Lattice effects and magnetic structure in the layered colossal magnetoresistance manganiteLa2−2xSr1+2xMn2O7, x=0.3

We report on the temperature dependence of the crystal and magnetic structure of the layered colossal magnetoresistive manganite, ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7,\hspace{0.5em}x=0.3\mathrm{}$. Neutron-diffraction measurements show that the insulator-metal (IM) transition ${(T}_{\mathrm{IM}})$ at ∼100 K is accompanied by a ferromagnetic (FM) ordering of spins within ${\mathrm{MnO}}_6$ bilayers (intrabilayer coupling), but with an antiferromagnetic coupling between neighboring bilayers (interbilayer coupling). Below $T_{\mathrm{IM}},$ 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 $0.3\lesssim x\lesssim 0.32$. 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 $d_{x^2-y^2}$ 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.