High-field ESR studies of the quantum spin magnet CaCu2O3

TL;DR

This study uses high-field ESR to investigate the magnetic properties of CaCu2O3, revealing a small excitation gap, the role of uncompensated spins, and the effects of strong magnetic fields on antiferromagnetic order.

Contribution

It provides new insights into the magnetic excitation gap and the influence of impurity spins in the quantum spin magnet CaCu2O3 using high-field ESR.

Findings

Detected a small magnetic excitation gap of 0.3-0.8 meV.

Observed ESR signals from uncompensated spins at imperfections.

Found that strong magnetic fields can suppress antiferromagnetic order.

Abstract

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 percent of ''extra'' spin states are coupled strongly to the bulk spins and are involved in the antiferromagnetic 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 CaCu2O 3 at weak magnetic fields H_{sf} ~ 3 T. Qualitative changes of the ESR response with increasing the field strength give indications that strong magnetic fields reduce the antiferromagnetic 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.