Coexistence of exchange bias effect and giant magnetoresistance in a Ni/NiO nanogranular sample

Abstract

We have studied the coexistence of exchange bias (EB) effect and spin-dependent magnetotransport in a Ni/NiO nanogranular sample by measuring the magnetization (M) and the magnetoresistance (MR) versus the magnetic field (H) in the 5-250 K temperature (T) range, both in zero-field-cooling (ZFC) and field-cooling (FC) conditions. The sample consisted of Ni nanocrystallites (mean size ∼13 nm) dispersed in a NiO matrix; the Ni volume fraction was ∼33%, above the percolation threshold for electrical conductivity, as revealed by the low resistivity (order of 10−3 Ωm) and by its growth with increasing T. The EB and magnetotransport phenomena appear strictly intertwined: the FC M(H) and MR(H) loops exhibit a similar horizontal shift, corresponding to an exchange field of ∼460 Oe at T = 5 K, which decreases with increasing T and disappears at ∼200 K. Both the EB and the magnetotransport properties have been explained, considering the presence of a structurally disordered component of the NiO matrix around the Ni nanocrystallites, whose spin-glass-like magnetic character rules the interface exchange interaction with the Ni phase and the spin-dependent conductivity.