具有不同交换偏置方向的外延FeGa/IrMn双层膜的磁各向异性与磁化翻转

Magnetic anisotropy and reversal in epitaxial FeGa/IrMn bilayers with different orientations of exchange bias

采用磁控溅射方法在MgO(001)单晶衬底上制备了交换偏置分别沿着FeGa[100]和[110]方向的FeGa/IrMn外延交换偏置双层膜,研究了交换偏置取向对磁化翻转过程与磁化翻转场的影响.铁磁共振场的角度依赖关系的测量与拟合,表明样品存在不同取向的四重对称磁晶各向异性、单向交换磁各向异性和单轴磁各向异性的叠加.矢量磁光克尔效应测量表明交换偏置沿着[100]方向的样品在不同磁场方向下表现矩形、非对称和单边两步磁滞回线;交换偏置沿着[110]方向的样品在不同磁场方向下表现单边两步和双边两步磁滞回线.考虑不同交换偏置方向的畴壁形核和位移模型,能够很好地解释磁化翻转路径随磁场方向的变化规律和拟合磁化翻转场的角度依赖关系,表明交换偏置方向的改变使得畴壁形核能发生显著变化.

Epitaxial FeGa/IrMn bilayers with exchange biases along the FeGa[100]and[110]directions are prepared on MgO(001)single crystal substrates by magnetron sputtering through controlling the orientation of the external field in situ applied during growth.The effect of the exchange bias orientation on the magnetic switching process and the magnetic switching field are studied.The X-rayφ-scan indicates that the FeGa layer is epitaxially grown with a 45°in-plane rotation on the MgO(001)substrate along the FeGa(001)[110]direction and the MgO(001)[100]direction.The measurements of the angular dependence of the ferromagnetic resonance field and the corresponding fitting to the Kittel equation show that the samples have a superposition of fourfold symmetric magnetocrystalline anisotropyK1,unidirectional magnetic exchange bias anisotropy K;,and uniaxial magnetic anisotropy K;with configuration of K;//[100]or K;//[110].The combined longitudinal and transverse magneto-optical Kerr effect measurements show that sample with K;//[100]exhibits square loops,asymmetrically shaped loops,and one-sided two-step loops in different external magnetic field directions.In contrast,the sample with K;//[110]exhibits one-sided two-step and two-sided two-step loops as the magnetic field orientation changes.Because the K;is superimposed by K;and K;,the in-plane fourfold symmetry of the magnetic anisotropy energy is broken.The local minima are no longer strictly along the in-plane<100>directions,but make a deviation angle which depends on the relative orientation and strength of magnetic anisotropy.A model based on the domain wall nucleation and propagation is proposed with considering the different orientations of K;,which can nicely explain the change of the magnetic switching route with the magnetic field orientation and fit the angular dependence of the magnetic switching fields,indicating a significant change of domain wall nucleation energy as the orientation of K;changes.