Unexpected Competition between Antiferromagnetic and Ferromagnetic States in Hf₂MnRu₅B₂: Predicted and Realized

Materials "design" is increasingly gaining importance in the solid-state materials community in general and in the field of magnetic materials in particular. Density functional theory (DFT) predicted the competition between ferromagnetic (FM) and antiferromagnetic (AFM) ground states in a ruthenium-rich Ti₃Co₅B₂-type boride (Hf₂MnRu₅B₂) for the first time. Vienna ab initio simulation package (VASP) total energy calculations indicated that the FM model was marginally more stable than one of the AFM models (AFM1), indicating very weak interactions between magnetic 1D Mn chains that can be easily perturbated by external means (magnetic field or composition). The predicted phase was then synthesized by arc-melting and characterized as Hf₂Mn_1-xRu_5+xB₂ (x = 0.27). Vibrating-scanning magnetometry shows an AFM ground state with T_N ≈ 20 K under low magnetic field (0.005 T). At moderate-to-higher fields, AFM ordering vanishes while FM ordering emerges with a Curie temperature of 115 K. These experimental outcomes confirm the weak nature of the interchain interactions, as predicted by DFT calculations.