Formation of Alternating Antiferromagnetic Heisenberg Chains in (CH_<b>3</b>)_<b>2</b>CHNH_<b>3</b>CuBr_<b>3</b>

Abstract

We report that antiferromagnetic and antiferromagnetic (AF-AF) alternating Heisenberg chains with S =1/2 are formed in (CH 3 ) 2 CHNH 3 CuBr 3 , which is in contrast to the almost isomorphous compound (CH 3 ) 2 CHNH 3 CuCl 3 , 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 H = -2 J 1 ∑ S 2 i · S 2 i +1 -2 J 2 ∑ S 2 i · S 2 i -1 are determined to be J 1 / k =-61 K and J 2 / k =-33 K. Applying the low temperature experimental data to the formula χ( T )∝ T -1/2 exp (-Δ/ k T ), 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.