Size effects on a first-order phase transition in submicron disks is demonstrated using a high-resolution imaging method. In MnAs disks on GaAs (001) substrates having sizes smaller than twice the film thickness, an individual disk consists of either entirely $\alpha \text{−}\mathrm{Mn}\mathrm{As}$ or entirely $\beta \text{−}\mathrm{Mn}\mathrm{As}$ as a consequence of the nucleation initiation of the first-order phase transition. A phase coexistence occurs on macroscopic scales as the two kinds of the disks can be present simultaneously. In slightly larger disks, the stripe structure due to the phase coexistence in films is modified to a core-shell-type phase segregation.

• Received 25 January 2006

DOI:https://doi.org/10.1103/PhysRevB.73.125324