What could possibly be new in MnO 46 years after the discovery of its antiferromagnetic transition?: a spin-glass transition in the (25-50)-K temperature range. The susceptibility of MnO powders obtained by various treatments (MnO aged 30 years in ambient atmosphere, ${\mathrm{MnO}}_2$ reduced at 500 and 1000°C in hydrogen, and MnO heated in air at 500°C) was measured in low dc and ac magnetic fields and in a high dc magnetic field. The spin-glass behavior is linked to a slight offstoichiometry: $\mathrm{Mn}{\mathrm{O}}_{1+x}$ with $x\le 0.01$. It is not believed that the susceptibility peak arises from a mixture of MnO and ${\mathrm{Mn}}_3$${\mathrm{O}}_4$. This conclusion is based on the shape of the susceptibility curves at both the spin-glass temperature ($T_{\mathrm{SG}}$) and the Néel temperature as well as on the various $T_{\mathrm{SG}}$ values which differ from the ferrimagnetic transition of ${\mathrm{Mn}}_3$${\mathrm{O}}_4$. It is more likely that the spin-glass interaction occurs either between ${\mathrm{Mn}}_3$${\mathrm{O}}_4$-like clusters or between defects typical in MnO such as an interstitial Mn surrounded by four Mn vacancies.

• Received 31 May 1984

DOI:https://doi.org/10.1103/PhysRevB.30.5167