Interplay of spin and orbital ordering in the layered colossal magnetoresistance manganite La2-2xSr1+2xMn2O7 (0.5<=x<=1.0)

The crystallographic and magnetic phase diagram of the n=2 layered manganite La2-2xSr1+2xMn2O7 in the region x=>0.5 has been studied using temperature dependent neutron powder diffraction. The magnetic phase diagram reveals a progression of ordered magnetic structures generally paralleling that of 3-D perovskites with similar electronic doping: A (0.5 C (0.75 G (0.90<=x<=1.0). However, the quasi-2-D structure amplifies this progression to expose features of manganite physics uniquely accessible in the layered systems: (a) a "frustrated" region between the A and C regimes where no long-range magnetic order is observed; (b) magnetic polytypism arising from weak inter-bilayer magnetic exchange in the Type-C regime; and (c) a tetragonal to orthorhombic phase transition whose temperature evolution directly measures ordering of d3y2-r2 orbitals in the a-b plane. This orbital-ordering transition is precursory to Type-C magnetic ordering, where ferromagnetic rods lie parallel to the b-axis. These observations support the notion that eg orbital polarisation is the driving force behind magnetic spin ordering. Finally, in the crossover region between Type-C and Type-G states, we see some evidence for the development of local Type-C clusters embedded in a Type-G framework, directly addressing proposals of similar short-range magnetic ordering in highly-doped La1-xCaxMnO3 perovskites.Comment: 32 pages, 13 figures, submitted to Phys. Rev.

Dissimilarities between cholinergic and dopaminergic turning elicited by nucleus accumbens stimulation in freely moving rats

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Targeted T1 Magnetic Resonance Imaging Contrast Enhancement with Extraordinarily Small CoFe2O4 Nanoparticles

Extraordinarily small (2.4 nm) cobalt-ferrite nanoparticles (ESCIoNs) were synthesised by a one-pot thermal decomposition approach to study their potential as MRI contrast agents. Fine size control was achieved using oleylamine alone, and annular dark-field scanning transmission electron microscopy revealed highly-crystalline cubic spinel particles with atomic-resolution. Ligand exchange with dimercaptosuccinic acid rendered the particles stable in physiological conditions with a hydrodynamic diameter of ∽12 nm. The particles displayed superparamagnetic properties, and a low r2/ r1 ratio suitable for a T1 contrast agent. The particles were functionalised with bile-acid which improved biocompatibility by significant reduction of reactive oxygen species generation and is a first step towards liver-targeted T1 MRI. Our study demonstrates the potential of ESCIoNs as T1 MRI contrast agents