4 Core/Shell Bimagnetic Nanoparticles
95
magnetization reversal mechanism. In spite of this, the authors call the attention on
the complexity of the system that should be taken into account to interpret the results.
On one hand, the growth process leads to a homogeneous CoO shell formation when
the coating is thin, while polycrystalline discontinuous shell is found for thicker
thicknesses. On the other hand, interdiffusion at the interface was found which leads
a CoFe 2 O 4 thin layer formation to be observed, only, in the thinner CoO shell. The
presence of this hard/soft interface could result in the huge coercivity increase and
the diminution of the exchange bias for the lower Co concentration.
Another parameter that plays a fundamental role in the exchange bias strength is
the lattice mismatch between core and the shell phases. Depending of the system,
different effects were reported. In the size dependence of Co-core/Co 3 O 4 -shell
nanoparticles study it was found that the exchange bias field and the vertical shift
present a maximum when the lattice mismatch is maximized. This result is associated
with the increase of the interfacial magnetic anisotropy with the strain ε Co = (a Co –
a Co-bulk )/a Co-bulk , and also it is related with the increase of pinned spins with the lattice
mismatch [8, 26, 51]. On the contrary, when the Co-core/CoO-shell nanoparticles
are embedded in Cu x O matrix, the exchange bias and the coercivity was optimized
by diminishing the mismatch between the AFM shell and the matrix. In this case,
by diminishing the mismatch, the highly anisotropic CoO phase is stabilized and
also an increase of the number of pinned and unpinned uncompensated spins as a
consequence of the interdiffusion of the Cu ions into the CoO shell is observed. This
strategy could be reaches by modulating the oxygen partial pressure and therefore
adjusting the Cu x O phases, been Cu, Cu 2 O, Cu 4 O 3 or CuO, of the matrix [17]. These
examples illustrate different approaches to maximize the anisotropy of the AFM
phase and as a consequence optimize the EB effects.
Most of the researches on core/shell exchange bias systems report ferromagnetic
coupling at the interface which is compatible with the negative H EB usually observed.
Different couplings were evaluated mainly from Monte Carlo simulations, however,
only recently could be experimentally detected. Bimagnetic core/shell nanoparticles
based on iron and manganese oxide were studied, and antiparallel coupling at the
interface was found with the corresponding positive exchange bias field [11]. Interestingly, it was proved that the sign and the magnitude of the exchange bias field can
be controlled by the cooling field, which give an additional tool to handle the EB
effect.
Up to here we have discussed the reversal process when the AFM anisotropy
energy is larger than the interface exchange energy, in the opposite situation: J EX
K AFM V AFM , the AFM and FM phases are rigidly coupled and an enhancement of the
magnetic anisotropy with the consequent coercivity increase is observed. From the
equilibrium condition of the free energy, in the approximation α~β the coercivity
field can be estimated:
H C =
2(K FM V FM + K AFM V AFM )
m FM V FM + m AFM V AFM
,
95
magnetization reversal mechanism. In spite of this, the authors call the attention on
the complexity of the system that should be taken into account to interpret the results.
On one hand, the growth process leads to a homogeneous CoO shell formation when
the coating is thin, while polycrystalline discontinuous shell is found for thicker
thicknesses. On the other hand, interdiffusion at the interface was found which leads
a CoFe 2 O 4 thin layer formation to be observed, only, in the thinner CoO shell. The
presence of this hard/soft interface could result in the huge coercivity increase and
the diminution of the exchange bias for the lower Co concentration.
Another parameter that plays a fundamental role in the exchange bias strength is
the lattice mismatch between core and the shell phases. Depending of the system,
different effects were reported. In the size dependence of Co-core/Co 3 O 4 -shell
nanoparticles study it was found that the exchange bias field and the vertical shift
present a maximum when the lattice mismatch is maximized. This result is associated
with the increase of the interfacial magnetic anisotropy with the strain ε Co = (a Co –
a Co-bulk )/a Co-bulk , and also it is related with the increase of pinned spins with the lattice
mismatch [8, 26, 51]. On the contrary, when the Co-core/CoO-shell nanoparticles
are embedded in Cu x O matrix, the exchange bias and the coercivity was optimized
by diminishing the mismatch between the AFM shell and the matrix. In this case,
by diminishing the mismatch, the highly anisotropic CoO phase is stabilized and
also an increase of the number of pinned and unpinned uncompensated spins as a
consequence of the interdiffusion of the Cu ions into the CoO shell is observed. This
strategy could be reaches by modulating the oxygen partial pressure and therefore
adjusting the Cu x O phases, been Cu, Cu 2 O, Cu 4 O 3 or CuO, of the matrix [17]. These
examples illustrate different approaches to maximize the anisotropy of the AFM
phase and as a consequence optimize the EB effects.
Most of the researches on core/shell exchange bias systems report ferromagnetic
coupling at the interface which is compatible with the negative H EB usually observed.
Different couplings were evaluated mainly from Monte Carlo simulations, however,
only recently could be experimentally detected. Bimagnetic core/shell nanoparticles
based on iron and manganese oxide were studied, and antiparallel coupling at the
interface was found with the corresponding positive exchange bias field [11]. Interestingly, it was proved that the sign and the magnitude of the exchange bias field can
be controlled by the cooling field, which give an additional tool to handle the EB
effect.
Up to here we have discussed the reversal process when the AFM anisotropy
energy is larger than the interface exchange energy, in the opposite situation: J EX
K AFM V AFM , the AFM and FM phases are rigidly coupled and an enhancement of the
magnetic anisotropy with the consequent coercivity increase is observed. From the
equilibrium condition of the free energy, in the approximation α~β the coercivity
field can be estimated:
H C =
2(K FM V FM + K AFM V AFM )
m FM V FM + m AFM V AFM
,