Abstract
We report that antiferromagnetic and antiferromagnetic (AF-AF) alternating Heisenberg chains with S=1/2 are formed in (CH3)2CHNH3CuBr3, which is in contrast to the almost isomorphous compound (CH3)2CHNH3CuCl3, because the Cl compound forms ferromagnetic-dominant ferromagnetic and antiferromagnetic (F-AF) alternating Heisenberg chains [H. Manaka et al.: J. Phys. Soc. Jpn. 66 (1997) 564]. From the analysis of the dependence of magnetic susceptibility on temperature, χ(T), the two different antiferromagnetic exchange interactions defined in the Hamiltonian {\cal H}= -2J1∑ S2i·S2i+1 -2J2∑S2i·S2i-1 are determined to be J1/k=-61 K and J2/k=-33 K. Applying the low temperature experimental data to the formula χ(T)∝ T-1/2exp(-Δ/kT), we estimate the energy gap Δ to be 98 K. We discuss why AF-AF and F-AF alternating chains are formed in the Br and the Cl compounds, respectively, in spite of their almost isomorphous crystal structures.
