(001)取向FePt薄膜的有序化过程及磁性

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

在加热到400°C的MgO(001)单晶基片上,用磁控溅射法沉积了25 nm厚的FePt薄膜,在Ta=[500°C,800°C]温度范围进行5 h的热处理.用X射线衍射仪、振动样品磁强计和可外加磁场的磁力显微镜分析了薄膜的结构和磁性.结果表明,未经热处理的薄膜能够在MgO(001)单晶基片的诱导下实现(001)取向生长,但仍处于无序的A1相,呈软磁性.Ta=500°C,薄膜结构没有明显改变.Ta=600°C,FePt发生部分有序化,薄膜中A1相和L10相(有序相)共存,形成一种具有磁各向异性的特殊硬磁-软磁复合体.软磁相的磁性主要表现在沿平行于膜面方向施加磁场的磁化曲线中,但矫顽力可以达到10 kOe(1Oe=103/4πA m-1),硬磁相的磁性主要表现在沿垂直于膜面方向施加磁场的磁化曲线中,矫顽力却只有5kOe.这说明薄膜中硬磁相和软磁相之间存在强烈的交换耦合,形成了磁性弹簧.当Ta提高到700°C,薄膜基本完成有序化,磁化易轴彻底转向垂直于膜面的方向,矫顽力大于20 kOe.原子力显微镜和磁力显微镜观察表明,薄膜由岛状颗粒构成,在Ta=700°C时大部分颗粒内部形成多磁畴结构,在不太大的磁场作用下依靠畴壁移动和消失变为单磁畴,磁化反转过程应该主要依靠形核.

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 （Ta） 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 A1 phase. The ordering did not occur until Ta was raised up to 600℃. At Ta =600℃, the films changed into a mixture of soft A1 phase and hard L10 phase due to the partial A1→L10 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.