The precise definition of laterally confined structures at the nanoscale with modern fabrication techniques has opened the possibility to create novel functional materials, whose properties transcend that of their constituent elements. This project has aimed to understand and, ultimately, control the magnetization dynamics in a class of geometrically frustrated systems called artificial spin ices, which consist of magnetostatically coupled nanomagnets, in view of developing functional materials that may find applications in novel nanoscale devices as well as in information and communications technologies.
First, the project has established that artificial spin ices, which had mainly been used as a model system for investigating fundamental effects of frustration, potentially constitute a new class of functional materials with potential technological applications. In particular, we have experimentally demonstrated a spin ice based active material – consisting in a repeating pattern of chiral units – in which energy is converted into unidirectional dynamics during thermal relaxation, thus functioning like a ratchet. This behavior is attributed to an asymmetric energy landscape generated by the magnetostatic field of the system and opens the possibility of implementing a Brownian ratchet, which may find applications in nanomotors, actuators or low-power memory cells. In a broader context, the study of the emergent statistical and thermodynamic properties of active matter – a general class of systems that converts free energy into systematic movement – is expected to lead to the creation of highly efficient functional materials.
Second, we have investigated the possibility of controlling the magnetization dynamics, and in particular the global state of the artificial spin ice array through the application of microwave magnetic fields as well as through changes to the shape of the system. Our experiments have demonstrated that the reversal of the magnetization can be effectively controlled and that it is possible to create specific magnetic configuration geometries, which can potentially be used as ‘channels’ for information transfer and processing. These represent essential steps towards implementing novel high speed, low power logical architectures based on the propagation of spin waves and is expected to contribute to the creation of magnonic metamaterials that permit the control of spin waves through frustration engineering.
Third, we have explored the possibility of extending artificial spin ice geometries into the third dimension. We have addressed two main challenges, namely the fabrication of three-dimensional magnetic structures and their measurement. To this effect, we have demonstrated two-photon lithography based fabrication of structures with feature sizes of ca. 100 nanometers and uniform magnetic coating of a few tens of nanometers. The physical and magnetic properties of these systems have respectively been measured using x-ray based tomography and ferromagnetic resonance. In addition, we have contributed to the first demonstration of x-ray based magnetic tomography, which represents a significant development for the characterization of three-dimensional magnetic thin-film and bulk structures with nanoscale resolution. Because magnetic elements form an essential part of modern technologies and their efficiency is often determined by their internal magnetic structure, this imaging technique may also find industrial applications, such as assessing the quality of magnets, which can lead to lead to improvements in their fabrication.
