ANOMALOUS MOSSBAUER FRACTION IN SMALL MAGNETIC PARTICLES DUE TO MAGNETOSTRICTION

The biological molecule ferritin and its proven synthetic counterpart polysaccharide iron complex (PIC) have been shown to contain small(<l0 0 Angstrom in diameter) antiferrimagnetic cores at their centers. Moss bauer studies of these molecules have revealed an anomalous drop in th e Mossbauer fraction (f-factor) as the temperature rises above 30 K fo r mammalian ferritin and 60 K for PIC. Above the blocking temperature, superparamagnetic relaxation results in the disappearance of hyperfin e splitting. Data that are treated with FFT procedures to eliminate th e thickness effect still exhibit this anomaly. We have investigated th e effect of superparamagnetic relaxation on the f-factor. Spin-lattice relaxation was excluded based upon a calculation of the rate of energ y transfer from the spin system to the lattice. We have found the foll owing process as a plausible explanation of the anomaly: Superparamagn etic relaxation brings about a dynamical displacement of the Mossbauer nucleus through magnetostriction. These displacements produce a Doppl er broadening of the Mossbauer spectrum that reduces the apparent f-fa ctor. The temperature dependence of the theoretically calculated f-fac tor agrees qualitatively with experiment. Finally, there is semi-quant itative agreement if the as yet unknown dimensionless magnetostriction constant were to be on the order of 10(-3).