Jason Gardner joined the Group in 1992 after completing his first degree here at Warwick and taking an M.Sc. at Keele University.
He began working on the magnetic, and structural properties of manganese oxide materials. In his second year he extended his work to include the study of the magnetic borocarbide superconductorsAfter graduating, Jason has worked in neutron science at a number of locations in North America including Los Alamos, Chalk River, the National Institute of Standards and Technology, Center for Neutron Research in Maryland. He lead the National Science Council of Taiwan's team of instrument scientists and engineers. and worked at Songshan Lake Materials Laboratory in China. Jason is currently based in the Materials Division at the Oak Ridge National Laboratory.
Ph.D. thesis summary
The thesis describes the magnetic properties of materials associated to the high temperature superconductors. Three compounds proposed as possible substrates (Sr2RuO4 , and SrRuO3 and NdGaO3) for the epitaxial growth of high Tc thin films and two families of materials (La2-xSrxNiO4 and La2-xSrxCoO4 ) that are isostructural to La2-xSrxCuO4 have been examined.
Single phase polycrystalline samples were prepared by the normal solid state diffusion process, but due to the anisotropic properties of these materials single crystals were required. Single crystals of La2-xSrxNiO4 and La2-xSrxCoO4 were grown by the floating zone method in an infra-red (IR) image furnace.
La2-xSrxNiO4 and La2-xSrxCoO4 have been investigated by room temperature X-ray diffraction a.c. and d.c. susceptibility, muon spin relaxation and neutron diffraction. Two magnetic transitions were observed in La2-xSrxCoO4 samples with up to 15% strontium doping. Evidence has been shown for a similar sucession of magnetic transitions in the lightly doped nickelates.
Neutron powder diffraction experiments were performed on two of the proposed high Tc substrates Sr2RuO4 and SrRuO3. Results showed that the lattice mismatch of Sr2RuO4 (0.15%) and SrRuO3 (0.2%) to YBa2 Cu3O7 is better than that of SrTiO3 (1.16%) a common substrate.
Inelastic neutron scattering and magnetic susceptibility measurments have been used to determine the crystalline electric field Hamiltonian for NdGaO3. A set of crystal field parameters have been derived that adequately explain the crystal field spectrum and susceptibility results of NdGaO3
Publications resulting from Ph.D. work
The Effect of Magnetic-Ordering On the Crystal-Field Levels of ErNi2B2C, J. S. Gardner, et al., Physica B 213, 136-138 (1995).
Powder Neutron Powder Diffraction Studies of Sr2RuO4 and SrRuO3, J. S. Gardner, G. Balakrishnan, D. Mc K. Paul, Physica C 252, 303-307 (1995).
Neutron diffraction and magnetisation studies of Sr2RuO4 below 2 K, J. S. Gardner, G. Balakrishnan, D. Mc K. Paul, C. Haworth, Physica C 265, 251-257 (1996).
Neutron diffraction study of a La2CoO4.15, Single Crystal, J. S. Gardner, D. M. Paul, B. Lebech, Physica B 234, 721 (1997).
gardnerjs at ornl dot gov