Synthesis and crystal growth

Abstract:

Advantage is taken of the wealth of experimental data relating to the evolution with temperature of spin states of Co(3+) in LaCoO₃ in order to undertake a detailed investigation of the mechanisms by which changes in electronic structure can influence strain, and elastic and anelastic relaxations in perovskites. The macroscopic strain accompanying changes in the spin state in LaCoO₃ is predominantly a volume strain arising simply from the change in effective ionic radius of the Co(3+) ions. This acts to renormalize the octahedral tilting transition temperature in a manner that is easily understood in terms of coupling between the tilt and spin order parameters. Results from resonant ultrasound spectroscopy at high frequencies (0.1-1.5 MHz) reveal stiffening of the shear modulus which scales qualitatively with a spin order parameter defined in terms of changing Co-O bond lengths. From this finding, in combination with results from dynamic mechanical analysis at low frequencies (0.1-50 Hz) and data from the literature, four distinctive anelastic relaxation mechanisms are identified. The relaxation times of these are displayed on an anelasticity map and are tentatively related to spin-spin relaxation, spin-lattice relaxation, migration of twin walls and migration of magnetic polarons. The effective activation energy for the freezing of twin wall motion below ~590 K at low frequencies was found to be 182 ± 21 kJ mol(-1) (1.9 ± 0.2 eV) which is attributed to pinning by pairs of oxygen vacancies, though the local mechanisms appear to have a spread of relaxation times. It seems inevitable that twin walls due to octahedral tilting must have quite different characteristics from the matrix in terms of local spin configurations of Co(3+). A hysteresis in the elastic properties at high temperatures further emphasizes the importance of oxygen content in controlling the properties of LaCoO₃.

Abstract:

We report on magnetization measurements of La 1.5Sr 0.5NiO 4+δ for δ=0.01 and δ=-0.01. Zero field-cooled and field-cooled bulk magnetization measurements were performed parallel to the Ni-O planes of single crystals, and on a polycrystalline δ=0.01 sample. Striking differences in the magnetization curves are observed between the two doping levels that are identifiable in polycrystalline δ=0.01. The bulk magnetization differences indicate a crossover in the magnetic properties of La 2-x Sr x NiO 4+δ at half doping level, which can be used as a simple non-destructive way to determine whether La 1.5Sr 0.5NiO 4+δ is under or over half doping. © 2010 Springer Science+Business Media, LLC.

Abstract:

The low-energy spin excitations of La2-xSrxNiO 4, x=0.275 and 1/3, have been investigated by unpolarized- and polarized-inelastic neutron scattering from single crystals. A pattern of magnetic diffuse scattering is observed in both compositions and is consistent with quasi-one-dimensional AFM spin correlations along the charge stripes. Analysis of the energy line shape for x=1/3 indicates that the diffuse scattering is inelastic with a characteristic energy of 1.40±0.07 meV. There is no discernible difference between the diffuse scattering from x=0.275 and x=1/3, suggesting that it is an intrinsic property of the charge stripes. © 2011 American Physical Society.

Abstract:

Single crystals of Na0.8-yAyCoO2 (A=Ca, Sr; y=0.10.3) have been grown using the optical floating-zone method for the first time. We have determined the optimum growth conditions to grow the highest quality single crystals. Columnar growth behavior has been observed for the higher doped samples. The magnetic and structural ordering temperatures of the doped samples were found to vary considerably. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

Abstract:

The recent discovery of 'magnetricity' in spin ice raises the question of whether long-lived currents of magnetic 'monopoles' can be created and manipulated by applying magnetic fields. Here we show that they can. By applying a magnetic-field pulse to a Dy 2 Ti 2 O 7 spin-ice crystal at 0.36 K, we create a relaxing magnetic current that lasts for several minutes. We measure the current by means of the electromotive force it induces in a solenoid coupled to a sensitive amplifier, and quantitatively describe it using a chemical kinetic model of point-like charges obeying the Onsager-Wien mechanism of carrier dissociation and recombination. We thus derive the microscopic parameters of monopole motion in spin ice and identify the distinct roles of free and bound magnetic charges. Our results illustrate a basic capacitor effect for magnetic charge and should pave the way for the design and realization of 'magnetronic' circuitry. © 2011 Macmillan Publishers Limited. All rights reserved.