4 Core/Shell Bimagnetic Nanoparticles
99
depending on the particular problem to be analyzed, or the magnetic response that
it wanted to be achieved. In most of the cited examples, the magnetic response
of bimagnetic nanoparticles is tuned by controlling the size of the core or shell
components or by combining materials with different magnetocrystalline anisotropy.
In the next section, we will discuss an alternative approach to control the magnetic
properties as a function of the interface exchange energy.
4.4 Tuning the Magnetic Properties by the Interface
Exchange Coupling
In Sect 4.3, the exchange bias effect was analyzed in AFM/FM(FiM) or
FM(FiM)/AFM core/shell systems by comparing the magnetic anisotropy energy
of the AFM phase with the interface exchange energy, where two limit situations
were considered K AFM V AFM J EX and K AFM V AFM J EX . Here, we present a
model system to analyze the evolution of the exchange bias effect as a function of
the interface coupling. With this aim CoO, AFM nanoparticles of ~3 nm of diameter
were synthesized by high-temperature decomposition of organometallic precursor
and encapsulated with a Co 1–x Zn x Fe 2 O 4 shell of ~4 nm thickness. It is expected
that by replacing the Co
2+ (3d
7 , S = 3/2) by Zn
2+ (3d
10 , S = 0) the strength of
the interface exchange coupling would be reduced, therefore a change from rigid
coupling (K AFM V AFM < J EX ) to exchange bias regime (K AFM V AFM > J EX ) could be
obtained. In order to perform a systematic study, the CoO cores were synthesized in
one step and split in five batches to overgrow the shells with different compositions,
and this step assures that the properties of the AFM component are comparable in
all the studied systems. Figure 4.7 shows TEM images of the five systems studied
named Zn-x, where x corresponds to the Zn concentration that changed nominally
as x = 0, 0.25, 0.50, 0.75, 1. All the systems present comparable size ~ 11 nm and
similar morphology, also notice that the shell is formed by several nanograins in
close contact as was observed in other core/shell system [36, 38].
From the FC and ZFC magnetization loops, the coercivity and the exchange bias
fields were obtained and they are shown in Fig. 4.8. Also, for comparison, the field
values for 8 and 5 nm CoO/CoFe 2 O 4 core/shell bimagnetic nanoparticles [29] and
the coercivity of (Co 1–x Zn x )Fe 2 O 4 single-phase nanoparticles [19, 18] are included.
Several features of this figure call the attention: the coercivity field monotonically
decreases with the Zn concentration, while the H EB presents a maximum H EB ~1500
Oe for x = 0.25; also, when the H C values are compared with the obtained for the
single-phase ferrite, the magnetic hardening of the system is evidenced. Moreover,
it is noteworthy that H C and H EB can be systematically changed without producing
a significant modification of the magnetization.
As it is known, the magnetocrystalline anisotropy is originated in the spin orbit
coupling. In the cobalt ferrite, the Co
2+ (3d
7 , S = 3/2) occupy the octahedral site of
the spinel structure resulting in a degenerate ground-state energy level with nonzero
99
depending on the particular problem to be analyzed, or the magnetic response that
it wanted to be achieved. In most of the cited examples, the magnetic response
of bimagnetic nanoparticles is tuned by controlling the size of the core or shell
components or by combining materials with different magnetocrystalline anisotropy.
In the next section, we will discuss an alternative approach to control the magnetic
properties as a function of the interface exchange energy.
4.4 Tuning the Magnetic Properties by the Interface
Exchange Coupling
In Sect 4.3, the exchange bias effect was analyzed in AFM/FM(FiM) or
FM(FiM)/AFM core/shell systems by comparing the magnetic anisotropy energy
of the AFM phase with the interface exchange energy, where two limit situations
were considered K AFM V AFM J EX and K AFM V AFM J EX . Here, we present a
model system to analyze the evolution of the exchange bias effect as a function of
the interface coupling. With this aim CoO, AFM nanoparticles of ~3 nm of diameter
were synthesized by high-temperature decomposition of organometallic precursor
and encapsulated with a Co 1–x Zn x Fe 2 O 4 shell of ~4 nm thickness. It is expected
that by replacing the Co
2+ (3d
7 , S = 3/2) by Zn
2+ (3d
10 , S = 0) the strength of
the interface exchange coupling would be reduced, therefore a change from rigid
coupling (K AFM V AFM < J EX ) to exchange bias regime (K AFM V AFM > J EX ) could be
obtained. In order to perform a systematic study, the CoO cores were synthesized in
one step and split in five batches to overgrow the shells with different compositions,
and this step assures that the properties of the AFM component are comparable in
all the studied systems. Figure 4.7 shows TEM images of the five systems studied
named Zn-x, where x corresponds to the Zn concentration that changed nominally
as x = 0, 0.25, 0.50, 0.75, 1. All the systems present comparable size ~ 11 nm and
similar morphology, also notice that the shell is formed by several nanograins in
close contact as was observed in other core/shell system [36, 38].
From the FC and ZFC magnetization loops, the coercivity and the exchange bias
fields were obtained and they are shown in Fig. 4.8. Also, for comparison, the field
values for 8 and 5 nm CoO/CoFe 2 O 4 core/shell bimagnetic nanoparticles [29] and
the coercivity of (Co 1–x Zn x )Fe 2 O 4 single-phase nanoparticles [19, 18] are included.
Several features of this figure call the attention: the coercivity field monotonically
decreases with the Zn concentration, while the H EB presents a maximum H EB ~1500
Oe for x = 0.25; also, when the H C values are compared with the obtained for the
single-phase ferrite, the magnetic hardening of the system is evidenced. Moreover,
it is noteworthy that H C and H EB can be systematically changed without producing
a significant modification of the magnetization.
As it is known, the magnetocrystalline anisotropy is originated in the spin orbit
coupling. In the cobalt ferrite, the Co
2+ (3d
7 , S = 3/2) occupy the octahedral site of
the spinel structure resulting in a degenerate ground-state energy level with nonzero