Impurity susceptibility and the fate of spin-flop transitions in lightly doped La2CuO4

We investigate the occurrence of a two-step spin-flop transition and spin reorientation when a longitudinal magnetic field is applied to lightly hole doped La2CuO4. We find that for large and strongly frustrating impurities, such as Sr in La2-xSrxCuO4, the huge enhancement of the longitudinal susceptibility suppresses the intermediate flop and the reorientation of spins is smooth and continuous. Contrarily, for small and weakly frustrating impurities, such as O in La2CuO4+y, a discontinuous spin reorientation (two-step spin-flop transition) takes place. Furthermore, we show that for La2-xSrxCuO4 the field dependence of the magnon gaps differs qualitatively from the La2CuO4 case, a prediction to be verified with Raman spectroscopy or neutron scattering.