Weak ferromagnetic component in goethite (α-FeOOH) and its relation with microstructural characteristics

Goethite is an antiferromagnetic material that frequently presents a superimposed weak ferromagnetic component. In this research, two goethite samples were prepared using distinct synthesis routes, being later subjected to heat treatment. Magnetization dynamics were analyzed as a function of applied magnetic field and temperature. Moreover, the magnetic measurements were correlated with the resulting microstructures, obtained through scanning and transmission electron microscopy. The results indicate an enhanced ferromagnetic behavior in those goethite samples, with greater presence of defects and imperfections. Besides that, it was observed a reduction of magnetic fluctuations after the healing of the samples via heat treatment. Despite what is vastly proposed in the literature to explain the origin of the magnetic component in goethite, our analysis showed that, for the studied samples, this effect is caused by a magnetic mismatch in the antiferromagnetic lattice. This mismatch is possibly caused by defects as grain boundaries, voids and grain rotations, which may lead to dislocations and consequently to magnetic mismatches in the lattice.