Crossover from S=1/2 to S=1 Haldane state in the ferromagnetic and antiferromagnetic alternating Heisenberg chain system (CH3)2CHNH3CuCl3 observed with EPR at 24 GHz

The compound (CH3)(2)CHNH3CuCl3, which consists of ferromagnetic-dominant ferromagnetic and antiferromagnetic alternating Heisenberg chains with S=1/2, is regarded as a Haldane system with S=1 at low temperatures because a pair of ferromagnetically coupled spins behaves as S=1. There should therefore be a crossover of the spin state from S=1/2 to S=1 when the temperature is decreased. With the expectation that the crossover causes a drastic change in spin dynamics, electron paramagnetic resonance (EPR) experiments were performed at 24 GHz on;single crystals of this compound over the region of 1.4-295K. The EPR spectra observed below 10 K were found to show characteristics clearly distinct from those above 10 K. That is, a single absorption line observed above 10 K was found to split into two lines below 10 K, and an additional weak line appeared at a position corresponding to half of the averaged resonance fields of the two Lines. The resonance fields of the two lines vary with the direction of the external field H; their angular dependence is a+/-b(1-3 cos(2)theta), where a and b are constants, and theta is the angle between the direction of H and one of the normals of the orthogonal crystal surfaces. The resonance field of the weak line that appeared at the half-field position was almost constant with respect to the direction of H. These experimental results observed below 10 K are explained when one considers that the S=1 state caused by pairs of ferromagnetically coupled two spins supersedes the S=1/2 state of the individual spins. Then the dipole-dipole interaction (H-DD') and the anisotropic exchange interaction (H-AE') between ferromagnetically coupled two spins act as a fictitious single ion anisotropy, and remove the threefold degeneracy of the triplet state of S=1, i.e., E-S,E-Sz = E-1,E-1,E-1,E-0, and E-1,E--1. As a result, the DeltaS(z)=+/-1 transitions, i.e., the transitions between E-1,E--1 and E-1;0, and between E-1,E-0 and E-1,E-1 bring about two absorption lines. That is why the two lines appear below 10 K. The weak half-held line is due to the DeltaS(z)=+/-2 transition, which is also caused by H-DD' and H-AE' between ferromagnetically coupled two spins because nondiagonal elements between /1,1] and /1,-1] are not 0.