ABSTRACT
A previously unreported tetragonal phase has been discovered in a epitaxially strained GdMnO3 thin films deposited on (001)-oriented SrTiO3 substrates by radio frequency (RF) magnetron sputtering. The tetragonal axis of the films grown up to a 35 nm thickness is perpendicular to the film surface and the basal lattice parameters are imposed by the cubic structure of the substrate. Furthermore, the emergence of a spontaneous electric polarization below ~32 K points to the stabilization of an improper ferroelectric phase at low temperatures, which is not observed in bulk GdMnO3. This work shows how strain engineering can be used to tailor the structure and properties of strongly correlated oxides.
ABSTRACT
This study provides a comprehensive insight into the effects of controlled off-stoichiometry on the structural and multiferroic properties of the hexagonal manganite LuMn1-xO3+δ (x = 0.02; δ â¼ 0), supported by neutron powder diffraction measurements confirming single phase P63cm symmetry and evidencing a relevant ferromagnetic component, below TN â¼ 90 K, which breaks the archetypal geometrically frustrated antiferromagnetic state typically ascribed to LuMnO3. The perturbations in the triangular disposition of spins prompt an additional electric polarization contribution and a clear enhancement of the magnetoelectric coupling which are in good agreement with the results of first principles calculations. In addition, Raman spectroscopy, dielectric permittivity, pyroelectric current and magnetic measurements as a function of temperature point out the precursor effects of the magnetic phase transitions involving a strong coupling between spins, lattice and electric order, even above the Néel temperature.
ABSTRACT
This study explores controlled off-stoichiometric LuMn1+zO3+δ (|z| < 0.1) compounds, intended to retain the utter LuMnO3 intrinsic hexagonal symmetry and ferroelectric properties. X-ray powder diffraction measurements evidenced a single phase P63cm structure. Thermo-gravimetric experiments show a narrow impact of oxygen vacancies while a distinguishable gas exchange at â¼700 K, a surprisingly lower temperature when compared to perovskite systems. A comparison of different nominal ceramics revealed pertinent structural and magnetic property variations owing to subtle self-doping effects. Deviations from the archetypal antiferromagnetic state were detected below â¼90 K suggesting local rearrangements of the nominal Mn(3+) ions matrix, breaking the ideal geometrical spin frustration, leading to a non-compensated magnetic structure.
ABSTRACT
The upwelling systems of the eastern boundaries of the world's oceans are susceptible to harmful algal blooms (HABs) because they are highly productive, nutrient-rich environments, prone to high-biomass blooms. This review identifies those aspects of the physical environment important in the development of HABs in upwelling systems through description and comparison of bloom events in the Benguela, California and Iberia systems. HAB development is dictated by the influence of wind stress on the surface boundary layer through a combination of its influence on surface mixed-layer characteristics and shelf circulation patterns. The timing of HABs is controlled by windstress fluctuations and buoyancy inputs at the seasonal, event and interannual scales. Within this temporal framework, various mesoscale features that interrupt typical upwelling circulation patterns, determine the spatial distribution of HABs. The inner shelf in particular provides a mosaic of shifting habitats, some of which favour HABs. Changes in coastline configuration and orientation, and bottom topography are important in determining the distribution of HABs through their influence on water stratification and retention. A spectrum of coastline configurations, including headlands, capes, peninsulas, Rías, bays and estuaries, representing systems of increasing isolation from the open coast and consequent increasing retention times, are assessed in terms of their vulnerability to HABs.
