Exchange bias properties and surface spin freezing in magnetic nanoparticles

ZFC and FC magnetization measurements versus field are carried out on manganese ferrite based nanoparticles with a mean diameter of 3.3 nm. The exchange bias field determined from the field shift of hysteresis loops, decreases as the cooling field increases. Magnetization measurements performed at constant applied field H as a function of temperature allows us to separate two H-dependent contributions. One is associated to the well ordered core which inflates as the field increases and the other is related to surface spins frozen in a disordered structure. The thermal dependence of this disordered surface contribution decreases exponentially with a freezing temperature Tf, which decreases as the applied field increases.