Antiferromagnetic Phase Diagram and Magnetic Band Gap Shift of NaCrS2

Abstract

The magnetization and differential susceptibility of the layer-structured compound NaCr${\mathrm{S}}_2$ have been measured parallel and perpendicular to the $c$ axis in pulsed magnetic fields up to 200 kOe as a function of temperature. Antiferromagnetic ordering was observed below 18°K. For fields applied parallel to the Cr layers, a spin-flop-type transition was observed at 20 kOe and a transition to the paramagnetic phase observed at higher fields. The temperature dependence of the latter transition is found to vary as the sublattice magnetization computed from the parallel susceptibility. The in-plane and out-of-plane anisotropies are due to intraplane dipolar interactions and are of comparable magnitude, ∼3 kOe. The optical absorption spectrum has been measured in the range 1-2.4 eV where the charge-transfer band gap occurs. Measurements as a function of temperature show that the $d-d$ crystal-field transitions observed between 1.5 and 1.9 eV, are only slightly affected on passing through the Néel temperature, whereas the band gap shows an anomalously large blue-shift. This is thought to be evidence against the recent theory that these magnetic shifts are due to deformation potentials and magnetoelastic coupling. An alternative explanation is presented in which the band gap shift is due to the different exchange interactions in the ground and excited states.