Professor Stephen Blundell

Abstract:

The crystal structure of the anion-deficient perovskite Ca2.5Sr0.5GaMn2O8 has been studied at 290 and 5 K by neutron diffraction (290 K; space group Pcm21, a = 5.4294(1), b = 11.3722(3), c = 5.2983(1) Å). The vacant oxide sites order to create a structure in which perovskite bilayers consisting of MnO6 octahedra are isolated from each other along [010] by a single layer of GaO4 tetrahedra. At 5 K the material is antiferromagnetic with an ordered magnetic moment of 3.09(1) μB per Mn cation. Magnetic susceptibility measurements suggest that short-range magnetic ordering within the bilayers occurs above 200 K, and muon spin relaxation data show that the transition to long-range magnetic order occurs between 150 and 125 K. The resistivity of Ca2.5Sr0.5GaMn2O8 decreases by an order of magnitude at 125 K, and ∼50% magnetoresistance is seen in a field of 80 kOe at 110 K.

Abstract:

Muon-spin relaxation (μSR) experiments on the conducting polymers poly(2, 3-dibutoxy-1, 4-phenylene vinylene) and poly(2, 5-bis(dimethyloctylsilyl)-1, 4-phenylene vinylene) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state. The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependences of which permit the extraction of intrachain and interchain diffusion rates. The intrachain diffusion rate decreases with temperature and can be fitted to a model of phonon-limited transport. The interchain diffusion rate increases with temperature and can be fitted to an activated temperature dependence.

Abstract:

Polycrystalline samples of Sr3MnRuO7 and Sr2Mn0.5Ru0.5O4 have been synthesized and characterized by neutron diffraction, dc magnetometry, and magnetotransport measurements. They are, respectively, n = 2 and n = 1 members of the Ruddlesden-Popper (RP) An+1BnO3n+1 family; both have tetragonal (I4/mmm) symmetry and a disordered distribution of Ru and Mn over the six-coordinate sites within the perovskite layers of the RP structure. Neither compound shows long-range magnetic order at 2 K, but a spin-glass transition is observed at 16 K (n = 1) or 25 K (n = 2). In the case of the n = 2 compound only, the magnetic transition is accompanied by a reduction in the zero-field electrical conductivity. A maximum magnetoresistance of ∼8% in 14 T is found in both compounds.