Magnetic fluctuations of goethite (α-FeOOH) analyzed through Al substituted samples

Goethite is a Fe3+ oxide-hydroxide very common in nature. At room temperature it shows an antiferromagnetic ordering, with Néel temperature of 400 K. Despite this, it is well known that it also presents magnetic fluctuations, whose origins are still under discussion in the literature. For this research, goethite samples were synthesized with and without Al substitutions. Micrographs obtained by transmission electron microscopy (TEM) showed the presence of defects, as kinks and steps, at the rod-shaped particles of samples with Al substitutions. With Electron Spin Resonance (ESR) and Mössbauer experiments, it was observed an increase in the magnetic fluctuations in these samples, due to imbalanced spins. Such effect was related to magnetic mismatch caused by the presence of Al ions at the goethite structure. This mismatch can be caused by the increase of crystalline defects, such as grain boundaries, or due to the proximity of Fe ions to diaspore-like clusters. •SEM/TEM analysis indicate that Al substitutions causes higher defects concentration.•ESR and Mössbauer showed increase of magnetic fluctuations in Al substituted samples.•The presence of Al in the goethite structure weakens the interaction among Fe ions.•The weakening of interactions can be caused by defects or to Fe by Al substitutions.