Magnetic anisotropies of ultrathin Co films on Cu(001) and Cu (1113) substrates (abstract)
Authors: Krams P. , Lauks F. , Stamps R. L. , Hillebrands B. , Güntherodt G. , Oepen H. P. 
Published: July 2002

The stabilization of long-range ferromagnetic order in two-dimensional systems at finite temperatures is presently discussed very controversially. Among the mechanisms dipolar interactions or magnetic anisotropies are currently most considered. For experimental clarification all relevant magnetic anisotropy constants of ultrathin Co layers on Cu(001) as well as on Cu(1 1 13) substrates using Brillouin light scattering.1,2 Due to the fourfold symmetry of Co(001) films the relevant anisotropy contributions are a fourfold in-plane anisotropy Kin-plane(4)=Kp(4)+2kp(4)/d and a perpendicular anisotropy Kperp=Ks+2ks/d with Kp(4) and Ks (kp(4) and ks) the in-plane and out-of-plane volume (surface) anisotropy constants, respectively, and d the film thickness. For Co films prepared on Cu(1 1 13) substrates an additional uniaxial in-plane anisotropy (Kin-plane(2)) is generated by the rotation of the (001) surface about the [11̄0]-in-plane axis by 6.2°. The symmetry axis of this anisotropy lies along the [11̄0] axis. Co films with film thicknesses between 1.5 and 14 monolayers (ML) were prepared in an ultrahigh vacuum (UHV) system and characterized with low-energy electron diffraction (LEED) and Auger spectroscopy as well as by in situ magneto-optic Kerr effect, as described elsewhere.1,2 The Brillouin light scattering experiments were performed in situ in the UHV system for Co/Cu (001) with the external field aligned parallel to the [100] hard axis

The light scattering measurements on Co/Cu (1 1 13) with a Cu overlayer were performed ex situ with the external field direction aligned along different in-plane directions. From a fit to the measured spin wave frequencies as a function of the applied field and the Co film thickness the anisotropy constants are obtained. The Co/Cu (001) system is discussed first. Ferromagnetic order is observed for film thickness d larger than dc=(1.6±0.3) ML for uncovered Co films and dc=(1.9±0.3) ML for Co films covered by a 2-ML thick Cu overlayer. From the fits to data for different film thicknesses it was found that Kp(4)=(−2.32±0.15)×106 erg/cm3 and kp(4)=(0.034±0.004)erg/cm2 for the uncovered films, and Kp(4)=(−2.17±0.15)×106 erg/cm3 and kp(4)=(0.031±0.003)erg/cm2 for the Co layers covered with 2-ML Cu.1 Due to their opposite sign, the contributions of Kp(4) and kp(4) to Kin-plane(4) cancel each other at dc*=(1.55±0.3)ML for the uncovered films and at dc*=(1.7±0.3)ML for the films covered with 2-ML Cu. The out-of-plane anisotropy constant ks was found as ks=(−1.06±0.17)erg/cm2 for the Co/vacuum interface and ks=(0.15±0.04)erg/cm2 for the Co/Cu interface. The negative sign indicates that the surface normal is a magnetic hard axis for this anisotropy. No volume out-of-plane contribution, Ks, was found.

From the observed agreement between the critical thickness for ferromagnetic order dc, with the thickness dc* at which the contributions to the in-plane anisotropy cancel, it can be concluded that the symmetry breaking interaction for stabilizing ferromagnetic order in Co(001) films at room temperature is indeed given by the magnetic in-plane anisotropy contribution. For Co/Cu(1 1 13) an additional large uniaxial in-plane anisotropy contribution of Kin-plane(2)=(−4.4±0.4)×105 erg/cm3 was found.2 The negative sign indicates that the easy axis is parallel to the step edges. The fourfold in-plane anisotropy constant, Kin-plane(4), is reduced by a factor of 2 to 4 compared to the (001)-oriented films. For the Co/Cu(1 1 13) system the uniaxial in-plane anisotropy can be described as magnetoelastic in origin: The determined value of Kin-plane(2) agrees well with the calculated in-plane magnetoelastic anisotropy constant of Kin-plane(2)=−3.5×105 erg/cm3.