Identification of magnetoelastic and magnetocrystalline anisotropy contributions in ultrathin epitaxial Co(110) films (abstract)

Abstract

Magnetic anisotropies of ultrathin transition-metal films are inherently related to their structural properties. In ultrathin films the large fraction of atoms located at the film interface generates strong interface anisotropies, whereas elastic strain fields caused by the forced registry of atoms at the substrate/film interface induce magnetoelastic anisotropy contributions. So far the experimental confirmation of the transition from these thin-film properties to bulk anisotropy properties, characterized by a dominating magnetocrystalline anisotropy, has not yet been presented. Magnetic anisotropies reflect, depending on their origin, both the crystallographic symmetry and the symmetry of the film geometry. For a clear separation between magnetoelastic, magnetocrystalline and Néel-type interface anisotropy contributions, the film symmetry and thickness must be chosen such that the respective different anisotropy contributions appear with different symmetries and film thickness dependencies. This is the case for (110)-oriented fcc Co films. In the present study we use the Brillouin light-scattering technique for the determination of the anisotropy contributions. An analysis of the spin-wave frequency measured as a function of the in-plane direction of the external field and the film thickness yields information about all relevant anisotropies. The samples used were molecular-beam-epitaxy grown in ultrahigh vacuum. Onto a Cu (110) single-crystal substrate a wedge-type sample and two staircase-shaped samples with distinct thicknesses in the range of 8–110 Å were grown. To obtain symmetric Co/Cu interfaces the Co layers were covered with a 12 Å Cu layer. Finally, a 25-Å-thick Au protective layer was deposited. Low-energy electron-diffraction studies were used to obtain the structural data of the films. All relevant anisotropy contributions—the magnetocrystalline anisotropy, and the uniaxial in-plane and out-of-plane anisotropy contributions—were determined. Three different anisotropy regimes are observed as a function of the Co layer thickness dCo. This thickness regime up to 13 Å is dominated by the magnetoelastic anisotropy contributions as a result of the pseudomorphic film growth of the Co layer. For Co layer thicknesses larger than 13 Å we find a reduction of the magnetoelastic anisotropy contributions. This is structurally correlated to an anisotropic relaxation of the in-plane Co lattice constant. In the regime of dCo≳50 Å we observe a thickness-independent value for the magnetocrystalline anisotropy contribution K1=−8.5×105 erg/cm3. This anisotropy contribution is largely suppressed for dCo<50 Å. This finding might either indicate a breakdown of the usually postulated linear superposition principle of magnetic anisotropy contributions to the free anisotropy energy, or it might point to a subtle modification of the electronic band structure. At the onset of the magnetocrystalline anisotropy we find a change in the easy magnetization direction from 〈001〉 for thin Co films to 〈111〉 for thicker ones. For a more detailed discussion see Ref. .