THEORY OF THE CURIE TEMPERATURE OF COBALT-BASED FERROMAGNETIC ULTRATHIN FILMS AND MULTILAYERS

Abstract

Abstract-Calculations of the Curie temperature and magnetization in cobalt-based ultrathin films and multilayers have been performed within the framework of the Green-function theory. The effect of the long-ranged dipolar interactions and of the strong interface magnetic anisotropy is fully taken into account. The influence of the interlayer coupling across the non magnetic spacer is also investigated. The results are discussed in connection with experimental data for Co/Au, Co/Pt, and Co/Pd multilayers. KEYWORDS: MULTILAYERS, CURIE TEMPERATURE, GREEN FUNCTION, MAGNETIC ANISO- TROPY, DIPOLE INTERACTIONS, INTERLAYER COUPLING INTRODUCTION In the last *few years, ferromagnetic ultra- thin films have been shown to exhibit remarkabie interface-induced properties. In particular strongly enhanced magnetic anisotropies [l] and magneto- optical effects [2] have been found in Co/Au, Co/Pd, and Co/Pt ultrathin films and multilay- ers. Cobalt-based multilayers with extremely low individual cobalt thicknesses (a few atomic layers) thus present a strong perpendicular anisotropy, to- gether with a large magneto-optical Kerr or Fara- day rotation; these properties are precisely the two major requirements for magneto-optical stor- age technology. Indeed cobalt-based multilayers have been proved to be very promising candidates as high-density magneto-optical storage materials [3-61. They present over the traditional rare-earth- transition-metal alloys the advantages of a higher chemical stability, together with larger magneto- optical effects at small wavelengths, thus allowing higher storage densities. The stability of the written information and the thermomagnetic writing process used in magneto-optical technology impose severe require- ments on the Curie temperature: an optimal value for the Curie temperature is in the range of 400- 600 K [q. In rare-earth-transition-metal alloys, the Curie temperature can be tuned by varying the al- loy composition. The Curie temperature of bulk cobalt is much too high for this purpose; fortu- nately it is much smaller in ultrathin films, and strongly thickness dependent. It can therefore be 15 adjusted to a