Study of the magnetic behavior of Mn$_{1-x}$Zn$_{x}$ Ferrite nanoparticles at low temperature

We report on magnetic spin resonance and magnetization hysteresis loops of Mn$_{1-x}$Zn$_{x}$ Ferrite nanoparticles with sizes ranging from 20nm to 50nm obtained via microemulsions. The samples were evaluated by VSM technique ranging from 10K$<$300K at ZFC and FC. EPR measurements at T=180K were carried out in the 0 $\le $ x $\le $ 0.75 range. Experimental results for the peak-to-peak linewidth, \textit{$\Delta $H}$_{pp}$, have been discussed by the existence of monodomain ferrimagnetic particles. The results indicate an increase of \textit{$\Delta $H}$_{pp}$ by increasing the Zn concentration. The EPR signal shows a second EPR pick for x$>$0.5 at g=4.10 associated at local magnetic fields produced for Mn chains. The anisotropy constant was calculated by means of a genetic algorithm for parameter optimization of the Jiles-Atherton model.