X-ray magnetic circular dichroism (XMCD) is a difference spectrum of two X-ray absorption spectra (XAS) taken in a magnetic field, one taken with left circularly polarized light, and one with right circularly polarized light.[1] By closely analyzing the difference in the XMCD spectrum, information can be obtained on the magnetic properties of the atom, such as its spin and orbitalmagnetic moment. Using XMCD magnetic moments below 10−5μB can be observed.[2]
In the case of transition metals such as iron, cobalt, and nickel, the absorption spectra for XMCD are usually measured at the L-edge. This corresponds to the process in the iron case: with iron, a 2p electron is excited to a 3d state by an X-ray of about 700 eV.[3] Because the 3d electron states are the origin of the magnetic properties of the elements, the spectra contain information on the magnetic properties. In rare-earth elements usually, the M4,5-edges are measured, corresponding to electron excitations from a 3d state to mostly 4f states.
The radial part is referred to as the line strength while the angular one contains symmetries from which selection rules can be deduced. Rewriting the product of three spherical harmonics with the 3-j symbol finally leads to:[4]
The 3-j symbols are not zero only if satisfy the following conditions giving us the following selection rules for dipole transitions with circular polarised light:[4]
^Helmut Kronmüller; Stuart S. P. Parkin, eds. (2007). Handbook of magnetism and advanced magnetic materials. Hoboken, NJ: John Wiley & Sons. ISBN978-0-470-02217-7. OCLC124165851.
^ abcde Groot, F.; Vogel, J. (2006). "Fundamentals of X-ray Absorption and Dichroism: The Multiplet Approach". Neutron and X-ray Spectroscopy. pp. 3–66. doi:10.1007/1-4020-3337-0_1. ISBN978-1-4020-3337-7.
^J. Stöhr; Y. Wu (1994). "X-ray Magnetic Circular Dichroism: Basic concepts and theory for 3d transition metal atoms". New Directions in Research with Third-Generation Soft X-Ray Synchrotron Radiation Sources. pp. 221–250. doi:10.1007/978-94-011-0868-3. ISBN978-94-010-4375-5.