Magnetic birefringence of two S=1/2 Heisenberg antiferromagnetic chains

The compounds CuX2⋅2DMSO (X=Cl,Br; DMSO=[(CH3)2SO]) are being studied as model compounds for the S= 1/2 Heisenberg antiferromagnetic linear chain system (HB AF). This report presents the results of birefringence measurements on both the bromide (CDB) and the chloride (CDC) between 3.5 and 250 K. The temperature derivative of the birefringence has been compared to the theoretical predictions for the magnetic specific heat of the model. Both protonated and deuterated versions were studied but no significant differences were observed. Both the (001) and (010) faces of CDB were studied; the results were different but both sets of data correspond to the specific heat of the HB AF with an exchange constant of JBr =−11.4±0.3 K. The exchange strength found for CDC was JCl =−8.1±0.3 K. These values are consistent with those found from magnetic measurements. It was necessary to account for nonmagnetic contributions to the birefringence. The presence of small amounts of exchange anisotropy could not be detected in t...