Exchange bias in macroporous Co/CoO

The magnetic properties of new magnetic material, macroporous cobalt have been investigated. The cobalt samples studied have FCC close packed, three-dimensional arrays of spherical pores (pore size roughly 300 nm). These samples were prepared by colloidal crystal templating. The hysteresis loops of the field-cooled samples of macroporous cobalt were shifted along the field axis with the shift consistent with the exchange bias phenomenon. The exchange bias is attributed to antiferromagnetic CoO, which most likely forms when the samples are exposed to the ambient atmosphere. The temperature dependence of the exchange bias and coercivity of these samples was investigated. The temperature dependence of the coercivity was found to be non-monotonic. We proposed a possible explanation of this behavior in terms of a spin-glass-like state of cobalt nanoparticles that forms below 45 K.