Coupling between exchange bias effect and giant magnetoresistance in a Ni/NiO nanogranular sample

We have studied the magnetotransport properties of a Ni/NiO nanogranular sample, showing exchange bias (EB) effect. The sample was obtained by milling precursor NiO powder for 30 hours to reduce the mean grain size to ~ 20 nm; then, the milled powder was annealed in H2 for 30 minutes at the temperature of 250 °C, to induce the partial reduction to metallic Ni. Typically, samples prepared by this procedure consist of Ni nanocrystallites dispersed in a nanocrystalline NiO matrix [1]. As for this sample, X-ray diffraction analysis reveals that the Ni phase is about 30 volume % and the mean grain size DNi = 14 nm. The magnetoresistance (MR) was measured as a function of T and H on a cold-compacted Ni/NiO specimen, using the standard four-probe technique, in a SQUID magnetometer operating in the 5-300 K temperature range at a maximum applied field H = 50 kOe. Magnetization loops, M(H), and MR(H) loops were measured both after zero-field-cooling (ZFC) and after field-cooling (FC) in Hcool = + 20 kOe, from T = 300 K down to selected measuring temperatures T. At T = 5 K, the sample exhibits EB effect, as revealed by the horizontal shift of the FC M(H) loop (the exchange field Hex ~ 460 Oe); it decreases with increasing T and is almost vanished at T = 250 K. The low value of the electric resistance R at T = 5 K (~ 0.5 Ohm) and its 20% increase with increasing T up to 300 K indicate the existence of a metallic conduction channel across the sample. We define MR = [R(H)-R(0)]/R(0), where R(0) is the highest resistance value. At T = 5 K, a negative MRmax ~ – 0.40 % is measured at H = 50 kOe, both in the mode with the current flowing perpendicular to H and in the mode with the current parallel to H. This implies that the effect is of the type usually referred to as Giant-MR and is caused by the electronic scattering from the nonaligned magnetic moments of the Ni nanocrystallites. Thus, despite the creation of a percolation metallic network allowing electronic transport, the Ni nanocrystallites, or at least a part of them, act as separated ferromagnetic entities and are not magnetically coupled. In fact, in this last condition, they would form a ferromagnetic network, unsuitable for the observation of Giant-MR. At T = 5 K, the FC MR(H) loop appears shifted along the H axis, in good agreement with the value of Hex, and the effect exhibits the same thermal dependence as derived from the M(H) loops. Hence, at all T, a strict coupling exists between EB and MR effects in the investigated sample. Finally, the MR effect is seen to increase with increasing T and, at T = 300 K, MRmax ~ – 0.57%. This anomalous behaviour is explained considering that a structural NiO disordered phase, with glassy magnetic properties, surrounds the Ni nanocrystallites [1]. At very low T, below ~ 50 K, the magnetic moments of this phase are completely frozen and even under the maximum H a poor alignment is attained. However, with increasing T, the magnetic moments become progressively unfrozen and, for applied field H > 20 kOe, they provide a substantial contribution to MR. [1] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M. Vittori Antisari, E. Piscopiello, Phys. Rev. B 77 (2008), 094408