Ferromagnetic-Dominant Alternating Heisenberg Chains with Ferromagnetic and Antiferromagnetic Interactions Formed in (CH_<b>3</b>)_<b>2</b>CHNH_<b>3</b>CuCl_<b>3</b>

Abstract

On measuring the dependence of the magnetic susceptibility of (CH 3 ) 2 CHNH 3 CuCl 3 on the temperature, we find that this compound forms ferromagnetic and antiferromagnetic alternating linear chains with S =1/2. The magnetic susceptibilities observed in external fields applied along three different surfaces of an as-grown single-crystal sample show broad peaks at around 24 K and steeply decrease to zero with further decreases in temperature toward 0 K. Two types of superexchange 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 =-23.5 K and J 2 / k =+54.1 K, which indicates that a pair of ferromagnetically coupled spins behaves as a spin of S =1 and thus this compound is regarded as antiferromagnetic linear chains with S =1. The present system can therefore be discussed in terms of the Haldane conjecture for integer spins. The energy gap is estimated to be 17.1∼18.1 K.