J. F. Mitchell

Structure and superconductivity of HgBa2CuO4+δ

Abstract We have used neutron powder diffraction to investigate the defect structure of HgBa 2 CuO 4+δ . An interstitial oxygen defect in the Hg plane is the primary doping mechanism. A superconducting transition temperature, T c onset , of 95 K is achieved when ≈0.06 oxygen atoms per formula unit are incorporated at this site by annealing the sample at 500°C in pure oxygen. Annealing in argon at 500°C lowers the oxygen content in this site to ≈0.01 and results in a T c of 59 K. The neutron powder diffraction data give evidence for a second defect in the Hg plane which we conclude involves the substitution of copper for about 8% of the mercury and the incorporation of additional oxygen (≈0.1 atoms per formula unit), presumably bonded to the copper defects. In the present samples, the concentration of this defect does not vary with synthesis conditions and its contribution to doping is, therefore, unclear. The structure of the compound is the same at room temperature and superconducting temperatures.

Structural and magnetic chemistry of NdBaCo2O5+δ

Abstract The crystallographic and magnetic structures of the oxygen-deficient perovskites NdBaCo 2 O 5+ δ ( δ =0, 0.38, 0.5, 0.69) have been studied as a function of temperature by neutron powder diffraction. Long-range G-type antiferromagnetic order is realized for all samples apart from that with δ =0.5. The lack of magnetic order for δ =0.5 can be understood on the basis of a crystal-field-induced spin-state ordering between low-spin and high-spin Co 3+ . Contrary to studies of similar materials with smaller lanthanides, the δ =0 material exhibits no evidence of long-range charge ordering. No evidence of a spin-state transition as observed in YBaCo 2 O 5 is found for any of our samples.

Phase separation and low-field bulk magnetic properties of Pr0.7Ca0.3MnO3

We present a detailed magnetic study of the perovskite manganite

Magnetic excitation spectra of Sr2IrO4 probed by resonant inelastic x-ray scattering: establishing links to cuprate superconductors.

{\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}

Competing magnetic interactions in the extended Kagome system YBaCo{sub 4}O{sub 7}

at low temperatures including magnetization and ac susceptibility measurements. The data appear to exclude a conventional spin glass phase at low fields, suggesting instead the presence of correlated ferromagnetic clusters embedded in a charge-ordered matrix. We examine the growth of the ferromagnetic clusters with increasing magnetic field as they expand to occupy almost the entire sample at

Orbital correlations in the pseudocubic O and rhombohedral R phases of LaMnO3.

H\ensuremath{\sim}0.5\mathrm{T}.

Mesoscopic and microscopic phase segregation in manganese perovskites

Since this is well below the field required to induce a metallic state, our results point to the existence of a field-induced ferromagnetic insulating state in this material.

Driving magnetic order in a manganite by ultrafast lattice excitation

Jungho Kim, D. Casa, M. H. Upton, T. Gog, Young-June Kim, J. F. Mitchell, M. van Veenendaal, M. Daghofer, J. van den Brink, G. Khaliullin, B. J. Kim Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7 Material Science Division, Argonne National Laboratory, Argonne, IL 60439, USA Department of Physics, Northern Illinois University, De Kalb, IL 60115, USA Institute for Theoretical Solid Sate Physics, IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany and Max Planck Institute for Solid State Research, Heisenbergstrae 1, D-70569 Stuttgart, Germany (Dated: December 11, 2012)

Nanomagnetic droplets and implications to orbital ordering in La1-xSrxCoO3

YBaCo{sub 4}O{sub 7} belongs to a new class of geometrically frustrated magnets like the pyrochlores, in which Co spins occupy corners of tetrahedra. The structure can be viewed as an alternating stacking of Kagome and triangular layers. Exactly half of the triangular units of the Kagome plane are capped by Co ions to form columns running perpendicular to the Kagome sheets. Neutron powder diffraction reveals a broad temperature range of diffuse magnetic scattering, followed by long-range magnetic ordering below 110 K. A unique low-temperature magnetic structure simultaneously satisfies an S=0 arrangement in the uncapped triangular units and antiferromagnetic coupling along the columns. A spin reorientation above 30 K tracks the relative strengths of the in-plane and out-of-plane interactions.

Magnetotransport of single crystalline YSb

The local and intermediate structure of stoichiometric LaMnO3 has been studied in the pseudocubic and rhombohedral phases at high temperatures (300-1150 K). Neutron powder diffraction data were collected and a combined Rietveld and high real space resolution atomic pair distribution function analysis was carried out. The nature of the Jahn-Teller (JT) transition around 750 K is confirmed to be orbital order to disorder. In the high-temperature orthorhombic (O) and rhombohedral (R) phases, the MnO6 octahedra are still fully distorted locally. More importantly, the intermediate structure suggests the presence of local ordered clusters of diameter approximately 16 A ( approximately 4 MnO6 octahedra) implying strong nearest-neighbor JT antiferrodistortive coupling. These clusters persist well above the JT transition temperature even into the high-temperature R phase.