Neutron diffraction studies on a ferromagnetic layered manganite La2-2xSr1+2xMn2O7 (x=0.3) prepared by a ceramic method

Low-temperature neutron diffraction data on a ceramic sample of layered perovskite type La2-2xSr1+2xMn2O7, x=0.3, are reported. The neutron diffraction data show that this compound undergoes a ferromagnetic transition at 100 K (Tc), but there is no antiferromagnetic signal as reported on a floating zone melt grown sample of the same nominal composition. The unit cell constants and cell volume for this nominal x=0.3 sample are slightly larger than those for samples which do show an antiferromagnetic component which indicates a higher Sr content. The magnetic structure was determined to have colinear Mn spins with a tilting angle against the c axis. The tilting angle abruptly increases from about 20° near Tc to about 50° at 75 K and then gradually decreases afterwards to 30° at 15 K. This anomalous tilting angle behavior correlates with a distortion index based on the variance of the Mn-O bond distances and can be understood qualitatively with a simple crystal field argument. Below Tc, there is a magnetostrictive effect on the c axis which correlates with the behaviors of the spin orientation and the Mn-O bond distances.