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D. Passeri et al.
standardized and allow one to accurately quantify magnetic properties of materials, i.e., to acquire their characteristic magnetization curve and to evaluate magnetic
parameters like saturation magnetization M s and coercivity H c . Due to their sensitivity, however, these techniques allow the analysis of samples containing a significant
number of NPs and thus the measured magnetic characteristics are averaged on
the whole ensemble of NPs. This approach is obviously effective in those cases in
which ‘macroscopic’ amounts of NPs are needed for specific applications and their
average magnetic properties have to be characterized. Conversely, the possibility
of retrieving only average magnetic properties may represent a severe limitation
when their dependence on physical parameters of the NPs are investigated, e.g., to
obtain a more comprehensive knowledge of magnetisms at the nanometer scale and
to design and optimize innovative magnetic nanomaterials. Indeed, the capability
to relate the obtained results to specific physical parameters of the NPs dramatically depends on the homogeneity of the NPs in the analyzed sample. Therefore,
the scientific and technological research in the field of characterization methods in
nanomagnetisms has been recently focused on the development of techniques capable to analyze magnetic properties of nanomaterials combining satisfying accuracy
and sensitivity with high spatial resolution. Ideally, these techniques should replicate the performances of standardized methods on individual nanomaterials, which
should be visualized, selected and probed with nanometer scale spatial resolution.
Different methods specifically developed to this purposed have been proposed by
miniaturizing standard techniques, e.g., in the case micro and nano-SQUID [32–35].
In these methods, however, a certain limitation may be related to the actual sensitivity
an to the capability to select the sample to be investigated. Methods have been also
developed based on beam microscopy (e.g., electron or X-ray) approaches, such as
off-axis electron holography [36, 37], differential phase contrast mode (DPC) [38],
X-ray holography [39], transmission X-ray microscopy [40–42], X-ray photoemission electron microscopy (XPEEM) [43, 44]. Overall and with the due distinctions,
the performances of these methods in terms of sensitivity and capability to image and
select the single nanomaterial to probe undoubtedly represent significant advantages.
However, some limitations may be related to the required characterization environment, e.g., vacuum or low temperature, to the sample preparation as well as to the
relative complexity of the experimental setups. Combining high resolution imaging
capabilities, possibility of probing a selected location on the sample with nanometer lateral resolution, flexibility of characterization conditions (e.g., air as well as in
liquid, inert gas, vacuum and at room or low temperature), simplicity of sample preparation, scanning probe microscopy (SPM) has been used as a platform to develop
advanced and very effective methods for magnetic characterization at the nanoscale,
such as MFM, magneto-optical scanning near field optical microscopy [45] or scanning magnetometry with nitrogen-vacancy color centers in diamond [46–49]. Among
them, standard MFM and advanced MFM-based methods are undoubtedly the most
widespread techniques due to relative popularity of AFM setups, in which MFM is
generally featured or can be easily included. While MFM has been widely used to
qualitatively image magnetic NPs, one of the most intriguing possibilities offered by
D. Passeri et al.
standardized and allow one to accurately quantify magnetic properties of materials, i.e., to acquire their characteristic magnetization curve and to evaluate magnetic
parameters like saturation magnetization M s and coercivity H c . Due to their sensitivity, however, these techniques allow the analysis of samples containing a significant
number of NPs and thus the measured magnetic characteristics are averaged on
the whole ensemble of NPs. This approach is obviously effective in those cases in
which ‘macroscopic’ amounts of NPs are needed for specific applications and their
average magnetic properties have to be characterized. Conversely, the possibility
of retrieving only average magnetic properties may represent a severe limitation
when their dependence on physical parameters of the NPs are investigated, e.g., to
obtain a more comprehensive knowledge of magnetisms at the nanometer scale and
to design and optimize innovative magnetic nanomaterials. Indeed, the capability
to relate the obtained results to specific physical parameters of the NPs dramatically depends on the homogeneity of the NPs in the analyzed sample. Therefore,
the scientific and technological research in the field of characterization methods in
nanomagnetisms has been recently focused on the development of techniques capable to analyze magnetic properties of nanomaterials combining satisfying accuracy
and sensitivity with high spatial resolution. Ideally, these techniques should replicate the performances of standardized methods on individual nanomaterials, which
should be visualized, selected and probed with nanometer scale spatial resolution.
Different methods specifically developed to this purposed have been proposed by
miniaturizing standard techniques, e.g., in the case micro and nano-SQUID [32–35].
In these methods, however, a certain limitation may be related to the actual sensitivity
an to the capability to select the sample to be investigated. Methods have been also
developed based on beam microscopy (e.g., electron or X-ray) approaches, such as
off-axis electron holography [36, 37], differential phase contrast mode (DPC) [38],
X-ray holography [39], transmission X-ray microscopy [40–42], X-ray photoemission electron microscopy (XPEEM) [43, 44]. Overall and with the due distinctions,
the performances of these methods in terms of sensitivity and capability to image and
select the single nanomaterial to probe undoubtedly represent significant advantages.
However, some limitations may be related to the required characterization environment, e.g., vacuum or low temperature, to the sample preparation as well as to the
relative complexity of the experimental setups. Combining high resolution imaging
capabilities, possibility of probing a selected location on the sample with nanometer lateral resolution, flexibility of characterization conditions (e.g., air as well as in
liquid, inert gas, vacuum and at room or low temperature), simplicity of sample preparation, scanning probe microscopy (SPM) has been used as a platform to develop
advanced and very effective methods for magnetic characterization at the nanoscale,
such as MFM, magneto-optical scanning near field optical microscopy [45] or scanning magnetometry with nitrogen-vacancy color centers in diamond [46–49]. Among
them, standard MFM and advanced MFM-based methods are undoubtedly the most
widespread techniques due to relative popularity of AFM setups, in which MFM is
generally featured or can be easily included. While MFM has been widely used to
qualitatively image magnetic NPs, one of the most intriguing possibilities offered by