Magnetic order in lightly doped La2-xSrxCuO4 -: art. no. 027001

We study long wavelength magnetic excitations in lightly doped La2-xSrxCuO4 (xless than or equal to0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferromagnetic (AF) phase. The second magnetic resonance, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state; it led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the Neel temperature in x=0.01.