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A. Kleibert
magnetic properties are averaged over large ensembles with distributions of particle
sizes, morphologies, structures, defects, and orientations. Therefore, it remains often
unclear whether the mean values obtained by integral measurements yield representative information about the actual nanoparticle properties or whether averaged data
mask a more complex distribution of properties within the ensembles.
In order to unambiguously address this issue it is necessary to be able to probe
the magnetism of individual nanoparticles and different experimental approaches are
available today for this purpose [6]. For instance, microSQUIDs have been used to
determine the magnetic anisotropy energy of isolated nanoparticles in great detail
[7], while electron holography is used to study the magnetic structure in isolated
or interacting nanoparticles [8]. In this chapter we focus on x-ray photoemission
electron microscopy (XPEEM), a magnetic spectromicroscopy technique suited to
the investigation of the magnetic properties and chemical composition of individual
nanoparticles under ultrahigh vacuum (UHV) conditions [9, 10]. Moreover, XPEEM
allows one to probe simultaneously many individual nanoparticles (typically a few
hundred) in large ensembles, giving direct insight into the distribution of the magnetic properties in typical samples. Further, magnetic and chemical properties can be
directly correlated with the actual size, shape, orientation and structure of the very
same nanoparticles by combining XPEEM with other microscopy techniques such
as scanning electron microscopy (SEM) or atomic force microscopy (AFM). Moreover, it is possible to follow the evolution of the magnetic properties as a function of
temperature, magnetic fields, and during in situ chemical reactions. Finally, XPEEM
provides not only access to the properties of ferromagnetically ordered systems,
but is also suitable to investigating the magnetic properties of antiferromagnetically
ordered nanoparticles.
XPEEM is a well established technique for the study of magnetic phenomena
in systems ranging from films to nanodevices [11–18]. The first XPEEM investigations on individual 3d transition metal containing nanoparticles likely date back to
2002 [19]. This study focused on chemically synthesized and surfactant capped γ -
Fe 2 O 3 particles dispersed on Si wafers. Somewhat later, an investigation of ambient
air exposed cobalt nanoparticles with a native oxide shell deposited on Si wafers
was reported [20]. Both experiments demonstrated that x-ray absorption spectra of
individual nanoparticles can be obtained, but the signal to noise ratio of the data
was low and no magnetism was detected. These issues were successfully addressed
in subsequent in situ experiments, where sample preparation, transfer and XPEEM
characterization were carried out under UHV conditions [21, 22]. This approach
yielded a significantly increased signal to noise ratio and the detection of magnetism
in individual nanoparticles. It also enabled the first investigations of the pristine
properties of individual 3d transition metal nanoparticles without the influence of
surfactants, oxide shells, or matrix materials.
To demonstrate the potential of XPEEM for nanoparticle research, we will review
the recent work on 3d transition metal nanoparticles with sizes ranging from 8 to
20 nm. We will mostly focus on iron nanoparticles, because they are often found as
spherical or cubic single crystalline nanoparticles with bulk-like bcc structure and
magnetic moments comparable to that of their bulk counterpart. These character-
A. Kleibert
magnetic properties are averaged over large ensembles with distributions of particle
sizes, morphologies, structures, defects, and orientations. Therefore, it remains often
unclear whether the mean values obtained by integral measurements yield representative information about the actual nanoparticle properties or whether averaged data
mask a more complex distribution of properties within the ensembles.
In order to unambiguously address this issue it is necessary to be able to probe
the magnetism of individual nanoparticles and different experimental approaches are
available today for this purpose [6]. For instance, microSQUIDs have been used to
determine the magnetic anisotropy energy of isolated nanoparticles in great detail
[7], while electron holography is used to study the magnetic structure in isolated
or interacting nanoparticles [8]. In this chapter we focus on x-ray photoemission
electron microscopy (XPEEM), a magnetic spectromicroscopy technique suited to
the investigation of the magnetic properties and chemical composition of individual
nanoparticles under ultrahigh vacuum (UHV) conditions [9, 10]. Moreover, XPEEM
allows one to probe simultaneously many individual nanoparticles (typically a few
hundred) in large ensembles, giving direct insight into the distribution of the magnetic properties in typical samples. Further, magnetic and chemical properties can be
directly correlated with the actual size, shape, orientation and structure of the very
same nanoparticles by combining XPEEM with other microscopy techniques such
as scanning electron microscopy (SEM) or atomic force microscopy (AFM). Moreover, it is possible to follow the evolution of the magnetic properties as a function of
temperature, magnetic fields, and during in situ chemical reactions. Finally, XPEEM
provides not only access to the properties of ferromagnetically ordered systems,
but is also suitable to investigating the magnetic properties of antiferromagnetically
ordered nanoparticles.
XPEEM is a well established technique for the study of magnetic phenomena
in systems ranging from films to nanodevices [11–18]. The first XPEEM investigations on individual 3d transition metal containing nanoparticles likely date back to
2002 [19]. This study focused on chemically synthesized and surfactant capped γ -
Fe 2 O 3 particles dispersed on Si wafers. Somewhat later, an investigation of ambient
air exposed cobalt nanoparticles with a native oxide shell deposited on Si wafers
was reported [20]. Both experiments demonstrated that x-ray absorption spectra of
individual nanoparticles can be obtained, but the signal to noise ratio of the data
was low and no magnetism was detected. These issues were successfully addressed
in subsequent in situ experiments, where sample preparation, transfer and XPEEM
characterization were carried out under UHV conditions [21, 22]. This approach
yielded a significantly increased signal to noise ratio and the detection of magnetism
in individual nanoparticles. It also enabled the first investigations of the pristine
properties of individual 3d transition metal nanoparticles without the influence of
surfactants, oxide shells, or matrix materials.
To demonstrate the potential of XPEEM for nanoparticle research, we will review
the recent work on 3d transition metal nanoparticles with sizes ranging from 8 to
20 nm. We will mostly focus on iron nanoparticles, because they are often found as
spherical or cubic single crystalline nanoparticles with bulk-like bcc structure and
magnetic moments comparable to that of their bulk counterpart. These character-