First-order nature of the ferromagnetic phase transition in (La-Ca)MnO3 near optimal doping

Article

关键词: SPIN DYNAMICS;    OXYGEN-ISOTOPE;    DOUBLE EXCHANGE;    COLOSSAL-MAGNETORESISTANCE;    MAGNETIC-PROPERTIES;    POLARON FORMATION;    LA1-XCAXMNO3;    STATES;    RESISTIVITY;    SCATTERING;   

DOI  :  10.1103/PhysRevB.70.134414

来源: SCIE

【 摘 要 】

Neutron scattering has been used to study the nature of the ferromagnetic transition in single crystals of La0.7Ca0.3MnO3 and La0.8Ca0.2MnO3, and polycrystalline samples of La0.67Ca0.33MnO3 and La5/8Ca3/8MnO3 where the naturally occurring O-16 can be replaced with the O-18 isotope. Small angle neutron scattering on the x=0.3 single crystal reveals a discontinuous change in the scattering at the Curie temperature for wave vectors below approximate to0.065 A(-1). 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 x=0.2 crystal, which is well away from optimal doping. There is a large oxygen isotope effect observed for the T-C in the polycrystalline samples, and the Curie temperature is decreased by 7 K by substituting 50% O-18 in the x=0.33 sample. For the optimally doped x=3/8 sample we observed T-C(O-16)=266.5 K and T-C(O-18)=261.5 K at 90% O-18 substitution. Although T-C is decreased by 5 K for the x=3/8 sample the temperature dependence of the spin-wave stiffness is found to be identical for the two samples. These results indicate that T-C 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 x=0.3 single crystal sharply enhances the polaron scattering at room temperature. Measurements of the phonon density-of-states show only modest differences above and below T-C and between the two different isotopic samples.

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