Multi-frequency ESR in NaCu₂O₂

NaCu₂O₂ is regarded as an S 1/2 zigzag chain with competing nearest neighbor and next nearest neighbor exchange interactions. The spin structure of NaCu₂O₂ below a Néel temperature T_N 13 K was reported to be helical with a propagation vector q (0.5, 0.227, 0). To obtain further information on the ordered state of NaCu ₂O₂, we performed multi-frequency electron spin resonance (ESR) and high field magnetization measurements on single crystal samples of NaCu₂O₂. We observed a large and some small ESR signals and a monotonical increase of magnetization with a change of the slope around 20 T. One of the ESR resonance modes from the large signal is roughly reproduced by one of the helical resonance modes calculated on the assumption of a conical spin structure.