Magnetization of La2-x Srx Ni O4+δ (0≤x≤0.5): Spin-glass and memory effects
Physical Review B - Condensed Matter and Materials Physics 73:1 (2006)
Abstract:
We have studied the magnetization of a series of spin-charge-ordered La2-x Srx Ni O4+δ single crystals with 0≤x≤0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature TF1 ∼50 K and a smaller irreversibility that persists up to near the charge-ordering temperature. We observed memory effects in the thermoremnant magnetization across the entire doping range. We found that these materials retain a memory of the temperature at which an external field was removed and that there is a pronounced increase in the thermoremnant magnetization when the system is warmed through a spin reorientation transition. © 2006 The American Physical Society.Oxide muonics: I. Modelling the electrical activity of hydrogen in semiconducting oxides
Journal of Physics Condensed Matter 18:3 (2006) 1061-1078
Abstract:
A shallow-to-deep instability of hydrogen defect centres in narrow-gap oxide semiconductors is revealed by a study of the electronic structure and electrical activity of their muonium counterparts, a methodology that we term 'muonics'. In CdO, Ag2O and Cu2O, paramagnetic muonium centres show varying degrees of delocalization of the singly occupied orbital, their hyperfine constants spanning 4 orders of magnitude. PbO and RuO 2, on the other hand, show only electronically diamagnetic muon states, mimicking those of interstitial protons. Muonium in CdO shows shallow-donor behaviour, dissociating between 50 and 150 K; the effective ionization energy of 0.1 eV is at some variance with the effective-mass model but illustrates the possibility of hydrogen doping, inducing n-type conductivity as in the wider-gap oxide, ZnO. For Ag2O, the principal donor level is deeper (0.25 eV) but ionization is nonetheless complete by room temperature. Striking examples of level-crossing and RF resonance spectroscopy reveal a more complex interplay of several metastable states in this case. In Cu2O, muonium has quasi-atomic character and is stable to 600 K, although the electron orbital is substantially more delocalized than in the trapped-atom states known in certain wide-gap dielectric oxides. Its eventual disappearance towards 900 K, with an effective ionization energy of 1 eV, defines an electrically active level near mid-gap in this material. © 2006 IOP Publishing Ltd.On the ordering of Na+ ions in NaxCoO2
AIP CONF PROC 850 (2006) 1213-1214
Abstract:
The influence of electrostatic interactions on the ordering of sodium ions in NaxCoO2 is studied theoretically through Monte-Carlo simulations. For large x small di- or tri-vacancy clusters are stable with respect to isolated Na vacancies. At commensurate fillings these small clusters order in triangular superstructures. These results agree with recent electron diffraction data at x = 1/2 and 3/4. We have performed neutron Laue diffraction experiments at higher x, which confirm the predictions of this simple model. The consequences on the properties of the electronic charges in the Co layers are discussed.Spin dynamics of half-doped La3/2 Sr1/2 Ni O4
Physical Review B - Condensed Matter and Materials Physics 71:17 (2005)
Abstract:
We report polarized- and unpolarized-neutron inelastic-scattering measurements of the magnetic excitation spectrum in the spin-charge ordered phase of La3/2 Sr1/2 Ni O4. Up to energies of ∼30 meV we observe broad magnetic modes characteristic of a near checkerboard ordering. A linear spin-wave model for an ideal checkerboard ordering with a single antiferromagnetic exchange interaction J′ =5.8±0.5 meV between next-nearest-neighbor spins on Ni2+ sites, together with a small XY -like single-ion anisotropy, provides a reasonable description of the measured dispersion. Above 30 meV the excitations are not fully consistent with the linear spin-wave model, with modes near the two-dimensional reciprocal space wave vector (0.5, 0.5) having an anomalously large intensity. Furthermore, two additional dispersive modes not predicted by spin-wave theory were observed, both of which are probably magnetic. One disperses away from (0.5, 0.5) in the energy range between 50-56 meV, and the other appears around (h,k) type positions (h,k=integer) in the energy range 31-39 meV. We propose a model in which these anomalous features are explained by the existence of discommensurations in the checkerboard ordering. At low energies there is additional diffuse scattering centred on the magnetic ordering wave vector. We associate this diffuse scattering with dynamic antiferromagnetic correlations between spins attached to the doped holes. © 2005 The American Physical Society.Direct observation of orbital ordering in layered manganites
J SUPERCOND 18:5-6 (2005) 687-691