College Park, Maryland      June 6 - 10 , 2004

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T3-A4 (4:45 PM): Spin dynamics of CMR manganites

T. Chatterji (Institut Laue Langevin), N. Shannon (Max Planck Institute for PKS, Dresden, Germany), L.P. Regnault ( CEA, Grenoble, France)

We have investigated the spin dynamics of the hole-doped infinite-layer and bilayer manganites, La_0.7Ba_0.3MnO₃ and La_1.2Sr_1.8Mn₂O₇, showing colossal magnetoresistive properties close to the ferromagnetic transition temperatures, by inelastic neutron scattering. The infinite-layer manganite La_0.7Ba_0.3MnO₃ is a three-dimensional high-T_C ferromagnet with T_C = 350 K whereas the bilayer manganite La_1.2Sr_1.8Mn₂O₇ is a quasi-two-dimensional low-T_C ferromagnet with T_C = 120 K. The spin dynamics of these ferromagnets at low temperature, show important deviations from that expected for a localized Heisenberg ferromagnet. The spin wave stiffness and bandwidth scaled to k_BT_C show that these ferromagnets are closer to the itinerant ferromagnets. The experimentally determined spin wave dispersions in both these ferromagets at low temperatures show zone-boundary softening. Also the spin waves are heavily damped especially close to the zone boundary. We developed a spin wave theory of these ferromagnets on the basis of minimal double exchange model. This model reproduces qualitatively both the softening and damping of the spin waves. According to this model retaded interaction between spin waves mediated by the charge susceptibility of the itinerant electrons generates interaction between non-nearest-neighbor spins, and so modifies the spin wave dispersion. Also the spin waves can decay into lower energy states by giving up energy to electron-hole pairs and so become damped. However, this model falls short to reproduce quantitatively the experimental results. We have measured the temperature dependence of the spin waves in these ferromagnets. The temperature dependence of the spin waves of the hole-doped ferromagnets shows deviations from that expected from the localized Heisenberg model. These deviations can be explained qualitatively from the minimal double exchange model. The spin excitations in these ferromagnets are found to persist above the ordering temperatures.

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