Nearly two decades ago, physicists detected a phenomenon known as the Spin Hall effect1,2, in which an electric field separates electrons in a material based on their intrinsic angular momentum, or spin. This discovery stimulated the field of spintronics, which is a branch of electronics that uses spin and electric charge to transfer and store data. Write inside NatureChoi et al.3 report the direct detection of a related phenomenon, called the orbital Hall effect, in which the field sorts the electrons according to their orbital angular momentum, which correlates with their rotational motion. The prospect of encoding data in orbitals has been dubbed orbitronics and could lead to the development of environmentally friendly electronic devices.
Conventional electronics use electrical charge to process information, but computer memories built this way are volatile. By incorporating both charge and spin, spintronics offer a more stable alternative. However, it requires information transmitted as charge to be converted into spin currents and vice versa. This is usually accomplished by making use of a phenomenon known as spin-orbital coupling, where an electron’s spin interacts with its orbital motion. In the presence of an electric field, this interaction causes the electrons to move in a direction that depends on their spin, thus generating a spin flux perpendicular to the electric current. This is the Spin Hall effect.
Read the article: Observation of the orbital Hall effect in a light metal Ti
This mechanism for converting spin to charge works best in metals that have strong spin-orbital coupling, such as gold, platinum, and tungsten. But these metals are scarce and expensive, and their extraction can cause significant environmental damage. Orbitronics aims to overcome these limitations by manipulating the magnetic moment produced by the orbital angular momentum of electrons (instead of their spin), thus bypassing the need for materials with strong spin-orbital coupling. This approach expands the range of materials in which the magnetic moment of electrons can be controlled electrically.
The orbital Hall effect was first proposed4 in 2005, but making use of it proved challenging. Part of the problem is that it is difficult to distinguish the magnetic moments arising from orbital angular momentum from those associated with spin. To get around this problem, Choi and colleagues used titanium, a lightweight metal with weak spin-orbital coupling and negligible spin Hall effect. Titanium itself has no substantial advantages over metals with strong spin-orbital coupling, other than that it is much more abundant in the earth’s crust than these metals. But the authors’ proof of principle shows the feasibility of orbitonics, which could be used to manipulate orbitals in any type of material, thus showing the potential of environmentally friendly electronics.
One reason why measuring the orbital Hall effect presents a challenge is that the generated orbital currents do not directly affect the magnetization of a material, so these currents can only be detected indirectly in magnetic materials, through spinorbit coupling. One possible solution to this problem involves using light reflected from the surface of a material to measure orbital magnetization through a phenomenon called the magneto-optical Kerr effect. To detect spin accumulation, this experimental technique requires spin-orbital coupling, but if this coupling is weak enough, the measurements are more sensitive to orbital angular momentum than to spin.
Choi et al. generated accumulations of magnetic moments with different orientations on opposite surfaces of a titanium sample by passing an electric current through it (Fig. 1). They then illuminated the sample with linearly polarized light; this means that the electric field of light is confined to a single plane and oscillates along a fixed direction. By analyzing the polarization of the beam reflected from the surface, they could detect magnetic moments because the magneto-optical Kerr effect rotates the polarization of light, and the degree of rotation is proportional to the magnetization of the surface. After confirming an orbital buildup resulting from the orbital Hall effect, the authors used complementary measurements to rule out other possible explanations.
Figure 1 | Detection of orbital Hall effect in titanium. AThe orbital Hall effect is a phenomenon in which an electric current causes electrons to separate based on their orbital angular momentum, which is related to their orbital motion. This leads to angular momentum on one surface of a different material than on the opposite surface and manifests as surface magnetization in opposite directions. Choi et al.3 induced this effect in titanium and then illuminated the sample with linearly polarized light, which has an electric field that oscillates in only one direction. The magnetization resulting from the orbital Hall effect rotated the polarization of the reflected beam, making it detectable. bWhen the direction of the electric field was reversed, the polarization rotated in the opposite direction, indicating that the surface magnetization had changed sign.
Although the data suggest a direct detection of the orbital Hall effect, the measured magnetic moment buildup was much less than predicted by theoretical calculations3. This discrepancy shows that the behavior of orbital angular momentum in solids is not yet fully understood. Estimating magnetic moment buildup requires an understanding of how orbital angular momentum spreads through a material and the different ways orbitals can lose their magnetic moment, a process known as relaxation. While many of these processes are known to spin, little is known about the mechanisms of orbital angular momentum and the interaction between spin and orbital relaxation processes5.
Furthermore, other scattering processes that electrons undergo in titanium could potentially lead to orbital relaxation, as well as other forms of the orbital Hall effect. The presence of impurities in a material with strong spin-orbital coupling can cause electrons to scatter, resulting in a different type of spin Hall effect than pure metals. It is not clear whether such scattering processes can also generate an orbital Hall effect.
Choi and colleagues work and other studies6 shed light on directions for exploring the potential of orbitronics. One of the most pressing challenges in this field is to understand how orbital dynamics interact with spin, light and phonons (collective atomic vibrations) and how these interactions can be used in new technologies. Exploring other orbittronic effects, such as the connection between orbital magnetic moments and electrical polarization in solids, could also have far-reaching implications for the future of electronics. But in detecting the orbital Hall effect, Choi et al. have taken a critical step toward developing methods to manipulate magnetic materials using only electric fields, eliminating the need for spinorbit coupling.
Conflicting interests
The author declares no competing interests.
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