Ion irradiation induced phase transition of Co in Co/Au multilayers

• A phase transition of Co from fcc to hcp has been observed, for the first time, using 1.5 MeV Au ion irradiation.
• This phase transition was explained on the basis of evolution of grains under increasing ion fluence.
• A clear correspondence between the phase transformation and the Debye temperature of Co has been shown.

We have studied the structural phase transformation in thin Co films using moderate energy Au ion irradiation. We found that local energy transfer along the track of the ion beam is an effective method for transforming fcc phase of Co into technologically appealing hcp phase. As a function of the irradiation fluence, a gradual crossover to the hcp phase was observed in X-ray diffraction data. Such phase transition was explained on the basis of evolution of grains under increasing ion fluence. Bloch-Grüneisen simulations to the four contact resistivity data showed a decrease in Debye temperature with evolution of hcp-phase Cobalt, consistent with the reduced modal freedom for phonons in the hcp structure compared to the fcc structure.