Exploring the microscopic origin of magnetic anisotropies with X-ray magnetic circular dichroism (XMCD) spectroscopy

Symmetry breaking and bonding at interfaces leads to a variety of anisotropy phenomena in transition metal sandwiches and multilayers. The charge density, the spin density and the orbital moment become anisotropic. These e!ects can be studied by the X-ray magnetic circular dichroism (XMCD) technique which senses the local anisotropy of charge, spin and angular momentum around an atom that is excited by the absorption of polarized X-rays. Here we brie#y review the principles of the technique and then apply it to the study of the thickness-dependent electronic and magnetic properties of a Co "lm sandwiched between Au. The experimental results are compared to those obtained by electronic structure calculations for a free Co monolayer and a Co monolayer sandwiched between Au. In particular, a simple ligand "eld model is developed which allows one to visualize the origin of the magnetocrystalline anisotropy in terms of the preferred direction of the orbital moment, corresponding to the direction of maximum size. The model supports the intuitive picture that the orbital moment on an atom becomes anisotropic through quenching e!ects by the anisotropic ligand "elds of the neighbors. ( 1999 Elsevier Science B.V. All rights reserved.