Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)Comment: 22 pages, 9 figure

Ground geomagnetic field and GIC response to March 17, 2015, storm

Abstract The St. Patrick’s Day geomagnetic storm on March 17, 2015, has been chosen by the space community for synergetic analysis to build a more comprehensive picture of the storm’s origin and evolution. This storm had an unusually long (~ 17 h) main phase. During this period, many substorm-like activations occurred. These activations resulted in bursts of geomagnetically induced currents (GICs) in power lines on the Kola peninsula. To examine the substorm activations in more detail, we apply various data processing techniques for the world-wide array of magnetometers: the virtual magnetograms, magnetic latitude–local time (MLT) snapshots, and magnetic keograms. These techniques are simple tools that are supplementary to more advanced facilities developed for the analysis of SuperDARN, IMAGE, and CARISMA arrays. We compare the global spatial localization and time evolution of the geomagnetic X-component disturbance and magnetic field variability measured by the Hilbert transform of time derivative dB/dt. The latitude-MLT mapping of these magnitudes shows that very often a region with highest magnetic variability does not overlap with a substorm “epicenter” but is shifted to its poleward or equatorward boundaries. Highest variability of the geomagnetic field, and consequently intense GICs, are caused by medium-scale fast varying structures. There is no one-to-one correspondence between substorm intensity and GIC magnitude

Geomagnetic and ionospheric response to the interplanetary shock on January 24, 2012

Abstract We have examined multi-instrument observations of the magnetospheric and ionospheric response to the interplanetary shock on January 24, 2012. Apart from various instruments, such as ground and space magnetometers, photometers, and riometers used earlier for a study of possible response to a shock, we have additionally examined variations of the ionospheric total electron content as determined from the global navigation satellite system receivers. Worldwide ground magnetometer arrays detected shock-induced sudden commencement (SC) with preliminary and main impulses throughout the dayside sector. A magnetic field compression was found to propagate through the magnetosphere with velocity less than the local Alfven velocity. Though the preliminary pulse was evident on the ground, its signature was not observed by the THEMIS and GOES satellites in the magnetosphere. The SC was accompanied by a burst of cosmic noise absorption recorded along a latitudinal network of riometers in the morning and evening sectors. The SC also caused an impulsive enhancement of dayside auroral emissions (shock aurora) as observed by the hyperspectral all-sky imager NORUSCA II at Barentsburg and the meridian scanning photometer at Longyearbyen (both at Svalbard). The VHF EISCAT radar (Tromsø, Norway) observed a SC-associated increase in electron density in the lower ionosphere (100–180 km). The system for monitoring geomagnetically induced currents (GICs) in power lines at the Kola Peninsula recorded a burst of GIC during the SC. A ≤10% positive pulse of the ionospheric total electron content caused by the SC in the dusk sector was found. On the basis of the multi-instrument information, a validated theory of the magnetosphere–ionosphere response to IP shock may be constructed. Graphical Abstract

Influence of perpendicular magnetic anisotropy on closure domains studied with X-ray resonant magnetic scattering

FePd thin-film samples with perpendicular magnetic anisotropy (PMA) have been studied with x-ray resonant magnetic scattering, both at the Fe and at the Pd L-3 edges. In these samples the competition between PMA and shape anisotropy leads to the formation of highly ordered striped domain patterns with a magnetization component perpendicular to the film plane. These striped domains give rise to magnetic satellite peaks in the diffraction pattern. Magnetic diffraction rod scans of these satellites were analyzed to obtain information about the magnetic depth profile of the films. It was found that flux closure occurs in samples with a low to medium PMA, while a high PMA impedes the formation of closure domains. Data analysis gives a depth of the closure domains extending to 85 Angstrom, with approximately half the magnetic moment aligned in plane