204
6 Nanocrystalline Deposits
order of concentration of the metal ions in the solution has to follow a reverse trend.
This is why the concentration ratio of Ni
2+ and Fe
2+ in the plating bath can be as
high as 800, but this extreme ratio still allows the deposition of Ni–Fe alloys with
a few per cent of iron. Due to the weaker deposition preference of Co against Ni,
their concentration ratio is usually rather moderate (c(Ni
2+ )/c (Co
2+ ): 1–50). The
mutual deposition preference is retained in ternary baths, too, in which the order of
component concentrations follows the opposite trend than the deposition preference
(c (Ni
2+ ) > c (Co
2+ ) > c (Fe
2+ )). In either of the baths, the increase in the current
density leads to the deposit enrichment with respect to the less preferred metal.
Concerning nc-Fe–Ni alloys [135–144], their synthesis often aims at the so-called
Permalloy composition (~19 wt.% Fe) at which the magnetostriction is zero and
hence, the minimum of the coercivity is expected. Although the early studies of the
electrodeposition of Fe–Ni alloys also dealt with this composition [145, 146], the
study of their grain size and mechanical properties started much later [135–137].
It turned out that Fe–Ni alloys with grain size less than 10 nm can be obtained
relatively easily with d.c. plating and by applying some suitable additive, in particular, saccharin. The modified Watts bath is the most common bath type for Fe–Ni
deposition, but the deposit composition shows little variation if either all-sulphate
or all-chloride baths are applied [140]. Studies dealing with the thermal stability of
nanocrystalline Permalloy showed [138, 139, 143] that the grain growth starts at
about 500 K, i.e., nearly at the same temperature as for Ni, but the growth rate is
smaller, due to the decelerating effect of the chemical disorder. The grain coarsening
upon annealing also leads to a texture change [138]. Although the improvement of
the mechanical properties of nc-Fe–Ni alloys has been well documented [135–137,
139, 141, 142], the major issue is the magnetic behaviour of these alloys [138, 140,
144]. In contrast to the composition, the bath components have an immense impact on
the magnetic properties of the deposits [140]. Compared with magnetron-sputtered
samples of the same composition and grain size, electroplated samples exhibit much
smaller coercivity than the counterparts prepared with physical deposition methods
[144].
Partly similar trends have been evidenced for the properties of nc-Ni–Co deposits
[140, 147–156] that for nc-Fe–Ni alloys. Pulse plating was customary for obtaining
nc-Ni–Co deposits. Due to the anomalous nature of the deposition process, the
increase in current density led to a decrease of Co content. In the phase composition of Ni–Co alloys, a phase transition from fcc to hcp is expected as the cobalt
content increases. However, for nc-Ni–Co alloys, strict evidence for this transition
was seldom found, and the fcc diffraction lines were dominant even for 75 at.% Co
concentration. The complication of the phase analysis stems from the overlap of
the major hcp diffraction lines with some fcc lines, and the rest of the diffraction
lines belonging to the hcp crystals are very weak. The impact of saccharin on the
deposit stress is more complex than for Ni. While the tensile-to-compressive stress
is unambiguous for pure Ni as saccharin is applied as bath additive, the same stress
reversal can be achieved up to a certain Co content only above which it does not take
place. Although the thermal stability of nc-Ni–Co deposits is better than that of Ni,
the grain growth at high temperature is influenced by the sulphur content the same
6 Nanocrystalline Deposits
order of concentration of the metal ions in the solution has to follow a reverse trend.
This is why the concentration ratio of Ni
2+ and Fe
2+ in the plating bath can be as
high as 800, but this extreme ratio still allows the deposition of Ni–Fe alloys with
a few per cent of iron. Due to the weaker deposition preference of Co against Ni,
their concentration ratio is usually rather moderate (c(Ni
2+ )/c (Co
2+ ): 1–50). The
mutual deposition preference is retained in ternary baths, too, in which the order of
component concentrations follows the opposite trend than the deposition preference
(c (Ni
2+ ) > c (Co
2+ ) > c (Fe
2+ )). In either of the baths, the increase in the current
density leads to the deposit enrichment with respect to the less preferred metal.
Concerning nc-Fe–Ni alloys [135–144], their synthesis often aims at the so-called
Permalloy composition (~19 wt.% Fe) at which the magnetostriction is zero and
hence, the minimum of the coercivity is expected. Although the early studies of the
electrodeposition of Fe–Ni alloys also dealt with this composition [145, 146], the
study of their grain size and mechanical properties started much later [135–137].
It turned out that Fe–Ni alloys with grain size less than 10 nm can be obtained
relatively easily with d.c. plating and by applying some suitable additive, in particular, saccharin. The modified Watts bath is the most common bath type for Fe–Ni
deposition, but the deposit composition shows little variation if either all-sulphate
or all-chloride baths are applied [140]. Studies dealing with the thermal stability of
nanocrystalline Permalloy showed [138, 139, 143] that the grain growth starts at
about 500 K, i.e., nearly at the same temperature as for Ni, but the growth rate is
smaller, due to the decelerating effect of the chemical disorder. The grain coarsening
upon annealing also leads to a texture change [138]. Although the improvement of
the mechanical properties of nc-Fe–Ni alloys has been well documented [135–137,
139, 141, 142], the major issue is the magnetic behaviour of these alloys [138, 140,
144]. In contrast to the composition, the bath components have an immense impact on
the magnetic properties of the deposits [140]. Compared with magnetron-sputtered
samples of the same composition and grain size, electroplated samples exhibit much
smaller coercivity than the counterparts prepared with physical deposition methods
[144].
Partly similar trends have been evidenced for the properties of nc-Ni–Co deposits
[140, 147–156] that for nc-Fe–Ni alloys. Pulse plating was customary for obtaining
nc-Ni–Co deposits. Due to the anomalous nature of the deposition process, the
increase in current density led to a decrease of Co content. In the phase composition of Ni–Co alloys, a phase transition from fcc to hcp is expected as the cobalt
content increases. However, for nc-Ni–Co alloys, strict evidence for this transition
was seldom found, and the fcc diffraction lines were dominant even for 75 at.% Co
concentration. The complication of the phase analysis stems from the overlap of
the major hcp diffraction lines with some fcc lines, and the rest of the diffraction
lines belonging to the hcp crystals are very weak. The impact of saccharin on the
deposit stress is more complex than for Ni. While the tensile-to-compressive stress
is unambiguous for pure Ni as saccharin is applied as bath additive, the same stress
reversal can be achieved up to a certain Co content only above which it does not take
place. Although the thermal stability of nc-Ni–Co deposits is better than that of Ni,
the grain growth at high temperature is influenced by the sulphur content the same