On the possibility to achieve population inversion in a magnetic nanoparticle system

Based on the fact that an intense magnetic field may group the orientations of easy magnetic axis (e.m.a.) of nanoparticles of a ferrofluid around the field direction, one can state that, by freezing, the grouping remains although the field is absent. By bringing the frozen ferofluid to saturation magnetization in the e.m.a. direction, then followed by the field's effect inversion, a population inversion state is created. Although for nanoparticles with low anisotropy the time of inversion is rather short (10−5 s), we have proved that for nanoparticles with higher anisotropy, this time can increase, even up to 105 s. One can assume that, during the inversion, the particle system represents an active medium for amplipfying electromagnetic radiations in microwaves field.