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

Abstract

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 La 5/8 Ca 3/8 MnO3 where the naturally occurring 16 O can be replaced with the 18 O 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 ≈0.065Å −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 TC in the polycrystalline samples, and the Curie temperature is decreased by 7 K by substituting 50%18 O in the x = 0.33 sample. For the optimally doped x = 3/8 sample we observed TC( 16 O) = 266.5 K and TC( 18 O) = 261.5 K at 90% 18 O substitution. Although TC is decreased by 5 K for the x = 3/8 sample the temperature dependence of the spinwave stiffness is found to be identical for the two samples. These results indicate that TC 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. 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 TC and between the two different isotopic samples. 75.47.Gk, 75.47.Lx, 75.30.Ds, 63.20.Dj

Field-induced avalanche to the ferromagnetic state in the phase-separated ground state of manganites

“…For the initial zero field-cooled FM state we obtained D = 126(6) meV-Å 2 , while we measured D = 134(5) meV-Å 2 for the FMM state. These values are consistent with other observations of D in the induced metallic state [8], in contrast to the ferromagnetic insulating state where D is about 1 3 of these values [28,29]. The statistically insignificant difference in spin stiffness between states indicates that the zero-field phase-separated FM state is metallic and identical to the FMM state.…”

Critical Behavior Near the Paramagnetic to Ferromagnetic Phase Transition Temperature in Sr1.5Nd0.5MnO4 Compound

“…On the other hand, an unexpected dependence of the order of the ferromagnetic transition on doping has been reported for La 1−x Ca x MnO 3 . The transition in La 0.8 Ca 0.2 MnO 3 is continuous [18], while that for La 0.7 Ca 0.3 MnO 3 was found to be of first order in various studies [19][20][21]. Quenched disorder in the Mn sublattice of La 0.67 Ca 0.33 MnO 3 introduced by Ga doping leads to a rounded magnetic phase transition characterized by the critical properties of a Heisenberg-like ferromagnet [22].…”

Intrinsic and extrinsic inhomogeneities in mixed-valence manganites

“…According to Refs. [19,20,21,22,23,24], paramagnetic-ferromagnetic (PM-FM) transition in this compound is of first order for the range 0.25 < x < 0.4. It is found in these compounds that FM metallic clusters are present well above T c , while some PM insulating clusters can persist down to a range far below T c [25,26,27].…”