Abstract
A Ni80 Fe20 α- Fe2 O3 thin film has been deposited using a dual ion-beam deposition technique where the α- Fe2 O3 is formed in situ by O2 bombardment of pure Fe. AC susceptibility results indicate moment fluctuations at temperatures above 150 K. This behavior combined with the thermal fluctuation driven temperature dependence of the negative field coercivity indicates that the α- Fe2 O3 nanocolumnar crystallite magnetism evolves from static single domain moments to superparamagnetic moments with warming. 25 kOe field cooling establishes exchange bias with an onset temperature of 50 K. The positive (Hc2) and negative (Hc1) field coercivities exhibit very different temperature dependencies, with Hc1 (T) well described by a thermally activated coherent domain wall reversal model, while Hc2 (T) is essentially constant except for the lowest temperatures. These results point towards different domain reversal mechanisms in positive and negative fields. © 2007 American Institute of Physics.