Effect of growth temperature on the magnetic, microwave, and cation inversion properties on NiFe_2O_4 thin films deposited by pulsed laser ablation deposition

摘要

First principles band structure calculations suggest that the preferential occupation of Ni~(2+) ions on the tetrahedral sites in NiFe_2O_4 would lead to an enhancement of the exchange integral and subsequently the Neel temperature and magnetization. To this end, we have deposited NiFe_2O_4 films on MgO substrates by pulsed laser deposition. The substrate temperature was varied from 700 to 900℃ at 5 mTorr of O_2 pressure. The films were annealed at 1000℃ for different times prior to their characterization. X-ray diffraction spectra showed either (100) or (111) orientation with the spinel structure dependent on the substrate orientation. Magnetic studies showed a magnetization value of 2.7 kG at 300 K. The magnetic moment was increased to the bulk value as a result of postdeposition annealing at 1000℃. The as produced films show that the ferromagnetic resonance linewidth at 9.61 GHz was 1.5 kOe, and it was reduced to 0.34 kOe after postannealing at 1000℃. This suggests that the annealing led to the redistribution of Ni~(2+) ions to their equilibrium octahedral sites. Further, it is shown that the magnetically preferred direction of H_a can be aligned perpendicular to the film plane when films are grown with a fixed oxygen pressure of 5 mTorr for films deposited at 700 and 900℃.