Antiferromagnetic Order in MnO Spherical Nanoparticles

TL;DR

This study investigates antiferromagnetic order in MnO nanoparticles of 8 nm and 13 nm, revealing size-dependent transition temperatures and magnetic moments, with most Mn ions ordered within individual nanoparticles.

Contribution

First detailed neutron diffraction analysis of antiferromagnetic order in monodisperse MnO nanoparticles of different sizes, showing core-shell magnetic structure.

Findings

8 nm MnO nanoparticles have a suppressed $T_N$ compared to bulk.

Magnetic moments are reduced in nanoparticles relative to bulk.

Approximately 80% of Mn ions in a nanoparticle are magnetically ordered.

Abstract

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8 nm sample, the antiferromagnetic transition temperature $T_N$ (114 K) is suppressed compared to the bulk material (119 K) while for the 13 nm sample $T_N$ (120 K) is comparable to the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2$\pm$0.3 $\mu_B$/Mn for the 8 nm sample and 3.9$\pm$0.2 $\mu_B$/Mn for the 13 nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80$%$ of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.