arXiv subjects

Orlando V. Billoni

Publications and source records attributed to Orlando V. Billoni.

23 records · Page 2Linked to original sources

arXiv2008

Distributions of switching times of single-domain particles using a time quantified Monte Carlo method

Using a time quantified Monte Carlo scheme we performed simulations of the switching time distribution of single mono-domain particles in the Stoner-Wohlfarth approximation. We considered uniaxial anisotropy and different conditions for the external applied field. The results obtained show the switching time distribution can be well described by two relaxation times, either when the applied field is parallel to the easy axis or for an oblique external field and a larger damping constant. We found that in the low barrier limit these relaxation times are in very good agreement with analytical results obtained from solutions of the Fokker-Planck equation related to this problem. When the damping is small and the applied field is oblique the shape of the distribution curves shows several peaks and resonance effects.

cond-mat.stat-mech↗

arXiv2008

Dipolar interactions and thermal stability of two-dimensional nanoparticle arrays

We show results of Monte Carlo simulations of an array of monodispersed magnetic monodomain particles, in a square lattice with dipolar interactions and perpendicular uniaxial anisotropy. We first show the equilibrium phase diagram of the system, which shows three phases, superparamagnetic (SP), out-of-plane antiferromagnetic and in-plane antiferromagnetic with a reorientation transition between the last two. We then employ a recently introduced time quantified Monte Carlo method to study the relaxation of autocorrelations of the particle array for different ratios of dipolar to anisotropy energies. In the non-interacting case we show that relaxation is exponential in time with characteristic times obeying a classic result by Brown. When dipolar interactions are switched on, the relaxation is very well described by stretched exponential forms in the whole time window and in both the SP and ordered phases. Relaxation times still obey a nearly Arrhenius behaviour, with a single effective energy barrier that decreases as the dipolar interaction increases, a result that must be interpreted within the dynamics protocol. No signs of glassy behaviour were found, in agreement with the absence of disorder in the model system.

cond-mat.mtrl-sci↗

arXiv2008

Exponential equilibration by slow cooling in the planar random-anisotropy magnet: experiments and simulations

Neutron measurements of the equilibration of the staggered magnetization in DyAs(0.35)V(0.65)O4 are compared with Monte Carlo simulations of spin dynamics in a planar random-anisotropy magnet. The simulation results are in agreement with striking observed relaxation phenomena: when cooled rapidly to a low temperature no magnetic ordering is observed, but when cooled in small steps an ordered magnetic moment appears which is found to equilibrate exponentially with time at temperatures through and below the transition temperature. In contrast to the freezing of spins in other random systems, the time scale of the relaxation in this system does not increase significantly even at the lowest temperatures.

cond-mat.mtrl-sci↗

arXiv2007

The Spin Reorientation Transition and Phase Diagram of Ultrathin Ferromagnetic Films

We show results from Monte Carlo simulations of a two dimensional Heisenberg model for ultrathin films with perpendicular anisotropy. A complete phase diagram is obtained as a function of anisotropy and temperature, spanning a wide range of behavior. We discuss our results in relation with experimental findings in different ultrathin films. We observe and characterize a line of Spin Reorientation Transitions . This transition from out of plane stripe order to in plane ferromagnetic order presents a paramagnetic gap in between in a finite region in parameter space, as reported in experiments. For large anisotropies direct transitions from a low temperature stripe phase to a paramagnetic or tetragonal phase with dominant perpendicular magnetization is observed, also in agreement with experiments. We also show the phase diagram for a system without exchange, i.e. with pure dipolar and anisotropy interactions. It shows a similar behavior to the ferromagnetic case with antiferromagnetic instead of stripe phases at low temperatures. A Spin Reorientation Transition is also found in this case.

cond-mat.dis-nn↗

arXiv2005

Spin-glass behavior in the random-anisotropy Heisenberg model

We perform Monte Carlo simulations in a random anisotropy magnet at a intermediate exchange to anisotropy ratio. We focus on the out of equilibrium relaxation after a sudden quenching in the low temperature phase, well below the freezing one. By analyzing both the aging dynamics and the violation of the Fluctuation Dissipation relation we found strong evidence of a spin--glass like behavior. In fact, our results are qualitatively similar to those experimentally obtained recently in a Heisenberg-like real spin glass.

cond-mat.dis-nn↗