Tuning Magnetic Ground States through Cation Order in FeReO₄

Magnetism in rutile oxides has recently received strong attention due to their predisposition to host altermagnetism. Altermagnetism is a controversial proposed novel phase of magnetism, characterized by antiparallel magnetic ordering with net zero magnetization as seen in antiferromagnets while also exhibiting time-reversal symmetry-breaking effects most common to ferromagnets. Crystal structure is intrinsic to altermagnetism and thus the rutile structure with point group 4/mmm positioned these materials as ideal candidates. FeReO₄ is a rutile oxide originially reported as antiferromagnetic with T_N = 194 K, thus positioning it as a new candidate altermagnet.

We demonstrate the novel synthesis of epitaxial, stoichiometric, single-phase thin films of FeReO₄ on TiO₂ by MBE, which has hitherto only been synthesized in bulk. Furthermore we report ferromagnetism with a T_c near room-temperature. The magnetic properties of FeReO₄ is shown to depend on the atomic ordering of Fe and Re, with antiferromagnetism being seen only in disordered samples of FeReO₄ while an ordering of Fe/Re atomic planes along the [100] direction brings on this new ferromagnetic phase.