arXiv subjects

A. T. Boothroyd

Publications and source records attributed to A. T. Boothroyd.

At least 109 records · Page 6Linked to original sources

arXiv2005

On the Ordering of Na$^+$ Ions in Na$_x$CoO$_2$

The influence of electrostatic interactions on the ordering of sodium ions in Na$_x$CoO$_2$ 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 Laüe 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.

cond-mat.str-el↗

arXiv2005

Spin gaps and magnetic structure of NaxCoO2

We present two experiments that provide information on spin anisotropy and the magnetic structure of NaxCoO2. First, we report low-energy neutron inelastic scattering measurements of the zone-center magnetic excitations in the magnetically ordered phase of Na0.75CoO2. The energy spectra suggest the existence of two gaps, and are very well fitted by a spin-wave model with both in-plane and out-of-plane anisotropy terms. The gap energies decrease with increasing temperature and both gaps are found to have closed when the temperature exceeds the magnetic ordering temperature T_m~22 K. Secondly, we present neutron diffraction studies of Na0.85CoO2 with a magnetic field applied approximately parallel to the c axis. For fields in excess of ~8T a magnetic Bragg peak was observed at the (0,0,3) position in reciprocal space. We interpret this as a spin-flop transition of the A-type antiferromagnetic structure, and we show that the spin-flop field is consistent with the size of the anisotropy gap.

cond-mat.str-el↗

arXiv2005

Magnetization of La(2-x)Sr(x)NiO(4+ delta) (0 < x < 0.5) and observation of novel memory effects

We have studied the magnetization of a series of spin-charge ordered La(2-x)Sr(x)NiO(4+delta) single crystals with 0 < x < 0.5. For fields applied parallel to the ab plane there is a large irreversibility below a temperature T(F1) ~ 50 K and a smaller irreversibility that persists up to near the charge ordering temperature. We observed a novel memory effect in the thermo-remnant 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 thermo-remnant magnetization when the system is warmed through a spin reorientation transition.

cond-mat.str-el↗

arXiv2005

High-energy Cu spin excitations in PrBa2Cu3O6+x

This paper describes high-energy neutron inelastic scattering measurements of propagating magnetic excitations in PrBa2Cu3O6+x (x approx 0.2 and 0.93). The measurements probe the acoustic and optic modes of the antiferromagnetically-ordered copper--oxygen bilayers in the energy range 50-150 meV. The observed magnon dispersion can be described satisfactorily in this energy range by a spin wave model including intra- and inter-layer nearest-neighbour exchange constants J_parallel and J_perp. We find J_parallel = 127 +/- 10 meV and J_perp = 5.5 +/- 0.9 meV. The value of J_parallel is virtually the same as that found in YBa2Cu3O6.2, but J_perp is a factor of two smaller. To within experimental error the values of J_parallel and J_perp for PrBa2Cu3O6+x do not vary with oxygen doping.

cond-mat.str-el↗

arXiv2005

Infrared Conductivity and Charge Ordering in NaCoO2

The infrared conductivity of NaxCoO2 is studied as a function of doping and temperature for x between 0.5 and 1. Charge localization in CoO2 layers shows up through a far-infrared peak (FIP) in the infrared conductivity which coexists with a small Drude contribution. Long-range ordering at x = 0.5 is confirmed to create a far-infrared gap, in addition to the FIP. At high x, the formation of a Spin-Density Wave reported below 22 K dramatically shifts the FIP to higher energy when x is incommensurate with the lattice, indicating an abrupt deepening of the localizing potential. The in-plane E1u phonon lifetime is shown to be sensitive to both "freezing" and ordering of the mobile Na+ ions. A comparison with the behavior of the FIP shows that such "freezing" is not the only origin of charge localization in the CoO layers.

cond-mat.str-el↗

arXiv2004

Orbital Bi-Stripes in High Doped Bilayer Manganites

We present high resolution high energy and resonant x-ray diffraction results from La2-2xSr1+2xMn2O7 for x=0.55,0.575 and 0.60. These compounds show superlattice reflections at wavevectors of (h+-δ,k+-δ,l) and (h+-2δ,k+-2δ,l), arising from orbital ordering with associated Jahn-Teller distortions and charge ordering respectively. We observe a phase transition between the x=0.55 and x=0.575 doping levels. Samples with x=0.55 display structural characteristics similar to those previously reported for x=0.5. Compared to this, the long range order in samples with x=0.55 and x=0.6 have a distinct change in wavevector and correlation. We attribute this to a new orbital bi-stripe phase, accompanied by weak, frustrated, charge ordering. The observed azimuthal dependence of the orbital order reflections supports the model proposed for this new phase.

cond-mat.str-el↗

arXiv2004

Resonant Soft X-ray Scattering Investigation of Orbital and Magnetic Ordering in $\rm La_{0.5}Sr_{1.5}MnO_4$

We report resonant x-ray scattering data of the orbital and magnetic ordering at low temperatures at the Mn $L_{\rm 2,3}$ edges in \lsmo. The orderings display complex energy features close to the Mn absorption edges. Systematic modeling with atomic multiplet crystal field calculations was used to extract meaningful information regarding the interplay of spin, orbital and Jahn-Teller order. These calculations provide a good general agreement with the observed energy dependence of the scattered intensity {for a} dominant orbital ordering of the $ d_{x^2 - z^2} / d_{y^2 - z^2} $ type. In addition, the origins of various spectral features are identified. The temperature dependence of the orbital and magnetic ordering was measured and displays a strong interplay between the magnetic and orbital order parameters.

cond-mat.str-el↗

arXiv2004

Spin Dynamics of Half-Doped La3/2Sr1/2NiO4

We report polarized- and unpolarized- neutron inelastic scattering measurements of the magnetic excitation spectrum in the spin-charge ordered phase of La3/2Sr1/2NiO4. 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-neighbour 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 wavevector (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 wavevector. We associate this diffuse scattering with dynamic antiferromagnetic correlations between spins attached to the doped holes.

cond-mat.str-el↗

arXiv2004

Three-Dimensional Spin Fluctuations in Na0.75CoO2

We report polarized- and unpolarized-neutron scattering measurements of magnetic excitations in single-crystal Na0.75CoO2. The data confirm ferromagnetic correlations within the cobalt-layers and reveal antiferromagnetic correlations perpendicular to the layers, consistent with an A-type antiferromagnetic ordering. The magnetic modes propagating perpendicular to the layers are sharp, and reach a maximum energy of ~12 meV. From a minimal spin wave model, containing only nearest-neighbour Heisenberg exchange interactions, we estimate the inter- and intra-layer exchange constants to be 12.2 +/- 0.5 meV and -6 +/- 2 meV, respectively. We conclude that the magnetic fluctuations in Na0.75CoO2 are highly three-dimensional.

cond-mat.str-el↗

arXiv2004

Cooperative Jahn-Teller Distortion in PrO2

We report neutron diffraction data on single crystal PrO2 which reveal a cooperative Jahn-Teller distortion at TD = 120 +/- 2 K. Below this temperature an internal distortion of the oxygen sublattice causes the unit cell of the crystallographic structure to become doubled along one crystal axis. We discuss several possible models for this structure. The antiferromagnetic structure below TN = 13.5 K is found to consist of two components, one of which shares the same doubled unit cell as the distorted crystallographic structure. We also present measurements of the magnetic susceptibility, the specific heat capacity and the electrical conductivity of PrO2. The susceptibility data show an anomaly at a temperature close to TD. From the specific heat capacity data we deduce that the ground state is doubly degenerate, consistent with a distortion of the cubic local symmetry. We discuss possible mechanisms for this. The conductivity shows an activated behaviour with an activation energy Ea = 0.262 +/- 0.003 eV.

cond-mat.mtrl-sci↗

arXiv2004

Field-Induced Magnetic and Structural Domain Alignment in PrO2

We present a neutron diffraction study of the magnetic structure of single crystal PrO2 under applied fields of 0-6 T. As the field is increased, changes are observed in the magnetic Bragg intensities. These changes are found to be irreversible when the field is reduced, but the original intensities can be recovered by heating to T > 122 K, then re-cooling in zero field. The antiferromagnetic ordering temperature TN = 13.5 K and the magnetic periodicity are unaffected by the applied field. We also report measurements of the magnetic susceptibility of single crystal PrO2 under applied fields of 0-7 T. These show strong anisotropy, as well as an anomaly at T = 122 +/- 2 K which coincides with the temperature TD = 120 +/- 2 K at which a structural distortion occurs. For fields applied along the [100] direction the susceptibility increases irreversibly with field in the temperature range TN < T < TD. However, for fields along [110] the susceptibility is independent of field in this range. We propose structural domain alignment, which strongly influences the formation of magnetic domains below TN, as the mechanism behind these changes.

cond-mat.str-el↗

arXiv2004

Ferromagnetic in-plane spin fluctuations in NaxCoO2 observed by neutron inelastic scattering

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies (~10 meV), which is localized in wavevector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.

cond-mat.str-el↗

arXiv2004

Phonon Dispersion Relations in PrBa2Cu3O6+x (x ~ 0.2)

We report measurements of the phonon dispersion relations in non-superconducting, oxygen-deficient PrBa2Cu3O6+x (x ~ 0.2) by inelastic neutron scattering. The data are compared with a model of the lattice dynamics based on a common interaction potential. Good agreement is achieved for all but two phonon branches, which are significantly softer than predicted. These modes are found to arise predominantly from motion of the oxygen ions in the CuO2 planes. Analogous modes in YBa2Cu3O6 are well described by the common interaction potential model.

cond-mat.supr-con↗

arXiv2004

Stripe Order and Magnetic Transitions in La(2-x)Sr(x)NiO(4)

Magnetic order has been investigated in stripe-ordered La(2-x)Sr(x)NiO(4) (x = 0.275, 0.37, 0.4) by d.c. magnetization and by polarized- and unpolarized-neutron diffraction. In the magnetically ordered phase, all three compositions exhibit a magnetic transition consistent with a spin reorientation in the ab plane. For x = 0.37, the spin axes rotate from an angle of 37.7 +/- 0.3 degrees to the stripe direction at 71 K, to 52.3 +/- 0.2 degrees at 2 K. The x = 0.275 and 0.4 compounds were found to undergo a similar spin reorientation. A spin reorientation has now been observed to occur for five different doping levels in the range x = 0.275 - 0.5, suggesting that this spin transition is an intrinsic property of the stripe phase.

cond-mat.str-el↗

arXiv2003

Magnetic Studies of Polycrystalline and Single-Crystal NaxCoO2

We have investigated the preparation and bulk magnetic properties of NaxCoO2 in the range 0.5<x<0.95, including those for single crystals grown by the floating-zone method in an image furnace. To characterize the samples and ascertain their quality we performed x-ray powder diffraction, electron-probe microanalysis (EPMA) and magnetization measurements. Based on our findings we are able to report conditions for the preparation of single-phase polycrystalline and single-crystal samples in the range 0.65<x<0.95. For lower concentration samples, however, i.e. x<0.65, residual Co3O4 impurities above the detectable limit remained, and conditions to eliminate this have still to be identified. Investigations of the anisotropic magnetization of NaxCoO2 single crystals show: (i) transitions at ~320 K, ~275K and ~22 K present for all Na doping levels investigated, (ii) a strong increase in magnetization below ~8 K, (iii) a crossover in sense for the anisotropy around 8-10 K, and (iv) a hysteresis in the magnetization for certain Na doping levels at temperatures below ~15 K. Notably, neither of the high temperature anomalies (275 K, 320 K) could be observed in the powder samples.

cond-mat.mtrl-sci↗

arXiv2003

Magnetization of Charge-ordered la(2-x)sr(x)nio(4+delta)

We report magnetization measurements on La(2-x)Sr(x)NiO(4+ delta) single crystals, with 0 < x < 0.5. Glassy behaviour associated with the formation of spin-charge stripes, and a separate spin-glass phase at low temperatures were observed. We have also found a `memory effect' in the magnetic field -- temperature history, which is found to be suppressed in the low temperature spin state of the x = 0.33 crystal.

cond-mat.str-el↗

arXiv2003

Magnetic excitations in La$_{1.5}$Sr$_{0.5}$CoO$_4$

We report magnetization and neutron scattering measurements of the half-doped compound La$_{1.5}$Sr$_{0.5}$CoO$_4$, which exhibits a checkerboard pattern of charge ordering below ~800K. In the antiferromagnetically-ordered phase below \~40K the spins are found to be canted in the ab plane. The spin excitation spectrum includes spin-wave excitations with a maximum energy of 16 meV, and diffuse magnetic modes at energies around 30 meV.

cond-mat.str-el↗

arXiv2003

Spin correlations among the charge carriers in an ordered stripe phase

We have observed a diffuse component to the low-energy magnetic excitation spectrum of stripe-ordered La5/3Sr1/3NiO4 probed by neutron inelastic scattering. The diffuse scattering forms a square pattern with sides parallel and perpendicular to the stripe directions. The signal is dispersive, with a maximum energy of ~10 meV. Probed at 2 meV the scattering decreases in strength with increasing temperature, and is barely visible at 100 K. We argue that the signal originates from dynamic, quasi- one-dimensional, antiferromagnetic correlations among the stripe electrons.

cond-mat.str-el↗