Spintronics is a field of research that comprises the generation, manipulation and detection of spin-polarized electrons in a wide range of materials including metals, insulators, and semiconductors. The most important spintronic materials are composed of artificially engineered atomically layered magnetic thin film structures with spin-dependent transport properties. The use of both spin-polarized charge currents and pure spin currents, generated by electric voltages, temperature gradients and / or optical means, to excite and manipulate magnetic moments is of great interest from both fundamental and technological perspectives. Spin-orbit interaction is a principle driving mechanism behind many topical phenomena including spin-orbit torques, current driven domain-wall motion, perpendicular magnetic anisotropy, spin Hall effect, anomalous and topological Hall effects, and topological magnetic structures including skyrmions.
Oxides and Interfaces
Complex oxides display an enormous variety of properties, ranging from insulators to metals and superconductors, from anti-ferromagnetism to ferromagnetism, and ferroelectric to paraelectric to name just a few. By forming heterostructures between thin films of distinct oxides, novel materials can be engineered with functionalities that are not otherwise seen in the individual materials. One example are magneto-electric phenomena in which magnetic fields can influence ferroelectric behavior or electric fields can control magnetic behavior.
Atomically engineered materials
Experimental methods to deposit layers just one atom or one unit cell thick include physical methods, such as molecular beam epitaxy, pulsed laser deposition and magnetron and ion beam sputtering, as well as chemical methods such as atomic layer deposition and chemical vapor deposition. Theoretical methods to simulate the growth of such materials and to calculate their properties are powerful and accurate enough that, by using these methods in tandem with experiment, rapid advances in atomically engineered materials with novel and technologically relevant properties is currently possible. Atomically engineered materials underpin the fields of spintronics, oxide electronics, cognitive devices and routes to room temperature superconductors.
Computational Materials Discovery
The theoretical modelling of structure-property relationships from the nano- to the meso-scale requires the synergistic use of various simulation methodologies. Electronic and magnetic properties of, e.g., molecules, nano-wires or oxide layers are studied by quantum mechanical methods, via the development and application of electronic density functional techniques. Molecular arrangements and properties on longer length scales of, e.g., macromolecules or magnetic model systems, are studied by Molecular Dynamics and Monte Carlo techniques. Heterogeneous structures on still larger scales are mapped onto field-theoretical descriptions and treated with tools of numerical mathematics. Different techniques are used to bridge multiple time- and length-scales to characterise and understand the functions of complex structures. The use of such computational methods to explore and predict the properties of novel, engineered materials is an emerging discipline of considerable current interest.
Cognitive Devices and Bio-Inspired Materials
It is clear that nature has devised methods of preparing materials that we can’t yet replicate and with properties that we can’t yet match. However, the very existence of such biological materials and functionalities serves as inspiration for research into novel ways of manufacturing materials and fabricating devices with such properties. A very important area of research is the development of cognitive computing devices that could support novel computing architectures that could carry out computations a million times more efficiently than conventional charge based devices.
Certain materials with topological character exhibit novel states that are protected by symmetry. Currently the most studied example are topological insulators that exhibit metallic surface or edge states with massless charge carriers. The conductivity of the carriers in these states is quantized and protected from many scattering mechanisms that would otherwise degrade the conductivity. By controlling these states, for example, with magnetic fields, novel devices are possible.
The properties of materials under non-equilibrium conditions can be quite distinct from those in thermal equilibrium. These “non-equilibrium” materials can be created, for example, on short time scales, by the application of short electric field pulses and by detection of their properties using electrical or optical probe techniques. An important example is the metallization of complex insulating oxide by electric fields on the picosecond time scale. On longer time scales trickle currents can lead to thermal effects that overwhelm the nature of the short time scale non-equilibrium states. Other means of creating non-equilibrium materials are by applying an AC bias, by generating shock waves, and by resonant excitation of special modes in a material using timed optical, electric, magnetic or thermal pulses.
Routes to Room Temperature Superconductivity
An ambitious goal is research into novel materials, perhaps made by atomic scale engineering, that could exhibit superconductivity at or near room temperature. While the mechanism that gives rise to superconductivity in the materials with the highest known superconducting transition temperatures today is still a matter of considerable debate, nevertheless there are several possible routes to room temperature superconductivity. One example is by analogy to the cuprate family of superconductors for which the parent compound is an insulating material that displays a transition to an antiferromagnetic state as the temperature is reduced below a Néel transition temperature. The strength of the antiferromagnetic exchange coupling is, in some theoretical models, related to the Cooper pairing energy of the superconducting state. Thus, one route to high temperature superconductors is the engineering of composite materials that exhibit large short-range antiferromagnetic exchange and that can support high densities of superconducting condensate.