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

Ni/NiO nanogranular samples have been prepared by annealing in H2, at selected temperatures (200 <T_ann < 300 °C), NiO powder previously milled for 5, 10, 20 and 30 hours. The structure and composition of the samples, analyzed by X-ray diffraction, can be satisfactorily controlled during the synthesis by varying T_ann and the milling time of the precursor NiO powders. In fact, the structure of the NiO phase, determined by the milling process, affects the nucleation and growth of the Ni nanocrystallites induced by the H2-treatments. Hence, T_ann being equal, the amount and the mean grain size D_Ni of the Ni phase vary substantially in samples with 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 Hex: the highest value (~ 940 Oe) has been measured at T = 5 K in a sample with ~7 wt % Ni and D_Ni = 19 nm. However, in Ni/NiO samples with different values of Hex at T = 5 K, Hex vanishes at the same temperature (~ 200 K). We consider that the thermal dependence of Hex is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and T_ann.