Mossbauer spectra of Fe 67 in a-Fe 2 0 3 have been measured as a function of particle size and temperature. Bulk a-Fe 2 03 shows a change in the sign of the quadrupole interaction in going through the Morin transition temperature, 263°K. Analyses of the spectra show that the magnetization vector is in the с plane above the transition temperature and parallel to the с axis below it. In contrast, finely divided a-FtzOz particles, sup ported on a high-area silica, do not undergo a Morin transition. The spectra show that the magnetization vector remains perpendicular to the с axis at least down to 10°K. It is evident that the spins are pinned by the surface in small particles. When the particle size is less than 135 A, the room-temperature spectrum con sists only of a quadrupole-split center line corresponding to superparamagnetic a-Fe 2 03. 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 с plane of (4.7db 1.1) X Ю 4 erg per 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 a-FezOz is determined. In addition, these data are used to calculate an independent value of the anisotropy constant of (4.1±1.1)X10 4 erg per cm 3 . Careful measurements on the series of samples with varying particle sizes show that the quad rupole splitting increases from the bulk value of 0.42 mm sec -1 to a value of 0.98 mm sec -1 for particles with an undetermined average size of less than 100 A.

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Mössbauer spectra of 57Fe in Fe2SiO4, which has iron atoms in two distinct crystallographic sites, have been obtained from 9°K to 300°K, a temperature range in which two magnetic transitions occur as indicated by magnetic susceptibility measurements. Above the first transition temperature, 66°K, the spectra consist of a pure quadrupole split line. Below this temperature Zeeman splitting is observed, resulting in composite spectra attributed to the two different iron sites. Magnetization curves indicate the presence of weak ferromagnetism between 66°K and the second transition temperature, 20°K. Below the second transition temperature Fe2SiO4 is an ideal antiferromagnet. The hyperfine spectra were analyzed by numerical solution of the complete interaction Hamiltonian. From this analysis values of the magnetic fields, the electric field gradients, and their relative orientations are obtained. These results are compared with the spin assignments that have been made on the basis of neutron diffraction studies.