Cation Order–Disorder Transition in Fe-Doped 6H-BaTiO₃ for Dilute Room-Temperature Ferromagnetism

Abstract

Understanding the fundamental spin coupling mechanisms in dilute magnetic oxides is not possible without exact atomistic understanding of the solids. This is even more important for few systems that show dilute ferromagnetism at room temperature and could potentially be used in spintronic and multiferroic technologies. Here, we show the first known example of a cation ordering in hexagonal 6H-perovskites. Fe 3+ ions in Fe-doped 6H-BaTiO 3 are initially randomly distributed over two Ti 4+ crystallographic sites but can be, by a prolonged heat treatment, ordered onto one of the sites. The cation ordering is driven by reduction in the electrostatic repulsion, which occurs when Fe 3+ substitutes the highly charged Ti 4+ ions that are located at unusual proximity to each other. We show that such a continuous order−disorder transition is associated with induction of the room-temperature dilute ferromagnetism.

X‐ray Absorption Spectroscopy Studies of the Room‐Temperature Ferromagnetic Fe‐Doped 6H–BaTiO3

“…Ti cations in 6H–BaTiO 3 have two different crystallographic sites: Ti(1) and Ti(2) atoms are situated in the corner‐sharing and face‐sharing octahedra, respectively. On both crystallographic sites Ti cations in 6H–BaTiO 3 are octahedrally coordinated, however, they have small, but significant differences in the local surroundings …”

“…EXAFS spectra were modeled with an ab initio FEFF calculation using simultaneous fit of the Fe EXAFS spectra of 10% Fe‐doped BaTiO 3 sample sintered at 1250°C for 5 h and 10% Fe‐doped BaTiO 3 sample additionally annealed at 1500°C for 10 h. EXAFS spectra have been fitted using similar model, as in our previous EXAFS study of 20% Fe‐doped BaTiO 3. Two FEFF models with iron atoms incorporated onto Ti(1) and Ti(2) crystallographic sites were constructed, based on the 6H–BaTiO 3 crystal structure with P 63 /mmc space group ( a = 5.7240 Å and c = 13.9650 Å) . Measured EXAFS spectra were modeled with a linear combination of both FEFF models, where relative amount of each model signal (X Fe(1) and X Fe(2) )—representing the occupancy of Fe on each Ti site was allowed to vary in the EXAFS fit.…”

“…Ti K‐edge XANES spectra were measured at the XAFS beamline of the ELETTRA synchrotron radiation facility in Trieste (Italy) in transmission detection mode. All experimental details are described in our previous study . The Fe and Ti XAS spectra were analyzed with the IFEFFIT program package .…”

“…Also, there are assumptions that Ti 4+ partially reduces to Ti 3+ , which can contribute to the ferromagnetic coupling . In our previous study on 20% Fe‐doped 6H–BaTiO 3 we showed that Fe 3+ ions undergo an ordering process with a prolonged high‐temperature annealing, which plays a crucial role in induction of the magnetic coupling. We showed that the same batch can be paramagnetic or exhibit some ferromagnetic coupling depending on the degree of the cation ordering.…”

“…In the light of the newly discovered diffusion processes in Fe‐doped 6H–BaTiO 3 , which are of crucial importance for its functional properties, we also decided to thoroughly investigate the low‐concentration range of this system, that is, the range of ≤10%. First, we made a clear distinction between properties of the Fe‐doped paramagnetic tetragonal BaTiO 3 and 6H–BaTiO 3 phases.…”