磁性超薄膜中的磁各向异性和磁畴结构

【作者】 陈宫；

【导师】 吴义政；

【作者基本信息】 复旦大学 ， 凝聚态物理， 2011， 博士

【摘要】 近二十年来,自旋电子学的出现和发展不断激发着人们对超薄膜磁学研究的兴趣。由于薄膜表面和界面是影响磁性薄膜磁学性质的关键因素,并与自旋电子学器件的性能密不可分,对薄膜表面和界面相关的研究便成为理解磁性薄膜磁学性质或改善自旋电子学器件性能的重要途径之一。本博士论文对五个磁学性质和薄膜界面相关的课题进行了系统研究,主要包括磁性薄膜磁各向异性的定量研究、条纹磁畴的自旋结构及磁畴随外加磁场的变化等。(1)利用磁光克尔效应(]magnetic optic Kerr effect, MOKE)和旋转磁场的磁光克尔效应(magnetic optic Kerr effect with rotating field, ROT-MOKE),系统地研究了Au/Fe/GaAs(001)体系的磁各向异性随Fe薄膜厚度的变化关系,并且定量分离了界面贡献和体贡献,观察到了易轴沿[110]方向的单轴磁各向异性的体贡献。Fe薄膜中的单轴磁各向异性对生长温度非常敏感,单轴磁各向异性的界面贡献和体贡献之间的关联有可能来源于Fe/GaAs界面的各向异性。(2)利用MOKE和ROT-MOKE系统地研究了当MgO中间层很薄时Fe/MgO/GaAs(001)系统的晶体结构和磁各向异性。当MgO薄膜的厚度小于1ML时,Fe薄膜的<100>晶向随MgO薄膜厚度的增加从沿GaAs<100>方向转到了沿GaAs<110>方向。当MgO薄膜的厚度小于1.2ML时,单轴磁各向异性随着MgO薄膜厚度的增加而显著减弱,而四度磁各向异性的易轴从沿GaAs<100>方向转到了沿GaAs<110>方向。(3)利用MOKE和ROT-MOKE系统地研究了FeMn/Co/Cu(001)体系中Co的磁各向异性随Co和FeMn厚度的变化关系。我们发现,FeMn反铁磁序在Co薄膜内诱导了一个面内四度磁各向异性,该磁各向异性比顺磁态FeMn/Co界面的磁各向异性要大一个量级,这个反铁磁序诱导的磁各向异性来源于FeMn/Co界面的自旋阻挫,其大小约为0.118 erg/cm2。(4)利用自旋极化低能电子显微镜(Spin-polarized low-energy electron microscopy, SPLEEM)首次在条纹磁畴中通过三维实空间成像发现了室温的旋轮(cycloidal)手性磁有序。通过调控薄膜界面,可以控制手性磁有序的手性方向。同时通过Monte Carlo模拟表明Dzyaloshinskii-Moriya相互作用很可能是手性磁有序的来源。之后分别介绍了螺旋(helical)手性磁有序、磁泡磁畴的自旋结构以及磁畴中自旋倾斜的研究。(5)利用SPLEEM研究了不同宽度的条纹磁畴随外加垂直磁场的演化行为。我们发现和磁场方向相反的条纹磁畴会先随磁场的增大而变窄(小磁场行为),之后会一条接一条的消失(大磁场行为)直到形成单畴。在小磁场下,方向相反的磁畴面积比在条纹开始依次消失之前可以由理论公式拟合得出饱和场,开始依次消失的磁畴面积比和理论计算的结果非常接近；同时体系磁畴面积的不对称性(总磁化强度)随着磁场和零场条纹宽度乘积之间呈线性关系。在大磁场下,Fe/Ni/Cu(001)条纹依次消失的临界磁场要比Fe/Cu(001)条纹到磁泡相变的临界磁场大,但条纹消失时临界面积比的数值对于不同条纹宽度基本不变；体系磁畴面积的不对称性在条纹开始依次消失后不再满足原来的线性关系；和磁场方向相反的磁畴宽度消失前在大磁场下基本是一个常数,对于不同宽度的条纹磁畴,零场的磁畴宽度和在大场下的最小磁畴宽度之间存在一个线性的关系。更多还原

【Abstract】 Recently, research of magnetic ultrathin film was more and more attractive because of the emergence and development of spintronics. Since the surface and interface of thin films playing a key role to influent the magnetic property of thin films and performance of spintronics devices, the study related to the surface and interface of thin films is an important way to understand the thin films magnetic property or improve the spintronics device performance. In this thesis, five topics related to interface of ultrathin film are introduced. It mainly contains quantitatively study of magnetic anisotropy in magnetic thin films, spin structure of magnetic stripe domain and evolution of magnetic stripe domain in perpendicular magnetic field.(1) The in-plane magnetic anisotropy in Fe films grown on GaAs(001) was investigated quantitatively by the magneto-optic Kerr effect with a rotating magnetic field (ROT-MOKE). The clear 1/dFe relation of the uniaxial magnetic anisotropy indicates a surprising volume contribution with easy axis along the GaAs [110] direction. Such volume anisotropy was found to be sensitive to the growth temperature and also strongly correlate with the interface anisotropy. Our results may introduce a new aspect for further understanding the origin of uniaxial magnetic anisotropy in Fe/GaAs(001) system.(2) The in-plane magnetic anisotropy in the Fe/MgO/GaAs(001) system has been carefully studied as a function of MgO thickness. The epitaxial relation is Fe(001)[110]//MgO(001)[100]//GaAs(001)[100] for dMgO>lmonolayer (ML). The interfacial uniaxial anisotropy was greatly reduced by the MgO interlayer, and the easy axis of the fourfold anisotropy was found to rotate from the GaAs(100) direction to the GaAs(110) direction. Such anisotropy transition happens within the 1.2ML MgO thickness range.(3) Single crystalline FeMn/Co bilayers were grown epitaxially on Cu(001) and investigated by magneto-optic Kerr effect (MOKE). By doing the MOKE measurement within a rotating magnetic field, we were able to retrieve quantitatively the anisotropy constant of the ferromagnetic Co layer. We show unambiguously that as the FeMn layer changes from paramagnetic (PM) to antiferromagnetic (AFM) states, it enhances the interfacial magnetic anisotropy at the FeMn/Co interface by an order of magnitude. A thickness dependent study of the magnetic anisotropy constantrevealed that this induced magnetic anisotropy may originate from the FeMn/Co interfacial spin frustration, and was determined to be 0.118 erg/cm2.(4) By using spin-polarized low-energy electron microscopy(SPLEEM), we show the first real-space observation of magnetic cycloidal chiral order in magnetic stripe domain at room temperature. Moreover, the chirality of magnetic chiral order could be controlled by reversing Fe/Ni interface. A Monte-Carlo (MC) simulation indicates that Dzyaloshinskii-Moriya interaction (DMI) is a possible origin of the chirality. The helical chiral order, spin structure of magnetic bubble domain and canted spin in stripe domain were also introduced.(5) The evolution of magnetic stripe domain with different stripe width in perpendicular magnetic field was carefully investigated by spin-polarized low-energy electron microscopy. We found the stripe width of stripe domain with opposite direction from magnetic field decreases with increasing magnetic field at small magnetic field, then the stripe domain will vanish one by one in large magnetic field and become single domain finally, and stripe width is almost a constant at large magnetic field. A linear relation was found between stripe width in zero field and large field. Saturate field fitted from fraction area in small magnetic field is more or less consistent with experiment saturate field. An universal behavior between fraction area and magnetic field multiplied by stripe width in zero field was found.更多还原