222
7 Composites
particles. The decrease in GMR at annealing temperatures above 450 °C is due to
the formation of larger ferromagnetic Co grains instead of very small particles with
superparamagnetic properties.
The application of pulse plating is also widespread for producing granular Cu–Co
deposits [6, 14–16]. Pulse-plated deposits show a smaller surface roughness than their
d.c.-plated counterparts. The morphological features of the pulse-plated deposits are
much more sensitive to the application of bath additives than in the case of the
d.c. plating. The reason for the enhanced additive sensitivity probably lies in that
the impact of the additives on the electrodeposition itself is less strong than on the
exchange reaction during the off-time. Concerning the composition, it was found that
the increase in the off-time leads to the decrease in the Co content of the deposit, which
is an obvious consequence of the occurrence of the spontaneous exchange between
Co and Cu
2+ . Depending on the choice of the pulse parameters, pulse-plated deposits
can exhibit a wide range of grain structure. Granular materials without homogeneous
columns or layers can be achieved if the on-time is short enough so that the grains
formed during the time of a single pulse do not coalesce. It is to be noted that twopulse plated Cu–Cu alloys show somewhat different morphological properties. A
clear layered-to-granular structural transition can be observed if either the current
density in the high-current pulse is decreased [17] or the Cu
2+ concentration in the
bath is enhanced while leaving other deposition conditions unchanged [18]. However,
there is always a Cu layer between the alloyed parts of the two-pulse plated deposit
when the deposition conditions are set in accord with the principles discussed for
multilayer deposits (see Chap. 5).
7.1.3 Ag(Co) Alloys
The Ag–Co pair is a typical example for immiscible metals. In contrast to the Cu–Co
system, the large difference in the natural nearest-neighbour lattice plane distances of
these two elements (14%) makes it impossible to match the two lattices to each other
with little strain. This renders the Ag–Co mixtures to be nanocrystalline due to the
mechanical stress that cannot be relaxed with a small deformation for large crystals.
Beside the large lattice mismatch, the formation of metastable mixtures from Ag and
Co is also much hindered. Therefore, the codeposition of Ag and Co leads to the
mechanical mixture of the pure phases of the constituents.
Due to the weak adherence between the Co and Ag crystals in a composite deposit,
the electroplating process often leads to porous deposits with insufficient mechanical
cohesion. The incompact character of the deposit verifies the versatile pursuits for
obtaining deposits of good quality. The most frequently mentioned bath contains
sodium chloride as supporting electrolyte whose concentration ranged from 2.4 M
[19, 20] to 3.5 M [21–23]. In these baths, the Ag
+ concentration is 1–2 mM, and the
Co
2+ concentration varies from one work to another. Concerning the bath composition, other works mention metal sulphates with sodium citrate [24], sodium gluconate
7 Composites
particles. The decrease in GMR at annealing temperatures above 450 °C is due to
the formation of larger ferromagnetic Co grains instead of very small particles with
superparamagnetic properties.
The application of pulse plating is also widespread for producing granular Cu–Co
deposits [6, 14–16]. Pulse-plated deposits show a smaller surface roughness than their
d.c.-plated counterparts. The morphological features of the pulse-plated deposits are
much more sensitive to the application of bath additives than in the case of the
d.c. plating. The reason for the enhanced additive sensitivity probably lies in that
the impact of the additives on the electrodeposition itself is less strong than on the
exchange reaction during the off-time. Concerning the composition, it was found that
the increase in the off-time leads to the decrease in the Co content of the deposit, which
is an obvious consequence of the occurrence of the spontaneous exchange between
Co and Cu
2+ . Depending on the choice of the pulse parameters, pulse-plated deposits
can exhibit a wide range of grain structure. Granular materials without homogeneous
columns or layers can be achieved if the on-time is short enough so that the grains
formed during the time of a single pulse do not coalesce. It is to be noted that twopulse plated Cu–Cu alloys show somewhat different morphological properties. A
clear layered-to-granular structural transition can be observed if either the current
density in the high-current pulse is decreased [17] or the Cu
2+ concentration in the
bath is enhanced while leaving other deposition conditions unchanged [18]. However,
there is always a Cu layer between the alloyed parts of the two-pulse plated deposit
when the deposition conditions are set in accord with the principles discussed for
multilayer deposits (see Chap. 5).
7.1.3 Ag(Co) Alloys
The Ag–Co pair is a typical example for immiscible metals. In contrast to the Cu–Co
system, the large difference in the natural nearest-neighbour lattice plane distances of
these two elements (14%) makes it impossible to match the two lattices to each other
with little strain. This renders the Ag–Co mixtures to be nanocrystalline due to the
mechanical stress that cannot be relaxed with a small deformation for large crystals.
Beside the large lattice mismatch, the formation of metastable mixtures from Ag and
Co is also much hindered. Therefore, the codeposition of Ag and Co leads to the
mechanical mixture of the pure phases of the constituents.
Due to the weak adherence between the Co and Ag crystals in a composite deposit,
the electroplating process often leads to porous deposits with insufficient mechanical
cohesion. The incompact character of the deposit verifies the versatile pursuits for
obtaining deposits of good quality. The most frequently mentioned bath contains
sodium chloride as supporting electrolyte whose concentration ranged from 2.4 M
[19, 20] to 3.5 M [21–23]. In these baths, the Ag
+ concentration is 1–2 mM, and the
Co
2+ concentration varies from one work to another. Concerning the bath composition, other works mention metal sulphates with sodium citrate [24], sodium gluconate