Contact Details Jenny now works at the National Physical Laboratory National Physical Laboratory Hampton Road Teddington Middlesex TW11 0LW

 

Ph.D. Research

After graduating from Bristol in 2003 I moved to Warwick to start work on cobalt oxide systems. This mainly consisted of single crystal growth (using the IR image furnaces we have here - not as easy as it looks....), sample characterisation (magnetic, heat capacity and resistivity measurements also done with the in-house equipment) and structure determination (x-ray and neutron scattering, models refined using GSAS and/or Fullprof).

My main research interest was Na_xCoO₂ (aka sodium cobalt oxide). It has an interesting magnetic phase diagram; two paramagnetic metallic phases are separated by a completely charge ordered phase at x = 0.5. The samples with higher sodium doping exhibit magnetic ordering below 22 K in the form of a spin density wave (SDW), the moments of which are ordered along the c-axis (see diagram below). At lower temperatures the magnetic ground state is further modified by the appearance of a weak ferromagnetic moment (also along c) and finally a pinning of the SDW onto the underlying Co lattice resulting in a 'glassy' ground state at around 4 K. These samples also show Na ordering transitions at ~340 K; this has been observed directly in a neutron diffraction experiment at the ILL. At the other end of the phase diagram (i.e. lower sodium concentrations) the crystal can superconduct with a critical temperature of 4.5 K provided sufficient water is intercalated between the CoO₂ planes, increasing the two dimensionality in the system. Single crystal samples have been studied using neutron diffraction experiments to ascertain the nature of the water ordering within the Na_xCoO₂.yH₂O superstructure. Click if you would like to know more about the magnetic ordering and here to find out what happens to the magnetism under pressure

I also worked on RBaCo₂O_5+d. It is a cobalt based oxygen deficient perovskite where R is a rare earth; the structure of the compound consists of layers of cobalt oxide that are separated by alternate layers of BaO and RO_d, which in the d=0 phase results in the Co cations sitting in a pyramidal oxygen environment (see figure below). As more oxygen is doped into the system (in the RO_d layer) the oxygen anions and vacant sites order into chains along the a axis so that the Co³⁺ ions have alternate octahedral (CoO₆) and pyramidal (CoO₅) coordination. The resulting crystal field splitting is of similar energy to Hund's coupling constant and consequently there are three possible spin states for the cobalt, which vary as a function of oxidation and temperature. This system has competing AFM-FM phases at temperatures lower than 260 K and a MI transition at 360 K; neutron diffraction experiments designed to look at the magnetic structure were problematic (!) and so x-ray experiments using circular dichroism are planned for the near future.

NaxCoO2	RBaCoO2O5+d
Blue = Co, Red = O, Yellow = Na	Pink = Ba, Black = R, Blue = CoO5(6) octahedra

 

After graduation in December 2006, Jenny moved to London, where she now works as a Research Scientist at the National Physcal Laboratory.

 

 

 

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Thesis Abstract

Superconductivity and magnetism in sodium cobaltate oxyhydrate

Thesis

Superconductivity and magnetism in sodium cobaltate oxyhydrate

Download Ph.D. Thesis 15.6 MB