Magnetic order in lightly doped La_2-xSr_xCuO_4

We study magnetic Raman scattering in La_2-xSr_xCuO_4 (x 0.03) detwinned crystals. The anisotropic dispersion with magnetic field of the lowest spin-wave gap around 17 cm-1 is mapped in La2CuO4. Only 1% Sr doping softens by 30% the sharp gap resonance which disappears beyond the boundary of antiferromagnetic order. In contrast, the zone center spin-gaps persist above the N'{e}el temperature. Different effects of temperature and doping are also observed in the low frequency magnetic Raman continuum. We discuss the relaxation of the two-magnon excitation around 3000 cm-1.