A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and ...switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.
Magnetoelectric coupling between magnetic and electrical properties presents valuable degrees of freedom for applications. The two most promising scenarios are magnetic-field sensors that could ...replace low-temperature superconducting quantum interference devices, and electric-write magnetic-read memory devices that combine the best of ferroelectric and magnetic random-access memory. The former scenario requires magnetically induced continuous and reversible changes in electrical polarization. These are commonly observed, but the coupling constants thus obtained are invalid for data-storage applications, where the more difficult to achieve and rarely studied magnetic response to an electric field is required. Here, we demonstrate electrically induced giant, sharp and persistent magnetic changes (up to 2.3×10−7 s m−1) at a single epitaxial interface in ferromagnetic 40 nm La0.67Sr0.33MnO3 films on 0.5 mm ferroelectric BaTiO3 substrates. X-ray diffraction confirms strain coupling via ferroelastic non-180 BaTiO3 domains. Our findings are valid over a wide range of temperatures including room temperature, and should inspire further study with single epitaxial interfaces.
ECN set up a cross testing project in which suppliers of electrically conductive adhesives (ECA) and conductive back-sheet (CBS) foils participated. In the component part of the project, combinations ...of adhesive and foil were characterised for peel strength and contact resistance. The separate components were tested on dot geometry (ECA) and surface structure (CBS). In the module manufacture and testing part, 12 combinations of ECA and CBS were used in 4-cell MWT modules. The modules were tested up to 2000hours in damp heat and 400 thermal cycles (both tests, 2 times the requirement of IEC61215).
Most combinations passed the 5% power loss criterion. One of the conductive adhesives performed well on three different types of back-sheet which is supportive of a robust technology. Other results show that it is important to thoroughly test interesting combinations of ECA and CBS at module level before adding them to the recommended bill of materials.
We have used optical modeling to calculate whether optical absorption and corresponding current densities in solar cells consisting of PF10TBT:PCBM (1:4) can be enhanced, either in single junctions ...by applying optical spacers or by going to a tandem structure. The current densities are calculated from the optical absorption using experimentally determined IQE values. When an optical spacer is used, the thickness of the transparent electron transport layer (ETL) and hole transport layer (HTL) is very important. The absorption, and thus current density, in a single junction solar cell could be enhanced by 10% by reducing the HTL (PEDOT) thickness and by inserting a thin ETL (30 nm ZnO). The current density in a tandem consisting of two PF10TBT:PCBM blend layers is strongly dependent on the thickness of these blend layers. An increase of 20% is possible for a layer combination of 150 nm and 250 nm, combined with appropriate ETL and HTL layer thicknesses, and assuming no further losses.
Magnetite, Fe3O4, is an extensively studied material because of several interesting properties. It is a ferri-magnet with a high Curie temperature of 858 K and electronically conducting with highly ...spin-polarized conduction electrons. Consequently, it is an interesting candidate for magnetic recording media or spin-valve applications. The present study is a comparison of the resistive and magnetic properties of ultrathin epitaxial Fe3O4 films grown on MgO(001) and MgAl2O4(001). Such films on MgO are known to contain anti-phase boundaries, which cause magneto-resistance in these films, an increased resistivity with respect to the bulk and superparamagnetic behaviour for films below 5 nm thickness. Ultrathin films on MgAl2O4 show a very similar behaviour as the resistivity is also increased with respect to the bulk, but is still lower than the resistivity of films grown on MgO. This indicates a larger domain size in the films grown on MgAl2O4. This is supported by the observed magneto-resistance that is larger than for films grown on MgO. Mossbauer spectroscopy analysis of a 1.7 nm thick film on MgO indicates that all magnetic moments in the film are fluctuating, whereas for the film on MgAl2O4 a sextet with a parabolic background is observed. This shows that some domains are small enough for the moment to fluctuate while in larger ones, the moments are fixed; this also indicates that the domains in the film on MgAl2O4 are larger than in the film grown on MgO.
We describe experimental difficulties, and associated artefacts, that arose during the study of thin epitaxial films of the proposed multiferroic materials BiFeO
3
and BiMnO
3
. The problems ...experienced include large leakage currents and charge injection from Nb-doped SrTiO
3
substrates used as bottom electrodes. Charge injection prevents ferroelectric hysteresis loop closure and contributes to the apparent polarization. Leakage currents also contribute to the apparent polarization, and may help explain the large range of polarization values reported in the literature. Convincing reports of a material that is ferromagnetic and ferroelectric at room temperature remain elusive, and indeed we fail to reproduce a previous work concerning Tb-substituted BiFeO
3
. Our magnetization measurements show this material to be only a weak ferromagnet (0.1 µ
B
/unit cell). Lastly, we show that magnetoelectric measurements are easily corrupted in the presence of large leakage currents.
Wang et al. recently reported multiferroic behavior, with ferromagnetic and ferroelectric polarizations that are both large at room temperature, in thin strained films of BiFeO3 (BFO). Although at ...room temperature, bulk BFO is ferroelectric and anti-ferromagnetic (-), Wang et al. reported that a 70-nm film shows both an enhanced ferroelectric polarization (90 kC cm-2) and a substantial magnetization (1 kB/Fe). This remains the only report of a robust room-temperature multiferroic and suggests the potential for novel devices that exploit the anticipated strain-mediated magnetoelectric coupling between the two ordered ground states. In this Comment, we argue that epitaxial strain does not enhance the magnetization and polarization in BiFeO3.
Magneto-resistance (MR) measurements on epitaxial Fe
3O
4 films grown on polished MgO have been performed. The measurements presented here are interpreted by a model that describes the MR behaviour ...as spin-polarised transport across anti-ferromagnetic (AF) interfaces. The Fe
3O
4 films consist of structural domains, separated by anti-phase boundaries where an AF coupling is present. These AF interfaces enhance the resistance of the films. Upon application of a magnetic field the AF-spins rotate towards each other and the resistance decreases. The AF interfaces are thus behaving as spin-valves. In agreement with the model, the observed magneto-resistance is negative and shows linear and quadratic field dependence up to the anisotropy field for fields applied parallel and perpendicular to the film plane respectively. Above the anisotropy field, the slopes of the two MR curves are expected to be equal, which is observed at 60 K. Above the Verwey transition, the shape of the normalised MR curves is independent of temperature. Below the Verwey transition the MR curve becomes more linear with decreasing temperature. A large difference between parallel and perpendicular MR is observed at the Verwey transition.
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