XMCD: X-ray Magnetic Circular Dichroism

X-ray Magnetic Circular Dichroism (XMCD) uses the differential absorption of left and right circularly polarised light in a magnetic field to examine magnetic materials and properties. This includes the behaviour of bulk materials, thin films and multilayers, magnetic nanostructures and clusters, and spin transport systems. It has been used to study phenomena such as exchange biasing and magnetic anisotropy. It is element-specific and surface sensitive. Used alongside X-ray absorption spectroscopy it can provide complementary information on electronic structure, which is of interest to a broad range of disciplines covering physics, chemistry, materials science, surface science, life sciences, earth sciences, bio-manufacturing and catalysis research.

Synchrotron radiation – particularly polarised soft X-rays – can offer a number of unique advantages based on the following principles: (i) Element-specificity is obtained by tuning the X-rays to the resonance energy of the core to valence transition. (ii) With the light interacting only on the orbital part of the wave function, spin and orbital properties can be separated. (iii) Electric-dipole transitions from the ground state reach only a limited subset of final states, thereby providing a fingerprint for the specific ground state. (iv) Instrumental resolution is of a similar order of magnitude as the core hole lifetime (typically a few hundred meV) allowing resolution of the multiplet structure and charge-transfer satellites. (v) X-ray spectroscopy and scattering can be made sensitive to the magnetic moments by using the strong polarisation dependence of the electric dipole transitions.

XMCD has had a major impact on our understanding of the physics of 3d transition metal, lanthanide and actinide systems. It is the only element-specific technique that can distinguish between the spin and orbital part of the magnetic moment. XMCD is used in many areas, such as engineering (magneto-electronics), chemistry (organo-metallic compounds), earth sciences (spinels, geomagnetism) and life sciences (metalloproteins, biomagnets).