ABSTRACT
The demagnetization and associated magnetocaloric effect (MCE) in strong-weak-strong ferromagnetic trilayers, upon a reorientation of the strong ferromagnets from parallel to antiparallel (AP) magnetization, is simulated using atomistic spin dynamics. The simulations yield non-trivial spin distributions in the AP state, which in turn allows entropy to be calculated directly. The influence of longer-range spin-spin interactions and of variable strength of the external switching field are investigated. Finally, we find that the MCE in the system can be significantly improved by allowing the local exchange to vary through the spacer, which in practice can be implemented by spatially tailoring the spacer's magnetic dilution.
ABSTRACT
Different processes governing magnetic properties of an ensemble of magnetic nanoparticles in the temperature region close to a transition from superparamagnetic to paramagnetic state are analyzed and the ways to separate them are suggested. Enhanced role of paraprocess in magnetization behavior near Curie temperature is stressed. A procedure to isolate paraprocess contribution and adequately determine spontaneous magnetization of the ensemble of superparamagnetic nanoparticles is proposed. Critical behavior of the spontaneous magnetization is experimentally determined for the ensemble of nanoparticles of lanthanum-strontium manganites, which are considered as promising materials for self-controlled magnetic nanohyperthermia. Effect of dispersion of magnetic parameters on effective magnetic characteristics of nanoparticles and their critical behavior is discussed. Theoretical background for the use of the 'effective Curie temperature for the ensemble of nanoparticles' concept is proposed for ensembles of particles with dispersion of their Curie temperature. Based on the results obtained, various strategies to develop novel biomedical applications, in particular those suitable for noninvasive temperature monitoring, are discussed.
ABSTRACT
Here we report a study of chain formation and the magnetic anisotropy induced by them in suspensions of slightly anisotropic Fe3O4 nanoparticles in water and in aqueous V2O5 suspensions. An investigation into the magnetization of the fluid and frozen suspensions, the application of dynamic light scattering techniques and the observation of the magnetic anisotropy in the frozen magnetically aligned samples allowed us to confirm the existence of chains of Fe3O4 in both suspensions. Our study shows that the magneto-induced anisotropy appearing in magnetic fields in colloids with Fe3O4 particles is mainly due to many particle (chain) magneto-induced anisotropy, but not due to single particle magneto-induced anisotropy connected with particle shape anisotropy. In other words, the single particle magneto-induced anisotropy is much smaller than the many particle (chain) anisotropy. The anisometry of the chains provides effective coupling with the nonmagnetic V2O5 component of the suspension and results in its strong sensitivity to the magnetic field.