Eg1spin-forbidden transition in NiBr2: temperature dependence and magnetoabsorption

A detailed study of the spin-forbidden ${}_{}{}^3{}_{}{}^{}A_{2g}^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}E_g^{}{}_{}{}^{}$ transition at low temperatures in antiferromagnetic Ni${\mathrm{Br}}_2$ is presented. The spectrum consists of a number of peaks, some of them rather narrow, three of which are of extrinsic origin. In the intrinsic part of the absorption, two peaks (at 8 and 38 ${\mathrm{cm}}^{-1\mathrm{}}$ from the lowest-energy sharp peak at 5 °K) have an interesting temperature dependence around 20 °K. At this temperature, in fact, the 8-${\mathrm{cm}}^{-1\mathrm{}}$ displaced sideband merges with the lowest-energy sharp peak, whereas the other peak grows much stronger above 20 °K. A magnetic field in the $c$-axis direction affects only the 8-${\mathrm{cm}}^{-1\mathrm{}}$ displaced sideband with an asymmetric splitting. These features are discussed in terms of spin waves of the magnetic structure of Ni${\mathrm{Br}}_2$, which has two phases, one collinear above 20 °K and one probably noncollinear and with the spins canted out of the basal plane, at low temperatures. It is suggested that the canting of the spins is due to competition between anisotropy and zero-point motion of the spins and the noncollinear order is due to temperature renormalization of competing exchange interactions.