Spin current

Researchers at the Max Born Institute in Germany recently discovered a method for inducing and controlling the flow of spin and valley currents in ultrafast timeframes, using specially designed laser pulses. This discovery could offer a novel perspective on the search for the next generation of information technologies.

Ultrafast laser control over the basic quantum properties of matter is a critical challenge that must be addressed to develop future information technologies beyond the semiconductor electronics that define our current era. Electron spin and valley index, an emergent property of two-dimensional materials related to quasiparticle momentum, are two promising quantum properties in this regard. Both spintronics and valleytronics offer many potential advantages over classical electronics in terms of data manipulation speed and energy efficiency. While spin excitations suffer from a dynamic loss of character due to spin-orbit-induced spin precession, the valley wavefunction represents a more stable "data bit" that is only threatened by intervalley scattering, a feature controllable by sample quality. Valleytronics thus presents a potentially robust platform for moving beyond classical electronics.

Researchers from Tohoku University, University of California Riverside and Massachusetts Institute of Technology (MIT) have highlighted a series of critical achievements in antiferromagnetic spintronics (including their own contributions), revealing an emerging frontier distinguished by the coherent spin dynamics of antiferromagnets. 

Within antiferromagnetic spintronics, scientists have exerted a lot of efforts on the switching and readout of static magnetic order. But coherent spin dynamics, the key to exploring the wave features of spins and integrating spintronics with quantum and neuromorphic technologies, has only received attention very recently. "The coherent spin dynamics of antiferromagnets exhibits a lot more intriguing features than that of ferromagnets," says Jiahao Han, a JSPS Research Fellow working at the Research Institute of Electrical Communication (RIEC), Tohoku University. "By harnessing this unique property, the team has been pursuing breakthroughs that eventually form a new chapter named coherent antiferromagnetic spintronics."

An international research team led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has developed a new method for the efficient coupling of terahertz waves with much shorter wavelengths, so-called spin waves.

The team's experiments, in combination with theoretical models, clarify the fundamental mechanisms of this process previously thought impossible. The results are an important step for the development of novel, energy-saving spin-based technologies for data processing.

Researchers from Germany's Martin Luther University Halle-Wittenberg (MLU) have demonstrated that strong alternating magnetic fields can be used to generate a new type of spin wave. This is the first time this was accomplished as the phenomenon was previously only theoretically predicted. Thee team reported on their work and provided the first microscopic images of these spin waves.

The basic idea of spintronics is to use a special property of electrons (spin) for various electronic applications. The Spin is the intrinsic angular momentum of electrons that produces a magnetic moment. Coupling these magnetic moments creates the magnetism that could ultimately be used in information processing. When these coupled magnetic moments are locally excited by a magnetic field pulse, this dynamic can spread like waves throughout the material. These are referred to as spin waves or magnons.

Scientists have been looking for efficient methods to generate spin current. The photogalvanic effect, a phenomenon characterized by the generation of DC current from light illumination, is particularly useful in this regard. Studies have found that a photogalvanic spin current can be generated similarly using the magnetic fields in electromagnetic waves. However, there's a need for candidate materials and a general mathematical formulation for exploring this phenomenon.

Now, Associate Professor Hiroaki Ishizuka from Tokyo Institute of Technology (Tokyo Tech), along with his colleague Masahiro Sato, addressed these issues. In their recent study, they presented a general formula that can be used to calculate the photogalvanic spin current induced by transverse oscillating magnetic excitations. They then used this formula to understand how photogalvanic spin currents arise in bilayer chromium (Cr) trihalide compounds, namely chromium triiodide (CrI₃) and chromium tribromide (CrBr₃).

A team of researchers, led by Igor Barsukov at the University of California, Riverside, in collaboration with researchers at Helmholtz-Zentrum Dresden-Rossendorf, the University of Utah, and the University of California, Irvine, has demonstrated efficient spin transport in an antiferromagnet/ferromagnet hybrid that remains robust up to room temperature. The researchers observed coupling of magnonic subsystems in the antiferromagnet and ferromagnet and recognized its importance in spin transport, a key process in the operation of spin-based devices. 

Antiferromagnets have zero net magnetization and are insensitive to external magnetic field perturbations. Antiferromagnetic spintronic devices hold great promise for creating future ultra-fast and energy-efficient information storage, processing, and transmission platforms, potentially leading to faster and more energy-efficient computers.  However, in order to be useful for applications impacting everyday life, the devices need to be able to operate at room temperature. One of the key factors in realizing antiferromagnetic spintronics is the injection of spin current at the antiferromagnetic interface. Previously, efficient spin injection at these interfaces was realized at cryogenic temperatures. 

A research team, led by the University of California, Berkeley, recently took a step toward a spin-based computer by demonstrating a way to switch spin currents on and off electrically.

The development of devices based on pure spin currents instead of charge currents is the goal of many scientists working in spin electronics, or spintronics. A subfield of spintronics, called magnonics, focuses on devices in which these spin currents are carried specifically by magnons—wave-like disturbances of the aligned spins in a magnetic material. Magnonics researchers face a challenge in that simply exciting magnons in a material is not enough to guarantee the creation of a spin current: when the magnons are uniformly distributed, the spin current is exactly equal to zero. The magnons must be controlled, and controlling magnons in insulating materials—ones that, because of the absence of charge currents, dissipate the least amount of energy—has proven difficult. In previous experiments, researchers have sought to achieve this control using large magnetic fields, but such fields can cause collateral heating, undermining the reason for pursuing magnonics in the first place.

Scientists have long since been trying to use electric fields to control spin at room temperature but achieving effective control has thus far been elusive. In a recent research work, a team from Rensselaer Polytechnic Institute and the University of California at Santa Cruz took a step forward in addressing the issue.

An electron has a spin degree of freedom, meaning that it not only holds a charge but also acts like a little magnet. In spintronics, a key task is to use an electric field to control electron spin and rotate the north pole of the magnet in any given direction. The spintronic field effect transistor harnesses the so-called Rashba or Dresselhaus spin-orbit coupling effect, which suggests that one can control electron spin by electric field. Although the method holds promise for efficient and high-speed computing, certain challenges must be overcome before the technology reaches its true, miniature but powerful, and eco-friendly, potential.

Researchers from Beihang University and University of British Columbia have found that spin flipping can be achieved by the valley-Zeeman SOF in monolayer tungsten diselenide (WSe₂) at room temperature, which manifests as a negative magnetoresistance in the vertical spin valve.

Manipulating spins can enable the development of ultralow power electronics, but previous approaches were limited by the strength of the effective field and high-quality structures. The team in this recent study explored a mechanism to manipulate spins at room temperature with monolayer tungsten diselenide, in virtue of a novel giant spin-orbit field.

A research team from the Technical University of Kaiserslautern (TUK) has been awarded a Consolidator Grant from the European Research Council (ERC) to develop spintronic devices.

Professor Dr. Mathias Weiler, lead of the study, will receive €2 million over the next five years. Scientists are working on spin waves and new spintronic devices that could drastically accelerate the storage, processing, and transmission of information.