Magnetic Phases of Rare Earth Hexagonal Manganites

We describe the magnetic phases of hexagonal rare earth manganites RMnO3 using Landau theory. A minimal model based on four one-dimensional magnetic order parameters is developed.Comment: 2 Pages, Proceedings of SCES'0

Mining environments: the good, the bad, and the ugly

Since the first scientific observations on mining environments over 400 years ago, we have gained some phenomenal knowledge on mine, land and waterways degradation and related environmental protection issues. Yet today, we continue to be faced with numerous challenges, including the recurring failure of mine waste repositories, the unconstrained production of acid rock drainage and the widespread dispersal of contaminants from mine sites into the environment. More than ever, environmental scientists have important contributions to make as they provide the data necessary for rational decision-making in critical areas such as resource development, environmental protection, waste management and remediation, as well as mine, land and waterway rehabilitation. The most urgent problem facing environmental scientists working on mining environments is the quantification of the interactions that control the distribution of contaminants in rocks, soils, sediments, waters and biota. We must precisely describe the chemistry and mineralogy of contaminants and understand their long-term behaviour. We need to drastically improve our scientific efforts to explain environmental processes at mine sites on all scales, including micro and macro scales as well as in 3-D and 4-D. In addition, we must improve our predictions on mine drainage, aquifer and final void water quality. While the rehabilitation of many mine sites and waste repositories is pursued by using best practices, we must continue to search for innovative, cost-effective remediation technologies and sustainable rehabilitation practices. Evaluations of recently rehabilitated mine sites could produce data on the successes and failures of rehabilitation efforts. Such studies should sharpen our ideas on the factors leading to contaminant dispersal and the development of new remediation technologies. The rehabilitation of mine sites and secure disposal of mine wastes require a new precision in the total description of mine sites and an understanding whether our current rehabilitation practices are sustainable in the long term. There is reason for optimism that the required progress is possible. Such optimism is based on the phenomenal advances in our ability to observe and describe mining environments. However, detailed studies of natural, mined, contaminated and rehabilitated environments are necessary if we are to quantify the variables controlling the containment and dispersal of contaminants and if we are to develop innovative remediation protocols. Our efforts could ensure that the 21st century goes down in history as that of “green technologies”

Biogeochemistry of inundated actual acid sulfate soils, Cairns, Australia

Tidal exchange is used to rehabilitate actual acid sulfate soils at East Trinity, Cairns, Australia, The aims of this study were to evaluate the biogeochemistry of inundated actual acid sulfate soils and to establish the uptake and exclusion of environmentally significant elements by plants colonising such soils. The survey was designed not only to test different native plant species, but also to assess different plant tissue, such as roots and the above-ground biomass. The chosen site was ideal for this research as the East Trinity site has undergone recent inundation with seawater. Hence, the selected site was particularly suitable for establishing the transfer of environmentally significant elements during the remediation process. The biogeochemical analysis indicate pronounced enrichment of Al and lesser concentrations of As, Co, Cr, Cu, Pb and Zn in the tissue of the mangrove fern Acrostichum speciosum and the grass species Paspalum vaginatum. In particular, the uptake of Al, As, Co, Cu and Zn by Paspalum vaginatum and Acrostichum speciosum increase linearly with EDTA- extractable soil metal concentrations. In comparison to background samples, the roots ans tems of Acrostichum speciousum and of Paspalum vaginatum display higher Co, Cr and Zn and higher Cu and Zn concentrations, respectively. In general the two plant species growing on the inundated soils have translocation factors (TF, metal concentration ratio of plant foliage to roots) for all elements less than one. These plants growing in inundated soils acquire higher metal concentrations despite their tendency to exclude metals from their biomass. Thus, the applied remediation technique at East Trinity promotes the transfer of environmentally significant elements (Co, Cd, Cu, Zn) into local plant species. Also, the Al concentrations in roots and stems of Paspalum vaginatum from inundated soils and background sites are distinctly elevated. Such Al concentrations exceed NRC (1980) animal feed guidelines, indicating that this plant species, prevalent in coast mangrove grasslands, poses a toxicity threat to farmed animals

Environmental review of the Radium Hill mine site, South Australia

The Radium Hill uranium deposit, in semi-arid eastern South Australia, was discovered in 1906 and mined for radium between 1906 and 1931 and for uranium between 1954 and 1961 (production of 969,300 t of davidite ore averaging 0.12 % U3O8). Rehabilitation was limited to removal of mine facilities, sealing of underground workings and capping of selected waste repositories. In 2002, gamma-ray data, plus tailings, uncrushed and crushed waste rock, stream sediment, topsoil and vegetation samples were collected to assist in the examination of the current environmental status of the mine site. The preliminary data indicate that capping of tailings storage facilities did not ensure the long-term containment of the low-level radioactive wastes due to the erosion of sides of the impoundments. Moreover, active wind erosion of waste fines from various, physically unstable waste repositories causes increasing radiochemical (up to 0.94 μSv/h) and geochemical (Ce, La, Sc, Th, U, V, Y) impacts on local soils and sediments. However, measured radiation levels of soils and sediments are at or below Australian Radiation Protection Standards (20 mSv/a averaged over five consecutive years). Additional capping and landform design of the crushed waste and tailings repositories are required in order to minimise erosion and impacts on surrounding soils and sediments

A rigorous formulation of the cosmological Newtonian limit without averaging

We prove the existence of a large class of one-parameter families of cosmological solutions to the Einstein-Euler equations that have a Newtonian limit. This class includes solutions that represent a finite, but otherwise arbitrary, number of compact fluid bodies. These solutions provide exact cosmological models that admit Newtonian limits but, are not, either implicitly or explicitly, averaged

Incompatible Magnetic Order in Multiferroic Hexagonal DyMnO3

Magnetic order of the manganese and rare-earth lattices according to different symmetry representations is observed in multiferroic hexagonal (h-) DyMnO$_3$ by optical second harmonic generation and neutron diffraction. The incompatibility reveals that the 3d-4f coupling in the h-$R$MnO$_3$ system ($R$ = Sc, Y, In, Dy - Lu) is substantially less developed than commonly expected. As a consequence, magnetoelectric coupling effects in this type of split-order parameter multiferroic that were previously assigned to a pronounced 3d-4f coupling have now to be scrutinized with respect to their origin

First-order multi-k phase transitions and magnetoelectric effects in multiferroic Co3TeO6

A theoretical description of the sequence of magnetic phases in Co3TeO6 is presented. The strongly first-order character of the transition to the commensurate multiferroic ground state, induced by coupled order parameters corresponding to different wavevectors, is related to a large magnetoelastic effect with an exchange energy critically sensitive to the interatomic spacing. The monoclinic magnetic symmetry C2' of the multiferroic phase permits spontaneous polarization and magnetization as well as the linear magnetoelectric effect. The existence of weakly ferromagnetic domains is verified experimentally by second harmonic generation measurements

Phase transition at finite temperature in one dimension: Adsorbate ordering in Ba/Si(111)3x2

We demonstrate that the Ba-induced Si(111)3x2 reconstruction is a physical realization of a one-dimensional antiferromagnetic Ising model with long-range Coulomb interactions. Monte Carlo simulations performed on a corresponding Coulomb-gas model, which we construct based on density-functional calculations, reveal an adsorbate-ordering phase transition at finite temperature. We show numerically that this unusual one-dimensional phase transition should be detectable by low-energy electron diffraction.Comment: 11 pages + 4 figures. Surf. Sci. Lett. (in press