Electronic and magnetic structure of Fe ions in NiCr2S4

Abstract

The magnetic semiconductor NixFe1−xCr2S4 (x=0.985, 0.97, 0.96) has been investigated over the temperature range from 12 to 600 K using a Mössbauer technique. The electronic structure of Fe ions in NiCr2S4 was calculated with the Hamiltonian incorporating free-ion term, axial and rhombic crystal field, spin-orbital couplings, and exchange interactions. The ground orbital state is separated by 9.64 ‖λ‖ from the first excited state, thereby making the quadrupole splitting somewhat insensitive to temperature. Using x-ray crystallographic data, the contribution of direct lattice sum to the electric-field gradient has been considered. In calculating the temperature dependence of quadrupole splitting, the axial field parameter Δ1=−3.0‖λ‖, the rhombic field parameter Δ2=−2.8‖λ‖, and the covalency factor α2=0.73 in Ni0.985Fe0.015Cr2S4 were determined. Magnetic hyperfine and quadrupole interactions in the antiferromagnetic state of Ni0.96Fe0.04Cr2S4 at 12 K have been studied, yielding the following results: H=147.8 kOe, 1/2e2qQ(1+1/3η2)1/2=−1.96 mm/s, θ=66°, φ=90°, and η=1.0. The line broadening which suggests the electron relaxation was observed with decreasing temperature.