Exchange bias and interface structure in the Ni/NiO nanogranular system

The exchange bias ( EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H(2), at different temperatures ( 200 <= T(ann) <= 300 degrees C), NiO powder previously ball-milled for 20 h. Typically, the samples consist of Ni nanoparticles ( mean size of 10 - 18 nm) embedded in a nanocrystalline NiO matrix. With increasing Tann, the Ni fraction varies from 4% up to 69%. The exchange field depends on the Ni amount, being maximum (similar to 600 Oe), at T = 5K, in the sample with 15% Ni. In all the samples, the EB effect vanishes at T = 200 K. The structural features of the samples have been investigated by x-ray diffraction, electron microscopy and extended x-ray absorption fine structure and the low-temperature magneto-thermal behaviour has been thoroughly analyzed. The results show the existence of a structurally and magnetically disordered NiO component, which plays the key role in the EB mechanism.