High field phase transitions of quasi one-dimensional S = 1/2 heisenberg antiferromagnet Cu₂(1,4-diazacycloheptane)₂Cl₄ observed by submillimeter wave ESR

Millimeter and submillimeter wave ESR measurements on a single crystal of an S = 1/2 Heisenberg ladder-like system Cu₂(C₅H₁₂N₂)₂Cl ₄ have been performed using a pulsed magnetic field up to 16 T. The gap is estimated to be 9.7 K from the temperature dependence of ESR intensity. The detailed frequency dependence of ESR in the frequency region from 40 to 420 GHz at 1.8 K suggests magnetic transitions at H_c1 = 7.5T, H_c2 = 10.1T and H_s = 13.2T. A transition at 10.1T is suggested for the first time. These results are discussed with previous magnetization measurements and theoretical calculations, and our results appear to be difficult to interpret according to the simple ladder model.