Lattice effects and magnetic structure in the layered colossal magnetoresistance manganite La2-2xSr1+2xMn2O7, x=0.3

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 degree 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 approx-lt x approx-lt 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. copyright 1999 The American Physical Society