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Spin Transport Beyond Electrons

Periodic Reporting for period 4 - SPINBEYOND (Spin Transport Beyond Electrons)

Reporting period: 2022-03-01 to 2022-08-31

All our electronic devices work because small particles, called electrons, travel around in conductors and transport charge. As they do so, they collide with impurities in the material which causes heating and makes devices less energy-efficient. Next to charge, electrons also have a property called "spin", of which one can think of as a built-in compass needle of the electron. This property can transport information and to develop applications based on the spin of the electron is the goal of the field called "spintronics". In spintronics, one is concerned with spin transport rather than charge transport.

Recently, experiments have shown that spin transport does not only involve moving electrons. Rather it can also involve magnetic fluctuations, i.e. so-called magnons or spin waves. The theoretical understanding of such experiments is still in its infancy. The goal of the project is to develop the theoretical description of spin transport for situations where this transport is done by spin waves or magnons.

The transport of spin by magnons or spin waves does not involve moving electrons and therefore is expected be more energy-efficient than electronic transport. The longterm goal of this research is to propose devices that are energy-efficient as compared to standard electronic devices.

The project focuses on three candidate material systems: magnetic insulators --- these are magnetic materials that do not conduct electricity, magnetic metals --- magnets that do conduct electrically, and antiferromagnets. The latter materials are magnetic materials of which the magnetic structure is more complex than that of ordinary magnets, so that they do not have a net magnetic moment. Their spin fluctuations, however, can still carry information. In fact, in this project we have shown that they can do so with small dissipation.
The project is theoretical in nature, and the PI has established a group of three PhD students and several post-docs to develop the theoretical description of spin transport in these novel materials. This together with the PI, this group of young researchers has published a great deal of peer-reviewed scientific papers that report on their results. Highlights are:
*) the demonstration, in collaboration with the group of Prof. Klaui (Mainz) that spin currents through antiferromagnets can travel with low dissipation. This discovery established antiferromagnetic insulators as a materials platform for energy-efficient devices.
*) the theoretical proposal that the quantum properties of magnons can be detected electronically; this proposal opens the way for “quantum magnonics”, i.e. exploiting the quantum properties of magnons for quantum information and computation.
*) the theoretical demonstration that lattice vibrations (“phonons”) are able to carry information over long distances if they interact with ferromagnets. This discovery establishes ordinary insulators as materials for energy-efficient devices.
*) The theoretical proposal that in graphene, spin can be transported by means of vorticity in the electron fluid.
*) The first concrete theoretical proposal for a spin-wave amplifier and laser. Together with Dr. Lavrijsen at Eindhoven University, the PI has obtained a NWO grant to develop this proposal experimentally.
*) The first theoretical proposal for spin-orbit-coupling effects on interactions in semi-conductors.
The main results mention in the previous part have all pushed the field beyond the state of the art. Before these results were achieved, long-range spin transport through insulators was established only in ferromagnets. One of the main results of the proposed research is to put forward a theory for spin transport through antiferromagnetic insulators. Moreover, the quantum properties of magnons were mostly studied by optical means, rather than electrically. The work of the proposal is the first to integrate quantum properties of magnons with on-chip electrical detection. Finally, the proposals for spin-wave amplification and lasing are the first concrete proposal for a spin-wave amplifier.