Dynamic and magnetic excitations in ultrafine particles

Mossbauer spectroscopy of 57 Fe nuclei has been employed to determine relative recoilless fractions and hyperfine fields in a number of ultrafine iron and iron oxide particles prepared under a variety of conditions. The f-factor data are compatible with a model that assumes that some of the microcrystals undergo complete recoil upon absorption or emission of the gamma ray; this number depends on the temperature and on the supporting medium. The iron oxide hyperfine fields fit a collective magnetic excitation model proposed earlier by Morup and Topsoe However, for alpha -iron particles, some as small as 10 nm, the hyperfine field is the same as the bulk value.