Understanding the ordering transition from A1 to L10 structure in FePt thin films is of great significance for developing L10-FePt films as ultrahigh density magnetic recording media. Here, the L10-ordering transition of FePt films has been investigated based on activation volume measurements. A large activation volume ΔV∗=10–11 Å3=(0.75–0.8) Ω, where Ω is average atomic volume of FePt, is determined for atomic diffusions in the L10-ordering transition, indicating a thermal-vacancy-assisted phase transition. This transition is suggested to be predominantly dependent on the diffusion of Fe atoms. These findings have direct implications for yielding L10-FePt films at low temperatures and optimizing their microstructures.

The structural changes of the high‐Tc superconductor YBaCuO between 100 and 300 K are studied by positron lifetime and Doppler broadening measurements as a function of temperature. There are anomalous changes in the positron lifetimes and Doppler broadening S‐parameters near 130 and 260 K, which is consistent with some reported experimental results. These anomalies may be cause by the lattice instabilities, such as the ordering readjustment of oxygen vacancies.