Underpotential Co-Deposition UPCD

The term UPCD is related to the deposition of alloys at potentials which are more positive than the equilibrium potential for the elemental phase of less noble component in the alloy, Eq. (1). The schematic of the potential range where UPCD occurs is shown in Fig. 5. The phenomenological reason for UPCD is that the equilibrium potential of less noble component B in AB alloy can be more positive than the corresponding equilibrium potential of its elemental phase AJ. The origin of this condition is more transparent if Eqs. (1) and (7) are combined as  

For alloys that obey ideal solution model, the activity of the less noble component B in the alloy is equal to its molar/atomic fraction, i.e. = Xb. The value of 

Xb in this case is always less than one and the conclusion expressed by Eq. (9) is straightforward. The typical system that does show this behavior is AuCu alloy. However, the systems that obey a nonideal solution model are more common, and the typical examples of these alloys are CoPt, FePt, CoPd, and FePd. Because of their high magnetic anisotropy introduced by incorporation of Pt, or Pd in to Co hep or Fe bcc lattice and ordering (LIO phase ) these alloys are studied intensively in the past several years as possible candidates for the bit pattern media application as well as MEMS and NEMS technologies. 

The departure of the alloy from ideality is modeled through the activity coefficient Xb where the activity of the less noble component in the alloy is expressed as 

Depending on the type of the alloy, different models can be used to calculate the activity coefficient. For electrodeposited CoPt and FePt alloys which show strong negative departure from ideality,an asymmetric regular solution model is used to define XB (B = Fe or Co)  

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