7.1 Composite Preparation by Codeposition of Metals
223
and thiourea as complexing agents beside metal perchlorates [25, 26], sodium thiosulphate as complexing agent [27] and a solution containing the cyanide complex
of silver, potassium pyrophosphate and oxalic acid [28]. Similarly to many other
structurally inconsistent deposits, it was shown that pulse plating with optimized
conditions reduces the surface roughness of the deposits [25].
In the majority of studies, the X-ray diffractograms show separate lines for Ag
and Co [22, 24, 28]. Nevertheless, it is difficult to perform a correct phase analysis
especially for low-cobalt deposits for two reasons. First, the scattering intensity of
Ag is much stronger than that of Co; hence, the Ag lines dominate the diffractogram.
Secondly, the peak positions for fcc and hcp cobalt are very close to each other,
which makes them nearly indistinguishable if the peak intensity is small and the
peak position is uncertain. Although the formation of pure Ag and Co crystals was
the general experience, one work mentioned the possibility of the formation of a
metastable CoAg 3 phase [26]. This new metastable phase showed hcp structure,
similar to the CeAg 3 and GaAg 3 intermetallic phases.
The crystallite size of the Ag(Co) deposits as obtained from X-ray diffractograms
mostly refers to the matrix. The average grain size of Ag was reported to be less
than 10 nm [20]. The estimate from the XRD for the Co grain size was 7 nm for a
relatively Co-rich deposit [20] but TEM offers a more accurate method to assess the
size of the randomly distributed Co particles. This was found to be 1–2 nm [22]. This
diameter range was well confirmed by the measurement of the so-called blocking
temperature of the paramagnetic Co grains, which yielded a mean Co grain diameter
of 3 nm [19]. The simultaneous application of the TEM and magnetization-based
methods revealed that the diameter of Co grains increases with the Co concentration
in the deposit [20]. Although the grain size may vary depending on the deposition
conditions, it seems to be a general trend that the size of the Co particles is smaller
than the crystallite size of the Ag matrix.
For the majority of the Ag(Co) deposits, magnetoresistance was measured. The
assembly of segregated nanoscale magnetic (mostly SPM) particles in the NM matrix
gives rise to a significant GMR effect. Both the magnetization and magnetoresistance
curves are of SPM character. The GMR as a function of the Co content of the
deposit shows a maximum. This is general for NM(SPM) materials as the increase
of GMR in the range of small concentration is due to the growing contribution of the
electron scattering of magnetic origin, while at high concentration, the percolation
of the magnetic entities cancels the independence of the local magnetizations. The
Co concentration with maximum room-temperature GMR depends on the deposition
conditions (like the current density) and falls in the interval of 20–50 at.% Co [20–24].
Annealing of Ag(Co) deposits leads exclusively to a grain growth [29], which is
accompanied with the decrease in GMR [21, 24]. The latter is explained with the
decrease in the number of independent magnetic particles and the concomitantly
diminished spin-dependent electron scattering contribution.
223
and thiourea as complexing agents beside metal perchlorates [25, 26], sodium thiosulphate as complexing agent [27] and a solution containing the cyanide complex
of silver, potassium pyrophosphate and oxalic acid [28]. Similarly to many other
structurally inconsistent deposits, it was shown that pulse plating with optimized
conditions reduces the surface roughness of the deposits [25].
In the majority of studies, the X-ray diffractograms show separate lines for Ag
and Co [22, 24, 28]. Nevertheless, it is difficult to perform a correct phase analysis
especially for low-cobalt deposits for two reasons. First, the scattering intensity of
Ag is much stronger than that of Co; hence, the Ag lines dominate the diffractogram.
Secondly, the peak positions for fcc and hcp cobalt are very close to each other,
which makes them nearly indistinguishable if the peak intensity is small and the
peak position is uncertain. Although the formation of pure Ag and Co crystals was
the general experience, one work mentioned the possibility of the formation of a
metastable CoAg 3 phase [26]. This new metastable phase showed hcp structure,
similar to the CeAg 3 and GaAg 3 intermetallic phases.
The crystallite size of the Ag(Co) deposits as obtained from X-ray diffractograms
mostly refers to the matrix. The average grain size of Ag was reported to be less
than 10 nm [20]. The estimate from the XRD for the Co grain size was 7 nm for a
relatively Co-rich deposit [20] but TEM offers a more accurate method to assess the
size of the randomly distributed Co particles. This was found to be 1–2 nm [22]. This
diameter range was well confirmed by the measurement of the so-called blocking
temperature of the paramagnetic Co grains, which yielded a mean Co grain diameter
of 3 nm [19]. The simultaneous application of the TEM and magnetization-based
methods revealed that the diameter of Co grains increases with the Co concentration
in the deposit [20]. Although the grain size may vary depending on the deposition
conditions, it seems to be a general trend that the size of the Co particles is smaller
than the crystallite size of the Ag matrix.
For the majority of the Ag(Co) deposits, magnetoresistance was measured. The
assembly of segregated nanoscale magnetic (mostly SPM) particles in the NM matrix
gives rise to a significant GMR effect. Both the magnetization and magnetoresistance
curves are of SPM character. The GMR as a function of the Co content of the
deposit shows a maximum. This is general for NM(SPM) materials as the increase
of GMR in the range of small concentration is due to the growing contribution of the
electron scattering of magnetic origin, while at high concentration, the percolation
of the magnetic entities cancels the independence of the local magnetizations. The
Co concentration with maximum room-temperature GMR depends on the deposition
conditions (like the current density) and falls in the interval of 20–50 at.% Co [20–24].
Annealing of Ag(Co) deposits leads exclusively to a grain growth [29], which is
accompanied with the decrease in GMR [21, 24]. The latter is explained with the
decrease in the number of independent magnetic particles and the concomitantly
diminished spin-dependent electron scattering contribution.