248
C. Binns et al.
beam while maintaining the parallel and antiparallel alignment of the photon polarisation and sample magnetisation will eliminate the term and (10.9) can be
used to determine the spin moment.
10.2 Example System: Fe@Cr Core–Shell Nanoparticles
As pointed out earlier, XMCD is particularly powerful when investigating samples
containing more than one element as it is able to chemically focus on the magnetic
orbital and spin moments in individual elements. Whereas conventional magnetometry measures the average magnetic behaviour of all the elements in the sample. In
recent years, the magnetic behaviour of ferromagnetic/antiferromagnetic interfaces
has received particular attention due to the appearance of exchange bias after fieldcooling, that is, a shift of the hysteresis loop of the magnetic material along the
applied field axis [13, 14]. The effect is due to the ‘pinning’ of magnetic moments
of the ferromagnetic material by exchange coupling with moments on the antiferromagnetic material at the interface and exchange bias is accompanied by an increase
in coercivity. The effect is not only of interest from a fundamental perspective but is
exploited in spin valves used for magnetic recording [15]. The majority of the work
has focused on thin film interfaces but more recently an investigation using XMCD
and magnetometry of the magnetic behaviour of Fe nanoparticles coated with one or
two layers of Cr was reported [16].
The core–shell Fe@Cr nanoparticles were synthesised in ultra-high vacuum
(UHV) in the gas-phase and matrix isolated in Ag films as illustrated in Fig. 10.5.
The substrates used were C-coated Cu TEM grids and the finished samples had an
X-ray transmission at the Fe L-edge in the range 10–90% and could be used for direct
absorption measurements as illustrated in Fig. 10.1a. Three types of nanoparticles
were prepared, that is, uncoated Fe nanoparticles, Fe cores with a single atomic layer
of Cr, and Fe cores with two atomic layers of Cr (see Fig. 10.5c) thus the evolution
of the magnetic behaviour as a function of the shell thickness could be studied.
The details of the analysis of the XMCD data is illustrated for the case of pure
Fe nanoparticles in Fig. 10.6. The raw absorption data in transmission is shown in
Fig. 10.6a and the same data after subtracting an integral background is plotted in
Fig. 10.6b. The final dichroism spectrum after subtraction of a weak Ag N-edge
absorption at 730 eV within the Fe L 2,3 absorption spectrum is shown in Fig. 10.6c.
This is in a suitable form for sum rule analysis as illustrated in Fig. 10.4. In addition
to obtaining the full XMCD spectrum, it is possible to obtain a magnetisation loop for
a given element in the sample by simply measuring the intensity of the L 3 absorption
edge as a function of the applied magnetic field intensity. The resulting curve is
on a background signal that can easily be removed to produce loops such as that
shown in Fig. 10.6d for pure Fe nanoparticles at a temperature of 204 K. The red
line is a Langevin function at the same temperature that shows an optimum fit to the
measured curve for a particle size of 2.68 nm (850 atoms). The inset in Fig. 10.6d
compares the magnetisation measured by XMCD (black line) with that measured
C. Binns et al.
beam while maintaining the parallel and antiparallel alignment of the photon polarisation and sample magnetisation will eliminate the
used to determine the spin moment.
10.2 Example System: Fe@Cr Core–Shell Nanoparticles
As pointed out earlier, XMCD is particularly powerful when investigating samples
containing more than one element as it is able to chemically focus on the magnetic
orbital and spin moments in individual elements. Whereas conventional magnetometry measures the average magnetic behaviour of all the elements in the sample. In
recent years, the magnetic behaviour of ferromagnetic/antiferromagnetic interfaces
has received particular attention due to the appearance of exchange bias after fieldcooling, that is, a shift of the hysteresis loop of the magnetic material along the
applied field axis [13, 14]. The effect is due to the ‘pinning’ of magnetic moments
of the ferromagnetic material by exchange coupling with moments on the antiferromagnetic material at the interface and exchange bias is accompanied by an increase
in coercivity. The effect is not only of interest from a fundamental perspective but is
exploited in spin valves used for magnetic recording [15]. The majority of the work
has focused on thin film interfaces but more recently an investigation using XMCD
and magnetometry of the magnetic behaviour of Fe nanoparticles coated with one or
two layers of Cr was reported [16].
The core–shell Fe@Cr nanoparticles were synthesised in ultra-high vacuum
(UHV) in the gas-phase and matrix isolated in Ag films as illustrated in Fig. 10.5.
The substrates used were C-coated Cu TEM grids and the finished samples had an
X-ray transmission at the Fe L-edge in the range 10–90% and could be used for direct
absorption measurements as illustrated in Fig. 10.1a. Three types of nanoparticles
were prepared, that is, uncoated Fe nanoparticles, Fe cores with a single atomic layer
of Cr, and Fe cores with two atomic layers of Cr (see Fig. 10.5c) thus the evolution
of the magnetic behaviour as a function of the shell thickness could be studied.
The details of the analysis of the XMCD data is illustrated for the case of pure
Fe nanoparticles in Fig. 10.6. The raw absorption data in transmission is shown in
Fig. 10.6a and the same data after subtracting an integral background is plotted in
Fig. 10.6b. The final dichroism spectrum after subtraction of a weak Ag N-edge
absorption at 730 eV within the Fe L 2,3 absorption spectrum is shown in Fig. 10.6c.
This is in a suitable form for sum rule analysis as illustrated in Fig. 10.4. In addition
to obtaining the full XMCD spectrum, it is possible to obtain a magnetisation loop for
a given element in the sample by simply measuring the intensity of the L 3 absorption
edge as a function of the applied magnetic field intensity. The resulting curve is
on a background signal that can easily be removed to produce loops such as that
shown in Fig. 10.6d for pure Fe nanoparticles at a temperature of 204 K. The red
line is a Langevin function at the same temperature that shows an optimum fit to the
measured curve for a particle size of 2.68 nm (850 atoms). The inset in Fig. 10.6d
compares the magnetisation measured by XMCD (black line) with that measured