88
E. L. Winkler and R. D. Zysler
the research on field was driven by looking for harder and softer magnetic materials,
and compounds for magnetic recording.
One important area in this field is the development of high-performance permanent
magnets which requires hard magnetic materials with large energy product (BH) max .
Most of the progress in this area was reached from the design of new alloys that
combines 3d ions coupled with rare-earth ions as SmCo 5 , Nd 2 Fe 14 B, Sm 2 Fe 17 N 3 , in
this way compounds with large remanent magnetization and large coercive field are
obtained. Novel ways to fabricate permanent magnets exploit the exchange length of
the magnetic materials by combining at nanoscale soft with hard magnetic materials,
composites with large magnetization, and also large magnetic anisotropy could be
obtained. This strategy also opens new possibilities to fabricate new compounds with
large energy products and rare-earth free magnets which would lower the cost of raw
materials for manufacturing, and also simplify production due to the limitations of
obtaining rare earth. The dimension of each component is determinant in this family
of nanostructures and as a consequence fine control of the morphology and structure
at nanoscale is mandatory.
The miniaturization of the magnetic devices and the increasing demand of highdensity data storage materials also have driven the research in material science. The
pushing toward high-density data storage in smaller dimensions has to face the wellknown superparamagnetic limits so other approaches based on exchange coupling
bimagnetic materials at nanoscale emerge as a possible solution. As it was mentioned
previously, the current strategy does not go through the search for new compounds,
although it cannot be ruled out to find a new phase with astonishing properties,
but it is looking to design and manufacture new compounds from the possibility of
manipulating and combining materials at the nanoscale. New synthesis and physical
fabrication methods give a huge impulse to the area. The possibility of combining in
a single nanoparticle two or more components, with controlled size and high quality
of interfaces, opens a wide range of new possibilities.
The presence of interfaces in core/shell bimagnetic nanoparticles introduces additional interactions that could radically modify the static and dynamic magnetic
behavior of the systems. The number of parameters that governs the magnetic
behavior grows enormously and the opportunity to manipulate, control, and understand the role playing by each one of them opens a wide range of possibility to design
novel materials with suited properties. The magnetic response changes depend on
the magnetic ordering and anisotropy of the phases, the core size and shell thickness,
the quality of the interface, and the strength of the interface exchange coupling. As a
consequence, different behaviors named exchange spring, exchange bias, magnetic
hardening, and proximity effects are observed. In this chapter, we are going to focus
on the advances reached in the fabrication and the new properties found in core/shell
bimagnetic nanoparticles, and we will discuss the main characteristics that have to
be taken into account to design a particular system. The chapter is organized as
follows: in Sect. 4.2, a brief summary of the most used synthesis methods is given.
In Sect. 4.3, we will focus on the phenomenology and the physical parameters that
determine the magnetic response of the core/shell system. In Sect. 4.4, we are going
to present a case study, CoO-core/Co 1–x Zn x Fe 2 O 4 -shell nanoparticles with x = 0–1,
E. L. Winkler and R. D. Zysler
the research on field was driven by looking for harder and softer magnetic materials,
and compounds for magnetic recording.
One important area in this field is the development of high-performance permanent
magnets which requires hard magnetic materials with large energy product (BH) max .
Most of the progress in this area was reached from the design of new alloys that
combines 3d ions coupled with rare-earth ions as SmCo 5 , Nd 2 Fe 14 B, Sm 2 Fe 17 N 3 , in
this way compounds with large remanent magnetization and large coercive field are
obtained. Novel ways to fabricate permanent magnets exploit the exchange length of
the magnetic materials by combining at nanoscale soft with hard magnetic materials,
composites with large magnetization, and also large magnetic anisotropy could be
obtained. This strategy also opens new possibilities to fabricate new compounds with
large energy products and rare-earth free magnets which would lower the cost of raw
materials for manufacturing, and also simplify production due to the limitations of
obtaining rare earth. The dimension of each component is determinant in this family
of nanostructures and as a consequence fine control of the morphology and structure
at nanoscale is mandatory.
The miniaturization of the magnetic devices and the increasing demand of highdensity data storage materials also have driven the research in material science. The
pushing toward high-density data storage in smaller dimensions has to face the wellknown superparamagnetic limits so other approaches based on exchange coupling
bimagnetic materials at nanoscale emerge as a possible solution. As it was mentioned
previously, the current strategy does not go through the search for new compounds,
although it cannot be ruled out to find a new phase with astonishing properties,
but it is looking to design and manufacture new compounds from the possibility of
manipulating and combining materials at the nanoscale. New synthesis and physical
fabrication methods give a huge impulse to the area. The possibility of combining in
a single nanoparticle two or more components, with controlled size and high quality
of interfaces, opens a wide range of new possibilities.
The presence of interfaces in core/shell bimagnetic nanoparticles introduces additional interactions that could radically modify the static and dynamic magnetic
behavior of the systems. The number of parameters that governs the magnetic
behavior grows enormously and the opportunity to manipulate, control, and understand the role playing by each one of them opens a wide range of possibility to design
novel materials with suited properties. The magnetic response changes depend on
the magnetic ordering and anisotropy of the phases, the core size and shell thickness,
the quality of the interface, and the strength of the interface exchange coupling. As a
consequence, different behaviors named exchange spring, exchange bias, magnetic
hardening, and proximity effects are observed. In this chapter, we are going to focus
on the advances reached in the fabrication and the new properties found in core/shell
bimagnetic nanoparticles, and we will discuss the main characteristics that have to
be taken into account to design a particular system. The chapter is organized as
follows: in Sect. 4.2, a brief summary of the most used synthesis methods is given.
In Sect. 4.3, we will focus on the phenomenology and the physical parameters that
determine the magnetic response of the core/shell system. In Sect. 4.4, we are going
to present a case study, CoO-core/Co 1–x Zn x Fe 2 O 4 -shell nanoparticles with x = 0–1,