Electron paramagnetic resonance in the singlet dimer system (CH3)(2)CHNH3CuBr3

Electron paramagnetic resonance measurements were performed at 24 GHz on th e magnetic compound (CH3)(2)CHNH3CuBr3, which consists of antiferromagnetic -antiferromagnetic alternating Heisenberg chains. The results are compared with those obtained for ferromagnetic-antiferromagnetic chains (CH3)(2)CHNH 3CuCl3 reported in Phys. Rev. B 62 (2000) 14270. The absorption intensity, after showing a broad maximum around 70 K, decreases to zero when tile temp erature approaches OK. Contrary to the case in the Cl compound in which the absorption line splits into two lines around 10K, a single absorption line persists down to 1.4K, the lowest temperature of these experiments. These results are interpreted to result from formation of singlet dimers at low t emperatures. The Dzyaloshinsky-Moriya exchange interaction is strongly sugg ested to be the main perturbation term from the analysis of the dependence of linewidth and resonance field on temperature.