FERROMAGNETIC-DOMINANT ALTERNATING HEISENBERG-CHAINS WITH FERROMAGNETIC AND ANTIFERROMAGNETIC INTERACTIONS FORMED IN (CH3)(2)CHNH3CUCL3

On measuring the dependence of the magnetic susceptibility of (CH3)(2) CHNH3CuCl3 on the temperature, we find that this compound forms ferrom agnetic and antiferromagnetic alternating linear chains with S = 1/2. The magnetic susceptibilities observed in external fields applied alon g three different surfaces of an as-grown single-crystal sample show b road peaks at around 24 K and steeply decrease to zero with further de creases in temperature toward 0 K. Two types of superexchange interact ions defined in the Hamiltonian H = -2J(1) Sigma S-2i. S2i+1-2J(2) Sig ma S-2i. S2i-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 be haves as a spin of S = 1 and thus this compound is regarded as antifer romagnetic 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 similar to 18.1 K.