Magnetic disorder and exchange bias in nanogranular metal / metal-oxide systems

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method which combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powders are ball-milled to reduce the grain size to the nanometric scale; subsequently, the milled powder is subjected to high temperature treatments in H2, inducing the reduction to metallic Ni. The prepared samples typically consist of Ni nanoparticles dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (TEM). The advantage of this method is that by varying the temperature and duration of the H2-treatment, we can well control the amount of Ni that forms. Moreover, we can predetermine, to some extent, the structural features of the NiO phase, by varying the milling parameters (milling time, ball-to-powder weight ratio). This presentation reports about the EB properties of a series of Ni/NiO samples obtained by annealing in H2, at different temperatures (200 < T