Morin temperature of annealed submicronic α-F2O3 particles

The Morin temperature (${T}_{M}^{0}$) of annealed, monodispersed, nearly spherical particles having diameters (d) between (0.07--0.62) \ensuremath{\mu}m 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=\ensuremath{\infty} gives ${T}_{M}^{0}$=264\ifmmode\pm\else\textpm\fi{}2 K, the Morin temperature of the bulk \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$. Furthermore, ${T}_{M}^{0}$=0 implies d=83\ifmmode\pm\else\textpm\fi{}5 A\r{}, which is the size limit for the onset of superparamagnetism in this system.