First XANES evidence of a disorder-order transition in a spinel ferrite compound: nanocrystalline ZnFe2O4

In situ Zn K-edge XANES experiments were performed to investigate the thermal evolution of the non-equilibrium state in nano-sized ZnFe2O4. The initially disordered ferrite was annealed under oxygen atmosphere and kept at temperatures of 673, 773 and 873 K. Modifications of the XANES features allowed the direct detection of the Zn local surrounding changes from Oh to Td symmetry. Quantitative analyses of these results were performed by using the principal-component analysis approach. The ferrite inversion does not change until the activation barrier is overcome at Ta = 585 K. Above Ta, the Zn ions continuously change their environment to their normal equilibrium state. Isothermal treatments confirm that the Zn transference follows a first-order kinetic process. In addition, the thermal treatment produces a partial recrystallization that increases the average grain size from 13 to 50 nm and reduces the microstrain. The room-temperature magnetic state changes from ferrimagnetic to paramagnetic, while the blocking temperature increases after the treatment.