Two-dimensional ferromagnetic correlations above T-c in the naturally layered CMR manganite La2-2xSr1+2xMn2O7 (x = 0.3-0.4)

Neutron diffuse scattering in the form of rod-like features has been observ ed in single crystals of the layered CMR material La2-2xSr1-2xMn2O7 (x=0.4, 0.36), consistent with the presence of 2D ferromagnetic spin correlations. These diffuse features are observed over a wide temperature region. However , their coherence length does not appear to diverge at T-C, although there is evidence of the development of three-dimensional correlations around fer romagnetic reflections of the 3D-ordered magnetic structure close to T-C. Q uasi-elastic neutron scattering on a ceramic sample of x=0.3 shows that the lifetime of these ferromagnetic correlations increases at T-->T-C. They ex hibit a spin-diffusion constant above T-C of similar to 5 meV Angstrom(2), much lower than that reported for La2/3Ca1/3MnO3. We discuss the relationsh ip of these magnetic correlations to models of the ferromagnetic transition in CMR compounds. (C) 1998 American Institute of Physics. [S0021-8979(98)4 3211-0].