High Field Phase Transitions of Quasi One-DimensionalS=1/2Heisenberg Antiferromagnet Cu2(1,4-diazacycloheptane)2Cl4Observed by Submillimeter Wave ESR

Abstract

Millimeter and submillimeter wave ESR measurements on a single crystal of an S =1/2 Heisenberg ladder-like system Cu 2 (C 5 H 12 N 2 ) 2 Cl 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 =7.5 T, H c2 =10.1 T and H s =13.2 T. A transition at 10.1 T 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.