Exchange bias properties and surface spins freezing in ferrite nanoparticles of magnetic nanocolloids

Abstract

We investigate magnetic nanocolloids based on 3.3 nm sized particles of manganese ferrite. The observation of shifted hysteresis loops after cooling the sample in high fields indicates the existence of a coupling between the ferrimagnetic ordered core and the disordered surface layer. The exchange bias field, determined from the field offset from the origin, decreases as the cooling field increases. Zero field cooling measurements of the thermal dependence of the high field magnetization allow separating two contributions. One is associated to the well ordered core and the other is related to surface spins frozen in a disordered spin glass like structure. As the applied field increases the freezing temperature decreases from 23 K to 12 K.