Magnetic Properties and Crystallography of Selected Co/Pt Multilayers with Rare-Earth Additions
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MAGNETIC PROPERTIES AND CRYSTALLOGRAPHY OF SELECTED Co/Pt MULTILAYERS WITH RARE-EARTH ADDITIONS G.A. BERTERO, R.L. WHITE, AND R. SINCLAIR Department of Materials Science and Engineering Stanford University, Stanford CA 94305
ABSTRACT We have sputter-deposited a series of Pt/Co multilayers with differing amounts of rareearth (RE) in an effort to improve the uniaxial perpendicular anisotropy in these structures. The present work investigates the influence on the magnetic properties of Tb and Ho incorporated both at the interfaces of Pt/Co multilayers and into the Co layers. The uniaxial anisotropy improved significantly only for those multilayers that showed poor perpendicular anisotropy in the undoped state, in particular, those with large (-30 A) bilayer periods. High resolution transmission electron microscopy was used to study these multilayers in cross-section. The multilayer structures present strong 1111] texture with grain sizes ranging from 200 to 600 A. It is found that the RE does not produce a significant change in the growth texture of the multilayers until a critical amount is introduced beyond which the multilayer structure amorphizes resulting in a compositionally modulated amorphous film.
INTRODUCTION Pt/Co multilayer thin films (with the [1111 out-of-plane crystallographic orientation) present maximum Kerr rotations at blue range wavelengths of the order of 0.5' or better depending on the overall composition [I], they have also been shown to display large magnetic coercivities and remanent perpendicular magnetization of about 100% [1-6]. In addition, Pt and Co have a natural tendency to deposit onto amorphous substrates with a [1111 texture under most deposition conditions. Thus, these structures are good candidates for use as high density magneto-optic recording media. The most promising combination of properties in the Pt/Co multilayer structures is achieved when the individual Co layer thickness is about 2 monolayers (-4 A for closed-packed planes) and the Pt layer thicknesses corresponding to 5 to 7 monolayers (- 12 to 16 A) [1-4]. Increasing the thickness of the Co layers, and maintaining the same Pt/Co ratio, results in a gradual decrease of the perpendicular magnetic anisotropy energy, K 1 , (K 1 = Ku-2n M2 ) [2] where Ku is the uniaxial crystal anisotropy and M is the magnetization in the film. For high density recording, the perpendicular anisotropy energy, KI, becomes increasingly important because it plays a crucial role in the stability of the magnetic domains. Rare earth-transition metal (RE-TM) alloys and multilayers are known to display large magnetic anisotropy [6-8]. A giant magnetostriction effect has also been reported in these systems [8] and therefore a contribution to K 1 can be expected from this effect if present. In addition, Tb and Ho have been alloyed with Co and an anti-parallel alignment of magnetic moments was reported [7]. In a previous publication [9] we had shown that the introduction of one third of a monolayer of Tb or Ho at the interfaces of Pt/Co multilayers pr
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