Magnetic moment distribution in nanosized antiferromagnetic NiO

Nanosized antiferromagnets show anomalously large magnetization and superparamagnetism, having complicated magnetic properties due to a competition between core and surface spins in addition to the surface-induced effect, the finite size effect, and the magnetic moment distribution. A significant distribution of magnetic moments, which are responsible for the superparamagnetism, can also exist in real ensembles of nanoparticles, making the analysis of magnetization difficult. It has been a key issue for the understanding of the complicated magnetic property to correctly separate a superparamagnetic component from an antiferromagnetic component that linearly depends on magnetic fields in the observed magnetization. Here, we report the separation of these two components observed in the magnetization of antiferromagnetic NiO nanoparticles by applying the scaling law in the magnetization of superparamagnets. The distribution of the magnetic moment was taken into account without a priori assumption of the distribution function. The magnetic moments causing superparamagnetism had a bimodal distribution with different mean values. The contribution of the two different superparamagnetic components could thus be newly clarified and was ascribed to the uncompensated spins in the core and on the surface. The magnetic structure that leads to the bimodal distribution is discussed in light of the inhomogeneity of the surface structure.