Epitaxy-Stabilized Structures

Abstract

Ultrathin films can be grown pseudomorphically on suitable substrates under certain growth conditions, even when the structure grown differs from the equilibrium structure of the bulk material. This provides a new method to stabilize nonequilibrium structures at room temperature and ambient pressure. The stabilized nonequilibrium structure can be a metastable structure if the substrate lattice matches the metastable structure. A metastable structure corresponds to a state of a local minimum of the free energy. Nevertheless, the energy of this structure is higher than that in the thermodynamic equilibrium state. Owing to this energy difference, there is a thickness limit for the metastable pseudomorphic film. With increasing thickness, the energy necessary to introduce defects or break interface bonds will become smaller than the energy difference between the metastable and the equilibrium phase. Hence the system will relax to the equilibrium state. In this chapter, we shall first present the theoretical predictions for various metastable phases of 3d metals (from V to Ni). This is followed by a summary of the experimental results for epitaxy-stabilized metastable 1 ultrathin films, including their growth, structure, and magnetic properties. 5.1 Theoretical Search for Metastable Structures Magnetic transition metals are particularly rich in metastable structures. This feature is reflected by the existence of various high-temperature phases in their thermodynamic equilibrium phase diagrams. For example, Mn occurs in four allotropic modifications, i.e., the α, β, γ and δ phases, and Fe has three modifications, i.e., α-Fe, γ-Fe, and δ-Fe, in the phase diagram. As a general rule, each of these different crystallographic phases possesses different magnetic properties. This occurs because the difference in electronic 1 In this book, we generally call a pseudomorphic ultrathin film with a Bra-vais lattice which differs from the bulk equilibrium phase an epitaxy-stabilized metastable film. However, one should realize that these epitaxy-stabilized metastable films often have a lower free energy than the bulk equilibrium structure , owing to the interface interaction. They are actually thermodynamic equilibrium states in the ultrathin-film limit. This distinguishes epitaxy-stabilized structures from quenched metastable bulk phases.