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 ∼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 μ₀H_sf ∼ 3 T. 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.