Tuning the exchange coupling in pulse laser deposited cobalt ferrite thin films by hydrogen reduction

Pulse laser deposited cobalt ferrite CoFe₂O₄ (CFO) thin films were transformed to CFO/Co-Fe solid solution hard/soft nanocomposite thin films by annealing the CFO films in hydrogen atmosphere. By controlling the annealing time, the level of transformation between the two phases was controlled thereby the exchange coupling and magnetic properties of the nanocomposite layer. X-ray diffraction patterns showed the formation of (0 0 l) orientation for the CFO hard phase and (0 l l) orientation for bcc Co-Fe solid solution soft phase in all samples. Atomic force microscopy images revealed that by increasing the annealing time, the surface roughness of the films increased. Magnetic measurements demonstrate that the more the annealing time, the higher the saturation magnetization and the lower their coercivity and magnetic anisotropy. Hysteresis loops of the films show that these samples behave like single-phase materials indicating the presence of exchange coupling between two different hard and soft phases. Using X-ray photoelectron spectroscopy (XPS), we determined the cations distribution variation in our samples thus a change in the spinel inversion parameter (y) from 0.70 for the as-deposited CFO to 0.49 for the films reduced in hydrogen for 60 min. The Raman spectroscopy confirms the XPS results in our samples. The present approach may help the development of methods for tuning the saturation magnetization, coercivity and magnetic easy axis direction in magnetic nanocomposite thin films and magnetic oxide-based spintronic devices.