Linear magnetic birefringence and double-excitonic transitions of the two-dimensional antiferromagnet BaNiF4

Abstract

The temperature dependence of the linear birefringence (LB) of BaNiF4 as measured between 5 and 600K proves to be identical for the (a,c), (b,c) and (a,b) planes. In agreement with a microscopical theory the LB consists of two terms being proportional to the energies of the lattice and of the nearly-two-dimensional Heisenberg spin system, respectively. By use of cp data of BaZnF4 and of high-temperature series expansion data of cm the complete cm curve is quantitatively derived from d mod Delta nac mod /dT and critically compared with results from caloric measurements. The T2 dependence at low temperatures is consistent with spin-wave theory. The 3D anomaly at TN=68.4K has a logarithmic divergence as a consequence of the crossover from 2D H to 2D I behaviour near tau =0.04. Essentially the same temperature dependence as for the LMB results for the intensity of double excitonic transitions in the near UV in agreement with the theory of Fujiwara et al. (1972). The transitions are identified by use of the single-exciton absorption spectrum being measured at LHeT and analysed in terms of the cubic crystal field scheme.