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Rectangular tungsten producing large spin currents

Electrons have two properties, "charge" and "spin. The technology to extract various functions by controlling the flow of charge (current) is well known as electronics. Recently, however, "spintronics," a technology to extract new functions by actively utilizing the degree of freedom of spin, has been attracting attention in addition to this technology. Electrons have two states: upward spin and downward spin. The physical phenomenon of "spin currents," in which these flow in opposite directions, is thought to have applications in information transmission and magnetic memory, and is being vigorously studied experimentally, theoretically, and computationally.

Spin currents are obtained in materials containing heavy elements, and since their magnitude depends greatly on the crystal structure, new materials that can generate large spin currents are being searched for. Tungsten, which means "heavy stone" in Swedish, has been observed to have two cubic crystal structures (cubic lattice structure) and is known to generate one of the largest spin currents among materials made of a single element. We focused on tungsten and searched for a crystal structure that further increases the spin current. Since crystal structures have a great number of degrees of freedom in terms of lattice geometry and atomic positions, a great deal of effort is required to find a stable structure that exhibits low energy. In this study, we used an "evolutionary algorithm," a method for finding optimal solutions inspired by Charles Darwin's theory of evolution. This algorithm was applied to tungsten in combination with first-principles electronic structure calculations, which can predict the physical properties of a system based on the principles of quantum mechanics without reference to experimental results, and found that two different types of rectangular crystal structures (rectangular structure with a unit lattice shape) were obtained, both of which generate a larger spin current than the cubic structure Both of these structures produce a larger spin current than the cubic structure. We also confirmed that the spin currents in other heavy metals such as molybdenum and tantalum are also enhanced by adopting these tetragonal structures.

Although the tungsten tetragonal structure is a theoretical material predicted by computer simulation, we believe that it is possible to create these tetragonal structures using modern deposition technology, which produces artificial materials by stacking thin films layer by atomic layer, and to demonstrate the enhancement of spin currents. If this can be achieved, it is expected to lead to the development of even more innovative spin current generating materials.

	Two types of tetragonal structures in tungsten predicted using evolutionary algorithms and first-principles electronic structure calculations. Spheres indicate tungsten atoms. The rectangular lattice, indicated by the boxed lines, is periodically aligned to form a crystal. Both structures produce a larger spin current than the cubic structure known so far.

This work was published in T. Ishikawa et al, Phys. Rev. Materials 7, 026202 (2023).

(Press release, February 15, 2023)

Published in Faculty of Science News May 2023

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