Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets. II. Raman spectroscopy
We compare the theoretical predictions of the previous paper on the field dependence of the magnetic spectrum in anisotropic two-dimensional and Dzyaloshinskii-Moriya layered antiferromagnets [ L. Benfatto and M. B. Silva Neto Phys. Rev. B 74 024415 (2006)], with Raman spectroscopy experiments in Sr2CuO2Cl2 and untwinned La2CuO4 single crystals. We start by discussing the crystal structure and constructing the magnetic point group for the magnetically ordered phase of the two compounds, Sr2CuO2Cl2 and La2CuO4. We find that the magnetic point group in the ordered phase is the m̱mm̱ orthorhombic group, in both cases. Furthermore, we classify all the Raman active one-magnon excitations according to the irreducible co-representations for the associated magnetic point group. We find that the in-plane (or Dzyaloshinskii-Moriya) mode belongs to the DAg co-representation while the out-of-plane (XY) mode belongs to the DBg co-representation. We then measure and fully characterize the evolution of the one-magnon Raman energies and intensities for low and moderate magnetic fields along the three crystallographic directions. In the case of La2CuO4, a weak-ferromagnetic transition is observed for a magnetic field perpendicular to the CuO2 layers. We demonstrate that from the jump of the Dzyaloshinskii-Moriya gap at the critical magnetic field Hc≃6.6 T one can determine the value of the interlayer coupling J⊥∕J≃3.2×10−5, in good agreement with previous estimates. We furthermore determine the components of the anisotropic gyromagnetic tensor as gsa=2.0, gsb=2.08, and the upper bound gsc=2.65, also in very good agreement with earlier estimates from magnetic susceptibility. For the case of Sr2CuO2Cl2, we compare the Raman data obtained in an in-plane magnetic field with previous magnon-gap measurements done by electron spin resonance (ESR). Using the very low magnon gap estimated by ESR (∼0.05 meV), the data for the one-magnon Raman energies agree reasonably well with the theoretical predictions for the case of a transverse field (only hardening of the gap). On the other hand, an independent fit of the Raman data provides an estimate for gs≃1.98 and gives a value for the in-plane gap larger than the one measured by ESR. Finally, because of the absence of the Dzyaloshinskii-Moriya interaction in Sr2CuO2Cl2, no field-induced modes are observed for magnetic fields parallel to the CuO2 layers in the Raman geometries used, in contrast to the situation in La2CuO4.