THERMAL LATTICE EXPANSION AND OUT-OF-PLANE RELAXATION OF BCC-CO IN FECO MULTILAYERS OBSERVED WITH PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY/

We report on the temperature dependence of the Cd-111 magnetic hyperfi ne field in bcc-Co as measured in Fe/Co multilayers. An anomalous incr ease of the hyperfine field with temperature is observed. Below the gr owth temperature of the samples, the behaviour can be explained by sim ple thermal expansion of the lattice. At 570 K a relaxation of the int erplanar distance accounts for the anomaly. This relaxation is interpr eted as an onset to the degeneration of the forced bcc structure to an energetically more favorable Co-phase.