Seminar: Orbitronics: A pathway to a sustainable future

The Spin Hall Effect (SHE) offers promising applications in spintronic devices, such as controlling magnetization switching in magnetic random-access memory (MRAM) [1,2], spintronic terahertz emitters [3], and magnetic sensors [4]. However, it relies on materials with high spin-orbit coupling (SOC), typically found among heavier elements in the periodic table, often expensive, and detrimental to the environment. My presentation proposes an alternative approach leveraging the orbital Hall effect (OHE), which facilitates the conversion of charge current into orbital current in spin-orbitronics devices [5,6], potentially reducing reliance on SOC materials and paving the way for a more sustainable future. I will address three questions: (1) How can we experimentally distinguish the SHE and the OHE [7]? (2) How does large OHE improve torque and reduce switching current density for Spin-Orbit Torque (SOT) MRAM devices [8]? (3). Does the orbital Hall effect exhibit the reciprocity phenomenon or not [9]?

In this context, we begin by examining the substantial orbital Hall torque in low-SOC Nb and Ru systems. We observed significant damping-like torque efficiencies in Nb/Ni and Ru/Ni bilayers compared to Nb/FeCoB, with unique sign reversals in Nb/Ni compared to Nb/FeCoB, highlighting distinctive orbital Hall properties. Additionally, our findings reveal a notable increase in torque in Ru/Ni as a function of Ni thickness, deviating from typical spin torque behavior and suggesting a unique long-range effect of the OHE [7].

To explore the impact of the OHE in SOT-MRAM, we demonstrate that Ru/Pt OHE layers achieve a 30% enhancement in orbital Hall torque and reduce switching current density by 25–30% compared to pure Pt layers associated with the SHE. Extensive testing on over 250 industrial-scale devices confirms the OHE's performance and environmental advantages in SOT-MRAM [8]. Lastly, we show that magnon generation, facilitated by the combination of the SHE and OHE, and its detection via the iSHE, is approximately 35% more efficient than the inverse process [9].

References:
1. J. Sinova et al., Rev. Mod. Phys. 87, 1213 (2015)
2. S. Husain, R. Gupta et al., Appl. Phys. Rev. 7, 041312 (2020)
3. R. Gupta et al., Adv. Optical Mater. 9, 2001987 (2021)
4. S. Koraltan, R. Gupta et al., Phys. Rev. App. 20, 044079 (2023)
5. D. Go et al., Phys. Rev. Lett. 121, 086602 (2018)
6. S. Ding et al., Phys. Rev. Lett. 125, 177201 (2020)
7. A. Bose, R. Gupta et al., Phys. Rev. B 107, 134423 (2023)
8. R. Gupta et al., Nat. Commun. 16, 130 (2025)
9. J. O. L. Martin, R. Gupta et al., arXiv:2411.07044 (2024)