Magnetic interactions in the quasi-two-dimensional magnet Ca₆FeN₅

Transition metal nitrides with distinct chemical and structural characteristics hold the promise for exotic magnetic properties. However, limited by the inherent stability of N₂ and competition with oxide formation, only a few transition metal ternary nitrides have been successfully synthesized. Here, we investigate the magnetic properties of Ca₆FeN₅, which features isolated planar [FeN₃]^6- triangular units stacked along the c-axis. Our magnetic susceptibility and neutron scattering data point to a robust antiferromagnetic (AF) ground state below 50K and feature AF interlayer exchange. This is quite surprising. Because the [FeN₃]^6- units are dispersed in dielectric [Ca₆N₂]^6- layers and being 6.11Å apart, Intriguingly, the interlayer exchange is expected to be extremely weak and should not support long-range ordering. Using density functional theory, we examine the electronic structure in detail and explain how σ- and π-bonding interactions contribute to the stability of the antiferromagnetic phase. We exclude the possible exchange mechanism of a direct exchange or indirect super-super-exchange (as those in 2D magnet CrI₃). The puzzling magnetism in this material is yet to be unraveled.