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Abstract:

Neutron diffraction and small-angle scattering on La1-xSrxCoO3 (0 < x ≤ 0.30) show that for x = 0.3 the system is near a two-phase percolation threshold and undergoes a transition from itinerant to polaronic conduction at the Curie temperature TC-The stabilization of superparamagnetic clusters on warming through TC is revealed by an anomalous thermal expansion of the volume and a deviation of the paramagnetic susceptibility from the Curie-Weiss law. The development of a temperature-dependent small-angle-scattering signal confirms that regions of short-range ferromagnetic order are present above TC.

Abstract:

We report on the temperature dependence of the crystal and magnetic structure of the layered colossal magnetoresistive manganite, La2-2xSr1+2xMn2O7, x = 0.3. Neutron-diffraction measurements show that the insulator-metal (IM) transition (TIM) at 100 K is accompanied by a ferromagnetic (FM) ordering of spins within MnO6 bilayers (intrabilayer coupling), but with an antiferromagnetic coupling between neighboring bilayers (interbilayer coupling). Below TIM, the Mn spins rotate from 45° inclination to the c axis until they are almost parallel to the c axis at 5 K. Coincident with this spin reorientation, a FM c axis component develops below 75 K. Evidence from both neutron and synchrotron x-ray-diffraction experiments suggest that the FM c-axis magnetic moment results from a second layered manganite phase with composition 0.3≲x≲0.32. This observation emphasizes the need for thorough examination of the homogeneity when measuring bulk properties (e.g., magnetization, transport) of nominally x=0.3 samples. Associated with the electronic and magnetic transitions, a pronounced lattice response along the c axis (observed in both phases) signals a transfer of charge into dx2-y2 orbitals in the low-temperature phase. That the lattice effects here are opposite in sign to those observed in the x=0.4 layered manganite points to the sensitivity of the spin-lattice-charge coupling to dopant concentration in these reduced-dimensionality manganites. © 1999 ThAmerican Physical Society.