Direct exchange-interactions boost magnetic frustration and a large zero-point entropy in magnetic diamond-lattice spinels*

Magnetic frustration is instrumental in the suppression of long range magnetic order. Spinels AB₂X₄, where magnetic ions exclusively occupy the tetrahedral A-sites, are examples of frustrated magnetic systems, some displaying exotic phenomena such as order-by-disorder and spiral spin-liquids, due to the presence of competing interactions among nearest-neighbors (J_nn) and next-nearest-neighbors (J_nnn) magnetic ions. E. Current understanding is based on the competing concurrence of Jⁱⁿ_nn and Jⁱⁿ_nnn indirect (super) exchange interactions. Here, we report structural, magnetic and calorimetric data indicating that in the new spinel nitride: MnTa₂N₄, N-Ta and N-Mn hybridization modulates the Jⁱⁿ_nn and Jⁱⁿ_nnn interactions but more importantly the direct Mn-Mn magnetic interactions are found to overrule the superexchange interactions and dramatically reinforce frustration. As a result, magnetic order only emerges at lower temperature, where only a fraction of Mn²⁺ spins order in a short-range degenerated spiral state. Calorimetric data reveals the presence of a large (30%) zero-point magnetic entropy. These observations, indicate an unexpected central role of direct magnetic interactions on frustration and suggest that spinel nitrides offer a new route for the research of quantum materials.