Structural Characterization of Pt/Co Multilayers for Magnetooptic Recording Using X-Ray Diffraction
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STRUCTURAL CHARACTERIZATION OF Pt/Co MULTILAYERS FOR MAGNETOOPTIC RECORDING USING X-RAY DIFFRACTION James A.Bain,* Bruce M. Clemens,* Sean Brennan" *Dept. of Materials Science and Engineering, Stanford University, Stanford CA, 94305-2205 "Stanford Synchrotron Radiation Lab, Stanford, CA 94309 ABSTRACT Structural features in magnetic multilayer films such as interfacial sharpness and in-plane stress are regarded as responsible for the perpendicular magnetic anisotropy observed in these films. The multilayers often consist of alternating magnetic and non-magnetic layers, and the degree of interfacial sharpness between the two is a critical component in producing perpendicular anisotropy. Additionally, in-plane stress affects the anisotropy through magnetostriction. In this work, we measure both the composition modulation and the stress in multilayers of Pt/Co with x-ray diffraction. Quantitative information about the composition modulation is extracted by recursively fitting a model of multilayer diffraction to the high angle superlattice lines. The model incorporates a composition modulation of variable amplitude, along with a statistical description of the layer thickness fluctuations. INTRODUCTION Two major (potential) contributions to the perpendicular magnetic anisotropy in Pt/Co multilayers are stress and interfacial anisotropy [1, 2]. In this work we examine the intrinsic stresses and interfacial sharpness in epitaxial multilayers of Pt/Co, using x-ray diffraction. Identical multilayer structures were deposited using DC magnetron sputtering at two different Ar pressures. We found that both stress and interfacial sharpness were affected by sputtering pressure, but that only the interfacial sharpness correlated with the changes in the perpendicular magnetic anisotropy. Magnetic Anisotropy Magnetic anisotropy is the dependence of the magnetic energy of a material on the direction of magnetization. For Pt/Co multilayers, the major contribution to the anisotropy is the term associated with single axis of easy magnetization parallel to the film normal, such that the magnetic energy, Ena-9, takes the form: E
K &sin
2
+C
(1)
where K& is the magnitude of this energy dependence, the magnetic anisotropy, 0 is the angle between the surface normal and the magnetization, and C is a constant. Stress can contribute to perpendicular magnetic anisotropy through an inverse magnetostrictive effect. For an cubic thin film with a (111) surface normal, K•, is given by: K.O= -3/2A 11 1a
(2)
where A111 is one of the two magnetostriction coefficients which relate magnetization and strain [3], and a is the in-plane stress. Specifically, A111 is defined as the change in length of a crystal in the [111] direction when it is magnetized in the [111] direction. Interfacial anisotropy is another contributor to the anisotropy. It's dependence on the structure is not so clear. A common approach is to assume each interface introduces a fixed anisotropy and to count interfaces [4, 5]. This leads to the "l/t" behavior, where t is the magnet
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