Origin of Electric Field Induced Magnetization in Multiferroic HoMnO3

We have performed polarized and unpolarized small angle neutron scattering experiments on single crystals of HoMnO3 and have found that an increase in magnetic scattering at low momentum transfers begins upon cooling through temperatures close to the spin reorientation transition at TSR ~ 40 K. We attribute the increase to an uncompensated magnetization arising within antiferromagnetic domain walls. Polarized neutron scattering experiments performed while applying an electric field show that the field suppresses magnetic scattering below T ~ 50 K, indicating that the electric field affects the magnetization via the antiferromagnetic domain walls rather than through a change to the bulk magnetic order

Multiferroic behavior in the new double-perovskite Lu2_2MnCoO6_6

We present a new member of the multiferroic oxides, Lu$_2$MnCoO$_6$, which we have investigated using X-ray diffraction, neutron diffraction, specific heat, magnetization, electric polarization, and dielectric constant measurements. This material possesses an electric polarization strongly coupled to a net magnetization below 35 K, despite the antiferromagnetic ordering of the $S = 3/2$ Mn$^{4+}$ and Co$^{2+}$ spins in an $\uparrow \uparrow \downarrow \downarrow$ configuration along the c-direction. We discuss the magnetic order in terms of a condensation of domain boundaries between $\uparrow \uparrow$ and $\downarrow \downarrow$ ferromagnetic domains, with each domain boundary producing a net electric polarization due to spatial inversion symmetry breaking. In an applied magnetic field the domain boundaries slide, controlling the size of the net magnetization, electric polarization, and magnetoelectric coupling

Spatial analysis of fluoride concentrations in drinking water and population at risk in Namibia

© 2017, South African Water Research Commission. All rights reserved. Namibia, the driest country in sub-Saharan Africa, is largely reliant on groundwater for its potable water demand and groundwater is a major source of naturally-occurring fluoride. This study assessed the spatial distribution of fluoride in potable water and appraised the population at risk for high fluoride intake. Analysis of fluoride levels used existing databases that include 28 000 borehole locations across the country, while the population data were based on the 2011 Census. Spatial analysis and spatial statistics methods employed included Moran’s I, local indicators of spatial association (LISA), basic Euclidian distance, analysis of variance (ANOVA) and spatial overlay in a GIS environment. Fluoride concentrations above the recommended limit of 1.5 mg/L occur in a fifth of the boreholes and as much as 8% of the population across the country is at risk. Although the number of people in Namibia who are exposed to high fluoride is relatively small at a global scale, it is significant at a national level. Preventative measures against high fluoride intake are thus necessary in the country

Detecting grave sites from surface anomalies: A longitudinal study in an Australian woodland.

Forensic investigations of single and mass graves often use surface anomalies, including changes to soil and vegetation conditions, to identify potential grave locations. Though numerous resources describe surface anomalies in grave detection, few studies formally investigate the rate at which the surface anomalies return to a natural state; hence, the period the grave is detectable to observers. Understanding these processes can provide guidance as to when ground searches will be an effective strategy for locating graves. We studied three experimental graves and control plots in woodland at the Australian Facility for Taphonomic Experimental Research (Sydney, Australia) to monitor the rate at which surface anomalies change following disturbance. After three years, vegetation cover on all grave sites and control plots had steadily increased but remained substantially less than undisturbed surroundings. Soil anomalies (depressions and cracking) were more pronounced at larger grave sites versus the smaller grave and controls, with leaf litterfall rendering smaller graves difficult to detect beyond 20 months. Similar results were observed in two concurrent burial studies, except where accelerated revegetation appeared to be influenced by mummified remains. Extreme weather events such as heatwaves and heavy rainfall may prolong the detection window for grave sites by hindering vegetation establishment. Observation of grave-indicator vegetation, which exhibited abnormally strong growth 10 months after commencement, suggests that different surface anomalies may have different detection windows. Our findings are environment-specific, but the concepts are applicable globally

Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)

We have studied the low temperature a.c. magnetic susceptibility of the diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges for x > 0.4 and persists to x = 1.98. This behavior can be understood as a non-monotonic dependence of the quantum spin relaxation time with dilution. The results suggest that the observed spin freezing is fundamentally a single spin process which is affected by the local environment, rather than the development of spin-spin correlations as earlier data suggested.Comment: 26 pages, 9 figure

Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the a.c. magnetic susceptibility. While the characteristic spin relaxation time is thermally activated at high temperatures, it becomes almost temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling dominates the relaxation process below that temperature. As the low-entropy spin ice state develops below Tice ~ 4 K, the spin relaxation time increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become highly correlated

Geometrical Magnetic Frustration in Rare Earth Chalcogenide Spinels

We have characterized the magnetic and structural properties of the CdLn2Se4 (Ln = Dy, Ho), and CdLn2S4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all compounds to be normal spinels, possessing a geometrically frustrated sublattice of lanthanide atoms with no observable structural disorder. Fits to the high temperature magnetic susceptibilities indicate these materials to have effective antiferromagnetic interactions, with Curie-Weiss temperatures theta ~ -10 K, except CdYb2S4 for which theta ~ -40 K. The absence of magnetic long range order or glassiness above T = 1.8 K strongly suggests that these materials are a new venue in which to study the effects of strong geometrical frustration, potentially as rich in new physical phenomena as that of the pyrochlore oxides.Comment: 17 pages, 5 figures, submitted to Phys Rev B; added acknowledgement