Evidence for two disparate spin dynamic regimes within Fe-substituted La0.7Pb0.3„Mn1−xFex...O3 (0ÏxÏ0.2) colossal magnetoresistive manganites: Neutron spin-echo measurements

The spin dynamics of substituted colossal magnetoresistive CMR manganites of general formula La0.7Pb0.3 Mn1−xFex O3, 0 x 0.2 is investigated by means of neutron spin-echo measurements. Substitution of Mn by Fe leads to a strong decrease of the temperature of macroscopic magnetic long-range ordering with a concomitant enhancement of the CMR effect. For x=0.2, a long-range-ordered state is not achieved as a result of the increase in antiferromagnetic interactions brought forward by Fe+3-Mn couplings. The results display two relaxations having well separated decay constants. A fast process with a relaxation time of about 10 ps within the paramagnetic phase is found for all compositions. It shows a remarkably strong dependence with temperature and sample composition as the apparent activation energy for spin diffusion as well as the preexponential term exemplify. The physical origin of such a fast relaxation is assigned to heavily damped or overdamped spin waves spin diffusion on the basis of some signatures of excitations having finite frequencies found for the parent compound La0.7Pb0.3MnO3 at temperatures just below Tc, together with preliminary data on the effect of Fe doping on the stiffness constant. A slower relaxation is present for all compositions. Its temperature dependence follows the behavior of the macroscopic magnetization, and its intensity grows within the ordered ferromagnetic state. Its physical origin is ascribed to collective reorientation of nanoscale ferromagnetic domains on the basis of the wave-vector dependence of its relaxation rate and amplitude.