Thickness dependence of the perpendicular magnetic anisotropy and magnetic-domain structures in nickel-ferrite films

Inverse spinel NiFe₂O₄ (NFO) films were prepared on (110) MgAl₂O₄ substrates by using pulsed laser deposition. The NFO films were deposited at a substrate temperature of 600 °C and an oxygen partial pressure of 1 mTorr, and their thicknesses were in the range of 20 nm - 240 nm. As the film's thickness increased, the positon of the (440) NFO peak moved to a higher angle, implying a relaxation of the lattice strain. For 240 nm, the lattice constant of the NFO film was about 8.39 Å, which corresponded to a lattice mismatch of about 0.6% with the lattice constant of the NFO bulk. The magnetic hysteresis measurement suggested an increasing tendency of the perpendicular magnetic anisotropy with increasing film thickness. For 160 nm, the ratio of the remanent magnetization to the saturation magnetization (M_rem./M_sat.) was maximally ∼0.22. The magnetic domain structure was observed by using magnetic force microscopy. The image contrast was improved with increasing film thickness, and it was the best for a film with a thickness of 160 nm.