Magnetic microstructure of Ni/NiO nanogranular samples studied by magnetoresistance measurements

A main issue in the magnetism of nanoparticles is gaining a fundamental understanding of the magnetic interaction and/or aggregation mechanisms with increasing their volume concentration. In this work, the study of the magnetoresistive properties, in the 5-250 K temperature range, has been used as a powerful tool to gain information on the magnetic microstructure of two Ni/NiO samples consisting of Ni nanocrystallites (mean size of the order of 10 nm) in a NiO matrix and differing for the Ni amount, ~ 33 and ~ 61 vol. %. They exhibit a metallic-type conduction mechanism indicating that the Ni content is above the percolation threshold for electrical conductivity, even if the electric resistivity is definitely smaller in the sample with higher Ni fraction (10-5 Ωm against 10-3 Ωm). An isotropic spin-dependent magnetoresistive response has been measured in the sample with lower Ni content, whereas both isotropic and anisotropic magnetoresistance phenomena coexist in the other case. The study of these behaviors, coupled to magnetization loop measurements and to the discussion of the exchange bias effect characterizing both samples [1], allows one to draw a picture for their magnetic configuration: in the sample with lower Ni content, neither the physical percolation of the Ni nanocrystallites nor the magnetic percolation (i.e., formation of a homogeneous ferromagnetic network associated with a magnetic-domain pattern) are achieved; in the other sample physical percolation is reached while magnetic percolation is still absent. In both descriptions, a key role is played by the NiO matrix that brings about a magnetic nanocrystallite/matrix interface exchange energy term and rules the direct magnetic exchange interaction among the Ni nanocrystallites and, consequently, the magneto-transport properties of both materials. [1] L. Del Bianco et al., Phys. Rev. B 77, 094408 (2008)