Xenon-ion-induced phase transition in thin Co films

We report on the observation of the ion-induced hcp --> fcc phase transition in 75 nm thick polycrystalline Co films, which were irradiated with 200 keV Xe ions to fluences of 2.5 x 10(13)-8 x 10(15) ions/cm(2) at a temperature of 300 K. Analyses by means of Rutherford Backscattering Spectroscopy (RBS), X-Ray Diffraction (XRD), Magneto-Optical Kerr Effect (MOKE) and Vibrating Sample Magnetometry (VSM) provided information on the film thickness and the implanted Xe profiles, the phase structure, the ion-induced lattice expansion and the magnetic hysteresis, respectively. After film deposition and irradiations up to 4 x 10(14) ions/cm(2), we predominantly found the hcp phase (XRD, uniaxial MOKE pattern) with an in-plane magnetization. The transition to fourfold in-plane magnetization typical of the fcc-phase occurred around a fluence of 2 x 10(15) ions/cm(2). We interpreted these findings as due to rapid cooling of thermal spikes into the metastable Co-fcc phase.