Moreover, our work reports the emergence of spin helicity below $T_N$ which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model. We confirm the electric control of spin helicity by simulating $P$-$E$ hysteresis loops at various temperatures.Comment: 6 pages, 8 figure

Induced side-branching in smooth and faceted dendrites: theory and Phase-Field simulations

The present work is devoted to the phenomenon of induced side branching stemming from the disruption of free dendrite growth. Therein, we postulate that the secondary branching instability can be triggered by the departure of the morphology of the dendrite from its steady state shape. Thence, the instability results from the thermodynamic trade-off between non monotonic variations of interface temperature, surface energy, kinetic anisotropy and interface velocity within the Gibbs Thomson equation. For purposes of illustration, the toy model of capillary anisotropy modulation is prospected both analytically and numerically by means of phase field simulations. It is evidenced that side branching can befall both smooth and faceted dendrites, at a normal angle from the front tip which is specific to the nature of the capillary anisotropy shift applied

Induced side-branching in smooth and faceted dendrites: theory and phase-field simulations

International audienceThe present work is devoted to the phenomenon of induced side branching stemming from the disruption of free dendrite growth. We postulate that the secondary branching instability can be triggered by the departure of the morphology of the dendrite from its steady state shape. Thence, the instability results from the thermodynamic trade-off between non monotonic variations of interface temperature, surface energy, kinetic anisotropy and interface velocity within the Gibbs–Thomson equation. For the purposes of illustration, the toy model of capillary anisotropy modulation is prospected both analytically and numerically by means of phase-field simulations. It is evidenced that side branching can befall both smooth and faceted dendrites, at a normal angle from the front tip which is specific to the nature of the capillary anisotropy shift applied. This article is part of the theme issue ‘Transport phenomena in complex systems (part 2)’

Antiferromagnetic thickness and temperature dependence of the exchange bias properties of Co/IrMn nanodots and continuous films: A Monte Carlo study

International audienceMotivated by the challenge of understanding the complex influence of the antiferromagnetic (AF) thickness and the temperature on exchange bias (EB) properties, and by the necessity of miniaturization of devices, we investigate EB properties of Co/IrMn nanodots and of continuous films by using kinetic Monte Carlo simulations. To that purpose, we use a granular model, which takes into account disordered interfacial phases in the AF layer and, in the case of nanodots, disordered phases at the edges in the AF layer. Our results show that the AF thickness dependence of the exchange field H E (measured at room temperature) in both nanodots and continuous films exhibits a maximum in agreement with experimental results. We explain these results in terms of superparamagnetic and blocked grains in the AF layer at room temperature and also not polarized AF grains during the initial field-cooling. The simulated values of H E in nanodots are smaller than that in continuous films for small AF thicknesses and larger for larger ones due to the contribution of the disordered phases at the edges in the AF layer. Also, we investigate the temperature and AF thickness effects on H E and on the coercive field H C. We found that H E slightly decreases at low temperatures due to the disordered interfacial phases. Importantly, at the maximum blocking temperature of the AF grains, H E vanishes and H C exhibits a maximum. Our numerical results are successfully compared to experimental data on Co/IrMn bilayers for various IrMn thicknesses and all temperatures. In addition, our results indicate that H E is smaller in nanodots at low measurement temperature due to the presence of disordered phases at the edges. Concerning H C , our data show that it can be either larger or smaller in nanodots depending on the measurement temperature

Antiferromagnetic thickness and temperature dependence of the exchange bias properties of Co/IrMn nanodots and continuous films: A Monte Carlo study

International audienceMotivated by the challenge of understanding the complex influence of the antiferromagnetic (AF) thickness and the temperature on exchange bias (EB) properties, and by the necessity of miniaturization of devices, we investigate EB properties of Co/IrMn nanodots and of continuous films by using kinetic Monte Carlo simulations. To that purpose, we use a granular model, which takes into account disordered interfacial phases in the AF layer and, in the case of nanodots, disordered phases at the edges in the AF layer. Our results show that the AF thickness dependence of the exchange field H E (measured at room temperature) in both nanodots and continuous films exhibits a maximum in agreement with experimental results. We explain these results in terms of superparamagnetic and blocked grains in the AF layer at room temperature and also not polarized AF grains during the initial field-cooling. The simulated values of H E in nanodots are smaller than that in continuous films for small AF thicknesses and larger for larger ones due to the contribution of the disordered phases at the edges in the AF layer. Also, we investigate the temperature and AF thickness effects on H E and on the coercive field H C. We found that H E slightly decreases at low temperatures due to the disordered interfacial phases. Importantly, at the maximum blocking temperature of the AF grains, H E vanishes and H C exhibits a maximum. Our numerical results are successfully compared to experimental data on Co/IrMn bilayers for various IrMn thicknesses and all temperatures. In addition, our results indicate that H E is smaller in nanodots at low measurement temperature due to the presence of disordered phases at the edges. Concerning H C , our data show that it can be either larger or smaller in nanodots depending on the measurement temperature

H-T magnetic phase diagram of CuCrO2: A Monte Carlo study based on a realistic 3D classical Heisenberg model

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

We investigate the thermal activated magnetisation reversal in a single ferromagneticnanoparticle with uniaxial anisotropy using Monte Carlo simulations. The aim of this work is toreproduce the reversal magnetisation by uniform rotation at very low temperature in the highenergy barrier hypothesis, thatis to realize the Néel-Brown model. For this purpose we have considered a simple cubicnanoparticle where each site is occupied by a classical Heisenberg spin. The Hamiltonian isthe sum of an exchange interaction term, a single-ion anisotropy term and a Zeemaninteraction term. Our numerical data of the thermal variation of the switching field arecompared to an approximated expression and previous experimental results on Co nanoparticles

Phase field modelling of snowflakes growth

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Size-Dependent Solute Segregation at Symmetric Tilt Grain Boundaries in α-Fe: A Quasiparticle Approach Study

International audienceIn the present work, atomistic modeling based on the quasiparticle approach (QA) was performed to establish general trends in the segregation of solutes with different atomic size at symmetric ⟨100⟩ tilt grain boundaries (GBs) in α-Fe. Three types of solute atoms X1, X2 and X3 were considered, with atomic radii smaller (X1), similar (X2) and larger (X3) than iron atoms, respectively, corresponding to phosphorus (P), antimony (Sb) and tin (Sn). With this, we were able to evidence that segregation is dominated by atomic size and local hydrostatic stress. For low angle GBs, where the elastic field is produced by dislocation walls, X1 atoms segregate preferentially at the limit between compressed and dilated areas. Contrariwise, the positions of X2 atoms at GBs reflect the presence of tensile and compressive areal regions, corresponding to extremum values of the σXX and σYY components of the strain tensor. Regarding high angle GBs Σ5 (310) (θ = 36.95°) and Σ29 (730), it was found that all three types of solute atoms form Fe9X clusters within B structural units (SUs), albeit being deformed in the case of larger atoms (X2 and X3). In the specific case of Σ29 (730) where the GB structure can be described by a sequence of |BC.BC| SUs, it was also envisioned that the C SU can absorb up to four X1 atoms vs. one X2 or X3 atom only. Moreover, a depleted zone was observed in the vicinity of high angle GBs for X2 or X3 atoms. The significance of this research is the development of a QA methodology capable of ascertaining the atomic position of solute atoms for a wide range of GBs, as a mean to highlight the impact of the solute atoms’ size on their locations at and near GBs