X-ray dichroism
X-ray dichroism is a generic term for several X-ray spectroscopic effects that are based on the coupling of (X-ray) photons to certain electron orbitals . Put simply, the absorption of X-rays in a material depends on the polarization .
X-ray absorption
Circular magnetic X-ray dichroism
With circular magnetic X-ray dichroism ( X-ray magnetic circular dichroism or X-MCD), the absorption of X-rays at an atom with a magnetic moment depends on whether the X-ray radiation is polarized to the left or right. The difference spectrum between the X-ray absorption spectrum of left-hand or right-hand circularly polarized radiation is relevant . When recording the polarized absorption spectra, the direction of magnetization and the helicity of the X-ray photons are aligned once parallel and once anti-parallel. The absorption spectra obtained are subtracted from one another and direct information about the element-specific spin moment and the orbital (orbit) moment of the valence band can be obtained from the difference by using the so-called sum rules .
Typically, X-MCD studies are carried out on magnetic transition metals such as iron , cobalt and nickel , as well as their alloys or compounds , including metal complexes. X-ray absorption spectra are measured at the 2p absorption edge. Here, transitions into the 3D shell take place, which can be strongly magnetic, i.e. are dependent on the magnetic field.
Linear magnetic X-ray dichroism
Linear magnetic X-ray dichroism is a magneto-optical effect similar to circular dichroism , but of a higher order (quadratic in the spin moment M) and therefore weaker. This form of magnetic X-ray dichroism is also useful for obtaining magnetic information, e.g. B. of antiferromagnets that do not have circular X-ray dichroism.
Linear (non-magnetic) X-ray dichroism
Linear (non-magnetic) X-ray dichroism is a consequence of crystal anisotropies of non-magnetic origin and is often difficult to separate from linear magnetic X-ray dichroism.
X-ray emission
The X-ray emission also shows circular and linear dichroism, which, however, is more difficult to interpret because no sum rules could be established so far.
Photoelectron emission
In photoelectron emission, the angular distribution shows a dependence on the helicity of the exciting X-rays and is known as dichroism based on the X-MCD effect.
literature
- Joachim Stöhr, Hans Christoph Siegmann: Magnetism: From Fundamentals to Nanoscale Dynamics . Springer Science & Business Media, 2006, ISBN 978-3-540-30283-4 .
Web links
- X-ray Magnetic Circular Dichroism (XMCD) at the Néel Institute