Concurrent Nd and Mn spin excitations in Nd0.6Sr0.4MnO3

The spin dynamics of Nd0.6Sr0.4MnO3 have been determined along [qqq], [qq0] , and [00q] propagation directions using inelastic neutron scattering. At T = 13 K, the low-q Mn-Mn mode energies varied quadratically with wave vecto r q, but exhibited marked softening on approach to the zone boundaries when compared with a Heisenberg nearest-neighbor exchange calculation. At the l arger q's the modes also show evidence of increasing spin wave energy width . The temperature dependence of the spin wave stiffness parameter D was mea sured up to T-c = 273 K and showed conventional behavior. The 0 K value of D = 150 meV Angstrom(2) yields D(0)/kT(c) = 6.3. No anomalous E = 0 central mode intensity was observed on approach to T-c. At low T additional modes were observed and are ascribed to Nd-Mn exchange interactions. These modes have q = 0 spin wave gap of 2.1 meV compared to 1.1 meV for the Mn-Mn excit ations. (C) 1999 American Institute of Physics. [S0021-8979(99)20308-8].