Mössbauer Effect in Antiferromagnetic Fine Particles

Mössbauer measurements were made on fine particles of antiferromagnetic α-FeOOH with various particle sizes to examine the Mössbauer spectra in fluctuating fields produced by the relaxation of electron spins. In ultra fine samples, the collapse of the internal field due to superparamagnetism was observed. The critical volume at 294°K was found to be 3× 10-17 cm3 and the critical temperature for the sample of 1× 10-17 cm3 to be 210°K. The effective anisotropy constant was estimated to be of the order of 104 erg/cm3. The transitional patterns from the ordinary magnetic splitting (6-line) to the superparamagnetic doublet are discussed and the assignment of the doublet is attempted in comparison with Blume’s prediction.