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Recent theories have predicted that unlike the spin Hall effect, the orbital Hall effect can be gigantic even in light metals because it does not require spin-orbit interaction [1]. This opens a novel route toward electric control of magnetic moments in spintronic devices by harnessing the angular momentum carried by orbital current [2]. As the choice of materials is not restricted to heavy elements, utilizing the orbital current offers great flexibility in choosing environment-friendly materials [3]. In recent years, so-called the orbital torque mechanism has been found in various magnetic heterostructures, demonstrating its potential for highly efficient spintronic devices [4-7]. So far, however, direct experimental evidence of the orbital current has still been missing. In this talk, we present a magneto-optical measurement of the orbital accumulation induced by the orbital Hall effect in a light metal Ti [8]. The orbital accumulation at the surface of Ti leads to ~ 50 nrad of Kerr rotation of a polarized light for current density ~107 A/cm2, which is of the same order of magnitude for the Kerr rotation induced by the spin Hall effect in Pt [9]. Our analysis based on semi-realistic calculations shows that this cannot be explained by the spin Hall effect, which is far too small in Ti, and the estimation of the Kerr angle caused by the spin accumulation is two orders of magnitude smaller than what we observe in experiment. Moreover, variations of the sign and magnitude of the Kerr angle agree with the theoretical model based on the orbital Hall effect while they significantly deviate from the model based on the spin Hall effect. As another evidence, we also measure the orbital torque in Ti/Co heterostructures. We find gigantic field-like component, which amounts to the orbital Hall angle of 0.3. Our experimental results, which are corroborated by theoretical analyses, provide a direct and strong evidence of the orbital Hall effect and serve as a solid ground for future directions of orbitronics research [10].
References:
[1] D. Go et al., Phys. Rev. Lett. 121, 086602 (2018).
[2] D. Go and H.-W. Lee, Phys. Rev. Res. 2, 013177 (2020).
[3] D. Jo, D. Go, and H.-W. Lee, Phys. Rev. B 98, 214405 (2018).
[4] S. Ding et al., Physical Review Letters 125, 177201 (2020).
[5] J. Kim et al., Phys. Rev. B 103, L020407 (2021).
[6] S. Lee et al., Commun. Phys. 4,234 (2021).
[7] D. Lee et al., Nat. Commun. 12, 6710 (2021).
[9] Y.-G. Choi et al., arXiv:2109.14847.
[9] C. Stamm et al., Phys. Rev. Lett. 119, 087203 (2017).
[10] D. Go et al., Europhys. Lett. 135, 37001 (2021).
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