Magnetism and spin dynamics of nanoscale FeOOH particles

Abstract

The nature of magnetism and electron paramagnetic resonance (EPR) spin dynamics in 30 Å particles of FeOOH have been investigated in the range 5–350 K. X-ray diffraction shows that the particles are neither α-Fe2O3 nor α-FeOOH based and they convert to Fe3O4 (α-Fe2O3) on heating to 800 K in vacuum (air). Magnetization M vs T data show a blocking temperature TB in the range 40 K<TB<100 K so that for T≥100 K, scaling of M with H/T for H up to 50 kOe verifies superparamagnetism. In EPR, both the linewidth ΔH and the g value decreases upon increasing T so that near 300 K, g≂2.05 (characteristic of Fe3+). A model of motional narrowing is used to explain the ΔH vs T behavior. The dramatic changes in M, ΔH, g value, and the recoilless fraction of Mössbauer spectroscopy observed near 50 K are all related to the thermal motion of the superparamagnetic nanoscale particles.