Unprecedented Robust Antiferromagnetism in Fluorinated Hexagonal Perovskites

The diversification of antiferromagnetic (AFM) oxides with high Neel temperature is of fundamental as well as technical interest if one considers the need for robust AFM in the field of spin-tronics (exchange bias, multiferroics, etc.). Within the broad series of so-called hexagonal perovskites (HP), the existence of face-sharing octahedral units drastically lowers the strength of magnetic exchanges as compared to corner-sharing octahedral edifices. Here, we show that the partial introduction of F- in several Fe-based HP types leads to a drastic increase of the AFM ordering close to the highest values reported in iron oxides (T-N approximate to 700 K). Our experimental results are supported by ab initio calculations. The T-N increase is explained by the structural effect of the aliovalent F- for O2- substitution occurring in preferred anionic positions: it leads to local changes of the Fe-O-Fe connectivity and to chemical reduction into predominant Fe3+, both responsible for drastic magnetic changes.