5.4 Properties of Electrodeposited CMAs
171
It would sound logical to apply additives to achieve a grain refinement and hence,
making the layer structure fully independent of the grain boundaries, as shown in
Fig. 5.8b. However, all such attempts failed for electrodeposited multilayer films
with GMR. Even if the grain refinement results in smoother layers in terms of the
parallel position of the layer interfaces both with each another and with the substrate,
the ultrafine-grained material has a large resistivity. This means that the zero-field
resistivity of the multilayer drastically increases, which decreases the magnetoresistance ratio as defined with Eq. 5.4. This happens with the application of surfactants,
stress relievers and also complexing agents. Saccharin as stress reliever, citric acid as
complexing agent and tensides as wetting agents are all detrimental for the magnetoresistance properties of electrodeposited multilayers [148]. The increase in pH also
impacts negatively the maximum magnetoresistance effect observed for multilayered
deposits, and the pH effect is essentially independent of the multilayer composition
[102, 159, 171]. This is thought to be related to the general tendency that the crystallite
size of the metals grown from acidic solution is larger than from neutral and basic
ones, although the logical chain between the structure and the magnetoresistance
properties is much less underpinned than that of the additive effect.
As an inherent feature of the electrodeposition technique, the FM metal (which is
also the LN component) always contains the NM one (which is the MN if we regard
its relative nobility). Depending on the NM metal content of the FM layer, the magnetoresistance curves exhibit various shapes. An example is shown in Fig. 5.15. The
NM metal content in the layer produced in the high-current pulse is determined by
the j FM /j Lim,NM ratio and j Lim,NM is proportional to c(NM
z+ ). Therefore, the increase
in the NM metal content can be achieved by either increasing its concentration in
the bath [154] or decreasing the current density in the high-current pulse [179].
Whichever way is chosen, the NM metal tends to segregate in the layer produced
during the high-current pulse, and so does the FM metal, too. This is particularly true
for Co–Cu/Cu multilayers because Co and Cu are not miscible in equilibrium. The
small Co-rich fragments are no longer ferromagnetic but they exhibit superparamagnetism. A superparamagnetic (SPM) entity is a single-domain particle (or zone) in
which the spin orientation of the magnetic atoms are aligned in a parallel manner due
to the exchange coupling; therefore, the spin ordering interaction is the same as in a
bulk FM metal. However, the SPM particle is so small that it cannot exhibit a fixed
magnetization direction because the thermal excitation easily changes the magnetic
orientation (in other words, the kT energy is much larger than the anisotropy energy
of the magnetic entity). The magnetization curve related to the SPM behaviour can
be described with the Langevin function:
L(x) = coth(x) − x
−1
(5.5)
where x = μH/kT, μ being the magnetic moment of the SPM entity (the other
symbols have their usual meaning). The magnetic saturation of an SPM particle
requires a much larger field than that of an FM domain, which lends to a sample with
171
It would sound logical to apply additives to achieve a grain refinement and hence,
making the layer structure fully independent of the grain boundaries, as shown in
Fig. 5.8b. However, all such attempts failed for electrodeposited multilayer films
with GMR. Even if the grain refinement results in smoother layers in terms of the
parallel position of the layer interfaces both with each another and with the substrate,
the ultrafine-grained material has a large resistivity. This means that the zero-field
resistivity of the multilayer drastically increases, which decreases the magnetoresistance ratio as defined with Eq. 5.4. This happens with the application of surfactants,
stress relievers and also complexing agents. Saccharin as stress reliever, citric acid as
complexing agent and tensides as wetting agents are all detrimental for the magnetoresistance properties of electrodeposited multilayers [148]. The increase in pH also
impacts negatively the maximum magnetoresistance effect observed for multilayered
deposits, and the pH effect is essentially independent of the multilayer composition
[102, 159, 171]. This is thought to be related to the general tendency that the crystallite
size of the metals grown from acidic solution is larger than from neutral and basic
ones, although the logical chain between the structure and the magnetoresistance
properties is much less underpinned than that of the additive effect.
As an inherent feature of the electrodeposition technique, the FM metal (which is
also the LN component) always contains the NM one (which is the MN if we regard
its relative nobility). Depending on the NM metal content of the FM layer, the magnetoresistance curves exhibit various shapes. An example is shown in Fig. 5.15. The
NM metal content in the layer produced in the high-current pulse is determined by
the j FM /j Lim,NM ratio and j Lim,NM is proportional to c(NM
z+ ). Therefore, the increase
in the NM metal content can be achieved by either increasing its concentration in
the bath [154] or decreasing the current density in the high-current pulse [179].
Whichever way is chosen, the NM metal tends to segregate in the layer produced
during the high-current pulse, and so does the FM metal, too. This is particularly true
for Co–Cu/Cu multilayers because Co and Cu are not miscible in equilibrium. The
small Co-rich fragments are no longer ferromagnetic but they exhibit superparamagnetism. A superparamagnetic (SPM) entity is a single-domain particle (or zone) in
which the spin orientation of the magnetic atoms are aligned in a parallel manner due
to the exchange coupling; therefore, the spin ordering interaction is the same as in a
bulk FM metal. However, the SPM particle is so small that it cannot exhibit a fixed
magnetization direction because the thermal excitation easily changes the magnetic
orientation (in other words, the kT energy is much larger than the anisotropy energy
of the magnetic entity). The magnetization curve related to the SPM behaviour can
be described with the Langevin function:
L(x) = coth(x) − x
−1
(5.5)
where x = μH/kT, μ being the magnetic moment of the SPM entity (the other
symbols have their usual meaning). The magnetic saturation of an SPM particle
requires a much larger field than that of an FM domain, which lends to a sample with