Unexpected surface superparamagnetism in antiferromagnetic Cr2O3 nanoparticles

Abstract

We report an unexpected superparamagnetic behavior of antiferromagnetic Cr₂O₃ nanoparticles. The Cr₂O₃ particle cores retain their original antiferromagnetic phase, while the surfaces of the particles become superparamagnetic. The X-ray diffraction results confirm that the sample has a corundum structure without any other phases. Through X-ray photoelectron spectroscopy characterization, the particle surfaces present three different oxidation states: Cr³⁺ (antiferromagnetic), Cr⁴⁺ (ferromagnetic), and Cr⁶⁺ (nonferromagnetic). A bimagnetic particle model with Cr³⁺ cores and higher Cr oxidation surface states is used to explain the experimental results. In addition, we observe that spin-flop transitions occur in the antiferromagnetic cores below the Néel temperature (292 K). The spin-flop transition field is uncommon compared with other research, this novel behavior is attributed to the presence of superparamagnetism in the surfaces through the exchange field. These findings reveal the significance of surface states in mediating the magnetic properties in antiferromagnetic materials.