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Neutron scattering has been used to study the nature of the ferromagnetictransition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, andpolycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where thenaturally occurring O-16 can be replaced with the O-18 isotope. Small angleneutron scattering on the x=0.3 single crystal reveals a discontinuous changein the scattering at the Curie temperature for wave vectors below ~0.065 A^-1.Strong relaxation effects are observed for this domain scattering, for themagnetic order parameter, and for the quasielastic scattering, demonstratingthat the transition is not continuous in nature. There is a large oxygenisotope effect observed for the T_C in the polycrystalline samples. For theoptimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5K at 90% O-18 substitution. The temperature dependence of the spin-wavestiffness is found to be identical for the two samples despite changes in T_C.Hence, T_C is not solely determined by the magnetic subsystem, but instead theferromagnetic phase is truncated by the formation of polarons which cause anabrupt transition to the paramagnetic, insulating state. Application ofuniaxial stress in the x=0.3 single crystal sharply enhances the polaronscattering at room temperature. Measurements of the phonon density-of-statesshow only modest differences above and below T_C and between the two differentisotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.

physical review. b, condensed matter and materials physics

First-order nature of the ferromagnetic phase transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mrow><mml:mo>(</mml:mo><mml:mi mathvariant="normal">La</mml:mi><mml:mtext>‐</mml:mtext><mml:mi mathvariant="normal">Ca</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mi mathvariant="normal">Mn</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>near optimal doping

First-order nature of the ferromagnetic phase transition in(La‐Ca)MnO3near optimal doping