High-field and high-frequency ESR study of the Haldane state formed in the ferromagnetic and antiferromagnetic alternating Heisenberg chain system (CH3)2CHNH3CuCl3 -: art. no. 144428

The compound (CH3)(2)CHNH3CuCl3 consists of ferromagnetic and antiferromagnetic alternating Heisenberg chains with S = 1/2. According to recent electron spin resonance (ESR) experiments at 24 GHz, a pair of ferromagnetically coupled two spins, which is expressed as (S) over cap equivalent to S-1 + S-2, behaves as a spin with (S) over cap = 1 below 10 K. and therefore the magnetic ground state in this compound is the Haldane state. Then the dipole-dipole interaction and the anisotropic-exchange interaction between ferromagnetically coupled two spins oct as a fictitious single-ion anisotropy, which removes the threefold degeneracy of the tripler state. To confirm the energy scheme of the nondegenerate triplet state, high-field and high-frequency ESR experiments up to 20 T and 540 GHz were performed on the single crystals. The absorption due to the (S) over cap (z) = -1 <----> (S) over cap (z) = 0 transition in the tripler excited levels was observed below 330 GHz and 10 K. The sign and the value of the fictitious single ion anisotropy were determined to be D*/g mu (B) = -0.24 to -0.48 T at 1.7 K. At 427.6 GHz and under an external field near 15 T, where the field-induced gapless state is formed, the absorption line that was thought to be antiferromagnetic resonance was observed at 1.7 K.