Measurement of the orbital Hall effect in Cu and Al by optically probed spin-torque- and microwave-ferromagnetic resonance

Orbital currents are useful for transferring angular momentum in solid-state devices. Recently, the orbital Hall effect (OHE) was reported in pure Cu and Al, which are two technologically attractive metals that have been traditionally used for interconnects. Interestingly, recent theoretical studies have reported a wide variation in their orbital Hall conductivities, with some even differing on the polarity of the effect. Here, we present an in‐depth study of the OHE in Cu and Al. We employ two optical techniques: an optically probed ferromagnetic resonance (FMR) and an optical spin‐torque FMR (STFMR) and compare them to an all‐electrical STFMR measurement. The two optical methods yield consistent effective OHE conductivities of ∼ 5200 (ℏ/e)(Ω cm)⁻¹ and ∼ −2700 (ℏ/e)(Ω cm)⁻¹ in 30 nm of Cu and Al, respectively. In contrast, the electrical STFMR measurement exhibits a much lower signal‐to‐noise ratio and requires dedicated devices depending on the material composition. Nonetheless, it confirms the main trends measured optically. Due to their high sensitivity and ability to mitigate issues, such as low resistance and shunting, optical detection methods are expected to be pivotal in unraveling the elusive nature of orbital currents in highly conductive metals.