Antiferromagnetic Spinmechatronics

While most people are familiar with the fact that electrons possess mass and charge, they also have a lesser-known third property called spin. The concept of electron spin was first demonstrated in the famous Stern-Gerlach experiment of 1922. In this experiment, a beam of particles was passed through an inhomogeneous magnetic field, resulting in distinct deflections of particles of opposite spins. Unlike charge and mass, which are scalar quantities, spin is a vector quantity, and each spin is accompanied by a magnetic moment. Consequently, electrons possess a small magnetic moment. In ferromagnetic materials, these moments align in the same direction, creating a net magnetization. Conversely, in antiferromagnetic materials, neighboring spins organize themselves in an ordered pattern, pointing in opposite directions, effectively canceling out the net magnetization. Spinmechatronics is a technology that harnesses the spin property of electrons to create nanoscale mechanical devices driven by the collective spin excitations – known as spin waves – of magnetic materials. Currently, this technology predominantly utilizes ferromagnetic elements. The primary objective of this research project has been to explore the utilization of antiferromagnets (AFs) in spinmechatronics. AFs possess distinctive properties that make them particularly attractive for integration into spinmechatronics, e.g., they have ultrafast spin dynamics and high stability against magnetic fields. The anticipated application potential of our research is to develop new, ultrafast, nanoscale spin mechatronics with improved stability and new forms of output signals. The project has concentrated on the following four research areas: 1) The equilibrium properties of AFs, 2) how spin waves and spin currents couple to magnetic domain walls, which are interfaces separating domains with different spin orientations, 3) how AFs can be utilized to pump spin currents, and 4) how AFs can be used to develop high-frequency generators that operate in the terahertz (THz) regime. Antiferromagnetic materials can be categorized into two main groups: collinear antiferromagnets (CAFs) and non-collinear antiferromagnets (NCAFs). The project has extensively investigated the equilibrium properties of both these material classes. In the case of CAFs with broken inversion symmetry, a notable finding of the project is the emergence of significant magnetization along the material's edges. This discovery is of particular significance as one of the primary challenges in spin electronics utilizing antiferromagnetic elements lies in the difficulty of controlling the antiferromagnetic spin order through external force fields such as magnetic fields. The presence of magnetization at the system's edges offers the potential for easier manipulation of AFs using electrical pulses and magnetic fields. In the context of NCAFs, the project has uncovered that broken inversion symmetry leads to deformations of the domain walls. This phenomenon gives rise to a novel coupling mechanism between spin currents and domain walls. Additionally, we have developed numerical software to study spin systems with arbitrary symmetry. Utilizing this software, we have demonstrated that antiferromagnetic domain walls can be manipulated via spin waves. Furthermore, the direction of movement of these domain walls can be controlled by adjusting the frequency of the spin waves. This discovery is of significant importance for a data storage technology known as "racetrack memory," which relies on the movement of these domain walls to read information (bits). The effect we have uncovered offers a simplified reading process and is particularly relevant for a class of non-collinear antiferromagnetic materials known as kagome AFs. Moreover, we have investigated the interaction between spin waves in kagome AFs and itinerant charge carriers. Remarkably, we have shown that spin waves in these materials can generate spin currents. This groundbreaking finding opens up the possibility of utilizing kagome AFs as spin current generators, with the added advantage of being able to adjust the spin currents' polarization direction by tuning the spin waves' frequency. Conventional electronics face challenges in generating and detecting electromagnetic radiation within the THz range, resulting in what is commonly referred to as the "THz technology gap." In this project, we have successfully demonstrated that kagome AFs can be used as THz frequency generators by employing direct electric currents to generate high-frequency alternating currents. Our findings reveal that high-frequency generators utilizing kagome AFs possess an exceptional bandwidth, enabling the adjustment of the alternating current from very low frequencies to frequencies far into the THz regime. These results indicate that NCAFs, such as kagome AFs, show great potential in developing unique electrical components for THz applications.

Terahertz (THz) teknologigapet refererer til et frekvensområde for elektromagnetisk stråling i THz-regimet hvor dagens teknologier er ineffektive til å generere og detektere stråling. Mens tradisjonell elektronikk fungerer bra for å produsere og registrere mikrobølger, og optikk vanligvis opererer i det infrarøde området, er det få enheter som kan operere i THz-området. Dette prosjektet har utviklet et helt nytt teoretisk rammeverk for å modellere den koblede dynamikken mellom antiferromagneter og spinn- og ladningsstrømmer. Hovedresultatene fra prosjektet inkluderer tre fundamentalt nye teorier som beskriver antiferromagnetisk spinn-dynamikk indusert av spinnbølger, spinnstrømmer og ladningsstrømmer. Disse teoriene har blitt anvendt for å oppdage nye spinn-mekatroniske effekter som potensielt kan spille en betydelig rolle i å fylle teknologigapet innenfor THz-teknologi. Nedenfor vil vi gi en mer detaljert gjennomgang av disse forskningsresultatene og beskrive deres potensielle virkninger og effekter. Det finnes to hovedklasser av antiferromagnetiske materialer: 1) kollineære antiferromagneter og 2) ikke-kollineære antiferromagneter. De mest sentrale resultatene i dette prosjektet har blitt oppnådd innenfor ikke-kollineære antiferromagneter. Ved å ta utgangspunkt i en mikroskopisk teori for ikke-kollineære antiferromagneter, har vi utviklet effektive teorier som beskriver hvordan den antiferromagnetiske spinnordningen kan manipuleres ved hjelp av spinnbølger, hvordan en spinnakkumulasjon kobler seg til antiferromagneter, og hvordan spinn-bane-kobling gir en vekselvirkning mellom spinnsystemet og ladningsstrømmer. Interaksjonen mellom spinnbølger og antiferromagneter kan spesielt benyttes i en teknologi som kalles «racetrack memory», der informasjon lagres i form av magnetiske domenevegger. Våre resultater viser at disse domeneveggene i ikke-kollineære antiferromagneter kan beveges i hvilken som helst retning ved hjelp av en enkelt spinnbølgekilde. Dette kan betydelig forenkle arkitekturen til racetrack memory. Videre har vi vist at koblingen mellom en spinnakkumulasjon og ikke-kollineære antiferromagneter kan benyttes til å produsere spinnbatterier som genererer THz-spinnstrømmer med vilkårlig polariseringsretning. I tillegg har vi oppdaget en alternativ kilde til elektriske THz-signaler gjennom den relativistiske spinn-bane vekselvirkningen mellom ladningsstrømmer og antiferromagneter. Denne vekselvirkningen kan, ved bestemte strømstyrker, konvertere en likestrøm til en vekselstrøm med frekvenser i THz-området. Både THz-spinnstrømmer og THz-ladningsstrømmer kan igjen konverteres til elektromagnetisk stråling. Forskningsresultatene kan derfor danne grunnlaget for ny elektronikk som kan generere elektromagnetiske bølger med frekvenser i THz-området. Slik teknologi forventes å ha stort anvendelsespotensial innen områder som medisinsk bildeteknologi og informasjons- og kommunikasjonsteknologi.

Spinmechatronics is a technology that exploits the spin property of the electron to produce nanoscale mechanical devices. Important examples of such devices include ultra-small magnetic engines and charge/spin pumps that are driven by the collective spin excitations of the magnetic material. So far, this technology has mainly concentrated on implementing ferromagnetic elements in spinmechatronic devices. The ultimate goal of the proposed research is to open a significant new area of research, which concentrates on the usage of antiferromagnets in spinmechatronics. We will refer to this new direction as antiferromagnetic spinmechatronics. The research project is developed in six work packages that cover the fundamental physics of antiferromagnets as well as the stability and functionality of actual spinmechatronic devices. The basic physics will concentrate on the equilibrium properties, theory development, and theoretical investigations of new experimental probes of antiferromagnetic spin dynamics and textures. With regard to the equilibrium properties, we will set particular focus on the possible existence of novel topological phases in noncollinear antiferromagnets. Motivated by the results and theory developed for the fundamental physical properties of antiferromagnets, we aim to explore charge and spin pumping phenomena and mechanical torques that are driven by antiferromagnetic spin precessions. The insights from these results will be used to theoretically demonstrate new ultra-fast nanoscale spinmechatronic devices with improved stability and novel forms of output signals.