Although the magnetoelectric effects - the mutual control of electric polarization by magnetic fields and magnetism by electric fields, have been intensively studied in a large number of inorganic compounds and heterostructures, they have been rarely observed in organic materials. Here we demonstrate magnetoelectric coupling in a metal-organic framework [(CH3)2NH2]Mn(HCOO)3 which exhibits an order-disorder type of ferroelectricity below 185 K. The magnetic susceptibility starts to deviate from the Curie-Weiss law at the paraelectric-ferroelectric transition temperature, suggesting an enhancement of short-range magnetic correlation in the ferroelectric state. Electron spin resonance study further confirms that the magnetic state indeed changes following the ferroelectric phase transition. Inversely, the ferroelectric polarization can be improved by applying high magnetic fields. We interpret the magnetoelectric coupling in the paramagnetic state in the metal-organic framework as a consequence of the magnetoelastic effect that modifies both the superexchange interaction and the hydrogen bonding.
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary collages in the world, and records a prolonged sequence of subduction‐accretion and collision processes. The Tarim Craton is located at the southernmost margin of the CAOB. In this study, the discovery of early Palaeozoic high‐pressure (HP) granulites from the Dunhuang block in the northeastern Tarim Craton is reported, and these rocks are characterized through detailed petrological and geochronological studies. The peak mineral assemblage of the HP mafic granulite is garnet + clinopyroxene + plagioclase + quartz + rutile, which is overprinted by amphibolite facies retrograde metamorphic assemblages. The calculated P–T conditions of the peak metamorphism are ∼1.4–1.7 GPa and ∼800 °C. The retrograde P–T conditions are ∼0.7 GPa and ∼700 °C. The metamorphic zircon grains from the HP mafic granulite show homogeneous CL‐images, low Th/U ratios and flat HREE patterns and yield a weighted mean 206Pb/238U age of 444 ± 5 Ma. The metamorphic zircon grains from the associated kyanite‐bearing garnet gneiss and garnet‐mica schist show a similar 206Pb/238U age of 429 ± 3 and 435 ± 4 Ma, respectively. The c. 440–430 Ma age is interpreted to mark the timing of HP granulite facies metamorphism in the Dunhuang block. The results from this study suggest that the Dunhuang block experienced continental subduction prior to the early Palaeozoic collisional orogeny between the northeastern Tarim Craton and the southern CAOB, and the Dunhuang area could be considered as the southward extension of the CAOB. It is suggested that the continental collision in the eastern part involving the Dunhuang block of the southern CAOB may have occurred c. 120 Ma earlier than in the western part involving the Tianshan orogen.
Beneath ultraslow‐spreading ridges, the oceanic lithosphere remains poorly understood. Using recordings from a temporary array of ocean bottom seismometers, we here report an ~17‐days‐long microearthquake study on two segments (27 and 28) of the ultraslow‐spreading Southwest Indian Ridge (49.2° to 50.8°E). A total of 214 locatable microearthquakes are recorded; seismic activity appears to be concentrated within the west median valley at Segment 28 and adjacent nontransform discontinuities. Earthquakes reach a maximum depth of ~20 km beneath the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. The relatively uniform brittle/ductile boundary beneath Segment 28 suggests that there is no focused melting in this region. The majority of earthquakes is located below the Moho interface, and a 5‐km‐thick aseismic zone is present beneath Segment 28 and adjacent nontransform discontinuities. At the Dragon Flag hydrothermal vent field along Segment 28, the presence of a detachment fault has been inferred from geomorphic features and seismic tomography. Our seismicity data show that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There is a virtual seismic gap beneath magmatic Segment 27, which may be connected to the presence of an axial magma chamber beneath the spreading center and focused melting; in this scenario, the increased magma supply produces a broad, elevated temperature environment, which suppresses earthquake generation.
Compressional seismic wave reflected off the Earth's inner core boundary (ICB) from earthquakes occurring in the Banda Sea and recorded at the Hi-net stations in Japan exhibits significant variations in travel time (from −2 to 2.5 s) and amplitude (with a factor of more than 4) across the seismic array. Such variations indicate that Earth's ICB is irregular, with a combination of at least two scales of topography: a height variation of 14 km changing within a lateral distance of no more than 6 km, and a height variation of 4-8 km with a lateral length scale of 2-4 km. The characteristics of the ICB topography indicate that small-scale variations of temperature and/or core composition exist near the ICB, and/or the ICB topographic surface is being deformed by small-scale forces out of its thermocompositional equilibrium position and is metastable.inner core growth | geodynamo | outer core convection T he Earth's inner core grows from the solidification of the liquid outer core (1). The solidification process releases latent heat and expels light elements, providing driving forces for the thermocompositional convection in the outer core (2, 3) and the geodynamo that is responsible for the Earth's magnetic field (4-6). Information about the inner core boundary (ICB) is thus the key to the understanding of driving forces in outer core convection. The ICB has always been thought to be flat, simple, and smooth, due to the presumed extremely small variation in temperature in the outer core (7). Although recent seismic observations provide indirect evidence for the existence of topography at the ICB based on the localized temporal changes of the inner core surface (8-10), direct seismic observations about the existence of inner core topography are still nonexistent. Here, we used PKiKP (a seismic compressional wave reflected from the ICB, Fig. 1A) recorded in precritical distances to study inner core topography. ResultsIn the precritical distances, PKiKP travel times and amplitudes are sensitive to topography, geometry, and property contrast across the ICB (11-16). We adopted PcP waves [a compressional wave that is reflected off the core-mantle boundary (CMB)] as a reference phase and used PKiKP-PcP differential travel time and relative amplitude to study the ICB property. The use of differential PKiKP-PcP travel time and relative PKiKP/PcP amplitude ratio minimizes the effects of shallow Earth's structure and uncertainties in source origin time, location, magnitude, and radiation pattern Fig. 1A).We collected all available PKiKP-PcP data from earthquakes occurring in the Banda Sea and recorded at the Hi-net stations in Japan during 2004-2010. The selected earthquakes have a depth range from 100 to 650 km, with magnitudes ranging between 5.8 and 7.6 (Table 1). We measured PKiKP-PcP differential times on the vertical components of seismic data. The seismic data were filtered with a two-pole causal Butterworth band-pass filter of 1-3 Hz and the worldwide standard seismic network short-period instrument response. These fi...
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