MAGNETIC MICROSTRUCTURE OF EXCHANGE BIASED Ni/NiO NANOGRANULAR SAMPLES INVESTIGATED BY MAGNETORESISTANCE MEASUREMENTS

We present the magnetotransport properties of two Ni/NiO nanogranular samples, produced in form of powder through a two-step procedure based on mechanical milling from coarse-grained NiO and subsequent thermal treatments in hydrogen, inducing the partial reduction to metallic Ni. Typically, samples prepared by this method consist of Ni nanocrystallites dispersed in a NiO matrix [1]. As for the selected samples, X-ray diffraction analysis revealed that the Ni volume fraction was ~ 33 % and ~ 61% and the Ni mean grain size was ~ 13 nm and ~15 nm, respectively (the two samples were labeled Ni30 and Ni60). The magnetoresistance (MR) was measured on cold-compacted pellets, using the standard four-probe technique, in a SQUID magnetometer. The field was applied parallel to the specimen surface and the current was applied in-plane, both parallel and perpendicular to the field. Magnetization loops, M(H), and MR(H) loops were measured in the 5-250 K range after both zero-field-cooling and field-cooling. In both samples, the Ni content is above the percolation threshold for electrical conductivity, as revealed by the low resistivity (of the order of 10-3 Ohm m in Ni30 and 10-5 Ohm m in Ni60) and by its growth with increasing T. However, Ni30 exhibits just isotropic spin-dependent magnetoresistance (GMR), whereas in Ni60 both GMR and anisotropic magnetoresistance (AMR) contributions are present. A key feature of these Ni/NiO samples is that they show exchange bias (EB) effect: it originates from the exchange interaction at the interface between the Ni phase and a structurally disordered component of the NiO matrix, which surrounds the Ni nanocrystallites and exhibits spin-glass like properties [1]. We will show that EB and MR phenomena are strictly intertwined so that both the GMR and, in Ni60, the AMR signals, measured in field-cooling, undergo a shift along the field axis as observed in the field-cooled M(H) loops, corresponding to exchange field values, at T = 5 K, of ~ 460 Oe and ~ 130 Oe for Ni30 and Ni60, respectively. This coupling between EB and MR allows gaining a deeper insight into the magnetic microstructure of the samples and its evolution with temperature. In fact, the results can be explained considering that the glassy NiO phase, giving rise to EB, is also present in the boundary region between adjacent Ni nanocrystallites and regulates the electronic transport as well as the transmission of the ferromagnetic exchange interaction to neighboring Ni nanocrystallites. Thus, depending on the Ni content, two different magnetic arrangements of the Ni nanocrystallites stem out (below and above the magnetic percolation threshold), determining the magnetotransport and EB properties. [1] L. Del Bianco, F. Boscherini, A.L. Fiorini, M. Tamisari, F. Spizzo, M. Vittori Antisari, E. Piscopiello, Phys. Rev. B 77 (2008) 094408