This Letter reports a remarkable influence of light on the magnetic properties of nanocrystalline α-Fe 2 O 3 . EPR spectroscopy shows that the α-Fe 2 O 3 is superparamagnetic, showing an intense narrow EPR signal at room temperature which shifts to a lower field and broadens to be barely detectable at 77 K or below. In situ irradiation of samples held in vacuo at 4 or 77 K with near-UV or visible light causes the appearance of an intense new EPR signal, which decays immediately after the irradiation is stopped. We suggest that the generation of conduction band electrons by irradiation into the band gap of the α-Fe 2 O 3 moderates the coupling between the magnetic domains in these nanocrystals.This effect is largely suppressed in the presence of adsorbed oxygen, which can scavenge conduction band electrons.
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Macdonald et al. (2010) studied this question.
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