Positive Exchange Bias and Upward Magnetic Relaxation in a Fe-film/CoO-Nanoparticle Hybrid System

Unusual positive exchange bias found in Fe/CoO x nanoparticle bilayer films is correlated to a characteristic magnetic spin-glass (SG) in CoO x , with the SG magnetization coupled antiparallel with the Fe magnetization upon field cooling. This SG magnetization has strong field- and time-dependence which displays unusual upward magnetic relaxation behavior in thermoremanent magnetization measurements. The antiparallel coupling is shown to result predominantly from the antiferromagnetic superexchange of the Fe 2+ -O-Co 3+ couple, of the oxygen-terminated CoO x at the interface. These experimental results reveal the possibility of manipulating the exchange bias effect via an indirect exchange coupling mechanism.