4 Core/Shell Bimagnetic Nanoparticles
89
where the tuning of the coercive field, the thermal stability and the exchange bias
will be attained by controlling the size and the interface exchange coupling. Finally,
in the last section, future perspectives on the field will be described.
4.2 Synthesis and Production of Core/Shell Nanoparticles
The advances in the chemical and physical fabrication methods have enabled a continuous production of novel multifunctional nanostructures. In the particular case of
bimagnetic nanoparticles, according to the technological challenge or basic research,
different core/shell structures, including FM(FiM)-core/AFM-shell [1, 5, 9], FMcore/FiM-shell [66], hard/soft FiM-core/FiM-shell [11, 39], inverted AFMcore/FiMshell [30, 36], doubly inverted AFM-core/FiM-shell (T N > T C ) [53], and multishell
nanoparticles [54], have been fabricated.
A physical approach to the fabrication of bimagnetic nanoparticles is based on
obtaining the initial nanoparticles by known methods as gas condensation, thermal
plasma, or spray pyrolysis. These initial particles can be formed, in general, by
simple metals, alloys, or oxides. A second step, consisting of a post-treatment
of partial reduction of oxides or oxidation of alloys (or overoxidation of certain
oxides), provides a core/shell architecture to the nanoparticles. An example of oxidation of metallic particles has been reported in the initial paper of the exchange
bias phenomena where cobalt particles were oxidized giving a Co/CoO ferromagnetic/antiferromagnetic core/shell structure [43]. Another example of oxidation
of seeded nanoparticles is given by the Ni/NiO (ferromagnetic/antiferromagnetic)
core/shell structures [23]. On the other hand, the reduction of transition metal oxides
by annealing in reducing atmosphere (e.g., H 2 ) can also be used to obtain the desired
core/shell nanostructures as the CoO (Co 3 O 4 )/Co antiferromagnetic/ferromagnetic
inverted core/shell structure [63] or CoFe 2 O 4 /CoFe 2 [20].
The concept to produce core/shell nanoparticles by chemical route is similar.
Single-phase nanoparticles are synthesized by a particular chemical method, and
using the same concept of superficial post-treatment, an oxidation (reduction) is
taking effect in order to obtain the core/shell structure. Many chemical routes give
high control of composition, crystallinity, and size. There exists a large diversity
of synthesis methods such as co-precipitation [13], sol–gel [64], cation exchange
process [59], and thermal decomposition [35, 45]. Although it is very simple to obtain
core/shell architecture by post-processing the surface, there are inherent drawbacks
to the method that limit its application, i.e., it is very difficult to control independently
the core and shell sizes, and the shell composition is determined (and limited) by the
core composition. On the other hand, these manufacturing methods produce a low
quality of the core/shell interface. The last point is a major problem in the observation
of the exchange bias effect and/or the enhancement of the coercive field.
Another interesting approach is the seed-mediated growth method. This procedure
consists in the synthesis of the core and shell in two stages which enable to control
independently both composition and size of each component This opens up many
89
where the tuning of the coercive field, the thermal stability and the exchange bias
will be attained by controlling the size and the interface exchange coupling. Finally,
in the last section, future perspectives on the field will be described.
4.2 Synthesis and Production of Core/Shell Nanoparticles
The advances in the chemical and physical fabrication methods have enabled a continuous production of novel multifunctional nanostructures. In the particular case of
bimagnetic nanoparticles, according to the technological challenge or basic research,
different core/shell structures, including FM(FiM)-core/AFM-shell [1, 5, 9], FMcore/FiM-shell [66], hard/soft FiM-core/FiM-shell [11, 39], inverted AFMcore/FiMshell [30, 36], doubly inverted AFM-core/FiM-shell (T N > T C ) [53], and multishell
nanoparticles [54], have been fabricated.
A physical approach to the fabrication of bimagnetic nanoparticles is based on
obtaining the initial nanoparticles by known methods as gas condensation, thermal
plasma, or spray pyrolysis. These initial particles can be formed, in general, by
simple metals, alloys, or oxides. A second step, consisting of a post-treatment
of partial reduction of oxides or oxidation of alloys (or overoxidation of certain
oxides), provides a core/shell architecture to the nanoparticles. An example of oxidation of metallic particles has been reported in the initial paper of the exchange
bias phenomena where cobalt particles were oxidized giving a Co/CoO ferromagnetic/antiferromagnetic core/shell structure [43]. Another example of oxidation
of seeded nanoparticles is given by the Ni/NiO (ferromagnetic/antiferromagnetic)
core/shell structures [23]. On the other hand, the reduction of transition metal oxides
by annealing in reducing atmosphere (e.g., H 2 ) can also be used to obtain the desired
core/shell nanostructures as the CoO (Co 3 O 4 )/Co antiferromagnetic/ferromagnetic
inverted core/shell structure [63] or CoFe 2 O 4 /CoFe 2 [20].
The concept to produce core/shell nanoparticles by chemical route is similar.
Single-phase nanoparticles are synthesized by a particular chemical method, and
using the same concept of superficial post-treatment, an oxidation (reduction) is
taking effect in order to obtain the core/shell structure. Many chemical routes give
high control of composition, crystallinity, and size. There exists a large diversity
of synthesis methods such as co-precipitation [13], sol–gel [64], cation exchange
process [59], and thermal decomposition [35, 45]. Although it is very simple to obtain
core/shell architecture by post-processing the surface, there are inherent drawbacks
to the method that limit its application, i.e., it is very difficult to control independently
the core and shell sizes, and the shell composition is determined (and limited) by the
core composition. On the other hand, these manufacturing methods produce a low
quality of the core/shell interface. The last point is a major problem in the observation
of the exchange bias effect and/or the enhancement of the coercive field.
Another interesting approach is the seed-mediated growth method. This procedure
consists in the synthesis of the core and shell in two stages which enable to control
independently both composition and size of each component This opens up many