252
C. Binns et al.
10.3 Future Perspectives
The emergence of the sum rules in the early 1990s was coincidental with an expansion
in the number of third-generation synchrotron radiation sources, which use insertion
devices to generate very high intensities of X-rays with variable polarisation. For
a review of the development of the technique, see ref [17]. This led to a proliferation of XMCD experiments and initially the focus was on static thin films and
nanostructures containing transition metals. The sum rules are equally applicable to
rare-earth systems though a more complex analysis is required to extract the magnetic
moments [18] and the field has matured and embraced complex rare-earth systems.
These including magnetically doped topological insulators, important for spintronics
applications [18] and DyFe/YFe exchange spring materials [4] that demonstrate giant
magnetoresistance (GMR). The method has also evolved to include measurements
with spatial and temporal resolutions and these developments are briefly illustrated
with some examples in this section.
10.3.1 Spatially Resolved XMCD: Domain Imaging
in Patterned Structures
XMCD can be combined with X-ray photoelectron microscopy (XPEEM) to provide
spatially resolved images of magnetisation in samples. The principle of X-PEEM is
illustrated in Fig. 10.8a and it consists of electron lenses that provide a magnified image of the sample at the image plane at which there is a two-dimensional
detector. The secondary electrons are excited by a soft X-ray beam and as illustrated in Fig. 10.1b, if the photons are tuned to an element absorption edge, there
will be a peak in the secondary electron yield at positions where that element is
present, providing chemical mapping of a surface. If, in addition, the photon beam is
circularly polarised, magnetic contrast at each position can be obtained by taking an
image on and off the edge. This will reveal which regions have their magnetisation
aligned with the photon spin (negative contrast) and which have their magnetisation
aligned antiparallel to the photon spin (positive contrast). The direction of magnetisation sampled can be controlled by changing the angle of incidence of the photon
beam. One of the major advantages of XMCD-XPEEM is that it combines magnetic
imaging with chemical sensitivity.
Figure 10.8b–d shows domain patterns in micro-patterned Co rectangles with a
thickness of 20 nm and aspect ratios of 1:1, 1:2, and 1:3. The Co squares show
predominantly a vortex state domain structure, while the others show a variety of
domain structures including symmetric and asymmetric vortex and antivortex states.
C. Binns et al.
10.3 Future Perspectives
The emergence of the sum rules in the early 1990s was coincidental with an expansion
in the number of third-generation synchrotron radiation sources, which use insertion
devices to generate very high intensities of X-rays with variable polarisation. For
a review of the development of the technique, see ref [17]. This led to a proliferation of XMCD experiments and initially the focus was on static thin films and
nanostructures containing transition metals. The sum rules are equally applicable to
rare-earth systems though a more complex analysis is required to extract the magnetic
moments [18] and the field has matured and embraced complex rare-earth systems.
These including magnetically doped topological insulators, important for spintronics
applications [18] and DyFe/YFe exchange spring materials [4] that demonstrate giant
magnetoresistance (GMR). The method has also evolved to include measurements
with spatial and temporal resolutions and these developments are briefly illustrated
with some examples in this section.
10.3.1 Spatially Resolved XMCD: Domain Imaging
in Patterned Structures
XMCD can be combined with X-ray photoelectron microscopy (XPEEM) to provide
spatially resolved images of magnetisation in samples. The principle of X-PEEM is
illustrated in Fig. 10.8a and it consists of electron lenses that provide a magnified image of the sample at the image plane at which there is a two-dimensional
detector. The secondary electrons are excited by a soft X-ray beam and as illustrated in Fig. 10.1b, if the photons are tuned to an element absorption edge, there
will be a peak in the secondary electron yield at positions where that element is
present, providing chemical mapping of a surface. If, in addition, the photon beam is
circularly polarised, magnetic contrast at each position can be obtained by taking an
image on and off the edge. This will reveal which regions have their magnetisation
aligned with the photon spin (negative contrast) and which have their magnetisation
aligned antiparallel to the photon spin (positive contrast). The direction of magnetisation sampled can be controlled by changing the angle of incidence of the photon
beam. One of the major advantages of XMCD-XPEEM is that it combines magnetic
imaging with chemical sensitivity.
Figure 10.8b–d shows domain patterns in micro-patterned Co rectangles with a
thickness of 20 nm and aspect ratios of 1:1, 1:2, and 1:3. The Co squares show
predominantly a vortex state domain structure, while the others show a variety of
domain structures including symmetric and asymmetric vortex and antivortex states.