Structure and magnetism in the layered CMR manganites La2-2xSr1+2xMn2O7 (x=0.3, 0.4)

In this paper we describe a detailed neutron diffraction investigation of t he crystal and magnetic structure of two layered CMR manganites La1.2Sr1.8M n2O7 (x = 0.4) and La1.4Sr1.6Mn2O7 (x = 0.3). In these materials of reduced dimensionality compared to the 3D perovskites, we find competing effects b etween charge-lattice and spin degrees of freedom. These effects can be inv estigated by studying the behaviour of crystal and magnetic structure as a function of temperature, composition and hydrostatic pressure. We find oppo site lattice responses to the onset of charge delocalisation and magnetic o rdering in these two layered compounds. Below the insulator-to-metal transi tion (T-IM), the lattice response suggests that charge is transferred to d( 3z2-r2) orbitals in La1.2Sr1.8Mn2O7 and to d(x2-y2) orbitals in La1.4Sr1.6M n2O7. We argue that these changes are too large to be due to chemical diffe rences. Instead we suggest that the orbital configuration of the Mn ion bel ow TIM is sensitive to electronic doping. In La1.2Sr1.8Mn2O7 We find that t he lattice response at T-IM to be driven by lattice displacements that rela x below TIM, consistent with polaronic degrees of freedom. We also note tha t the competition between super- and double-exchange to be significant in r educed dimensions. This is manifested in the change in the sign of the apic al 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 arran gement of Mn spins. Interestingly the magnetism in reduced dimensions in th ese materials can be Varied from relative simple structures that show ferro magnetic inter-bilayer coupling as observed in La1.2Sr1.8Mn2O7 to structure s with antiferromagnetic inter-bilayer coupling as found in La1.4Sr1.6Mn2O7 .