Magnetic x-ray standing waves

by Michal Kaminski (FS-PETRA-D (FS-PET-D Fachgruppe P24 (Chem.Cryst.)))


Magnetic x-ray standing waves (MXSW) - a combination of x-ray standing waves (XSW) and x-ray magnetic circular dichroism (XMCD) - is a new method for direct investigation of magnetic structure of crystals and thin films on the atomic level. In the regular XSW technique a standing wave emerging in the region where incoming and Bragg reflected waves interfere is employed to study atomic positions in element specific manner. The standing wave has a periodicity of the lattice and moves by half of its period as the sample is rocked through the reflection domain. This movement across the lattice causes modulations in the amount of emitted fluorescence - their character is characteristic for a distribution of given atomic kind. This gives - in contrary to diffraction methods - a direct, element specific structural information. In MXSW, additional magnetic sensitivity is achieved by using circularly polarised incoming wave and magnetising the sample. The normalised difference between fluorescence yields recorded for each helicity/magnetic field orientation (XMCD signal) is proportional to the distribution of magnetic atoms and their magnetic moments. This makes MXSW site, element and magnetic sensitive method. The theoretical framework of MXSW method is based on dynamical theory of x-ray diffraction and time-dependent perturbation theory.

The first experiment aiming at proving the feasibility of the method and confirming the established theory was performed at PETRA III synchrotron on the single crystalline sample of Pt3Co alloy. A clear variation in XMCD signal of the magnetic origin was observed. Since the platinum atoms in the platinum-cobalt alloy do not form any sublattices, and the MXSW signal for this system  originates purely from the variations in wavefield polarisation in the reflection domain, the second experiment was performed on the ferrimagnetic yttrium-iron-garnet crystal. The recorded variations in XMCD are related to the ferrimagnetic arrangement of the iron atoms.

In the presentation a foundations of the new MXSW method will be presented. First experimental results and their relation to the established theory will be also discussed.