Kerr microscopy study of thermal and athermal training effects in a Co/CoO exchange bias system

Abstract

The present work reports the magnetic domain evolution during the magnetization reversal and the training effect in a polycrystalline Co/CoO exchange bias system. Co/CoO bilayers with different cobalt (Co) layer thicknesses are being studied. The measurements are carried out using the Kerr microscopy at different temperatures (≥ 80 K) after the field-cooling across the Néel temperature of the antiferromagnetic (AFM) CoO layer. It is observed that with the increasing ferromagnetic (FM) Co layer thickness, the exchange bias reduces and the training effect becomes weaker. Analysis of the temperature variation of the exchange bias field indicates an increasing disorder at the FM-AFM interface with the decreasing FM thickness. Two different training effects, viz., athermal and thermal mechanisms, are observed depending on the thickness of the FM layer. For the lower FM thickness an athermal training effect is observed, whereas for the thicker FM layer thermal training effect is dominated. The domain structure of FM layers drastically changes during the athermal training effect with the cycling and domain size significantly decreasing during the magnetization reversal process below blocking temperature (TB), whereas in the thermal training mechanism no significant changes are found in the magnetic domain evolution throughout the temperature range with cycling. These results are expected to provide key inputs to various theoretical models that are being used to study the exchange bias phenomena in the recent literature.