Atomic-scale investigation on the origin of in-plane variants in L10-FePt nanoparticles embedded in a single-crystalline MgO matrix

In this work, we present an atomic-scale investigation of L1(0)-FePt particles that are partly or fully embedded in a single-crystalline MgO matrix. Hundreds of particles in different orientations, of different sizes, and with different side contact facets are statistically and quantitatively analyzed. It is found that the presence of side contact facets does not introduce any misorientation in particles with the c axis out-of-plane (OP), even in those of small sizes. In addition, a markedly higher proportion of in-plane (IP) variants is found in smaller particles of sizes less than 10 nm, and most IP variants and the IP part in multi-variants have a large area of side contact facets or are even fully embedded. Those results can be explained by the fact that the transformation strain and interface strain jointly affect the particle orientation, and the competition between total strain energy in the film plane and in lateral planes plays a key role in determining particle orientation. Thus, a suggestion on the L1(0)-FePt nanoparticle film production can be proposed that a moderate area of side contact facets may help keep a perfect OP orientation in the OP particles without increasing the proportion of IP variants. Additionally, our work can provide reference information on the variant orientation preference during a post-annealing process in nanoparticle films on a rough substrate or embedded in a matrix as well as in core-shell material systems. (C) 2022 Author(s)