Local magnetic inhomogeneity and weak ferromagnetism in Al-substituted goethite (α-Fe1-xAlxOOH)

Goethite (α-FeOOH) is an antiferromagnetic iron oxyhydroxide that frequently exhibits a weak ferromagnetic component, a behavior widely discussed in the literature but still not completely understood at the microscopic level. In Al-substituted goethites, this magnetic response becomes even more complex due to the combined effects of magnetic dilution and local structural perturbations introduced by the substitution of Fe 3+ with non-magnetic Al 3+ ions. In this study, we investigated the magnetic behavior of synthetic goethites with different Al contents through magnetization measurements as a function of the field and temperature, in combination with compositional and structural characterization using Wavelength Dispersive X-ray Fluorescence (WDXRF) and Raman spectroscopy. The results reveal a progressive reduction in the Néel temperature with increasing Al concentration, accompanied by pronounced magnetic irreversibility at low fields and a decoupling between the temperature evolution of the coercive field and the remanent magnetization. In contrast to pure goethite, where these quantities evolve more coherently, the Al-substituted samples show a faster suppression of remanence, while coercivity remains finite and close to the magnetic transition temperature, indicating distinct mechanisms governing magnetization reversal and magnetic remanence. When considered together with previous spectroscopic results reported for the same samples, these findings support the scenario of intrinsic magnetic inhomogeneity. The magnetic behavior is consistent with the diamagnetic-island model, in which Al incorporation leads to the formation of magnetically weakened regions within the goethite matrix, giving rise to local perturbations of antiferromagnetic order. The present results provide new magnetic evidence supporting the model previously proposed for Al-substituted goethites, while remaining compatible with literature models that attribute weak ferromagnetism in pure goethite to defect- and microstructure-related magnetic disorder.