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)₂CHNH₃CuCl₃, 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 1.4–295 K. 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(1-3cos²θ), where a and b are constants, and θ 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,Sz=E_1,1,E_1,0, and E_1,-1. As a result, the ΔS_z=±1 transitions, i.e., the transitions between E_1,-1 and E_1,0, and between E_1,0 and E_1,1 bring about two absorption lines. That is why the two lines appear below 10 K. The weak half-field line is due to the ΔS_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.