Cu Diffusion in Electrodeposited Permalloy Films

The diffusion of Cu into electrodeposited Ni-19% Fe films has been studied over the temperature range 325°−426°C by measuring the loss of magnetic flux associated with the interdiffusion of Cu and NiFe. The Permalloy films, nominally 4800 Å thick, were electroplated on top of a 1-μ-thick layer of Cu, which had previously been plated on an 0.005-in.-diam wire. Precision measurements of flux were made with a vibrating sample magnetometer using an applied field of 8 kOe along the wire axis. Principally, the flux loss and hence the penetration depth at all temperatures varies as the square root of the diffusing time in agreement with classical diffusion theory. There is, however, an initial flux loss due to a relatively rapid process which occurs with a relaxation time described by τ=τ0 exp (Q/RT), where τ0=8×10−14 sec and Q=51.6 kcal/mole. In view of this high activation energy Q, it is proposed that the initial flux loss is due to an oxidation reaction which occurs from oxygen trapped in the film during plating. The diffusion process is characterized by a diffusion coefficient D=5.2 exp (−47 300/RT). The Q obtained is considerably higher than that previously reported (34 kcal/mole) for diffusion of Cu into evaporated Permalloy films. To check the validity of the technique, the interdiffusion of electroplated Cu and Ni was investigated with the technique described above. A similar two-stage process is observed with Q=33 kcal/mole for the diffusion process, close to the Q=35.4 kcal/mole reported for diffusion of Cu in bulk Ni.