PRESS RELEASE
Source: Tokyo Institute of Technology
For immediate release: September 5, 2022
Investigating Magnetic Excitation-induced Spin Current in Chromium Trihalides(Tokyo, September 5) A general formula that can calculate the spin current induced byoscillating magnetic fields in magnetic materials and aid our understanding of novel
spintronics functionality has been developed by researchers from Tokyo Tech and Chiba
University. The formula predicts large spin currents arising from a hitherto unknown
contribution in antiferromagnetic chromium trihalides, opening doors to material designfor novel spintronics devices.
An ingenious approach toward developing low-power, high-speed, and high-density memory
devices is based on spintronics, an emerging frontier in technology that harnesses a degreeof freedom of electrons known as “spin.” Put simply, electrons, along with their negative
charge, possess a “spin” whose orientation can be controlled using magnetic fields. This isparticularly relevant for magnetic insulators, in which the electrons cannot move around, but
the “spin” remains controllable. In these materials, the magnetic excitations can give rise toa “spin current,” which forms the basis of spintronics.
Scientists have been looking for efficient methods to generate the spin current. The“photogalvanic effect,” a phenomenon characterized by the generation of dc current fromlight 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, we currently lack candidate materials and a general mathematical formulation forexploring this phenomenon.
Now, Associate Professor Hiroaki Ishizuka from Tokyo Institute of Technology (Tokyo Tech),along with his colleague, has addressed these issues. In their recent breakthrough publishedin Physical Review Letters, they presented a general formula that can be used to calculate thephotogalvanic spin current induced by transverse oscillating magnetic excitations. They thenused this formula to understand how photogalvanic spin currents arise in bilayer chromium(Cr) trihalide compounds, namely chromium triiodide (CrI3) and chromium tribromide (CrBr3).
“Unlike past studies that considered longitudinal oscillating magnetic fields for generating
spin currents, our study focuses on transverse oscillating magnetic fields. Based on this, wefound that processes involving one magnon (quantum of spin wave excitations) band as wellas two magnon bands contribute to the spin current,” elaborates Dr. Ishizuka, speaking of
their findings.
Using their formula, the duo found that both CrI3 and CrBr3 showed a large photogalvanic spincurrent for magnetic excitations corresponding to electromagnetic waves at gigahertz andterahertz frequencies. However, the current only appeared when the spins showedantiferromagnetic ordering, i.e., successive spins were anti-parallel, as opposed toferromagnetic ordering (successive spins were parallel). Moreover, the spin current direction
was governed by the orientation of the antiferromagnetic ordering (whether the spins on thefirst and second layers were arranged up-down or down-up). Additionally, they pointed out
that, unlike previous findings that attributed the spin current to only the two-magnon process,their formula showed that a large response was, in general, possible with the single magnon
process.
These results suggest that bilayer CrI3 and CrBr3 are strong candidates for investigating themechanism associated with photogalvanic spin current generation. “Our study not only
predicts unforeseen contributions to the spin current but also provides a guideline for thedesign of novel materials driven by the photogalvanic effect of magnetic excitations,”
highlights Dr. Ishizuka.
Indeed, the work done by the duo could open up new horizons in spintronics!
Reference

Authors:
Title of original
paper:
Journal:DOI:
Affiliations:

Hiroaki Ishizuka1,* and Masahiro Sato2, 3Large photogalvanic spin current by magnetic resonance in bilayer Cr trihalides
Physical Review Letters10.1103/PhysRevLett.129.107201
1
Department of Physics, Tokyo Institute of Technology
2
Department of Physics, Ibaraki University
3
Department of Physics, Chiba University*Corresponding author’s email: ishizuka@phys.titech.ac.jp
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