Abstract
Neutron scattering has been used to study the nature of the ferromagnetic transition in single crystals of and , and polycrystalline samples of and where the naturally occurring can be replaced with the isotope. Small angle neutron scattering on the single crystal reveals a discontinuous change in the scattering at the Curie temperature for wave vectors below . Strong relaxation effects are observed for this domain scattering, for the magnetic order parameter, and for the quasielastic scattering, demonstrating that the transition is not continuous in nature, in good agreement with the temperature dependence of the central component of the magnetic fluctuation spectrum, the polaron correlations, and the spin stiffness reported previously. This behavior contrasts with the continuous behavior observed for the crystal, which is well away from optimal doping. There is a large oxygen isotope effect observed for the in the polycrystalline samples, and the Curie temperature is decreased by by substituting 50% in the sample. For the optimally doped sample we observed and at 90% substitution. Although is decreased by for the sample the temperature dependence of the spin-wave stiffness is found to be identical for the two samples. These results indicate that is not solely determined by the magnetic subsystem, but instead the ferromagnetic phase is truncated by the formation of polarons which cause an abrupt transition to the paramagnetic, insulating state. The application of uniaxial stress in the single crystal sharply enhances the polaron scattering at room temperature. Measurements of the phonon density-of-states show only modest differences above and below and between the two different isotopic samples.
6 More- Received 2 June 2004
DOI:https://doi.org/10.1103/PhysRevB.70.134414
©2004 American Physical Society