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Register a new userThermal simulation of magnetization reversals for size-distributed assemblies of core-shell exchange biased nanoparticles
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A temperature dependent coherent magnetization reversal model is proposed for size-distributed assemblies of ferromagnetic nanoparticles and ferromagnetic-antiferromagnetic core-shell nanoparticles. The nanoparticles are assumed to be of uniaxial anisotropy and all aligned along their easy axis. The thermal dependence is included by considering thermal fluctuations, implemented via the Neel-Arrhenius theory. Thermal and angular dependence of magnetization reversal loops, coercive field and exchange-bias field are obtained, showing that F-AF size-distributed exchange-coupled nanoparticles exhibit temperature-dependent asymmetric magnetization reversal. Also, non-monotonic evolutions of He and Hc with T are demonstrated. The angular dependence of Hc with T exhibits a complex behavior, with the presence of an apex, whose position and amplitude are strongly T dependent. The angular dependence of He with T exhibits complex behaviors, which depends on the AF anisotropy and exchange coupling. The resulting angular behavior demonstrates the key role of the size distribution and temperature in the magnetic response of nanoparticles.
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We present an atomistic model of a single nanoparticle with core/shell structure that takes into account its lattice strucutre and spherical geometry, and in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and interfacial regions. By means of Monte Carlo simulations of the hysteresis loops based on this model, we have determined the range of microscopic parameters for which loop shifts after field cooling can be observed. The study of the magnetic order of the interfacial spins for different particles sizes and values of the interfacial exchange coupling have allowed us to correlate the appearance of loop asymmetries and vertical displacements to the existence of a fraction of uncompensated spins at the shell interface that remain pinned during field cycling, offering new insight on the microscopic origin of the experimental phenomenology.
We present a numerical simulation study of the exchange bias (EB) effect in nanoparticles with core/shell structure aimed to unveil the microscopic origin of some of the experimental phenomenology associated to this effect. In particular, we have focused our study on the particle size and field cooling dependence of the hysteresis loop shifts. To this end, hysteresis loops after a field cooling process have been computed by means of Monte Carlo simulations based on a model that takes into account the peculiar properties of the core, shell and interfacial regions of the particle and the EB and coercive fields have been extracted from them. The results show that, as a general trend, the EB field $h_{EB}$ decreases with increasing particle size, in agreement with some experimental observations. However, closer inspection reveals notable oscillations of $h_{EB}$ as a function of the particle radius which we show to be closely related to the net magnetization established after field cooling at the interfacial shell spins. For a particle with ferromagnetic interface coupling, we show that the magnitude and sign of $h_{EB}$ can be varied with the magnetic field applied during the cooling process.
Some of the main experimental observations related to the occurrence of exchange bias in magnetic systems are reviewed, focusing the attention on the peculiar phenomenology associated to nanoparticles with core/shell structure as compared to thin film bilayers. The main open questions posed by the experimental observations are presented and contrasted to existing theories and models for exchange bias formulated up to date. We also present results of simulations based on a simple model of a core/shell nanoparticle in which the values of microscopic parameters such as anisotropy and exchange constants can be tuned in the core, shell and at the interfacial regions, offering new insight on the microscopic origin of the experimental phenomenology. A detailed study of the of the magnetic order of the interfacial spins shows compelling evidence that most of the experimentally observed effects can be qualitatively accounted within the context of this model and allows also to quantify the magnitude of the loop shifts with striking agreement with the macroscopic observed values.
We present the results of Monte Carlo simulations of the magnetic properties of a model for a single nanoparticle consisting in a ferromagnetic core surrounded by an antiferromagnetic shell. The simulations of hysteresis loops after cooling in a magnetic field display exchange bias effects. In order to understand the origin of the loop shifts, we have studied the thermal dependence of the shell and interface magnetizations under field cooling. These results, together with inspection of the snapshots of the configurations attained at low temperature, show the existence of a net magnetization at the interface which is responsible for the bias of the hysteresis loops.
The utility of nanoscaled ferromagnetic particles requires both stabilized moments and maximized switching speeds. During reversal, the spatial modulation of the nanoparticle magnetization evolves in time, and the energy differences between each new configuration are accomodated by the absorption or emission spin waves with different wavelengths and energy profiles. The switching speed is limited by how quickly this spin wave energy is dissipated. We present here the first observation of dispersing spin waves in a nanoscaled system, using neutron scattering to detect spin waves in the CoO shells of exchange biased Co core- CoO shell nanoparticles. Their dispersion is little affected by finite size effects, but the spectral weight shifts to energies and wave vectors which increase with decreasing system size. Core-shell coupling leads to a substantial enhancement of the CoO spin wave population above its conventional thermal level, suggesting a new mechanism for dissipating core switching energy.
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