Ordering and magnetic properties of FePt film with (001) texture

Abstract

FePt films with a thickness of 25 nm were deposited by magnetron sputtering onto the MgO(001) substrates at 400℃, and subsequently annealed at temperatures (<italic>T</italic>_a) between 500℃-800℃ for 5 hours. The structures and magnetic properties of FePt films were analyzed by X-ray diffraction, vibrating sample magnetometer, and in-field magnetic force microscope. The as-deposited films show a (001) epitaxy induced by the MgO(001) substrates, but a disordered soft magnetic <italic>A</italic>1 phase. The ordering did not occur until <italic>T</italic>_a was raised up to 600℃. At <italic>T</italic>_a =600℃, the films changed into a mixture of soft <italic>A</italic>1 phase and hard<italic> L</italic>1₀ phase due to the partial <italic>A</italic>1→<italic>L</italic>1₀ transformation, and a special soft-hard phase composite with magnetic anisotropy can be obtained. The in-plane magnetization is mainly contributed by the soft phase with a coercivity exceeding 10 kOe; whereas the out-of-plane magnetization is dominant by the hard phase with a coercivity of about 5 kOe. These results indicate the existence of strong coupling between the soft and hard phases, leading to the formation of exchange springs. After annealing at 700℃, the structure of FePt films became almost ordered. The easy magnetization axis turned completely to the out-of-plane direction and a large coercivity higher than 20 kOe was observed. The atomic force microscope images show that the films treated at 700℃ are composed of island-like grains. Additionally, the in-field magnetic force microscope images indicates that most of grains initially having multiple domains change into single domain by domain wall displacing and vanishing at low field. The nucleation is supposed to be the main mechanism of magnetization reversal of grains.