Morin temperature of annealed submicronic α-${\mathrm{F}}_2$${\mathrm{O}}_3$ particles

The Morin temperature ($T_M^0$) of annealed, monodispersed, nearly spherical particles having diameters (d) between (0.07–0.62) μm have been investigated. A plot of $T_M^0$ versus 1/d is linear. The extrapolation of this plot between 230 and 262 K to d=∞ gives $T_M^0$=264±2 K, the Morin temperature of the bulk α-${\mathrm{Fe}}_2$${\mathrm{O}}_3$. Furthermore, $T_M^0$=0 implies d=83±5 Å, which is the size limit for the onset of superparamagnetism in this system.