Controlling Electric Signals with Insulating Antiferromagnets and Insulating Ferromagnets

Project Information

INSULATRONICS

Grant agreement ID: 669442

DOI

10.3030/669442

Project closed

EC signature date 2 July 2015

Start date 1 December 2015

End date 30 November 2020

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 2 140 502,50

EU contribution

€ 2 140 502,50

Coordinated by

NORGES TEKNISK-NATURVITENSKAPELIGE UNIVERSITET NTNU
Norway

Periodic Reporting for period 4 - INSULATRONICS (Controlling Electric Signals with Insulating Antiferromagnets and Insulating Ferromagnets)

Reporting period: 2020-06-01 to 2020-11-30

Summary of the context and overall objectives of the project

Electric charge transport powers conventional electronics- and spintronics-based logic and memory devices, interconnects, and microwave oscillators. These systems inherently dissipate power due to ohmic losses. The research seeks to determine the extent to which “Insulatronics” has the potential to control the electric and thermal signal generation, transmission, and detection in more power-efficient ways.

Insulatronics may have the potential to become a revolutionary technology in the daily lives of typical technology consumers. Conventional electronics function by manipulating electrons through voltage differences. When the electrons move, they release energy to their surroundings. The associated excessive heating in tiny devices prevents a further decrease in the feature size of integrated circuits, wireless transmitters, and memory devices. The electromagnetic waves, photons, carry signals for radio, TV, and other devices. Insulatronics may be capable of replacing moving charges with low-dissipation spin waves and their quanta, magnons, in magnetic insulators in contact with conventional electronic circuits.

The project aims to facilitate a revolution of information and communication technologies by controlling electric signals with antiferromagnetic insulators and ferromagnetic insulators.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Our core focus is on fundamental challenges facing Insulatronics. While charges cannot flow through insulators, spins can flow through magnetic insulators.

The coupling between electric currents in metals and spin-excitations in magnetic insulators is via spin-pumping and spin-transfer torques. Spin-pumping is the emission of spin-currents from dynamical spins in insulating or conducting magnetic materials to adjacent metals. The reciprocal process, spin-transfer, transfers spin angular momentum from the itinerant spin-polarized electrons in metals to the magnetic materials' spins. In several articles covering a wide range of ferromagnetic and antiferromagnetic insulators, we have made predictions for the strength, nature, and extent of these couplings. In doing so, we have fulfilled one of the main goals of our projects.

We first predicted that spins could flow across several micrometers in common antiferromagnetic insulators. The long-range transport is feasible both at high temperature via the semi-classical diffusions of spins and at low-temperatures via spin superfluidity. The spin current flow can be controlled by tuning the antiferromagnetic resonance frequency with an external magnetic field. Subsequently, with experimental colleagues, we realized the flow of spin information across tens of micrometers in the common antiferromagnetic insulator hematite, the rust's main ingredient. The results open doors towards the realization of ultra-fast, low-power antiferromagnet-insulator-based spin-logic devices that operate at room temperature.

Experimental colleagues have demonstrated spin-pumping from insulating antiferromagnets into metal, the emission of spin currents from a precessing antiferromagnet. The experimental results are in agreement with our earlier developed theory. Our results open the door to the unprecedented controlled generation of coherent, pure spin currents at terahertz frequencies.

In another collaboration with experimentalists, we have achieved efficient spin-wave propagation in thin films of the antiferromagnetic insulator hematite with large magnetic domains. Zero field spin transport is achieved across micrometers, as required for device integration.

We have also explored the hitherto less known physics related to magnon condensation, superfluidity, and superconductivity mediated by magnons.

Antiferromagnets may exhibit spin superfluidity since the dipole interaction is weak. We investigate nonlocal spin transport in a planar antiferromagnetic insulator with a weak uniaxial anisotropy. The anisotropy hinders spin superfluidity by creating a substantial threshold that the current must overcome. Nevertheless, we show that applying a high magnetic field removes this obstacle near the antiferromagnet spin-flop transition. Notably, the spin superfluidity can then persist across many micrometers, even in dirty samples.

Additionally, we explore routes to realize electrically driven Bose-Einstein condensation of magnons in insulating antiferromagnets. Even in insulating antiferromagnets, the localized spins can get coupled strongly to itinerant spins in adjacent metals via spin-transfer torque and spin pumping. We describe the formation of steady-state magnon condensates controlled by a spin accumulation polarized along the staggered field in an adjacent metal. Two types of magnons, which carry opposite magnetic moments, exist in antiferromagnets. Consequently, and in contrast to ferromagnets, Bose-Einstein condensation can occur for either sign of the spin accumulation. This condensation may occur even at room temperature when the metal interaction is fast compared to the antiferromagnet's relaxation processes. In antiferromagnets, the operating frequencies of the condensate are orders of magnitude higher than in ferromagnets.

We have demonstrated that magnons can mediate superconductivity in thin metal films sandwiched between magnetic insulators. The exchange coupling causes electrons within the metal to interact with magnons in the insulators at the interfaces. This electron- magnon interaction induces electron-electron interactions, which in turn can result in p-wave superconductivity. In yttrium iron garnet (YIG)-Au-YIG trilayers, superconductivity sets in at temperatures somewhere in the interval between 1 and 10 K.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Insulatronics has the potential to control the electric and thermal signal generation, transmission, and detection in antiferromagnetic insulators and ferromagnetic insulators in more efficient ways. In such devices, the information is transferred via the electron spin. Achieving long-range transport is essential. We have demonstrated that spins can be transported across long distances, several micrometers, both at high temperatures via diffusive spin transport and at lower temperatures via superfluidity.

At high temperatures, magnons scatter frequently and can diffusive even in insulators. We have shown that the spin transport in antiferromagnetic insulators at certain magnetic fields is much longer ranged than previously expected.

At low temperatures, coherent magnon phenomena become important. Antiferromagnets may exhibit spin superfluidity, transport of spins with very little loss. We have demonstrated that spin superfluidity can persist across many micrometers, even in dirty samples, in biaxial antiferromagnetic insulators.

The realization of long-range spin transport in antiferromagnetic insulators paves the way to ultra-fast, low-power antiferromagnetic insulator-based spin-logic devices. Furthermore, the realization of spin-pumping from an insulating antiferromagnet opens the door to the controlled generation of coherent, pure spin currents at terahertz frequencies.

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