A Delicate Balance between Antiferromagnetism and Ferromagnetism: Theoretical and Experimental Studies of A 2 MRu 5 B 2 (A=Zr, Hf; M=Fe, Mn) Metal Borides.

Metal-rich borides with the Ti ₃ Co ₅ B ₂ -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 A ₂ MRu ₅ B ₂ (A=Zr, Hf, M=Fe, Mn) are presented. Total energy calculations show that the phases Zr ₂ FeRu ₅ B ₂ and Hf ₂ FeRu ₅ B ₂ 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 Zr ₂ MnRu ₅ B ₂ and for FM in Hf ₂ MnRu ₅ B ₂ . 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.
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