90
E. L. Winkler and R. D. Zysler
possibilities for manufacturing new systems, where proper materials can be chosen
to optimize the exchange interaction between the two magnetic phases, i.e., by minimizing the crystalline mismatch, selecting compounds with particular magnetization
and magnetic anisotropy, etc. The seed-mediated high-temperature thermal decomposition method has shown high efficiency in producing monodispersed nanoparticles with high quality of crystalline order and well-defined interfaces. These characteristics make it one of the most used methods which enabled the fabrication of many
novel core/shell bimagnetic nanoparticles, e.g., CoO/CoFe 2 O 4 [36], CoO/NiFe 2 O 4
[31], CoFe 2 O 4 /MnFe 2 O 4 [57], Fe 3 O 4 /γ-Mn 2 O 3 [24], ZnFe 2 O 4 /CoFe 2 O 4 [42]. Also,
two-step co-precipitation methods have proven to be appropriate for creating
core/shell architecture. Examples of that are γ-Fe 2 O 3 /CoO [55], SmCo/Co [65],
and CoFe 2 O 4 /Fe 3 O 4 [16]. The mentioned chemical routes, among many others [15],
show an enormous versatility and easy to produce a great variety of bimagnetic
core/shell nanoparticles, which make this field promising for the design and fabrication of new systems for fundamental studies or for the development of nanoparticles
for a wide range of applications.
4.3 Interface Coupling Phenomenology and Models
As the size of a magnetic material is reduced to nanometric scale, the surface-tovolume ratio increases and, as a consequence, the surface effects became more relevant. For example, approximately 1% of the atoms are located at the surface of
single-phase nanoparticles of 100 nm, while the proportion increases over the 60%
when the size diminishes to 3 nm. Surface defects, broken bonds, variation in the interatomic distance and surface local anisotropy induce magnetic disorder and magnetic
frustration. In bimagnetic core/shell nanoparticles, besides the surface effects, the
presence of the interface between the two magnetically ordered phases introduces
additional interactions that could radically modify the static and dynamic magnetic
behavior of the systems. This interface interaction can generate a new behavior that
is not present in any of the original components as exchange bias effect, exchange
spring, and anisotropy enhancement, which present characteristic hysteresis loops
as illustrated in Fig. 4.1. There are good and complete reviews in the literature that
discuss in detail the models that describe these effects [14, 22, 25, 39, 46, 48], below
we are going to mention the main characteristics that has to be taken into account to
predict the behavior and for designing a particular system.
4.3.1 Exchange Bias Effect
In bimagnetic nanostructures with FM/AFM interfaces, the exchange bias effect (E B )
is typically manifested by a field shift (H EB ) of the hysteresis loop when the material
is field cooled from temperatures higher than the Néel temperature of the AFM (T N )
E. L. Winkler and R. D. Zysler
possibilities for manufacturing new systems, where proper materials can be chosen
to optimize the exchange interaction between the two magnetic phases, i.e., by minimizing the crystalline mismatch, selecting compounds with particular magnetization
and magnetic anisotropy, etc. The seed-mediated high-temperature thermal decomposition method has shown high efficiency in producing monodispersed nanoparticles with high quality of crystalline order and well-defined interfaces. These characteristics make it one of the most used methods which enabled the fabrication of many
novel core/shell bimagnetic nanoparticles, e.g., CoO/CoFe 2 O 4 [36], CoO/NiFe 2 O 4
[31], CoFe 2 O 4 /MnFe 2 O 4 [57], Fe 3 O 4 /γ-Mn 2 O 3 [24], ZnFe 2 O 4 /CoFe 2 O 4 [42]. Also,
two-step co-precipitation methods have proven to be appropriate for creating
core/shell architecture. Examples of that are γ-Fe 2 O 3 /CoO [55], SmCo/Co [65],
and CoFe 2 O 4 /Fe 3 O 4 [16]. The mentioned chemical routes, among many others [15],
show an enormous versatility and easy to produce a great variety of bimagnetic
core/shell nanoparticles, which make this field promising for the design and fabrication of new systems for fundamental studies or for the development of nanoparticles
for a wide range of applications.
4.3 Interface Coupling Phenomenology and Models
As the size of a magnetic material is reduced to nanometric scale, the surface-tovolume ratio increases and, as a consequence, the surface effects became more relevant. For example, approximately 1% of the atoms are located at the surface of
single-phase nanoparticles of 100 nm, while the proportion increases over the 60%
when the size diminishes to 3 nm. Surface defects, broken bonds, variation in the interatomic distance and surface local anisotropy induce magnetic disorder and magnetic
frustration. In bimagnetic core/shell nanoparticles, besides the surface effects, the
presence of the interface between the two magnetically ordered phases introduces
additional interactions that could radically modify the static and dynamic magnetic
behavior of the systems. This interface interaction can generate a new behavior that
is not present in any of the original components as exchange bias effect, exchange
spring, and anisotropy enhancement, which present characteristic hysteresis loops
as illustrated in Fig. 4.1. There are good and complete reviews in the literature that
discuss in detail the models that describe these effects [14, 22, 25, 39, 46, 48], below
we are going to mention the main characteristics that has to be taken into account to
predict the behavior and for designing a particular system.
4.3.1 Exchange Bias Effect
In bimagnetic nanostructures with FM/AFM interfaces, the exchange bias effect (E B )
is typically manifested by a field shift (H EB ) of the hysteresis loop when the material
is field cooled from temperatures higher than the Néel temperature of the AFM (T N )