Magnetic X-ray Circular Dichroism
MXCD measurements are now routinely made on stations 1.1 and 3.4 of the Synchrotron Radiation Source (SRS). Studies are in progress into a variety of fundamentally important and topical areas such as magnetic multilayers and thin films, giant magneto-resistance (GMR) systems, and nanoscale (so-called "cluster") magnetic materials.
So, how does the MXCD effect arise? This is illustrated diagramatically in figure 2 below. For magnetic materials in the presence of an applied magnetic field, spin up and spin down bands are not equally populated. For an applied field in the up direction, there will be some empty spin up 3d states. In this case, because of conservation of spin, only the 2p electrons with up spin can be excited into the 3d states. When the orbital motion of the 2p states is in the same sense as the circular motion of the incident light the transition probability is larger; when the two motions are in opposite directions the transition probability is smaller.
 Figure 2 The Magnetic Circular Dichroism Effect As a result, for light of a particular handedness, the L3 peak in the absorption spectrum, arising from the excitation of the 2p3/2 electrons, may be enhanced, whilst the L2 peak, arising from the excitation of the 2p1/2 electrons, may be reduced. When either the magnetisation direction or the polarisation direction are reversed the effects on the size of the L3 and L2 peaks is also reversed. Figure 3 below shows the normalised absorption spectrum (as measured in total electron yield) from a 500 Angstrom film of cobalt deposited on a silicon wafer and capped with 21 Angstroms of gold. The degree of circular polarisation of the light incident on the sample was 75%. The sample was first magnetised with a 4 Tesla field resulting in the curve shown in red, then the field direction was reversed and the curve shown in blue was obtained.  Figure 3 Dichroism Seen in a Cobalt Film Figure 4 below shows the difference between the two spectra. Evaluation of the areas under the L3 and L2 peaks in the dichroism (difference) spectrum, and under the L3+L2 peaks in the spectrum formed by summing the 2 curves together, allows the spin and orbital magnetic moments for each individual cobalt atom to be evaluated. Thus the technique is element specific, in contrast to other techniques such as those based on the Magneto-Optical Kerr Effect (MOKE) or the Faraday Effect.  Figure 4 The Dichroism Spectrum Although as explained above a dichroism spectrum can be produced by reversing either the magnetisation of the sample or the polarisation of the light, in practice on a synchrotron bending magnet it is usually easier and more accurate to reverse the magnetisation direction. This is because the photons of opposite polarisation in the beamline follow different optical paths, possibly resulting in energy shifts and/or resolution changes. It is also difficult to guarantee that the degree of circular polarisation remains the same whilst being reversed in sign.
In addition to the standard MXCD mode of measurements described so far, two further variations have been successfully adopted:
|
