XMCD

X-ray magnetic circular dichroism (XMCD) is a spectroscopy technique that probes the magnetic properties of materials with element specificity by measuring the difference in X-ray absorption of circularly polarized light with opposite helicities. The technique relies on the interaction of matter with polarized X-rays in the presence of a magnetic field or magnetization, and it reveals information about the spin and orbital magnetic moments of the absorbing atoms.

Experimentally, XMCD spectra are obtained by recording the X-ray absorption near an absorption edge while switching the helicity of the incident X-rays (or the magnetization direction). The dichroic signal Δμ(E) = μ+(E) − μ−(E) is typically measured at the L2/L3 edges of transition metals (2p for 3d metals) or at the M edges of rare earths, though other edges can be used. The measurements are often performed at synchrotron facilities using detectors such as total electron yield or fluorescence yield, and require circularly polarized X-rays produced by specialized beamlines.

XMCD spectra can be analyzed with sum rules that relate the integrated dichroic signals to the spin and orbital magnetic moments of the absorbing element. These sum rules provide access to orbital moment ⟨Lz⟩ and spin moment ⟨Sz⟩ per atom, enabling quantitative comparisons across materials.

Applications of XMCD span magnetism research in thin films, multilayers, nanostructures, ferrimagnets, and oxide materials, as well as studies of spin-polarized interfaces and anisotropy. The technique is highly sensitive to small magnetic signals and provides element-specific magnetization information, but it requires careful treatment of instrumental effects, self-absorption, saturation in detection modes, and interpretation via sum rules.

XMCD emerged in the early 1990s, with foundational work by Carra, Thole, Sette, and colleagues, and has since become a standard tool in condensed matter and materials science, complementing other magnetic spectroscopies.