Lattice effects and magnetic structure in the layered colossal magnetoresistance manganite La_2-2xSr_1+2xMn₂O₇, x = 0.3

We report on the temperature dependence of the crystal and magnetic structure of the layered colossal magnetoresistive manganite, La_2-2xSr_1+2xMn₂O₇, x = 0.3. Neutron-diffraction measurements show that the insulator-metal (IM) transition (T_IM) 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 T_IM, 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 d_x²-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.