224
A. Kleibert
ciently diluted samples with a typical density of a few nanoparticles per µm
2 on the
surface are required.
Magnetic investigations of ferromagnetically ordered systems in XPEEM are
based on the spatially resolved detection of the x-ray magnetic circular dichroism
(XMCD) effect occuring for instance at the L 2,3 edges of the 3d transition metals
or the M 4,5 edges of the rare earth elements [11–18]. In investigations on 3d transition metal nanoparticles, elemental and magnetic contrast maps recorded at the
respective L 3 edges are used to visualize the spatial distribution of the nanoparticles
and their individual magnetization, as illustrated in Fig. 9.1b, c [5]. In this example,
the nanoparticles appear as bright spots with a diameter that corresponds to the spatial resolution of the instrument, which is larger than the actual nanoparticle size.
Magnetic contrast maps yield grey tones for the individual nanoparticles, which can
range from bright to dark depending on the orientation of the magnetization m relative to the propagation vector k of the incident circularly polarized x-rays according
to m · k [21]. The latter relation allows one to quantitatively determine the actual orientation of m when varying the azimuthal sample orientation [21, 35]. Further details
regarding the data acquisition in XPEEM and processing can be found for instance
in [5]. Another important aspect in XPEEM investigations of magnetic nanoparticles
is the time τ x required to record a magnetic contrast map. This time ranges typically
between a few tens of seconds to several minutes. Only nanoparticles with a magnetic
relaxation time τ r > τ x can exhibit a detectable magnetic contrast in XPEEM [38].
9.2.3 Sample Requirements
Substrates for nanoparticles studies using XPEEM should possess a flattness in the
nanometer range in order to achieve high spatial resolution (to avoid electric field
perturbations at surface protrusions [39]). Further, the substrates should be sufficiently conductive to avoid charging of the sample. For instance, Si(100) wafers
passivated with a native, amorphous oxide layer with a thickness of 1–2 nm have
been proven to be suitable substrates for many studies. The native oxide layer is
chemically inert and thermally stable up to about 1000 K. Lithographically prepared
gold marker structures are ideally suited for the identification of the same nanoparticles in different microscopes, see Fig. 9.2. Marker structures further aid sample
alignment and focusing of the instrument. For investigations of individual nanoparticles, the particle density on the substrate should not exceed a few nanoparticles
per μm
2 . In XPEEM, organic cappings, oxide shells or residuals of solvents on the
sample usually attenuate the detectable signal significantly and, therefore, should be
either thin (1–2 nm) or completely avoided.
A. Kleibert
ciently diluted samples with a typical density of a few nanoparticles per µm
2 on the
surface are required.
Magnetic investigations of ferromagnetically ordered systems in XPEEM are
based on the spatially resolved detection of the x-ray magnetic circular dichroism
(XMCD) effect occuring for instance at the L 2,3 edges of the 3d transition metals
or the M 4,5 edges of the rare earth elements [11–18]. In investigations on 3d transition metal nanoparticles, elemental and magnetic contrast maps recorded at the
respective L 3 edges are used to visualize the spatial distribution of the nanoparticles
and their individual magnetization, as illustrated in Fig. 9.1b, c [5]. In this example,
the nanoparticles appear as bright spots with a diameter that corresponds to the spatial resolution of the instrument, which is larger than the actual nanoparticle size.
Magnetic contrast maps yield grey tones for the individual nanoparticles, which can
range from bright to dark depending on the orientation of the magnetization m relative to the propagation vector k of the incident circularly polarized x-rays according
to m · k [21]. The latter relation allows one to quantitatively determine the actual orientation of m when varying the azimuthal sample orientation [21, 35]. Further details
regarding the data acquisition in XPEEM and processing can be found for instance
in [5]. Another important aspect in XPEEM investigations of magnetic nanoparticles
is the time τ x required to record a magnetic contrast map. This time ranges typically
between a few tens of seconds to several minutes. Only nanoparticles with a magnetic
relaxation time τ r > τ x can exhibit a detectable magnetic contrast in XPEEM [38].
9.2.3 Sample Requirements
Substrates for nanoparticles studies using XPEEM should possess a flattness in the
nanometer range in order to achieve high spatial resolution (to avoid electric field
perturbations at surface protrusions [39]). Further, the substrates should be sufficiently conductive to avoid charging of the sample. For instance, Si(100) wafers
passivated with a native, amorphous oxide layer with a thickness of 1–2 nm have
been proven to be suitable substrates for many studies. The native oxide layer is
chemically inert and thermally stable up to about 1000 K. Lithographically prepared
gold marker structures are ideally suited for the identification of the same nanoparticles in different microscopes, see Fig. 9.2. Marker structures further aid sample
alignment and focusing of the instrument. For investigations of individual nanoparticles, the particle density on the substrate should not exceed a few nanoparticles
per μm
2 . In XPEEM, organic cappings, oxide shells or residuals of solvents on the
sample usually attenuate the detectable signal significantly and, therefore, should be
either thin (1–2 nm) or completely avoided.