Double perovskite oxides have attracted much interest since the discovery of spin-polarized conduction and large low field magnetoresistances in Sr<sub>2</sub>FeMoO<sub>6</sub>. This behaviour exists to high temperatures, making Sr<sub>2</sub>FeMoO<sub>6</sub> and related materials candidates for application in data storage devices. This thesis describes the synthesis, structures and properties of some new double perovskite oxides. These can be divided into the (LaM)BNbO<sub>6</sub> (M = Ca, Sr, Ba; B = Mn, Co) and LaSrFeNbO<sub>6</sub> materials that have one magnetic transition metal (TM) cation, and the (La<sub>1+x</sub>M<sub>1-x</sub>)CoRuO<sub>6</sub> (M = Ca, Sr; -0.25 £ x £ 0.25 x = 1) materials that have two magnetic TM cations. The (LaM)CoNbO<sub>6</sub> double perovskites crystalize with the monoclinic P2<sub>1/2</sub> (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, which have rocksalt ordered arrangements of Co<sup>2+</sup> (3d<sup>7</sup>) and Nb<sup>5+</sup> (4d<sup>0</sup>), with almost no Co/Nb inversion (<4%). All three phases are anti-ferromagnetic with Neel temperatures of 17 K (M = Ca), 16 K (M = Sr) and 10K (M = Ba). The magnetic ordering is described by the magnetic propagation vector k = ( ½ 0 ½ ) for all materials. For M = Ca and Sr, two “non-collinear” and two “collinear” magnetic structures are found. These are experimentally indeterminate because of the small monoclinic angle (b = 89.9<sup>0</sup>). The M = Ba material has a collinear antiparallel magnetic structure. The saturated cobalt moment decreases from 2.97(2) m<sub>B</sub> to 2.52 (3) m<sub>B</sub> to 1.85 (5) m<sub>B</sub> for M = Ca, Sr and Ba, respectively. The frustration due to competing 90<sup>0</sup> exchange interactions. The (LaK)MnNbO<sub>6</sub> materials also have the monoclinic P2<sub>1</sub>/n (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, with <3% inversion of Mn<sup>2+</sup> (3d<sup>5</sup>) and Nb<sup>5+</sup> (4d<sup>0</sup>). All materials are antiferromagnetic with Neel temperatures of 9 K (M = Ca), 8 K (M = Ba) and 6.5 K (M = Ba). The magnetic Bragg reflections are indexed by k = (0 ½ ½ ), and the possible magnetic models are identical to those for (LaM)CoNbO<sub>6</sub>. The manganese saturation moment decreases from 4.77(3) m<sub>B</sub> (M = Ca) to 4.43 (4) m<sub>B</sub> (M = Sr) to 41.9 (4) m<sub>B</sub> (M = Ba). (LaSr)FeNbO<sub>6</sub> has the monoclinic P2<sub>1</sub>/n superstructure with 14% inversion of Fe<sup>2+</sup> (3d<sup>6</sup>) and Nb<sup>5+ </sup>(4d<sup>0</sup>). This material is antiferromagnetic (T<sub>N</sub> = 22 K), and has a ( ½ 0 ½) magnetic propagation vector (m<sub>Fe</sub> = 3.45(3) m<sub>B</sub>). The lattice parameters, antisite disorder, Neel temperatures and resistivities of the (La<sub>1-x</sub>M<sub>1-x</sub>)CoRuO<sub>6</sub> double perovskites are found to vary in a systematic manner. The Co/Ru antisite disorder is minimal for x = 0, showing that this disorder is driven by the mixed Co or Ru electronic states at x ≠ 0 rather than by La/M cation mixing. The x = 0 materials have T<sub>N</sub> = 96K (M = Ca) and T<sub>N</sub> = 85 K (M = Sr), and are variable range hopping semiconductors. Hole doping )x < 0) results in a rapid reduction in the resistivity while the Neel temperature increases only moderately, whereas electron doping (x > 0) results in a significant reduction in T<sub>N</sub> but has little effect on the resistivity. X-ray absorption near edge spectroscopy shows that for x < 0, the Co<sup>2+</sup> states are selectively hope doped while for x > 0 only the Ru<sup>5+</sup> states are electron doped, which is in keeping with the strongly asymmetric doping response.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:596791 |
| Date | January 2005 |
| Creators | Bos, J.-W. G. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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