Competing Magnetic Ground States in Ilmenite-Type High-Entropy Oxides

High-entropy oxides containing more than five different cations have emerged as platforms for diverse functionalities, including magnetism. We synthesized a high-entropy oxide with an ilmenite-type structure (Mg0.2Mn0.2Co0.2Ni0.2Zn0.2)TiO3 and revealed that it undergoes long-range antiferromagnetic ordering below TN ∼ 9.5 K despite strong cation disorder. Neutron diffraction measurements showed that the magnetic structure comprises ferromagnetic honeycomb planes that stack antiferromagnetically, analogous to those of the parent CoTiO3 and NiTiO3 ilmenites. Derivatives of the high-entropy ilmenite were obtained by substituting one of the divalent cations of (Mg0.2Mn0.2Co0.2Ni0.2Zn0.2)TiO3 with an equimolar amount of Li+ ions and trivalent cations. The antiferromagnetic order remains robust against aliovalent doping that introduces extra disorder owing to cations with different valence states. In contrast, when the cation ratio was tuned to a Mn-rich composition of (Mg0.2Mn0.3Co0.15Ni0.15Zn0.2)TiO3, spin-glass freezing was observed, most likely because of the enhanced competition between the in-plane magnetic interactions. This suggests that long-range magnetic ordering in high-entropy materials is formed by a subtle balance of competing interactions, and fine-tuning of constituent magnetic ions provides control over their magnetic ground states.