5 Magneto-Plasmonic Nanoparticles
115
and particle growth are involved [13, 33, 35, 44]. Crystal growth, shape and composition of the magnetic moiety can be different than bare magnetic NPs. While most
studies compare bare nanoparticles with hybrid ones, in which the nucleation process
is different, recent studies [78–81] have compared hybrid systems with the corresponding NPs or hollow NPs obtained after the elimination of the plasmonic moiety.
Even if the morphology is similar, the two nanomaterials exhibit different magnetic
properties, such as saturation magnetization and hysteresis [81]. This demonstrates
the critical role in the magnetic properties of the plasmonic-magnetic interface.
More deeply investigated cases correspond to heterostructures as CS nanoparticles
and HDs in which Ag and Au are combined with Fe oxide. The first effect is the modification of the magnetization of the magnetic moiety, consisting in a strong decrease
of saturation magnetization of Fe 3 O 4 in comparison to the bare nanoparticles. This
decrease is typically correlated to the presence of a magnetic disordered layer, called
dead layer, at the free surface or at the Au-interface of the magnetic moiety [45,
82–87]. In the case of CS the presence of antiferromagnetic phase boundaries in
the intergranular regions of the multigrain or petals of Fe 3 O 4 can also give rise to
a decrease of the magnetization [61, 88]. In addition, the presence of an antiferromagnetic FeO wustite layer in the interface region between the Fe 3 O 4 and Au
moieties could also cause of the decrease of magnetization [61, 89, 90]. However,
such behavior is not always observed as in some studies the specific magnetization is
near the bulk value (80–90 emu/g), [91–93] suggesting the above proposed problem
could be overcome. Further studies on the physico-chemical properties and chemical
stability of these structures are required.
A significant question that illustrates the complexity of the magnetic order in
hybrid system is the magnetic observation of the Verwey temperature. This is a
specific fingerprint of magnetite, Fe 3 O 4 , not present in other Fe oxides, which corresponds to a metal-to-insulator transition around 120 K. The change of the electric
nature of the Fe 3 O 4 at this transition corresponds also with changes in the magnetic
and optical properties of the oxide and hence plasmonic coupling features in a hybrid
system could be influenced. In many studies, including both HDs and CS [81, 89,
93–95] structures the Verwey transition is observed at or below the bulk temperature
value, while in other cases it is absent [45, 61, 85, 87, 96, 97]. The absence of this
transition could be a fingerprint of the size effects in the oxide. However, changes in
the stoichiometry or the partial oxidation of the Fe 3 O 4 , composed of Fe
2+ and Fe
3+ ,
to maghemite (γ-Fe 2 O 3 ) or other oxides composed of only Fe
3+ are more possibly
the origin of these changes.
MP hybrid nanostructures exhibit also differences in the reversal process mechanism of the magnetization. In principle, most of the investigated MPs have the
magnetic moieties with a dimension below the 20–30 nm, being near or below the
single domain range [11]. In such case, the dominant mechanism of reversal of
the magnetization is the coherent rotation of all the spins. The hysteresis loops,
described by the Stoner-Wohlfarth model [11, 98], should have open loops with
maximum coercive field correlated to the anisotropy field of the material. However,
local and global surface and size effects determine mainly the reversal process and
all the magnetic properties of the magnetic particles [11, 98, 99]. In addition, thermal
115
and particle growth are involved [13, 33, 35, 44]. Crystal growth, shape and composition of the magnetic moiety can be different than bare magnetic NPs. While most
studies compare bare nanoparticles with hybrid ones, in which the nucleation process
is different, recent studies [78–81] have compared hybrid systems with the corresponding NPs or hollow NPs obtained after the elimination of the plasmonic moiety.
Even if the morphology is similar, the two nanomaterials exhibit different magnetic
properties, such as saturation magnetization and hysteresis [81]. This demonstrates
the critical role in the magnetic properties of the plasmonic-magnetic interface.
More deeply investigated cases correspond to heterostructures as CS nanoparticles
and HDs in which Ag and Au are combined with Fe oxide. The first effect is the modification of the magnetization of the magnetic moiety, consisting in a strong decrease
of saturation magnetization of Fe 3 O 4 in comparison to the bare nanoparticles. This
decrease is typically correlated to the presence of a magnetic disordered layer, called
dead layer, at the free surface or at the Au-interface of the magnetic moiety [45,
82–87]. In the case of CS the presence of antiferromagnetic phase boundaries in
the intergranular regions of the multigrain or petals of Fe 3 O 4 can also give rise to
a decrease of the magnetization [61, 88]. In addition, the presence of an antiferromagnetic FeO wustite layer in the interface region between the Fe 3 O 4 and Au
moieties could also cause of the decrease of magnetization [61, 89, 90]. However,
such behavior is not always observed as in some studies the specific magnetization is
near the bulk value (80–90 emu/g), [91–93] suggesting the above proposed problem
could be overcome. Further studies on the physico-chemical properties and chemical
stability of these structures are required.
A significant question that illustrates the complexity of the magnetic order in
hybrid system is the magnetic observation of the Verwey temperature. This is a
specific fingerprint of magnetite, Fe 3 O 4 , not present in other Fe oxides, which corresponds to a metal-to-insulator transition around 120 K. The change of the electric
nature of the Fe 3 O 4 at this transition corresponds also with changes in the magnetic
and optical properties of the oxide and hence plasmonic coupling features in a hybrid
system could be influenced. In many studies, including both HDs and CS [81, 89,
93–95] structures the Verwey transition is observed at or below the bulk temperature
value, while in other cases it is absent [45, 61, 85, 87, 96, 97]. The absence of this
transition could be a fingerprint of the size effects in the oxide. However, changes in
the stoichiometry or the partial oxidation of the Fe 3 O 4 , composed of Fe
2+ and Fe
3+ ,
to maghemite (γ-Fe 2 O 3 ) or other oxides composed of only Fe
3+ are more possibly
the origin of these changes.
MP hybrid nanostructures exhibit also differences in the reversal process mechanism of the magnetization. In principle, most of the investigated MPs have the
magnetic moieties with a dimension below the 20–30 nm, being near or below the
single domain range [11]. In such case, the dominant mechanism of reversal of
the magnetization is the coherent rotation of all the spins. The hysteresis loops,
described by the Stoner-Wohlfarth model [11, 98], should have open loops with
maximum coercive field correlated to the anisotropy field of the material. However,
local and global surface and size effects determine mainly the reversal process and
all the magnetic properties of the magnetic particles [11, 98, 99]. In addition, thermal