Tailoring the exchange bias of Ni/NiO nanogranular samples by the structure control

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H(2), at selected temperatures (200 <= T(ann)<= 300 degrees C), NiO powder previously milled for 5, 10, 20 and 30h. Both the as-milled NiO powders and the Ni/NiO samples have been analyzed by X-ray diffraction and the exchange bias properties have been investigated in the 5-200 K temperature range. The structure and the composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both T(ann) and the milling time of the precursor NiO powders. In particular, by increasing this last parameter, the mean grain size of the NiO phase reduces down to the final value of 16 nm and the microstrain increases, which is consistent with an enhancement of the structural disorder. The structure of the milled NiO matrix strongly affects the process of nucleation and growth of the Ni nanocrystallites induced by the H(2) treatments, so that, T(ann) being equal, the amount and the mean grain size D(Ni) of the Ni phase vary substantially in samples having different milling times. Such features of the Ni phase determine the extent of the Ni/NiO interface and consequently the magnitude of the exchange field H(ex): the highest value(similar to 940 Oe) has been measured at T = 5 K in a sample containing similar to 7 wt% Ni and with D(Ni) = 19nm. However, in Ni/NiO samples with very different structural characteristics and different values of H(ex) at T = 5 K, the EB effect vanishes at the same temperature (similar to 200K) and the same thermal dependence of H(ex) is observed. We consider that the evolution of the EB effect with temperature is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and Tann. (C) 2009 Elsevier B. V. All rights reserved.