9 Magnetism of Individual Nanoparticles Probed by X-Ray …
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revealed magnetic energy barriers that were still much lower when compared to
the experimental values [27]. The authors assigned this discrepancy to an underestimated contribution of the Néel surface anisotropy in their calculations. Also in
[38] it was shown by size and shape analysis that magnetic anisotropy contributions
due to shape effects were too low to account for the observed magnetic energy barriers, which could similarly hint at an underestimated surface contribution to the
magnetically blocked nanoparticles.
9.3.2 In Situ Oxidation of Iron Nanoparticles—The Role
of the Surface
In a number of experiments it was observed that the effective magnetic anisotropy
constants of iron nanoparticles increase with decreasing nanoparticle size [23, 24,
27]. In [24] this observation was explained by an effective surface anisotropy contribution which scales inversely with size and which adds to a bulk-like volume
contribution. However, the large spread of the experimental data in the different
reports does not permit the determination of an effective surface anisotropy constant
which could be used to successfully predict the effective magnetic anisotropy of iron
nanoparticles. This lack of understanding is partially related to the different surface
states of the iron nanoparticles in the various experiments and the particular chemical
reactivity of iron nanoparticles, which makes the control of their surface properties
very difficult. For instance, the iron nanoparticles studied in [23] were covered with
a native oxide layer, whereas the nanoparticles investigated in [27] were embedded
in a niobium matrix, while pure iron nanoparticles were investigated in an inert gas
atmosphere in [24]. Photoemission electron microscopy permits one to study samples under UHV conditions and under controlled exposure to reactive gases and has
therefore been used to investigate chemical surface reactions [50–52]. By taking
advantage of these capabilities, XPEEM was also used to investigate the evolution of
magnetic properties and chemical states in iron nanoparticles directly during in situ
oxidation experiments [53]. The latter work assumed that surface oxidation would
drastically alter the electronic and magnetic properties of the free surface of the
metal nanoparticles and would therefore be a direct probe of the role of the particle surface for the enhanced magnetic energy barriers of the magnetically blocked
nanoparticles [38]. On the other side, the lattice mismatch between metallic iron and
its oxides could give rise to significant strain in both the metallic core and the growing oxide shell and lead to sizeable modifications of the magnetic anisotropy energy
due to magneto-elastic contributions, which in turn could lead to significantly altered
properties in both superparamagnetic and magnetically blocked nanoparticles [54].
For the XPEEM in situ oxidation experiments, the iron nanoparticles were
deposited on silicon wafers as in [38]. The pristine state of the samples revealed therefore a similar distribution of magnetically blocked and superparamagnetic nanoparticles. X-ray absorption (XA) spectra of individual nanoparticles recorded at the Fe
229
revealed magnetic energy barriers that were still much lower when compared to
the experimental values [27]. The authors assigned this discrepancy to an underestimated contribution of the Néel surface anisotropy in their calculations. Also in
[38] it was shown by size and shape analysis that magnetic anisotropy contributions
due to shape effects were too low to account for the observed magnetic energy barriers, which could similarly hint at an underestimated surface contribution to the
magnetically blocked nanoparticles.
9.3.2 In Situ Oxidation of Iron Nanoparticles—The Role
of the Surface
In a number of experiments it was observed that the effective magnetic anisotropy
constants of iron nanoparticles increase with decreasing nanoparticle size [23, 24,
27]. In [24] this observation was explained by an effective surface anisotropy contribution which scales inversely with size and which adds to a bulk-like volume
contribution. However, the large spread of the experimental data in the different
reports does not permit the determination of an effective surface anisotropy constant
which could be used to successfully predict the effective magnetic anisotropy of iron
nanoparticles. This lack of understanding is partially related to the different surface
states of the iron nanoparticles in the various experiments and the particular chemical
reactivity of iron nanoparticles, which makes the control of their surface properties
very difficult. For instance, the iron nanoparticles studied in [23] were covered with
a native oxide layer, whereas the nanoparticles investigated in [27] were embedded
in a niobium matrix, while pure iron nanoparticles were investigated in an inert gas
atmosphere in [24]. Photoemission electron microscopy permits one to study samples under UHV conditions and under controlled exposure to reactive gases and has
therefore been used to investigate chemical surface reactions [50–52]. By taking
advantage of these capabilities, XPEEM was also used to investigate the evolution of
magnetic properties and chemical states in iron nanoparticles directly during in situ
oxidation experiments [53]. The latter work assumed that surface oxidation would
drastically alter the electronic and magnetic properties of the free surface of the
metal nanoparticles and would therefore be a direct probe of the role of the particle surface for the enhanced magnetic energy barriers of the magnetically blocked
nanoparticles [38]. On the other side, the lattice mismatch between metallic iron and
its oxides could give rise to significant strain in both the metallic core and the growing oxide shell and lead to sizeable modifications of the magnetic anisotropy energy
due to magneto-elastic contributions, which in turn could lead to significantly altered
properties in both superparamagnetic and magnetically blocked nanoparticles [54].
For the XPEEM in situ oxidation experiments, the iron nanoparticles were
deposited on silicon wafers as in [38]. The pristine state of the samples revealed therefore a similar distribution of magnetically blocked and superparamagnetic nanoparticles. X-ray absorption (XA) spectra of individual nanoparticles recorded at the Fe