Nonlinear magnetic dynamics of itinerant ferromagnets: "colossal" magnetoresistive manganites

A review is given focusing on the nonlinear magnetic dynamics of itinerant ferromagnets in the spin wave and relaxational regimes. The Landau-Lifshitz equation is shown to hold in the spin wave regime approximately, in the li mit of weak inhomogenities. Different mechanisms of magnon damping are disc ussed, caused by four-magnon scattering, two-maganon scattering by itineran t electrons and by longitudinal spin fluctuations (SF). A nonlinear mechani sm of overdamped SF caused by their emission (absorption) by a mag-non is d iscussed and is shown to give a quasielastic peak in the longitudinal SF sp ectrum, which strongly depends on temperature and magnetic field. It is sho wn that quasielastic SF are suppressed at low temperatures and develop at e levated temperatures. dominating near the Curie point. The magnetic field o f a few tesla is shown to strongly suppress them. These features of quasiel astic SF, essentially different from the paramagnon properties founding the basis for the conventional SF theory of Moriva, were observed in many itin erant ferromagnets, which strongly suggests the nonlinear nature of their q uasielastic SF. The results are applied to the "colossal" magneto resistive manganites La1-xBxMnO3 (where B = Ca, Sr, Ba) to explain their SF spectrum and to show that scattering of electrons by the nonlinear quasielastic SF may be a possible mechanism for magnetoresistance in these compounds, espec ially in La0.67Ca0.33MnO3. (C) 2001 Elsevier Science B.V. All rights reserv ed.