A Delicate Balance between Antiferromagnetism and Ferromagnetism: Theoretical and Experimental Studies of A₂ MRu₅ B₂ (A=Zr, Hf; M=Fe, Mn) Metal Borides

Metal-rich borides with the Ti₃ Co₅ B₂ -type structure represent an ideal playground for tuning magnetic interactions through chemical substitutions. In this work, density functional theory (DFT) and experimental studies of Ru-rich quaternary borides with the general composition A₂ MRu₅ B₂ (A=Zr, Hf, M=Fe, Mn) are presented. Total energy calculations show that the phases Zr₂ FeRu₅ B₂ and Hf₂ FeRu₅ B₂ prefer ground states with strong antiferromagnetic (AFM) interactions between ferromagnetic (FM) M-chains. Manganese substitution for iron lowers these antiferromagnetic interchain interactions dramatically and creates a strong competition between FM and AFM states with a slight preference for AFM in Zr₂ MnRu₅ B₂ and for FM in Hf₂ MnRu₅ B₂ . Magnetic property measurements show a field dependence of the AFM transition (T_N ): T_N is found at 0.1 T for all phases with predicted AFM states whereas for the predicted FM phase it is found at a much lower magnetic field (0.005 T). Furthermore, T_N is lowest for a Hf-based phase (20 K) and highest for a Zr-based one (28 K), in accordance with DFT predictions of weaker AFM interactions in the Hf-based phases. Interestingly, the AFM transitions vanish in all compounds at higher fields (>1 T) in favor of FM transitions, indicating metamagnetic behaviors for these Ru-rich phases.