Exchange and magnetic order in bulk and nanostructured Fe₅Si₃

The Curie temperature of bulk and nanostructured Fe₅Si₃ is investigated using experiments, density-functional simulations, and many-body model calculations. The bulk intermetallic, which crystallizes in the hexagonal D8₈ structure, exhibits several intriguing features: it does not exist as a room-temperature equilibrium phase, is close to the onset of ferromagnetism, and exhibits two crystallographically very different Fe sites. The samples, produced by rapid quenching (bulk) and cluster deposition (nanoparticulate thin films), have Curie temperatures of about 400 K. Interatomic exchange constants are calculated using the Kohn-Korringa-Rostoker (KKR) method and used to solve the multisublattice mean-field problem for the system. The Vienna ab initio simulation package (VASP) is employed to study the dependence of the Fe moment on the thermally induced spin misalignment, and a model calculation yields an estimate for quantum-spin-liquid corrections. The theory includes Heisenberg exchange but overestimates the Curie temperature, and a discussion is given regarding additional approaches to handle weakly ferromagnetic multisublattice intermetallic compounds.