Orbital torque and efficient magnetization switching using ultrathin Co|Al light-metal interfaces: Experiments and modeling

TL;DR

The study demonstrates that orbital torque (OT) at Co|Al interfaces is driven by orbital momentum locking and the orbital Rashba Edelstein effect (OREE), enabling strong field-like torques with light elements and a significant damping-like component via orbital channels. By combining second-harmonic Hall measurements, DFT/Kubo linear-response theory, and semi-classical modeling, the authors show that pure spin-Hall effect (SHE) mechanisms cannot account for the observed torques, and that Pt interlayers suppress the OML and OREE, reducing OT/ SOT efficiencies. First-principles calculations reveal a large OML on Co|Al that rapidly collapses with Pt insertion, while the OREE response is substantial for Co|Al and much smaller when Pt is present; torkance calculations indicate the orbital channel dominates the FL torque and contributes to the DL torque. These insights enable OT-assisted magnetization switching in nanoscale pillars, with reduced switching currents, and establish design rules for engineering interfacial orbital polarization to enhance low-power spintronic devices.

Abstract

The emergence of the orbital degree of freedom in modern orbitronics offers a promising alternative to heavy metals for the efficient control of magnetization. In this context, identifying interfaces that exhibit orbital-momentum locking and an orbital Rashba-Edelstein response to an external electric field is of primary importance. In this work, we experimentally investigate the Co/Al system and extend the study to Co/Pt/Al structures. We show that inserting ultrathin Pt layers between Co and Al can significantly modify the orbital properties, highlighting the critical role of Co/Al orbital bonding in generating orbital polarization. We further model the orbital response of these systems using semi-phenomenological approaches and linear-response theory within the framework of density-functional theory.

Figures (6)

• Figure 1: AHE resistances measured in series of multilayers. (a) Two series of samples were measured varying Co thickness in $\mathrm{Ta(5)|Pt(8)|Co(}t_{Co}\mathrm{)|Al(1.4 \text{or} 3)|Pt(3)}$. We then measured AHE (b) in samples with Pt inserted between Co and Al ($\mathrm{Ta(5)|Pt(8)|Co(0.9)|Pt(}t_{Pt}\mathrm{|Al(3)|Pt(3)}$). The thickness of bottom Pt layer is varied in $\mathrm{Ta(5)|Pt(}t_{Pt}\mathrm{)|Co(0.9)|Al(5)}$ (c).(d) AHE resistance as a function of Al thickness in $\mathrm{Ta(5)|Pt(8)|Co(0.9)|Al(}t_{Al}\mathrm{)|Pt(3)}$.
• Figure 2: Co thickness dependence of the damping like torque efficiency (a), the field like torque efficiency (b) and the ratio between the two torques (c). For each of the plots, we compare two series of sample with different Al thickness: Ta(5)|Pt(8)|Co($t_{Co}$)|Al(1.4-3)|Pt(3). The orange plot represents the reference sample replacing Al by Cu. The green star represents the sample Ta(5)|Pt(8)|Co($t_{Co}$)|Al(4)Ox free of the top Pt. The black dot line represent the line without any Rashba contribution. Data from Al(1.4) series are adapted from Ref. krishnia2023large.
• Figure 3: Anisotropy (a), damping like torque effective field (b) and field like torque effective field (c) measured as a function of the thickness of Pt between Co and Al in $\mathrm{Ta(5)|Pt(8)|Co(0.9)|Pt(}t_{Pt}\mathrm{|Al(3)|Pt(3)}$. In each plot this series of samples is compared to sample without Pt between Co and Al and a sample without Al: $\mathrm{Ta(5)|Pt(8)|Co(0.9)|Pt(3)}$.
• Figure 4: Expectation values of the $\braket{\hat{L}_x}$ (left panel) and $\braket{\hat{L}_y}$ (right panel) projected OAM onto the Fermi surface of Co(12)|Al(12) interfaces displaying an orbital moment transverse to the direction of the electronic wavevector $k$: the OML.
• Figure 5: Expected values of (a) $\braket{l_x}$ and (b) $\braket{l_y}$ projected over the Brillouin zone in Al|Pt(1)|Co. c) Integrated orbital moment density (accumulation) in response over the full Co thickness in Co(12)|Al(12) and CO(12)|Pt(1,2,3)|Al interfaces. numbers refers to the stacked atomic plane.