Abstract
Mössbauer spectra of antiferromagnetic goethite(α-FeOOH) particles usually show an asymmetric line broadening, which increaseswith increasing temperature, although the magnetic anisotropy is expected to beso large that magnetic relaxation effects should be negligible. By use of highresolution transmission electron microscopy we have studied a sample of goethiteparticles and have found that the particles contain many defects such as low anglegrain boundaries, in accordance with previous studies of other samples of goethiteparticles. Such defects can result in a magnetic mismatch at the grain boundariesbetween nanometer-sized grains, leading to a weakened magnetic coupling betweenthe grains. We show that the Mössbauer data of goethite can be explained byfluctuations of the sublattice magnetization directions in such weakly coupled grains. Itis likely that the influence of defects such as low angle grain boundaries alsoplays a role with regards to the magnetic properties in other antiferromagneticnanograin systems. We discuss the results in relation to Mössbauer studies ofα-Fe2O3 and α-Fe2O3/NiO nanoparticles.
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