Two properties are particularly sought after in materials for technology (for a variety of devices from sensors to computer memory, etc.): magnetism and ferroelectricity. Obtaining materials with both qualities is highly desirable. At the present time, these properties have shown to be almost entirely mutually exclusive, but a new study conducted by SISSA/Northwestern University introduces an innovative method which may soon become reality. Magnetism and ferroelectricity: two properties which are particularly important for technology. The former is well known in empirical uses: it makes the needle of the compass point towards the North Pole, a magnetic field can align magnetic moments called spin of the electrons that make up the material. The latter is the electric form of magnetism. Ferroelectric materials maintain electric polarization even after the electrical field that caused it is removed. The two properties are extremely useful, and would be even more so if they coexisted in the same material. At the moment one precludes the other: a material is either ferroelectric or magnetic. Things may soon change. A new study conducted by SISSA and Northwestern University (Illinois, USA) published in ("Design of a Mott Multiferroic from a Nonmagnetic Polar Metal"), proposes a completely new model for creating these “multiferroic” materials. 
The Multiferroic Sandwich (Image: James Rondinelli) “Ours is certainly not the first attempt at obtaining a material of this kind, but up to this point there has been little in terms of satisfying results,” notes Massimo Capone, SISSA researcher and one of the authors of the study. “Our method is based on a surprising system.” Capone and his colleagues’ work is a theoretical study which will serve as a guide for developing the material itself. “Our approach is based on creating a sort of sandwich with layers of Lithium Osmate, a ferroelectric metallic material, alternating with insulating material. Adding insulation causes magnetic properties to emerge from two non-magnetic materials. This arrangement, which we refer to in jargon as heterostructures, slows down electrons in the system, and it is this phenomenon that leads to the emergence of magnetism,” explains Gianluca Giovanetti, SISSA/CNR IOM researcher, and one of the authors of the study. “Our theoretical model shows a clear effect, and furthermore, we show that it is possible to control ferroelectricity with magnetism, another important property,” concludes Capone. “The next step will be to test the material itself.”
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The Multiferroic Sandwich (Image: James Rondinelli) “Ours is certainly not the first attempt at obtaining a material of this kind, but up to this point there has been little in terms of satisfying results,” notes Massimo Capone, SISSA researcher and one of the authors of the study. “Our method is based on a surprising system.” Capone and his colleagues’ work is a theoretical study which will serve as a guide for developing the material itself. “Our approach is based on creating a sort of sandwich with layers of Lithium Osmate, a ferroelectric metallic material, alternating with insulating material. Adding insulation causes magnetic properties to emerge from two non-magnetic materials. This arrangement, which we refer to in jargon as heterostructures, slows down electrons in the system, and it is this phenomenon that leads to the emergence of magnetism,” explains Gianluca Giovanetti, SISSA/CNR IOM researcher, and one of the authors of the study. “Our theoretical model shows a clear effect, and furthermore, we show that it is possible to control ferroelectricity with magnetism, another important property,” concludes Capone. “The next step will be to test the material itself.”
Organic light-emitting diodes (OLED) can be produced easily and at low cost. Thanks to compostable materials, they are also made sustainable. (Photo: Karlsruhe Institute for Technology) So far, this has not been the case for conventional printed electronics, such as organic light-emitting diodes (OLED). “We call all synthetic materials that are based on carbon “organic”. But this term does not tell us anything about environmental compatibility,” Dr. Hernandez-Sosa explains. For instance, the carrier foil of OLED – the paper equivalent for electronic inks – is made of the same plastic material as conventional beverage bottles. The Biolicht group only uses easily biodegradable materials that can be found in nature. Starch, cellulose, or chitin are suited as carrier foils, for instance. The scientists hardly use metals or metalloids, such as silicon. The advantages of plastic materials: They are bendable, cheap, and can be processed into miles of printing foil. By means of this technology, it is possible to produce on the industrial scale e.g. stickers with an electronic traffic light indicating shelf life or band aids with incorporated sensors to monitor the healing process. First, electronic components have to be printed onto the compostable foils similar to letters onto paper. Their function depends on the ink used: Instead of dye particles, conducting, semiconducting or non-conducting, i.e. insulating, materials are dissolved in the ink. Upon application, the liquid solvent dries and the remaining layer forms the corresponding component. Work of the Young Investigator Group is aimed at developing biodegradable inks adapted to the new foil material and suited for printing with existing equipment. ”Manufacturers of organic electronics can swap to environmentally compatible materials without having to exchange their printer arsenal,” Dr. Hernandez-Sosa says.
Measuring the voltages between different pairs of electrodes around a thin film topological insulator could reveal how electron spins behave under magnetic and electric fields. The thick white arrows represent spin-resolved current flow, while the thin black arrows represent the spin polarization. The imbalance between the spin-up and spin-down currents indicates that a net spin current flows along the edges of the topological insulator thin films. (© American Physical Society) Conventional microelectronic circuits process data by shuttling and manipulating the electric charges of electrons. But electrons have another property that is highly promising as a basis for data processing and storage—spin. Circuits based on spin have the potential to be much faster, more efficient and smaller. However, it is much harder to control electron spin than charge, and very specific material systems are required. Naoto Nagaosa of the RIKEN Center for Emergent Matter Science and his colleagues from the RIKEN Condensed Matter Theory Laboratory have been studying how electron spins behave in an unusual class of materials known as topological insulators. Such materials are electrically insulating internally but highly conductive on their surfaces. One consequence of this character is that the pattern and flow of spins at the surface could be manipulated relatively easily. “Topological insulators could be an ideal laboratory for spintronics,” says Nagaosa. To investigate, the research team performed calculations related to the exposure of a thin film to a strong magnetic field at very low temperatures. This induces the formation of discrete spin-dependent energy levels and results in a voltage that increases in steps as the magnetic field rises—a phenomenon known as the quantum Hall effect. These conditions are predicted to open up a ‘channel’ along the edges of a topological insulator film, along which electrical charges and spins could flow. The team also found that electric fields could be used to generate more spins in the channels. “These edge channels can support interesting charge- and spin-transport phenomena, which we believe will provide many useful spintronics functions,” notes Nagaosa. The researchers propose measuring these effects in an experiment involving a thin film of a topological insulator with an electrode at each corner (Fig. 1). The voltage between two of these electrodes would register charge transport, while the voltage between two sides of one of these electrodes would reveal the spin current. Applying an external electric field would then generate spins that could be recorded by the other two electrodes. “Quantum Hall effects in topological insulator thin films have already been observed by a couple of experimental groups, including one at RIKEN,” says Nagaosa “so we hope that spintronic functions in these thin films will be achieved in the near future.”