Large coercivity and surface anisotropy in MgO/Co multilayer films

The multilayer films of (30ÅMgO/12-65ÅCo)₃₀ were deposited on oxide-coated Si(100) wafers in a UHV chamber by rf and dc sputtering. Structural analyses reveal that the films with Co layer thickness (t_Co) above 16 Å have well-defined layer structure, however, Co layers become discontinuous to some extent when t_Co is down to 12 Å. MgO layers are (100) oriented, and Co layers have fcc phase with fcc(200) parallel to MgO(200) when t_Co is below 20 Å, but the hcp phase is stabilized in thicker Co layers (25Å < ∼ t_Co <∼ 65Å) with hcp(110) parallel to MgO(200). There is no evidence of exchange bias stemming from the oxidation of Co, and all films exhibit planar magnetization. Coercivity of higher than 12 kOe was observed at 10 K for t_Co = 12Å, which is far beyond the effective magnetocrystalline anisotropy field of fcc or hcp Co. The thickness dependence of coercivity at 10 K follows a linear scale as 1/t_Co. With the increase of the temperature, the coercivity decreases, and the maximum value at room temperature is about 420 Oe for t_Co = 25Å. A proposal to evaluate the surface anisotropy via initial susceptibility measurement along the film normal direction for a film with planar magnetization is presented. Surface anisotropy is thus determined to be about 1.3 erg/cm² at 10 K and 0.4 erg/cm² at room temperature with the film normal as the hard axis. Surface anisotropy has been demonstrated to be a dominant term to determine the magnetic behavior of MgO/Co multilayer films, especially at low temperature.