We study the spin dynamics of the manganites La 0.9 Ca 0.1 MnO 3 and La 0.91 Sr 0.09 MnO 3 in magnetic field. Previous studies in zero field have shown that the magnetic ground state is inhomogeneous with ferromagnetic ͑F͒ droplets embedded in a canted antiferromagnetic (CAF) matrix. The spin dynamics consists of two spin wave branches in a high and a low-energy range, respectively. However, the assignment of these two branches either to a mean CAF medium or to F droplets embedded in an AF matrix, was uncertain. The present study shows that these two branches do not follow the mean-field calculation for a homogeneous CAF state and do not correspond either to a phase separation between AF and F states. The progressive spin-flop transitions observed for H ʈ b and H ʈ a + b, give rise to strong changes in the dispersion of the spin waves. The low-energy branch, only visible around F zone centers at zero field, appears at any q-value in magnetic field. Moreover, as soon as the AF spin component deviates from its easy axis, leading to a decrease of the magnetic anisotropy, the high-energy branch is no more defined near the zone boundary. Its intensity is transferred to a higher flat level, likely related to defects in the CAF matrix. These new results strongly support a picture where these magnetic excitations arise from a modulated canted antiferromagnetic ground state consisting of two coupled F and AF components.
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