Magnetic X-ray absorption spectroscopy

Abstract

Basic principles of the magnetisation dependent effects of the absorption of circularly polarised X-rays are discussed. At the inner core absorption edges the effect is manifested in a difference of absorption coefficients for the right and left circularly polarised quanta in ferromagnetic media and is called X-ray magnetic circular dichroism (XMCD). It originates from the interplay of spin orbit interaction in the initial state and angular momentum conservation in the absorption process and reflects the local spin and orbital polarisation of the final states. An outstanding feature of XMCD is the applicability of the so-called ‘sum rules’ which can be used to determine spin and orbital contributions to the local magnetic moments separately. Its inherent element selectivity and a high sensitivity makes it especially useful in a study of technologically important multicomponent materials. Magnetic contributions occur also in the extended X-ray absorption fine structure (EXAFS) range and are called magnetic EXAFS (MEXAFS). They originate from an increased (decreased) absorption coefficient due to the constructive (destructive) interference of the outgoing polarised photoelectron wave with the wave backscattered by the adjacent magnetic atoms. The backscattering amplitude gives a quantitative measure of the local magnetic spin moments of the nearest neighbours. Some experimental data for the rare earth–3d element intermetallics, magnetic thin films, multilayers and oxide compounds in the hard and soft X-ray regime are selected to illustrate the potential of XMCD and MEXAFS. A very promising perspective of application of XMCD as a contrast mechanism in various established techniques using X-rays, e.g. imaging of magnetic domains in the magnetic X-ray transmission microscopy is discussed.