Introduction
The bulk nickel oxide (NiO), due to its electronic structure and chemical bonding, is a Mott insulator and shows up an easy-plane antiferromagnetic (AFM) ordering of type-II, which has made it a challenge for the theory for decades. In the paramagnetic phase above the Néel temperature TN = 523 K [1], NiO has a cubic rock-salt crystal structure (Fm-3m). Below TN, the magnetic ordering results in that the spins of the Ni2+ ions order ferromagnetically in {111} planes [2, 3], and the structure of NiO undergoes a week cubic-to-rhombohedral distortion (R-3m) due to the magnetostriction effect [1, 4]. The magnetic structure of NiO nanoparticles can differ from the bulk magnetic structure, because of the influence of surface effects [5]. For example, a complex magnetic structure with as many as eight sublattices has been observed in NiO nanoparticles [6], in contrast to the bulk NiO which has a simple two-sublattice structure.
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