Magnetism and lattice dynamics of FeNCN compared to FeO

Abstract

Three-dimensional non-oxidic extended frameworks offer the possibility to design novel materials with unique properties, which can be different from their oxide analogues. Here, we present first experimental results concerning unusual magnetic properties of FeNCN, investigated using Mössbauer spectroscopy and magnetometry between 5 and 380 K. This study reveals an unconventional behaviour of the magnetic parameters below the Néel temperature of 350 K, i.e., the hyperfine field on iron decreases with decreasing temperature. At room temperature, quadrupole and hyperfine magnetic field interaction energies are comparable in magnitude, which leads to a rare five-line absorption spectrum. We suggest that these features in the hyperfine field are caused by the combination of a small Fermi contact term and a temperature-dependent contribution from the orbital momentum and the dipole term. One additional spectral component is observed, which exhibits a magnetic relaxation behaviour and slows down at low temperatures to yield a sextet. The magnetometry data suggest that the antiferromagnetic FeNCN is rich in structural distortions, which results in a splitting of the field-cooled and zero-field-cooled curves. The lattice dynamics of FeNCN were investigated using nuclear inelastic scattering. The comparison of the obtained data with literature data of iron monoxide reveals very similar iron phonon modes with a small softening and a slightly reduced sound velocity.