Magnetism and spin dynamics of nanoscale FeOOH particles

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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<TBT≥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.