Super Insulator Spintronics

We aim to realize groundbreaking insulator spintronics. To this end, we aim for a breakthrough in the fundamental understanding of the electric signal generation, manipulation, transmission, and detection of spin-waves (magnons) in magnetic insulators. The insulator spintronics will be developed to integrate with conventional electronics. The most critical R&D challenge is to develop an understanding of how spin waves in magnetic insulators are coupled to electric currents in adjacent normal metals. Because insulator spintronics do not contain moving charges, they transmit signals at little or even zero power. Given the importance of electronics in today's society, we anticipate a radical paradigm shift in equipment development and ICT, as well as radically decreased energy usage. In the first reporting period, we have considered the transfer of spin information between antiferromagnetic insulators and ferromagnetic insulators. Recent experiments demonstrate that spins can be transferred or pumped from a precessing ferromagnet via an antiferromagnet in contact with a normal metal. We develop a microscopic description of the spin transfer across the interface and will use this model to gain a better understanding of state-of-the-art experiments. In the second reporting period, we have explored how dynamical antiferromagnets pump spins into adjacent conductors. The high antiferromagnetic resonance frequencies represent a challenge for experimental detection, but we demonstrate how magnetic fields can reduce these resonance frequencies. Near the spin-flop transition, there is a significant enhancement of the dc spin pumping and inverse spin Hall voltage for the uniaxial antiferromagnets MnF2 and FeF2. In the third reporting period, we predict there is a strong nonlocal coupling between antiferromagnets and ferromagnets in cavities. Microwaves couple to magnetic moments in both ferromagnets and antiferromagnets. Although the magnons in ferromagnets and antiferromagnets radically differ, they can become entangled via strong coupling to the same microwave mode in a cavity. The equilibrium configuration of the magnetic moments crucially governs the coupling between the different magnons, because the antiferromagnetic and ferromagnetic magnons have opposite spins when their dispersion relations cross. We derive analytical expressions for the coupling strengths and find that the coupling between antiferromagnets and ferromagnets is comparable to the coupling between two ferromagnets. Our findings reveal a robust link between cavity spintronics with ferromagnets and antiferromagnets. In the fourth reporting period, we utilize the recent discovery of magnetism in two-dimensional van der Waals systems that opens the door to discovering exciting physics. We investigate how a current can control the ferromagnetic properties of such materials. Using symmetry arguments, we identify a recently realized system in which the current- induced spin torque is particularly simple and powerful. In Fe3GeTe2, a single parameter determines the strength of the spin-orbit torque for a uniform magnetization. The spin-orbit torque acts as an effective out-of-equilibrium free energy. The contribution of the spin-orbit torque to the effective free energy introduces new in-plane magnetic anisotropies to the system. Therefore, we can tune the system from an easy-axis ferromagnet via an easy-plane ferromagnet to another easy-axis ferromagnet with increasing current density. This finding enables unprecedented control and provides the possibility to study the Berezinskii-Kosterlitz-Thouless phase transition in the 2D XY model and its associated critical exponents.

The research results by SpInS are expected to provide significant guidance for experiments studying interfaces between antiferromagnetic insulator spintronics and conventional electronics. The results provide a clear guideline for which materials and experimental conditions are the most optimal for use in experiments involving electrical injection and detection of spin currents in antiferromagnetic insulators. Antiferromagnetic insulator spintronics shows promise for future applications in nanocomputing, with the main selling points being a low energy consumption and operations on a time-scale that can be 100-1000 times faster than current computer-processor speeds. Our results are expected to be of value for experiments investigating the electrical generation and detection of spin signals in antiferromagnetic insulators, which will further develop our understanding of these signals.

To realize our long term vision of groundbreaking insulator spintronics, we aim for a breakthrough in the fundamental understanding of electric signal generation, manipulation, transmission, and detection of spin-waves (magnons) in magnetic insulators. The insulator spintronics will be developed to integrate with conventional electronics. The most critical R&D challenge is to develop an understanding of how spin waves in magnetic insulators are coupled to electric currents in adjacent normal metals. First, we will treat spin-waves as classical variables. We will develop a quantitative understanding of how electric currents can induce spin transfer from normal metals to magnetic insulators that in turn excite dynamic magnetization modes. This will widen the applicability of first intriguing experimental results on the transmission of electric signals by spin-wave interconversion in magnetic insulators, published in Nature in 2010, but not yet reproduced by other experimental groups. Furthermore, we will address how long spin waves propagate by modelling the interaction between magnons and lattice vibrations (phonons) and study if phonons can be used to manipulate the magnetic order. Second, we will explore the ultimate quantum limit of magnonic supercurrents. At sufficient density, magnons condense into a single Bose quantum state which supports dissipationless transport over macroscopic distances. Spectroscopically generated magnon condensates have been observed at room temperature. We will elucidate if magnon condensation can be electrically induced and controlled at room temperature so that it can enable dissipationless spin-transport over large distances. Because insulator spintronics do not contain moving charges, they transmit signals at little or even zero power. Given the importance of electronics in today's society, we anticipate a radical paradigm shift in equipment development and ICT, as well as radically decreased energy usage.