X-ray magnetic circular dichroism

Abstract

X-ray Magnetic Circular Dichroism is a technique that uses the polarization properties of X-rays to probe microscopic magnetism. Intense linearly or circularly polarized X-rays produced in synchrotron sources have enabled new probes of the magnetic structure of materials to be developed. Magnetic resonant scattering and magnetic X-ray dichroism use linearly or circularly polarized photons [1] with energy near an absorption edge. X-ray Magnetic Circular Dichroism is the difference between left and right circularly polarized X-ray absorption crosssection of a ferromagnetic or ferrimagnetic material. The interest of using circular polarized light comes from the fact that the magnetic absorption cross-section is directly proportional to M, the mean value of the macroscopic magnetic moment. Two models generally describe magnetism in •?The localized model where the electrons responsible for the magnetic moment are quasi-atomic. This model will describe the magnetism of the 4f orbitals in rare earth compounds and d orbitals in some transition metal compounds. These orbitals are probed by "soft X-ray" absorption (< 3 keV). •?The itinerant model where the spin-polarized electrons are considered to be delocalized. In this case the exchange and correlation interactions yield magnetism because of the different filling of the spin-up and spin-down conduction bands. These conduction bands are probed by absorption of "hard X-rays". In that range (2→20 keV) the final states (or the projection of the final states) of X-ray absorption spectroscopy are of p symmetry for 3d transition metals and of 5d symmetry for 4f rare earths. 3d bands of 3d transition metal are an intermediate case. They are usually described by rigid band models with two sub-bands of opposite spin direction, split by exchange energy. 3d states are of course delocalized states. Nevertheless the LII-III absorption edges of 3d transition metals (2pö3d transition) are usually described in the literature with good accuracy by atomic models (see section 5). In the hard and soft energy ranges circular magnetic X-ray dichroism is a selective probe of the element for both the itinerant and localized models of magnetism. The selectivity concerns both the nature of the probed element and the symmetry of the probed band. In this lecture we will focus on the origin of the XMCD effect in core-hole spectroscopy. Readers not familiar with core-hole spectroscopy can, for example, read the related chapter in reference [2]. © 2006 Springer.