Some Properties of Supported Smallα−Fe2O3Particles Determined with the Mössbauer Effect

Abstract

M\"ossbauer spectra of ${\mathrm{Fe}}^{57}$ in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ have been measured as a function of particle size and temperature. Bulk $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ shows a change in the sign of the quadrupole interaction in going through the Morin transition temperature, 263\ifmmode^\circ\else\textdegree\fi{}K. Analyses of the spectra show that the magnetization vector is in the $c$ plane above the transition temperature and parallel to the $c$ axis below it. In contrast, finely divided $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ particles, supported on a high-area silica, do not undergo a Morin transition. The spectra show that the magnetization vector remains perpendicular to the $c$ axis at least down to 10\ifmmode^\circ\else\textdegree\fi{}K. It is evident that the spins are pinned by the surface in small particles. When the particle size is less than 135 \AA{}, the room-temperature spectrum consists only of a quadrupole-split center line corresponding to superparamagnetic $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. As the particle size is gradually increased, a 6-line hyperfine spectrum of increasing intensity appears. This transition from superparamagnetic to ferromagnetic behavior, coupled with a measurement of the average particle size by x-ray line broadening, leads to a calculated value of the crystalline anisotropy constant in the $c$ plane of (4.7\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}${10}^{4}$ erg per ${\mathrm{cm}}^{3}$. A sample with an average particle size just below that required for ferromagnetic behavior at room temperature, when cooled, displays an increasing fraction of ferromagnetic material as the temperature is decreased. From these data the particle size distribution of the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ is determined. In addition, these data are used to calculate an independent value of the anisotropy constant of (4.1\ifmmode\pm\else\textpm\fi{}1.1)\ifmmode\times\else\texttimes\fi{}${10}^{4}$ erg per ${\mathrm{cm}}^{3}$. Careful measurements on the series of samples with varying particle sizes show that the quadrupole splitting increases from the bulk value of 0.42 mm ${\mathrm{sec}}^{\ensuremath{-}1}$ to a value of 0.98 mm ${\mathrm{sec}}^{\ensuremath{-}1}$ for particles with an undetermined average size of less than 100 \AA{}.