In this first period, our research focused mainly on three aspects: the realization and optimization of the experimental setup for phase nanoengineering, the proof-of-principle demonstration of the phase nanoengineering methodology in different systems, the demonstration of the three-dimensional imaging of spin waves.
Phase nanoengineering setup and materials. In order to perform phase nanoengineering, we employed the thermal scanning probe lithography (tSPL) technique. In tSPL, a heatable scanning probe is brought in contact with the surface and scanned with nanometric precision. The key parameter in tSPL is the temperature reached in correspondence of the surface, which determines the efficacy of the phase nanoengineering patterning process. In fact, since most thermally-induced reactions, such as ionic migration or crystallization, occur only above a certain threshold temperature, we worked on the maximization of the temperature by optimizing both the material and the experimental setup. In the first case, we worked on several materials and compositions, ranging from metallic thin film to transition metal oxides, optimizing both the thickness of the thin films (from a few nanometers up to a few hundreds of nanometers) and the substrate materials, in order to reduce the thermal dissipation towards the substrate and hence reach higher temperature. Regarding the setup we worked on several aspects. First of all, we combined tSPL with direct laser writing (DWL) using a focused ultraviolet laser. This proved an extremely effective combination because it allowed us to combine the sub-10 nm spatial resolution of tSPL, with the high power and faster throughput of the laser. As a matter of fact, the laser writing allowed us to explore an even larger range of systems, and proved an effective tool for rapid testing of different materials characterized by higher phase-transition temperatures. Finally, we integrated the system with a global heater for increasing the global temperature of the material during patterning. This proved very effective in increasing the patterning effectiveness by reducing thermal dissipation, and for studying the effects of combined global heating and laser irradiation. The patterning setup was successfully tested using sample systems with well-known response, and the materials were successfully optimized specifically for the phase nanoengineering process.
Demonstration of phase nanoengineering. In this first period of the project, we investigated a wide range of materials, including transition metal oxides and metallic magnetic multilayers. Transition metal oxides were chosen due to their interesting and highly tunable electronic properties, while metallic multilayers were chosen due to their magnetic properties, which allowed the stabilization of complex spin textures and the study of spin waves. Spin waves are oscillation of the magnetization which propagate in magnetically ordered materials, and are extremely promising for building unconventional computing architectures based on spin-wave interference. In order to demonstrate phase nanoengineering in such systems, the first tests were performed by patterning simple structures, varying systematically the patterning parameters and measuring the patterned structures via scanning probe techniques, which employ a nanoscopic tip for measuring different surface properties such as electrical resistance or magnetic domain structure. Patterning and characterization were performed in a closed-loop fashion in order to optimize the phase-nanoengineering parameters aiming to maximize the effectiveness of the patterning process. Phase nanoengineering was successfully demonstrated in all the investigated systems, with sizeable nanoscale modifications of the electronic transport and magnetic properties. In particular, we successfully patterned the magnetization configuration in ultrathin magnetic films and thick magnetic multilayers. In the first case we demonstrated the full nanoscale control of the hysteresis loop of the ferromagnet, by patterning reconfigurable skyrmion crystals (i.e. stable nanoscale vortex-like configurations of the magnetization) with different geometry. In the second case, we stabilized non-uniform magnetization configurations to be used as emitters of spin waves, in 100-nanometer thick films, with the aim of studying the propagation of spin waves in three-dimensions. In summary, phase nanoengineering was successfully demonstrated in several materials systems, with excellent results both regarding the spatial modulation of the electronic transport properties in TMOs, and the control of the spin configuration in magnetic materials to be used in spin-wave experiments.
Three-dimensional spin-wave imaging. The goal of this work was to observe experimentally for the first time spin waves in three dimensions. For doing so, we used 100-nm thick magnetic multilayers, where complex spin configurations were stabilized via both phase nanoengineering and conventional nanofabrication. In order to study spin waves in this system, we used a recently developed technique, Time-Resolved X-Ray Laminography, based on synchrotron X-Ray imaging available at the Swiss Light Source, PSI, Switzerland. This seminal experiment allowed us for the first time to image spin waves in three-dimensions, revealing the full three-dimensional distribution of the spin-wave mode across the thickness of the multilayer, and an intrinsically three-dimensional spin-wave interference figure. This marks a milestone in condensed matter physics, and opens the way to study experimentally for the first time complex tree-dimensional spin-wave modes, and to develop three-dimensional spin-wave devices for computing.
