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Exploring high-frequency DYNAmics in artificial MAGnetic frustrated systems

Periodic Reporting for period 1 - DYNAMAG (Exploring high-frequency DYNAmics in artificial MAGnetic frustrated systems)

Reporting period: 2016-05-22 to 2018-05-21

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.
The results obtained during the project cover three main topics:

i) The investigation of the stochastic behavior of artificial spin ices;
ii) The investigation of the high-frequency response of artificial spin ices;
iii) The possibility of extending spin ice geometries into three dimensions.

i) The results obtained for the first topic pertain to the investigation of the thermally-induced dynamics in an artificial spin ice geometry consisting of a chiral pattern. Time-resolved measurements performed using x-ray photoemission electron microscopy show that following saturation by an external field, thermal relaxation proceeds through the rotation of the average magnetization in a unique sense, namely clockwise. State-of-the-art micromagnetic simulations have been employed to determine the energy landscape of the system: these show that the emergent ratchet-like dynamics is driven by the magnetostatic field at the boundaries of the nanomagnet array, resulting in an asymmetric energy landscape.

This work establishes a connection between artificial spin ice and the field of thermal engines, providing the essential reproducible ratchet motion that is a prerequisite – and the most challenging aspect – for the realization of such systems (S. Gliga, et al., Nat. Mater. 16, 1106 (2017)).

Further dissemination was achieved through:
- Invited talks at major international conferences (American Physical Society meeting, Intermag, International Conference on Magnetism).
- Press releases.
- A scientific illustration, which received the Judges’ Choice Award in the ‘Magnetism at Art Showcase’ at the ICM2018 conference.

ii) The results on this topic pertain to the investigation of the dynamic response of artificial spin ice to MHz and GHz magnetic excitations. This has been carried out using scanning transmission x-ray microscopy as well as micromagnetic simulations both on the square ice geometry and on the ‘chiral’ ice geometry described in topic (i). The results show that the collective magnetization reversal sequence of the nanomagnets can be effectively influenced by AC fields of well-defined frequencies.

iii) The results on this topic concern the fabrication and characterization of three-dimensional micron-sized structures with the aim of studying magnetization dynamics in 3D. Samples were fabricated using two-photon lithography and their physical properties investigated using x-ray based tomography. The magnetic properties have been investigated by mapping the ferromagnetic resonance response in three dimensions. A second aspect of this topic consisted in the demonstration of a novel x-ray based magnetic tomography technique. The technique allows measuring the internal magnetic configuration in a micron-sized magnetic pillar with a resolution of ca. 100 nm (Publications in: Nature 547, 328 (2017) and New J. Phys. 20, 083009 (2018)).

Further dissemination was achieved through:
- Press releases;
- A vulgarization article (Phys. Unserer Zeit 48, 266 (2017)) targeted towards general readers who are interested in scientific issues.
Impact of the research:

- The demonstration of an active material based on spin ices has shown that spin ices can be used as functional materials, thus opening a new direction of research, beyond the fundamental study of frustration.

- The exploration of the high frequency behavior of spin ices has shown that the magnetic state could potentially be controlled to create channels for information propagation in the form of spin waves. Such systems could potentially be used as part of low-power logical devices.

- The fabrication and measurement of 3D magnetic structures with nanoscale resolution opens vastly new possibilities for probing magnetism, both in technologically-relevant bulk micromagnets as well as in novel curved thin film geometries.
A ratchet made of tiny magnets