9 Magnetism of Individual Nanoparticles Probed by X-Ray …
227
9.3.1 Enhanced Magnetism and Metastable Properties in
Iron Nanoparticles
We start the discussion with XPEEM investigations of iron nanoparticles in contact
with non-magnetic silicon substrates passivated with a native, amorphous oxide layer
[5, 38]. The iron nanoparticles, with sizes ranging from 8 to 20 nm, are generated
in a gas phase condensation process using a UHV-compatible arc cluster ion source
(ACIS) [46] and then deposited under soft-landing conditions on clean silicon substrates. Figure 9.3a, b display typical elemental and magnetic contrast maps of these
samples recorded at room temperature using XPEEM. The data reveal that a sizable
fraction of nanoparticles is in a magnetically blocked state and exhibits stable magnetic contrast over many hours with grey tone levels ranging from black to white
such as nanoparticles “B” and “C” in Fig. 9.3, while other nanoparticles exhibit no
magnetic contrast as for instance nanoparticle “A”. A detailed quantitative analysis
of the magnetic contrast levels of the magnetically blocked nanoparticles reveals
a random orientation of their magnetic moments [38]. This finding is consistent
with the random crystallographic orientation of the nanoparticles on the substrate,
which was confirmed by means of in situ reflection high energy electron diffraction
(RHEED) experiments [5]. The RHEED investigations showed in addition that the
iron nanoparticles possess the bcc crystal lattice known from bulk iron.
The magnetic state of the nanoparticles was further investigated by recording
magnetization curves in XPEEM using an external magnetic field. It was found that
nanoparticles which initially exhibited no magnetic contrast were easily magnetized
with magnetic fields of the order of a few mT, see Fig. 9.3c. This behaviour showed
that these nanoparticles were also ferromagnetically ordered, but in a superparamagnetic state. This finding revealed that magnetically blocked and superparamagnetic
iron nanoparticles coexist on the silicon substrates. A similar coexistence of superparamagnetic and magnetically blocked iron nanoparticles was also inferred from
earlier ensemble measurements [30], where a critical size of 9 nm was estimated
for the onset of superparamagnetic behavior within the samples. A direct determination of this critical size can be achieved by combining XPEEM with structural
characterization by means of SEM and AFM. When correlating the magnetic state,
magnetically blocked or superparamagnetic, with the size of the very same nanoparticles, no critical size was observed [38]. Instead, the analysis showed that iron
nanoparticles can exist in both states at any size in the investigated range from 8
to 20 nm. In case of the iron nanoparticles in the as deposited state on silicon substrates, about one half of the nanoparticles were found in a magnetically blocked
state, while the other half was found in a superparamagnetic state, with both fractions exhibiting nearly the same distribution of sizes as shown in Fig. 9.6g. Another
surprising finding was that some of the magnetically blocked nanoparticles changed
their behavior spontaneously and developed a superparamagnetic behaviour during
the experiments, carried out at room temperature in a time span of several hours, see
for instance Fig. 9.3e and f. This transition could be promoted by thermal treatment
of the sample, in particular heating 420 K led to superparamagnetic behavior at room
227
9.3.1 Enhanced Magnetism and Metastable Properties in
Iron Nanoparticles
We start the discussion with XPEEM investigations of iron nanoparticles in contact
with non-magnetic silicon substrates passivated with a native, amorphous oxide layer
[5, 38]. The iron nanoparticles, with sizes ranging from 8 to 20 nm, are generated
in a gas phase condensation process using a UHV-compatible arc cluster ion source
(ACIS) [46] and then deposited under soft-landing conditions on clean silicon substrates. Figure 9.3a, b display typical elemental and magnetic contrast maps of these
samples recorded at room temperature using XPEEM. The data reveal that a sizable
fraction of nanoparticles is in a magnetically blocked state and exhibits stable magnetic contrast over many hours with grey tone levels ranging from black to white
such as nanoparticles “B” and “C” in Fig. 9.3, while other nanoparticles exhibit no
magnetic contrast as for instance nanoparticle “A”. A detailed quantitative analysis
of the magnetic contrast levels of the magnetically blocked nanoparticles reveals
a random orientation of their magnetic moments [38]. This finding is consistent
with the random crystallographic orientation of the nanoparticles on the substrate,
which was confirmed by means of in situ reflection high energy electron diffraction
(RHEED) experiments [5]. The RHEED investigations showed in addition that the
iron nanoparticles possess the bcc crystal lattice known from bulk iron.
The magnetic state of the nanoparticles was further investigated by recording
magnetization curves in XPEEM using an external magnetic field. It was found that
nanoparticles which initially exhibited no magnetic contrast were easily magnetized
with magnetic fields of the order of a few mT, see Fig. 9.3c. This behaviour showed
that these nanoparticles were also ferromagnetically ordered, but in a superparamagnetic state. This finding revealed that magnetically blocked and superparamagnetic
iron nanoparticles coexist on the silicon substrates. A similar coexistence of superparamagnetic and magnetically blocked iron nanoparticles was also inferred from
earlier ensemble measurements [30], where a critical size of 9 nm was estimated
for the onset of superparamagnetic behavior within the samples. A direct determination of this critical size can be achieved by combining XPEEM with structural
characterization by means of SEM and AFM. When correlating the magnetic state,
magnetically blocked or superparamagnetic, with the size of the very same nanoparticles, no critical size was observed [38]. Instead, the analysis showed that iron
nanoparticles can exist in both states at any size in the investigated range from 8
to 20 nm. In case of the iron nanoparticles in the as deposited state on silicon substrates, about one half of the nanoparticles were found in a magnetically blocked
state, while the other half was found in a superparamagnetic state, with both fractions exhibiting nearly the same distribution of sizes as shown in Fig. 9.6g. Another
surprising finding was that some of the magnetically blocked nanoparticles changed
their behavior spontaneously and developed a superparamagnetic behaviour during
the experiments, carried out at room temperature in a time span of several hours, see
for instance Fig. 9.3e and f. This transition could be promoted by thermal treatment
of the sample, in particular heating 420 K led to superparamagnetic behavior at room