The Spiral Spin State in LiCu2O2

Abstract

We studied the spiral spin state of LiCu2O2. Our calculation is based on the physical picture of classical spiral spins and their quantum fluctuation. Comparison with the spectra of the electron spin resonance (ESR) shows that the excitations come from the spin waves. If the magnetic field is applied along the b-axis of the crystal, there are two branches of excitations. The resonance frequency \(\nu _{1} \sim 30\) GHz corresponds to the spin wave states of wave vectors k = ±Q where Q is the wave vector of spiral spins. However, when the magnetic field is parallel to c-axis, there is only one branch and it comes from the spin wave state of wave vectors k = 0. Our calculation also shows that the former come from the spin waves in spiral spin configuration and the latter from the collinear spins. We have also analyzed the data of dM/dH. Our calculation shows that the spiral spin axis is parallel to b-axis if there is no applied field. When magnetic field is applied along a- or c-axis the spiral axis remains more or less the same until a critical field is reached. When the field strength reaches the second critical field, all the spins become collinear. These results confirm the assumption that there are two critical fields, Hc1 and Hc2. We further discuss the roles played by anisotropy exchange interaction and Dzyaloshinskii–Moriya interaction and how the spiral spin state in LiCu2O2.