High-field ESR studies of the quantum spin magnet CaCu2O3

We report an electron spin resonance (ESR) study of the s = 1/2 Heisenberg pseudo-ladder magnet CaCu2O3 in pulsed magnetic fields up to 40 T. At sub-terahertz frequencies we observe an ESR signal originating from a small amount of uncompensated spins residing presumably at the imperfections of the strongly antiferromagnetically correlated host spin lattice. The data give evidence that these few per cent of 'extra' spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic (AF) ordering at T-N = 25 K. By mapping the frequency/resonance field diagram we have determined a small gap for magnetic excitations below T-N of the order of similar to 0.3-0.8 meV. Such a small value of the gap explains the occurrence of the spin-flop transition in CaCu2O3 at weak magnetic fields mu H-0(sf) similar to 3T. Qualitative changes of the ESR response with the increasing field strength give indications that strong magnetic fields reduce the AF correlations and may even suppress the long-range magnetic order in CaCu2O3. ESR data support scenarios with a significant role of the 'extra' spin states for the properties of low-dimensional quantum magnets.