6 Hollow Magnetic Nanoparticles
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6.3 Magnetic Properties of Hollow MNPs
Hollow MNPs exhibit interesting magnetic properties, which are rather different from
those of their solid counterparts. In general, the nanoparticles’ magnetic response is
based on the combination of surface effect, finite-size effect, inter and intra-particles
interactions, etc. In the case of hollow MNPs, increase in the surface area due to the
appearance of the inner surface can give rise to distinct magnetic properties, including
reduced magnetization, spin canting, and a strong increase in effective anisotropy
and exchange bias [15, 20]. Therefore, hollow MNPs provide an excellent model to
study these effects.
6.3.1 Basic Magnetic Behavior
The magnetic response of the hollow MNPs as a function of the temperature and
the applied magnetic field presents several clear differences in comparison with their
solid counterparts.
In the zero-field-cooled/field-cooled ZFC/FC curves obtained from measuring
the magnetization as a function of temperature (Fig. 6.4a), hollow iron oxide MNPs
normally present a maximum at low temperatures, corresponding to the magnetic
blocking of the hollow MNPs, followed by a continuous decrease of the magnetization with increasing temperature. For sizes size below 10 nm, the blocking usually
takes place at very low temperatures <50 K. As the size of these MNPs increases,
so it does their blocking temperature. This value of the blocking temperature tends
to be appreciably smaller than the one corresponding to solid MNPs of equivalent
volume. This indicates that the blocking process in hollow MNPs is more related to
the individual blocking of the nanograins that compose the hollow shell, than to the
overall blocking of the whole nanoparticle, as it is usually the case for solid MNPs.
However, even for the smallest hollow MNPs, the blocking temperature obtained
tends to be larger than the one that would correspond to the isolated nanograins.
This can be related to the presence of magnetic interactions among these nanograins
and/or an enhanced value of the anisotropy energy due to the spins surface disorder.
On the other hand, for the hollow nanoparticles, some separation between the ZFC
and FC magnetization branches can also be typically observed. The irreversibility
between the ZFC and FC curves can persist even at 300 K, far above the blocking
temperature, and it can be attributed to increased anisotropy in the system and the
effects of inter-particle and intra-particle interactions.
Concerning the magnetic hysteresis, M–H, loops (see Fig. 6.4b, c), at very low
temperatures (~5 K), hollow MNPs usually present an elongated shape with a
pronounced increase of the coercivity, decrease of the remanence, and a high field
slope in comparison with solid ones. The M–H loop for the hollow MNPs, especially
at low temperatures, resembles those of frustrated and disordered random anisotropy
magnets. This is related to the high magnetic frustration present in the spins of these
143
6.3 Magnetic Properties of Hollow MNPs
Hollow MNPs exhibit interesting magnetic properties, which are rather different from
those of their solid counterparts. In general, the nanoparticles’ magnetic response is
based on the combination of surface effect, finite-size effect, inter and intra-particles
interactions, etc. In the case of hollow MNPs, increase in the surface area due to the
appearance of the inner surface can give rise to distinct magnetic properties, including
reduced magnetization, spin canting, and a strong increase in effective anisotropy
and exchange bias [15, 20]. Therefore, hollow MNPs provide an excellent model to
study these effects.
6.3.1 Basic Magnetic Behavior
The magnetic response of the hollow MNPs as a function of the temperature and
the applied magnetic field presents several clear differences in comparison with their
solid counterparts.
In the zero-field-cooled/field-cooled ZFC/FC curves obtained from measuring
the magnetization as a function of temperature (Fig. 6.4a), hollow iron oxide MNPs
normally present a maximum at low temperatures, corresponding to the magnetic
blocking of the hollow MNPs, followed by a continuous decrease of the magnetization with increasing temperature. For sizes size below 10 nm, the blocking usually
takes place at very low temperatures <50 K. As the size of these MNPs increases,
so it does their blocking temperature. This value of the blocking temperature tends
to be appreciably smaller than the one corresponding to solid MNPs of equivalent
volume. This indicates that the blocking process in hollow MNPs is more related to
the individual blocking of the nanograins that compose the hollow shell, than to the
overall blocking of the whole nanoparticle, as it is usually the case for solid MNPs.
However, even for the smallest hollow MNPs, the blocking temperature obtained
tends to be larger than the one that would correspond to the isolated nanograins.
This can be related to the presence of magnetic interactions among these nanograins
and/or an enhanced value of the anisotropy energy due to the spins surface disorder.
On the other hand, for the hollow nanoparticles, some separation between the ZFC
and FC magnetization branches can also be typically observed. The irreversibility
between the ZFC and FC curves can persist even at 300 K, far above the blocking
temperature, and it can be attributed to increased anisotropy in the system and the
effects of inter-particle and intra-particle interactions.
Concerning the magnetic hysteresis, M–H, loops (see Fig. 6.4b, c), at very low
temperatures (~5 K), hollow MNPs usually present an elongated shape with a
pronounced increase of the coercivity, decrease of the remanence, and a high field
slope in comparison with solid ones. The M–H loop for the hollow MNPs, especially
at low temperatures, resembles those of frustrated and disordered random anisotropy
magnets. This is related to the high magnetic frustration present in the spins of these