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 anisotrop y phenomena in transition metal sandwiches and multilayers. The charge dens ity, the spin density and the orbital moment become anisotropic. These effe cts can be studied by the X-ray magnetic circular dichroism (XMCD) techniqu e which senses the local anisotropy of charge, spin and angular momentum ar ound an atom that is excited by the absorption of polarized X-rays. Hen we briefly review the principles of the technique and then apply it to the stu dy of the thickness-dependent electronic and magnetic properties of a Co hi m sandwiched between Au. The experimental results are compared to those obt ained by electronic structure calculations for a free Co monolayer and a Co monolayer sandwiched between Au. In particular, a simple ligand field mode l is developed which allows one to visualize the origin of the magnetocryst alline anisotropy in terms of the preferred direction of the orbital moment , corresponding to the direction of maximum size. The model supports the in tuitive picture that the orbital moment on an atom becomes anisotropic thro ugh quenching effects by the anisotropic ligand fields of the neighbors. (C ) 1999 Elsevier Science B.V, All rights reserved.