Abstract

In this paper we describe a detailed neutron diffraction investigation of the crystal and magnetic structure of two layered CMR manganites La_1·2Sr_1·8Mn₂O₇ (x = 0·4) and La_1·4Sr_1·6Mn₂O₇ (x = 0·3). In these materials of reduced dimensionality compared to the 3D perovskites, we find competing effects between charge-lattice and spin degrees of freedom. These effects can be investigated by studying the behaviour of crystal and magnetic structure as a function of temperature, composition and hydrostatic pressure. We find opposite lattice responses to the onset of charge delocalisation and magnetic ordering in these two layered compounds. Below the insulator-to-metal transition (TIM), the lattice response suggests that charge is transferred to d_3z2-_r2 orbitals in La_1·2Sr_1·8Mn₂O₇ and to d_x2-_y2 orbitals in La_1·4Sr_1·6Mn₂O₇. We argue that these changes are too large to be due to chemical differences. Instead we suggest that the orbital configuration of the Mn ion below TIM is sensitive to electronic doping. In La_1·2Sr_1·8Mn₂O₇ we find that the lattice response at TIM to be driven by lattice displacements that relax below TIM, consistent with polaronic degrees of freedom. We also note that the competition between super- and double-exchange to be significant in reduced dimensions. This is manifested in the change in the sign of the apical Mn-O bond compressibilities above and below TIM. Finally, we describe the magnetic structure of these two different layered manganites. We find that electronic doping also results in significant changes to the ordered arrangement of Mn spins. Interestingly the magnetism in reduced dimensions in these materials can be varied from relative simple structures that show ferromagnetic inter-bilayer coupling as observed in La_1·2Sr_1·8Mn₂O₇ to structures with antiferromagnetic inter-bilayer coupling as found in La_1·4Sr_1·6Mn₂O₇.