The microstructure role in the NiO nanoparticles exchange bias bahavior.

NiO is an antiferromagnetic (AF) material in its bulk configuration. However, fine particles of this material can present complex magnetic properties, increasing the appeal for investigating the mechanisms involved in this change and possible use in technological purposes. Properties such as the superparamagnetic relaxation of spin lattices and weak ferromagnetism (FM) emerge from controlling the surface-to-volume ratio and structural disorder. Indeed, they may be responsible for surface effects that allow interactions between the AF and FM interfaces [1-2]. In order to study the evolution of magnetic behavior, as a function of size and structural properties of the particles, we prepared a series of NiO nanoparticles obtained by decomposition of Ni(OH)$_2$ at different temperatures (250ºC - 700ºC). Preliminary magnetic measurements using a PPMF -Evercoll II magnetometer reveal an FM long-range order which increases for smaller particles. We also observe a shift of the hysteresis loops (MxH curves) for T < 220K measures, showing a sign of AF-FM interface interaction (exchange bias). Using Transmission Electron Microscopy (TEM), the particle average sizes were determined, ranging from 3 to 87 nm. X-ray Profile Analysis (XPA) provided further structural analysis by combining Rietveld refinement and Convolutional Multiple Whole Profile fitting (CMWP) [3]. By assuming that the defects structure mainly arises from the dislocation and the crystalline size distribution (coherent domain), we fit the diffractions patterns. Thus, we may confirm that the higher magnetic moment magnitude and exchange bias for the smaller nanoparticles result from the increased density of defects and the smaller particle size. The unit cell parameters also increased for smaller particles, which appears to determine the higher Ni-O-Ni superexchange.

[1] R. Patta, K. Bhagaban, P. Alagarsamy, Aip Advances, v. 5, n. 8, p.087116, (2015).
[2] B. Lucia, et al, Physical Review B, v. 77, n. 9, p. 094408, (2008).
[3] R. Gábor. U. Tamás, G. Jenõ, Journal of Applied Crystallography, v. 34, n. 5, p. 669–676, (2001).