6 Hollow Magnetic Nanoparticles
155
The high surface area of these hollow MNPs is also advantageous for chemical
and gas sensing. Sensors are widely used in industrial process control and safety
applications. In the case of hollow MNPs, the enhanced porosity and the cavity of
these MNPs seems to facilitate the diffusion of analytes into the MNPs leading to an
increase in their sensitivity. In most of the cases, the sensing with these materials is
carried out through measurements of change of resistivity produced by adsorption–
desorption of a targeted analyte on the surface of the MNPs. For example, Wu et al.
have reported that hematite composite hollow nanostructures exhibited high gas
sensitivity toward formaldehyde and ethanol at room temperature, improving the
results obtained with solid hematite MNPs [65].
One of the major potential applications of MNPs is in the field of data storage
(hard drives, DVDs, etc.). The underlying idea is to replace the large randomly
oriented magnetic grains employed in conventional media by single MNPs, in order
to exponentially increase the data storage density. The smaller the MNPs, the higher
the density. To attain this, the magnetic moment of the MNPs has to be very stable with
time. To this regard, FePt MNPs have been proposed as ideal candidates due to their
high anisotropy and stability [78]. However, as the MNPs become smaller, the thermal
disorder tends to overcome the anisotropy of the MNPs and their magnetic moments
are no longer stable. This is the so-called “superparamagnetic limit” and, in principle,
imposes restrictions on our capacity to increase the data storage density. However, in
the last few years, a way to overcome this limit has been proposed, based on the use of
“exchange bias” (EB) [12]. The EB effect generates an additional effective anisotropy
that allows reducing the size of the MNPs below the “superparamagnetic limit,” and
still obtaining a stable magnetization. As we explained before, hollow MNPs exhibit
enhanced EB effect, making them promising candidates for data storage and similar
applications [56]. Despite the promising results obtained in hollow MNPs, for now,
the EB effect is mainly appreciable at very low temperatures (<50 k), and therefore,
further work is needed to develop hollow magnetic nanostructures with appreciable
EB at temperatures closer to ambient.
These are only a few examples of applications of hollow MNPs, but they already
provide an overall idea of the wide use of these MNPs, and their potential applicability
in other promising research areas such as spintronics or magneto-optical devices.
6.7 Summary and Future Outlook
In this book chapter, we have reviewed the main characteristics of hollow magnetic
nanostructures in general, and iron-oxide-based hollow MNPs in particular. We have
shown that the hollow morphology is particularly interesting because the presence of
the additional surface area in these nanostructures gives rise to enhanced magnetic
phenomena toward improved applications.
We have shown that hollow MNPs can be synthesized using either physical or
chemical methods, but in the case of iron oxide ones, thermal decomposition and
similar chemical synthesis routes are often employed. These synthesis methods let
155
The high surface area of these hollow MNPs is also advantageous for chemical
and gas sensing. Sensors are widely used in industrial process control and safety
applications. In the case of hollow MNPs, the enhanced porosity and the cavity of
these MNPs seems to facilitate the diffusion of analytes into the MNPs leading to an
increase in their sensitivity. In most of the cases, the sensing with these materials is
carried out through measurements of change of resistivity produced by adsorption–
desorption of a targeted analyte on the surface of the MNPs. For example, Wu et al.
have reported that hematite composite hollow nanostructures exhibited high gas
sensitivity toward formaldehyde and ethanol at room temperature, improving the
results obtained with solid hematite MNPs [65].
One of the major potential applications of MNPs is in the field of data storage
(hard drives, DVDs, etc.). The underlying idea is to replace the large randomly
oriented magnetic grains employed in conventional media by single MNPs, in order
to exponentially increase the data storage density. The smaller the MNPs, the higher
the density. To attain this, the magnetic moment of the MNPs has to be very stable with
time. To this regard, FePt MNPs have been proposed as ideal candidates due to their
high anisotropy and stability [78]. However, as the MNPs become smaller, the thermal
disorder tends to overcome the anisotropy of the MNPs and their magnetic moments
are no longer stable. This is the so-called “superparamagnetic limit” and, in principle,
imposes restrictions on our capacity to increase the data storage density. However, in
the last few years, a way to overcome this limit has been proposed, based on the use of
“exchange bias” (EB) [12]. The EB effect generates an additional effective anisotropy
that allows reducing the size of the MNPs below the “superparamagnetic limit,” and
still obtaining a stable magnetization. As we explained before, hollow MNPs exhibit
enhanced EB effect, making them promising candidates for data storage and similar
applications [56]. Despite the promising results obtained in hollow MNPs, for now,
the EB effect is mainly appreciable at very low temperatures (<50 k), and therefore,
further work is needed to develop hollow magnetic nanostructures with appreciable
EB at temperatures closer to ambient.
These are only a few examples of applications of hollow MNPs, but they already
provide an overall idea of the wide use of these MNPs, and their potential applicability
in other promising research areas such as spintronics or magneto-optical devices.
6.7 Summary and Future Outlook
In this book chapter, we have reviewed the main characteristics of hollow magnetic
nanostructures in general, and iron-oxide-based hollow MNPs in particular. We have
shown that the hollow morphology is particularly interesting because the presence of
the additional surface area in these nanostructures gives rise to enhanced magnetic
phenomena toward improved applications.
We have shown that hollow MNPs can be synthesized using either physical or
chemical methods, but in the case of iron oxide ones, thermal decomposition and
similar chemical synthesis routes are often employed. These synthesis methods let