MAGNETISM AND SPIN DYNAMICS OF NANOSCALE FEOOH PARTICLES

The nature of magnetism and electron paramagnetic resonance (EPR) spin dynamics in 30 particles of FeOOH have been investigated in the range 5-350 K. X-ray diffraction shows that the particles are neither alpha -Fe2O3 nor alpha-FeOOH based and they convert to Fe3O4 (alpha-Fe2O3) o n heating to 800 K in vacuum (air). Magnetization M vs T data show a b locking temperature TB in the range 40 K<T(B)<100 K so that for T grea ter-than-or-equal-to 100 K, scaling of M with HIT for H up to 50 kOe v erifies superparamagnetism. In EPR, both the linewidth DELTAH and the g value decreases upon increasing T so that near 300 K, g congruent-to 2.05 (characteristic of Fe3+). A model of motional narrowing is used to explain the DELTAH vs T behavior. The dramatic changes in M, DELTAH , g value, and the recoilless fraction of Mossbauer spectroscopy obser ved near 50 K are all related to the thermal motion of the superparama gnetic nanoscale particles.