Zero-field-cooled and field-cooled magnetization of individual nanomagnets and their assembly

Abstract

Zero-field-cooled magnetization MZFC and field-cooled one M FC are measured for nearly uniform ironnitride nanomagnets, which are homogeneously dispersed in solidified kerosene at various density. For the isolated nanomagnets, super-paramagnetic behavior, MZFC = M FC, is observed at higher temperatures, while blocking phenomena, MZFC(T) ∼ 0 and MFC(T) = const., are seen at lower temperatures. The relative width of the transitional range between these regimes is almost the same with that of the volume distribution, as expected simply. As the density increases, the transitional range for MFC(T) is narrowed down to a point. This indicates that the blocking becomes cooperative owing to the dipolar interactions. On the other hand, the analysis of MZFC(T) shows that the transitional range broadens out with the density. This suggests that the interactions diversify the barrier heights for blocking. These inconsistent features can be explained by a conversion of the mechanism from individual blocking to spin-glass-like freezing. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.