Spin current

In 2023, EPFL researchers succeeded in sending and storing data using charge-free magnetic waves called spin waves, rather than traditional electron flows. The team from the Lab of Nanoscale Magnetic Materials and Magnonics, led by Dirk Grundler, used radiofrequency signals to excite spin waves enough to reverse the magnetization state of tiny nanomagnets. When switched from 0 to 1, for example, this allows the nanomagnets to store digital information; a process used in computer memory, and more broadly in information and communication technologies. This work was a big step toward sustainable computing, because encoding data via spin waves (whose quasiparticles are called magnons) could eliminate the energy loss, or Joule heating, associated with electron-based devices. But at the time, the spin wave signals could not be used to reset the magnetic bits to overwrite existing data.

Now, Grundler's lab at EPFL, in collaboration with colleagues from Beihang University, ETH Zurich, Japan Atomic Energy Agency, Chinese Academy of Sciences and China's International Quantum Academy, have published a study that could make such repeated encoding possible. Specifically, they report unprecedented magnetic behavior in hematite: an iron oxide compound that is earth-abundant and much more environmentally friendly than materials currently used in spintronics.

Researchers from Beihang University and Freie Universität Berlin have developed a spintronic THz emitter with a microscale stripe pattern that enables the modulation of chirality during THz wave generation. Unlike traditional THz sources that rely on external optical components, this emitter incorporates polarization tuning directly into its design, streamlining the technology and enhancing its capabilities.

The emitter comprises thin-film layers of tungsten, cobalt-iron-boron, and platinum. When exposed to ultrafast laser pulses, the material generates a spin current, which is converted into an electrical charge through the inverse spin Hall effect.

A joint research team, led by Professor Jeong Myung-hwa from Sogang University and Professors Lee Kyung-jin and Kim Gap-jin from the Korea Advanced Institute of Science and Technology (KAIST), has captured, for the first time, the phenomenon of quantum mechanical spin pumping occurring at room temperature.

With charge current,  as current flows, electrons collide with atoms inside the material, generating heat and increasing energy consumption. This lowers the efficiency of current generation. To address this, researchers worldwide are conducting studies on creating electronic devices using spin current. The research team focused on the spin pumping phenomenon where spin moves from a ferromagnet to a non-magnetic material due to precession.

A University of Tokyo research team has shown that the direction of a spin-polarized current can be restricted to only one direction in a single-atom layer of a thallium-lead alloys when irradiated at room temperature. 

This discovery defies conventions as single-atom layers have been thought to be almost completely transparent, in other words, negligibly absorbing or interacting with light. The one-directional flow of the current observed in this study could enable functionality beyond ordinary diodes, paving the way for more environmentally friendly data storage and ultra-fine two-dimensional spintronic devices. 

Researchers at CIC nanoGUNE BRTA, Charles University in Prague and IKERBASQUE have designed a new complex material with unique properties that could be beneficial for spintronics.

Twist engineering has emerged as a fascinating approach for modulating electronic properties in van der Waals heterostructures. While theoretical works have predicted the modulation of spin texture in graphene-based heterostructures by twist angle, experimental studies are lacking. In this recent work, by performing spin precession experiments, the team demonstrates tunability of the spin texture and associated spin–charge interconversion with twist angle in WSe₂/graphene heterostructures.

Researchers use ultrashort laser pulses to trigger a spin-aligned electron flow on the few-femtosecond timescale—opening up a possible path toward faster spintronic devices.

Spintronics technology requires a rapid, controlled way to create spin currents. To that end, the researchers have demonstrated that short laser pulses can create spin currents within a few femtoseconds (10–15 s) — about 30 times faster than previous techniques. The method, they believe, should provide a more flexible and precise way to generate spin currents by taking advantage of the control that physicists have over laser light.

Researchers from ETH Zurich recently developed a method to control the quantum states of single electron spins using spin-polarized currents, which could enhance quantum computing technologies. The new technique offers more precise, localized control compared to traditional methods using electromagnetic fields, potentially improving the manipulation of quantum states in devices like qubits. 

Control over quantum systems is typically achieved by time-dependent electric or magnetic fields. Alternatively, electronic spins can be controlled by spin-polarized currents. In their recent work, the team demonstrated coherent driving of a single spin by a radiofrequency spin-polarized current injected from the tip of a scanning tunneling microscope into an organic molecule. With the excitation of electron paramagnetic resonance, the scientists established dynamic control of single spins by spin torque using a local electric current. In addition, their work highlights the dissipative action of the spin-transfer torque, in contrast to the nondissipative action of the magnetic field, which allows for the manipulation of individual spins based on controlled decoherence.

Spintronics relies on the transport of spin currents for computing and communication applications. New device designs would be possible if this spin transport could be carried out by both electrons and magnetic waves called magnons. But spin transport via magnons typically requires electrically insulating magnets—materials that cannot be easily integrated with silicon electronics. Recently, a novel way to bypass that requirement has been developed by researchers at ETH Zürich, Bavarian Academy of Sciences, Technical University of Munich, University of Konstanz, Munich Center for Quantum Science and Technology (MCQST) and Autonomous University of Madrid.

The researchers say that this finding could have important implications for both spintronic applications and fundamental research on spin transport. To demonstrate their concept, the scientists placed two magnetic, metallic strips—each hosting coupled electrons and magnons—on a nonmagnetic, insulating substrate. In the first strip, the researchers converted electron charge currents to electron spin currents. These spin currents were transferred first to the magnons in the same strip, then across the substrate to the magnons in the second strip, and finally to the electrons in the second strip. The researchers detected this spin transport by converting the electron spin currents in the second strip to charge currents.

Researchers at North Carolina State University, University of Pittsburgh, University of Illinois at Urbana-Champaign, Chinese Academy of Sciences and Beijing Normal University have studied how the spin information of an electron, called a pure spin current, moves through chiral materials. 

They found that the direction in which the spins are injected into chiral materials affects their ability to pass through them. These chiral “gateways” could be used to design energy-efficient spintronic devices for data storage, communication and computing.

Researchers from Tohoku University, University of Tokyo, Australian Nuclear Science and Technology Organization, High Energy Accelerator Research Organization and Comprehensive Research Organization for Science and Society have found that the spin current signal changes direction at a certain magnetic temperature and diminishes at lower temperatures.

Spintronics uses electrons’ intrinsic spin, which is vital to the field, to regulate the flow of the spin degree of freedom, that is, spin currents. Scientists are continually exploring new ways to manage spintronics for future uses. Detecting spin currents is quite complicated and necessitates the use of macroscopic voltage measurement, which examines the entire voltage fluctuations across a material. However, a major stumbling block has been a lack of understanding of how the spin current flows or propagates inside the material.