Scientific contributions to the Yearbook of the Max Planck Society

Precision graphene nanoribbons (GNRs) represent promising candidates for next-generation nanoelectronics and spintronics due to their intriguing and tailorable electronic properties. The Department of Synthetic Materials and Functional Devices at the MPI of Microstructure Physics engages in developing synthetic strategies to precisely engineer GNRs with specific structural topologies. This approach aims to fine-tune the bandgaps, charge carrier mobilities, and topological states of GNRs, facilitating their seamless integration into a myriad of nanoelectronics and spintronics applications.

The Department of Nanophotonics, Integration, and Neural Technology at the MPI of Microstructure Physics is developing wafer-scale photonic circuit technologies to miniaturize and increase the integration density of optical systems. Such microchip technologies can transform numerous applications, such as displays, quantum information, and sensing.  The Department is using these capabilities to create a set of multifunctional implantable chips that interface with the brain to advance neuroscience. The systems are being deployed to neuroscience labs for exploratory and health research.

The physics of low-dimensional systems has been a topic of great interest. Recently, two-dimensional (2D) materials exhibiting long-range magnetic order have been in the spotlight. Using state-of-the art molecular beam epitaxy, we constructed the first large-area 2D ferromagnet - a single CrCl₃ monolayer on Graphene-on-Silicon-Carbide substrate - which exhibits an easy-plane magnetic anisotropy (2D-XY universality class). This discovery offers a suitable platform to observe exotic phenomena with application potential, such as 2D spin superfluidity and topologically protected magnetic textures.

Two-dimensional (2D) ferroics displaying magnetic, ferroelectric, or ferroelastic order have recently been discovered. These materials are attracting tremendous interest in the research community both because of the novel physics they host, as well as their potential for next generation nanoelectronics. Our institute explores, synthesizes and characterizes novel 2D ferroics with the aim of using them in innovative energy-efficient devices.

Spintronics is a field of research that focuses on the fundamental physics and applications of spin-based phenomena. To date spintronics has played a key role in the development of recording heads that are used in magnetic disk drives and in a high-performance, solid-state, non-volatile magnetic random access memory. A third spintronics technology, the magnetic Racetrack memory, has the potential to supplant magnetic disk drives: this article discusses the discovery of several novel magnetic nano-objects, so-called “skyrmions” that could encode the data within Racetrack Memory.

The ability to alter the magnetic configuration of a material on femtosecond timescales would allow computers to operate a thousand to a million times faster. In this work we report on a new quantum-mechanical phenomenon predicted by our simulations, which we named OISTR. The OISTR mechanism uses ultrastrong, ultrashort, optical laser pulses to locally magnetize and de- magnetize particular atoms in just a few femtoseconds. These predictions have since been verified by experiments.

Density functional theory, the most widely used method for calculating the properties of molecules and solids, is limited by its reliance on the Born-Oppenheimer approximation – the assumption that nuclei move infinitely more slowly than electrons. Research conducted at the Max Planck Institute of Microstructure Physics has overcome this limitation, exploiting recent advances in the concept of Berry curvature to establish a density functional theory that fully accounts for nonadiabatic coupled electron-nuclear motion.

Two-dimensional iron islands, some thousand atoms small, exhibit a novel magnetic order on the nanometer scale, which was discovered by spin-polarized scanning tunneling microscopy. The local magnetization direction of iron rotates continuously over five nearest neighbor distances by 360 degrees. For iron, this magnetic order is unusual, and it is ascribed to the reduced dimensionality of the iron nanostructure. Structural relaxation within the nanostructure modifies the spin-dependent interaction between electrons, and a non-collinear spin alignment results.

Magnons are the wave-like motions of the magnetic moments in a magnetically ordered solid. Similar to other waves, magnons may also be used for information processing. The study of wavelength, frequency and lifetime of magnons in magnetic solids is an important area of research. At the Max Planck Institute of Microstructure Physics the properties of magnons excited at ferromagnetic surfaces are investigated using spin-polarized electron spectroscopy.

By nanostructuring silicon its thermal conductivity can be significantly reduced. Such a reduction can potentially induce a corresponding increase of the thermoelectric efficiency so that the transformation of heat into electric power could be improved. Therefore porous silicon layers were produced by electrochemical etching and the thermoelectric properties of the nanostructured material investigated. These investigations show that the thermal conductivity is indeed strongly reduced but that due to competing effects only moderate increases of the thermoelectric efficiency can be achieved.

Magnetoelectric coupling allows changing the magnetic state of a material by applying an electric field. To date, this phenomenon has mainly been observed in insulating materials. Metallic bulk systems do not exhibit this effect, because applied electric fields are screened by conduction electrons. We have been able to switch the magnetic order in a metallic nanostructure reversibly between two stable magnetic states using magnetoelectric coupling induced by an applied electric field.

The investigation of possibilities to influence electrons in solids and at surfaces on the femtosecond (10^-15 s) time scale is an important area of research. This is also relevant for the steering of magnetic switching processes by ultrashort laser pulses and for the control of the spin of excited electrons. At the MPI of Microstructure Physics, the control of the spin of optically excited photoelectrons is investigated by the absorption of multiple photons at metal surfaces.

Non-volatile solid state memories of high memory density are a promising research field, both under technological and fundamental aspects. Since the size of a single memory cell must be clearly below 100 nanometer, and the properties of storage materials can be considerably modified at such low size, the development of suitable preparation methods and the property analysis of the thus prepared memory cells represent considerable challenges. Such investigations are part of the research on nanostructured materials at Max Planck Institute of Microsctructure Physics in Halle.

The surface alloy Bi/Ag( 111) exhibits a giant spin splitting of its surface electronic structure due to Rashba spin-orbit coupling. Electronic structure calculations prove that the effect is brought about by an in-plane structural inversion asymmetry in the surface layer, in interplay with the conventional Rashba effect. These findings pave the way for testing theoretical predictions for spin-orbit split two-dimensional electron gases.

Nanostructures are in the focus of research in technology, medicine and biology. The Max Planck Institute of Microstructure Physics in Halle currently develops a major research project which deals with the deposition of thin inorganic films, biological, organic and inorganic nanostructures and the exploitation of such functionalized materials, for applications in medicine, electronics, catalysis, and sensing.

Using surface sensitive x-ray diffraction the geometric structure of the Fe/MgO/Fe(001) Tunneling-Magneto-Resistance (TMR) device was investigated. Evidence for the formation of an FeO-like interface layer could be provided, which significantly influences the magnitude of the TMR-effect

Modern spin-electronics defines high requirements for the design and fabrication of new materials. Dilute magnetic semiconductors take an important position among the materials that can fulfill these requirements. Here we report on the results of some of our first-principles studies on these materials. We show that partially filled electronic bands play an important role in the magnetism of these systems. The long-range exchange interaction appears as a compromise between the delocalization of the hole states from the 3d impurities and the strength of the 3d-hole interaction. The properties of this compromise depend strongly on the system studied and can be determined only within the framework of the realistic first-principles density-functional-theory calculations

Semiconductor nanowires represent a promising research field where basic research and technology meet. The analysis of their growth mechanism, their properties and their possible applications are part of the research at the Max Planck Institute of Microstructure Physics in Halle. Several different growth methods have been applied to fabricate semiconductor nanowires, which were characterized using electron microscopy. Further investigations, e.g. of the electrical and optical properties are currently carried out.