Professor Paolo G. Radaelli OSI

Abstract:

Few recent examples are discussed where neutron diffraction had a major impact in rapidly developing areas of materials science and that illustrate advances in experimental technique and data analysis. Emphasis is put on imaging structural information across a multidimensional parameter space. High-speed diffractometers with low background have enabled impressive structural studies on small samples previously thought to be beyond the capability of neutron diffraction. Precision measurements of structural parameters have provided critical tests of theoretical models for the behavior of novel materials, and extreme sample environments have opened new experimental frontiers. The impact of neutron diffraction on condensed-matter science is highlighted.

Abstract:

The structural, magnetic and transport phase diagrams of the manganese perovskites (A1-xA'xMnO3) are characterized by several phenomena, including high-temperature polaronic behavior, charge-orbital and magnetic ordering and colossal magnetoresistance (CMR). These properties can be tuned by changing the doping level, the electronic bandwidth, and the A-site disorder. To demonstrate this, the recent x-ray synchrotron and neutron diffraction data on the crystallographic and magnetic modulation in La0.33Ca0.67MnO3 are presented.

Abstract:

Neutron powder diffraction measurements of perdeutero durene in the temperature range from 1.5 K to 290 K have been performed. The lowest temperature structure is the starting point for calculations of the methyl group tunneling and librational dynamics. Ab initio methods and atom-atom potentials are used to determine rotational single particle and coupling potentials. Tunneling splittings and librational bands are calculated by numerical solution of Schrödinger's equation for a system of many coupled methyl groups. High-resolution inelastic neutron scattering measurements of methyl tunneling and molecular vibrations have been repeated, the tunneling results resolving an inconsistency with earlier NMR work. Quantum molecular dynamics provide a stringent test of the numerical methods and the data are ultimately well reproduced. These results are also discussed in the context of optical measurements of dye molecules in a host lattice of durene. © 1999 American Institute of Physics.

Abstract:

The crystallographic structure and the magnetic order of the distorted perovskite La1-xSrxCoO3 (0.10≤x ≤0.30) has been studied by neutron diffraction, high-resolution electron microscopy, and magnetic-susceptibility measurements. The results give direct evidence for an inhomogeneous distribution of the Sr2+ ions and the segregation of the material into hole-rich ferromagnetic regions and a hole-poor semiconducting matrix at lower values of x. The holes introduced by Sr doping are attracted to the Sr2+ ions where they stabilize to lowest temperatures an intermediate-spin state at neighboring trivalent cobalt. The antibonding e electrons so stabilized increase the mean unit-cell volume and are delocalized over the cobalt atoms of the cluster where they couple the localized t5 configurations ferromagnetically. Long-range ferromagnetic order between clusters is realized even for Sr doping as low as x=0.10. The transition to a spin glass state is observed only for Sr concentrations smaller than 0.10. The volume of a hole-rich cluster grows in a magnetic field, and the origin of the large negative magnetoresistance observed near Tc for 0.15 ≤ x ≤ 0.25 appears to be due to a growth of the clusters to a percolation threshold. For x=0.30, the σ* band of the intermediate-spin state below Tc is at the threshold of a transition from itinerant to polaronic conduction and, above Tc, the system transforms smoothly to a cluster state. © 1999 The American Physical Society.

Abstract:

In this paper we describe a detailed neutron diffraction investigation of the crystal and magnetic structure of two layered CMR manganites La1.2Sr1.8Mn2O7 (x = 0.4) and La1.4Sr1.6O7 (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 d3z2-r2 Orbitals in La1.2Sr1.8Mn2O7 and to dx2-y2 orbitals in La1.4Sr1.6Mn2O7. 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 La1.2Sr1.8Mn2O7 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 La1.2Sr1.8Mn2O7 to structures with antiferromagnetic inter-bilayer coupling as found in La1.4Sr1.6Mn2O7. © CSIRO 1999.