Computational and Experimental Investigations of Osmium-Rich Borides Hf 2 MOs 5 B 2 (M = Mn, Fe, Co): From Spin Glass to Room-Temperature Magnetic Behaviors.

The metal borides, Hf2 MOs5 B2 (M = Mn, Fe, Co), which are the first Os-rich quaternary variants of the prolific Ti3 Co5 B2 structure type, were investigated computationally and experimentally. In their crystal structures, osmium builds a network of prisms, in which the other elements are located. The magnetic M elements are found in face-connected Os8 square prisms leading to M-chains with intra- and interchain distances of about 3.0 and 6.5 Å, respectively. Density functional theory (DFT) showed that magnetic ordering is hugely favored for M = Mn and Fe but only slightly favored for M = Co. Experimental investigations then confirmed and extended the DFT predictions as a metamagnetic behavior was found for the M = Mn and Fe phases, whereby the antiferromagnetic interactions ( T N = 19 and 90 K) found at low magnetic fields change to ferromagnetic at higher fields. A very broad transition ( T N = 45 K) is found for M = Co, suggesting spin-glass behavior for this phase. For M = Fe, a hard-magnet hysteresis at 5 K is found with a 40 kA/m coercivity, and even at room temperature, a significant hysteresis is found. This study paves the way for the discovery of Os-based magnets in this structure type and other intermetallics.