Size-dependent antiferromagnetism and direct observation of Néel axes in NiO nanoparticles

A comprehensive understanding of antiferromagnetism in nanostructures confined in three dimensions remains elusive. This work addresses this fundamental issue by studying samples of highly crystalline single-phase NiO nanoparticles of 6, 20, and 34 nm average size, prepared by an optimized two-step synthesis. All the samples exhibit prominent antiferromagnetic behaviour with an overlapping superparamagnetic contribution due to uncompensated spins at the particle surface and at the crystallite boundaries within the particles, which becomes nearly undetectable for 34 nm particles. Using synchrotron X-ray photoemission electron microscopy combined with magnetic linear dichroism, a determination of the antiferromagnetic Néel axis was obtained for a subset of individual 34 nm particles. No thermal fluctuations of the Néel axes are observed at room temperature; instead, they are stochastically aligned along easy directions compatible with the particle crystal facets resting on the substrate. Consequently, single domain states of two sublattices appear to prevail in this size range, in contrast with a seminal model predicting multi-sublattice arrangements. These findings provide significant insights into antiferromagnetism in nanostructures and open up new possibilities for data storage based on specific states of Néel axes.