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 H2, at different temperatures (200 < Tann < 300 °C), NiO powder previously ball-milled for 20 hours. 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 (~ 600 Oe), at T = 5 K, 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 analysed. The results show the existence of a structurally and magnetically disordered NiO component, which plays the key role in the EB mechanism.