Goethite (α-FeOOH) magnetic transition by ESR, Magnetometry and Mossbauer

A natural sample of the mineral goethite was characterized by X-band Electron Spin Resonance (ESR), vibrating sample Magnetometry and Mossbauer spectroscopy techniques, with the main objective of studying the magnetic transition from the antiferromagnetic to the paramagnetic state that this mineral undergoes upon reaching a certain critical temperature (Neel temperature). Although an ESR signal was not expected in goethite samples at room temperature, due to its antiferromagnetic arrangement, a resonance line was observed. This behavior was attributed to the existence of vacancies in the mineral structure. Increasing the temperature from RT to 352 K no significant change occurred in the ESR spectra. From 352 K, the goethite spectrum showed an additional ESR line, the intensity of this line grew until it stabilized at around 444 K. The appearance of this new resonance line, and its evolution with temperature, was attributed to two subsequent effects: firstly the spin flop effect from 352 to 372 K; and secondly, the transition to the paramagnetic state at T-N = 372 K. Magnetometry and Mossbauer measurements corroborated this assumption and the transition temperature was identified by magnetometry as T-N = 370 K. Simulations of the goethite ESR spectra in both antiferromagnetic and paramagnetic states were performed by deconvolution of the resonance lines into two contributions, one from species with vacancies in their vicinity, and another with complete vicinity. (C) 2016 Elsevier B.V. All rights reserved.