Magnetic materials with high magneotcrystalline anisotropy are required in both thin film and bulk forms for a wide spectrum of applications including emergent spintronic devices, computer storage media, motors for hybrid vehicles and generators for wind turbines. In addition to a high magnetocrstalline anisotropy, MnAl has recently been shown to exhibit novel magneto-transport effects. These aspects, combined with the fact that the material contains no rare earth elements and has low raw materials costs, make MnAl highly attractive for the applications mentioned above. The MnAl materials produced to date typically exhibit excellent values of some but not all the required magnetic properties. For example in thin films of MnAl, high coercivity with low saturation magnetisation has been reported, both of which are required in spintronic devices, but little is known about the (anisotropic) magnetoresistance. In bulk materials, high values of saturation magnetisation have been reported but the coercivity is significantly lower than that reported in thin films. High coercivity and magnetisation are crucial for permanent magnets. As the microstructure of MnAl thin films has not been studied and details of the microstructure in bulk MnAl are still emerging, the mechanisms leading to the contrasting magnetic properties in different forms of this material are not understood. The influence of some doping elements on the magnetic properties of bulk MnAl has been tentatively explored but much further work is required to clarify the effects. Very few studies have been carried out on doped MnAl films. Solving these problems is the key step which will allow the production of MnAl materials with magnetic properties tailored to applications in spintronics and permanent magnets.The development of thin films and bulk materials often takes place separately but in this project, synergies between thin films and bulk materials will be exploited in order to progress rapidly in understanding the magnetic properties and the effect of doping in MnAl. For example, by analysing highly coercive MnAl thin films and comparing their microstructure with that of MnAl bulk materials, the conditions necessary for high coercivity will be elucidated. Novel processing routes will be developed which reproduce these conditions in bulk materials. The solubility of the doping elements and phase equilibria will be studied in bulk materials, using the advantage that, unlike in thin films, no substrate is present which could influence the results. The knowledge gained from bulk will be transferred in order to accelerate the successful fabrication of doped films. The magnetic and magneto-transport properties of the materials produced will be studied in detail in order to elucidate the controlling mechanisms and assess their suitability for application.

Yttrium iron garnet (Y3Fe5O12, YIG) is an electrically insulating ferrimagnet with an ordering temperature of 560 K. Due to the extremely low magnetic damping (Gilbert damping parameters down to a few 10 ppm) and the small coercive fields, YIG and related magnetic garnets are now used in various high-frequency applications. In addition, these materials are very interesting because of their outstanding properties for basic research in the field of magnonics and spin electronics. However, mostly YIG single crystals and planar thin films are currently used. The fabrication and properties of more complex, 3-dimensional (e.g. curved or tube-like) thin-film structures of YIG or YIG-based heterostructures have not been studied experimentally, although very interesting properties have been predicted.In the framework of this project, we explore this interesting field by establishing the synthesis of YIG layers by means of atomic layer deposition (ALD). ALD is based on successive, self-limited surface reactions and, thus, allows the conformal coating of arbitrarily shaped surfaces. While the ALD of (binary) oxide thin films is already routinely used in the semiconductor industry, two ALD cycles must be combined to produce ternary compounds such as Y3Fe5O12. Therefore, the successful atomic layer deposition of YIG–or more generally of garnet thin films–is also an interesting technological challenge. By combining established ALD processes, we want to produce nano-laminates in which chemically stable binary constituents (Y2O3 and Fe2O3 for YIG) are stacked in a thin-film heterostructure. After a heat treatment, the nano-laminates are converted into the desired material. First, we will establish the nano-laminate process for Y3Al5O12 (YAG), since Al2O3 ALD is particularly robust; it is considered the ALD reference process. Afterwards, we will transition from YAG to YIG. The magnetic properties of the layers are quantified by magnetometry and magnetic resonance investigations. In combination with the ALD process for metallic Pt, YIG/Pt multilayers can be deposited on arbitrarily shaped surfaces. Such heterostructures are very interesting for experiments with pure spin currents. Here, we want to investigate the influence of local curvature or topology on spin transport by means of spin-Hall magnetoresistance experiments.In summary, the proposed atomic layer deposition of yttrium iron garnet provides the base for the production of modern, three-dimensional nanostructures from this special material and, thus, enables a large number of future (spin transport) experiments.

The project addresses the general problem to directly relate (magneto-)transport properties to the nanoscopic details of the underlying magnetic textures of Skyrmions and related topological solitons determined in a transmission electron microscope. These two aspects are key to an in-depth understanding of the nature of these magnetic nano objects. To date, transport measurements are usually conducted on (macroscopic or mesoscopic) samples, whose size and morphology differ substantially from those investigated in a microscope. Since the stability of topological spin solitons sensitively depends on the size and thickness of the investigated materials, such ex-situ comparisons of samples of different dimensions are inherently problematic. Also the general question as to whether the tolological Hall and Nernst effects have their origin in the presence of Skyrmions is still under debate. These difficulties highlight the necessity to directly probe such correlations in in-situ experiments.So far, electrically biased in-situ experiments mainly aim at the mere observation of the current or field-induced motion of, e.g., Skyrmions, in order to phenomenologically understand their dynamics. These studies, however, do either not address the microscopic details of the topological objects, or the methodologically limited resolution does not allow for it. Within the framework of the first funding period, we have successfully performed the quantitative determination of spin textures in high-resolution 2D volume projections and 3D mappings in the transmission electron microscope. Consequently, we now aim at extending this work by the establishment of an experimental platform for in-situ measurements of magnetotransport properties in the microscope for the second funding period. Embedding these in-situ experiments in the priority program SPP-2137 creates substantial added value to the endeavor, as the research network of the program allows to interconnect our work with related (ex-situ) approaches in partnering projects, exchange both bulk and thin film samples, and thus connect the mesoscopic with the nanoscopic world down to even atomic length scales.

We will experimentally demonstrate that a new class of antiferromagnets with spin-split electronic energy bands and vanishing magnetization can complement or replace ferromagnets in the detection and manipulation of magnetic states in spintronic devices. The project is based on our recent discovery of strongly polarized conserved spin-currents generated by these spin-split collinear antiferromagnets. We will demonstrate that our antiferromagnets can facilitate the prominent spintronic functionalities which were earlier thought to be exclusive merits of ferromagnets. Simultaneously, the introduction of antiferromagnets will increase robustness and decrease multilayer-material complexity of the devices. We will also show that semimetallic of semiconducting spin-split antiferromagnets open a prospect of efficient thermo-electric energy harvesting and magnetic imaging. Our international team has a previous track record of successful collaborations and the complementary expertise within the team ideally matches the objectives of the project. The aim of the project is the demonstration of electrical and thermoelectric manipulation and detection of magnetic states by spin transfer torque and giant magnetoresistance using collinear spin-split antiferromagnets and the imaging of antiferromagnetic domains by the anomalous Nernst effect.