9 Magnetism of Individual Nanoparticles Probed by X-Ray …
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istics seem to suggest a relatively simple behavior with predictable size dependent
properties such as magnetic structure and anisotropy energy. However, as we will
discuss in more detail below, ensemble measurements reveal a large variety and sometimes contradictory findings [23–32], making it impossible to establish an ambiguous
understanding of the size-dependent magnetic properties of iron nanoparticles. By
combining magnetic characterization of a large number of iron nanoparticles using
XPEEM with structural characterization using SEM and AFM of the very same particles, it was possible to demonstrate that the apparently contradictory experimental
observations can be actually explained by the existence of iron nanoparticles in two
distinct magnetic states: one state with strongly enhanced magnetic anisotropy and
another one with bulk-like magnetic anisotropy energy, irrespective of the size of the
particles. Moreover, the enhanced magnetic anisotropy is not due to shape, surface
or interface contributions, but is rather attributed to local structural defects in the
particle volume, such as dislocations.
Similarly, integral measurements on cobalt and nickel nanoparticles revealed a
wide scattering of magnetic properties. However, in contrast to the case of iron
nanoparticles, cobalt and nickel nanoparticles are known to exist or coexist in a rich
variety of different structures ranging from hcp to fcc single crystalline and multiply
twinned structures, often with defects such as stacking faults, making the interpretation of averaged data even more difficult. First XPEEM experiments on fcc cobalt
nanoparticle samples confirm the existence of at least two types of nanoparticles
with distinct magnetic properties, similar to the observations in the case of bcc iron
nanoparticles, while in fcc nickel nanoparticle samples so far only one type with
low magnetic anisotropy was found. However, to determine the intrinsic magnetic
properties of the different structural motifs and the impact of the different defects in
cobalt and nickel nanoparticles requires one to combine magnetic characterization by
means of XPEEM with techniques capable of structural characterization with atomic
resolution, such as high resolution TEM, of the very same nanoparticles, which is
the goal of future work.
This chapter is organized as follows: In Sect. 9.2 we will present the basics of
XPEEM with focus on nanoparticle experiments, including instrumentation, sample
requirements, and the fundamentals of chemical and magnetic nanoparticle characterization in XPEEM. In Sect. 9.3 we will review the recent XPEEM investigations
on iron, cobalt, nickel and iron–cobalt-alloy nanoparticles with sizes ranging from
8 to 20 nm, and Sect. 9.4 concludes the chapter with a perspective for XPEEM in
future nanoparticle research and an outlook for remaining challenges.
9.2 X-Ray Photoemission Electron Microscopy
XPEEM is a special case of low energy electron microscopy (LEEM) [9]. The samples
in XPEEM are usually investigated under UHV conditions employing synchrotron
x-ray radiation for illumination. An electron optical set-up is used to obtain an amplified image of the sample by detecting the x-ray photo-excited electrons [9]. XPEEM
221
istics seem to suggest a relatively simple behavior with predictable size dependent
properties such as magnetic structure and anisotropy energy. However, as we will
discuss in more detail below, ensemble measurements reveal a large variety and sometimes contradictory findings [23–32], making it impossible to establish an ambiguous
understanding of the size-dependent magnetic properties of iron nanoparticles. By
combining magnetic characterization of a large number of iron nanoparticles using
XPEEM with structural characterization using SEM and AFM of the very same particles, it was possible to demonstrate that the apparently contradictory experimental
observations can be actually explained by the existence of iron nanoparticles in two
distinct magnetic states: one state with strongly enhanced magnetic anisotropy and
another one with bulk-like magnetic anisotropy energy, irrespective of the size of the
particles. Moreover, the enhanced magnetic anisotropy is not due to shape, surface
or interface contributions, but is rather attributed to local structural defects in the
particle volume, such as dislocations.
Similarly, integral measurements on cobalt and nickel nanoparticles revealed a
wide scattering of magnetic properties. However, in contrast to the case of iron
nanoparticles, cobalt and nickel nanoparticles are known to exist or coexist in a rich
variety of different structures ranging from hcp to fcc single crystalline and multiply
twinned structures, often with defects such as stacking faults, making the interpretation of averaged data even more difficult. First XPEEM experiments on fcc cobalt
nanoparticle samples confirm the existence of at least two types of nanoparticles
with distinct magnetic properties, similar to the observations in the case of bcc iron
nanoparticles, while in fcc nickel nanoparticle samples so far only one type with
low magnetic anisotropy was found. However, to determine the intrinsic magnetic
properties of the different structural motifs and the impact of the different defects in
cobalt and nickel nanoparticles requires one to combine magnetic characterization by
means of XPEEM with techniques capable of structural characterization with atomic
resolution, such as high resolution TEM, of the very same nanoparticles, which is
the goal of future work.
This chapter is organized as follows: In Sect. 9.2 we will present the basics of
XPEEM with focus on nanoparticle experiments, including instrumentation, sample
requirements, and the fundamentals of chemical and magnetic nanoparticle characterization in XPEEM. In Sect. 9.3 we will review the recent XPEEM investigations
on iron, cobalt, nickel and iron–cobalt-alloy nanoparticles with sizes ranging from
8 to 20 nm, and Sect. 9.4 concludes the chapter with a perspective for XPEEM in
future nanoparticle research and an outlook for remaining challenges.
9.2 X-Ray Photoemission Electron Microscopy
XPEEM is a special case of low energy electron microscopy (LEEM) [9]. The samples
in XPEEM are usually investigated under UHV conditions employing synchrotron
x-ray radiation for illumination. An electron optical set-up is used to obtain an amplified image of the sample by detecting the x-ray photo-excited electrons [9]. XPEEM