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

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by an original method that combines mechanical milling and hydrogen partial reduction of NiO. In this procedure, precursor NiO powder is ball-milled to reduce the grain size to the nanometric scale; then, the milled powder is subjected to high-temperature treatments in H2, inducing the reduction to metallic Ni. Typically, the samples consist of Ni nanocrystallites (size of the order of 10 nm) dispersed in a nanocrystalline NiO matrix, as observed by electron microscopy (HRTEM) [1]. In particular, Ni/NiO samples have been prepared by annealing in H2, at selected temperatures (200 < Tann < 300 °C), NiO powder previously milled for 5, 10, 20 and 30 hours. The structural features of the samples have been investigated by X-ray diffraction and the low-temperature magneto-thermal behavior and EB properties have been analyzed by SQUID magnetometry. The structure and composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both Tann and the milling time of the precursor NiO powders [2]. 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, so that, Tann being equal, the amount and the mean grain size DNi 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 Hex: the highest value (~ 940 Oe) has been measured at T = 5 K in a sample containing ~7 wt % Ni and with DNi = 19 nm. However, in Ni/NiO samples with very different structural characteristics and different values of Hex at T = 5 K, the EB effect vanishes at the same temperature (~ 200 K) and the same thermal dependence of Hex 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. [1] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M. Vittori Antisari, E. Piscopiello, Phys. Rev. B 77 (2008) 094408 [2] L. Del Bianco, F. Spizzo, M. Tamisari, J. Magn. Magn. Mater. (2009, in press