[Paper Review] Interfacial Dzyaloshinskii-Moriya interaction, surface anisotropy energy,and spin pumping at spin orbit coupled Ir/Co interface

Kim, Nam-Hui, Jung, Jinyong+6 more|arXiv (Cornell University)|Jan 10, 2016

Magnetic properties of thin films76 citations

TL;DR

This study uses Brillouin light scattering (BLS) to investigate interfacial Dzyaloshinskii-Moriya interaction (iDMI), surface anisotropy, and spin pumping at Ir/Co interfaces. Contrary to prior reports, it finds the iDMI sign at Ir/Co is the same as in Pt/Co systems, with a smaller iDMI energy density (0.7 mJ/m²) despite Ir's strong spin-orbit coupling. The saturation magnetization and perpendicular magnetic anisotropy are significantly enhanced, and spin pumping increases the Gilbert damping constant from 0.012 to 0.024 for 1.5 nm Co due to strong spin-orbit coupling at the interface.

ABSTRACT

The interfacial Dzyaloshinskii-Moriya interaction (iDMI), surface anisotropy energy, and spin pumping at the Ir/Co interface are experimentally investigated by performing Brillouin light scattering. Contrary to previous reports, we suggest that the sign of the iDMI at the Ir/Co interface is the same as in the case of the Pt/Co interface. We also find that the magnitude of the iDMI energy density is relatively smaller than in the case of the Pt/Co interface, despite the large strong spin-orbit coupling (SOC) of Ir. The saturation magnetization and the perpendicular magnetic anisotropy (PMA) energy are significantly improved due to a strong SOC. Our findings suggest that an SOC in an Ir/Co system behaves in different ways for iDMI and PMA. Finally, we determine the spin pumping effect at the Ir/Co interface, and it increases the Gilbert damping constant from 0.012 to 0.024 for 1.5 nmthick Co.

Motivation & Objective

• To experimentally determine the sign and magnitude of interfacial Dzyaloshinskii-Moriya interaction (iDMI) at Ir/Co interfaces, challenging prior conflicting reports.
• To investigate the role of strong spin-orbit coupling (SOC) in Ir/Co on perpendicular magnetic anisotropy (PMA), saturation magnetization, and spin pumping.
• To measure spin pumping effects and their impact on Gilbert damping using Brillouin light scattering (BLS).
• To clarify the distinct roles of SOC in iDMI, PMA, proximity-induced magnetization, and spin pumping at Ir/Co interfaces.

Proposed method

• Employed wedge-shaped Ta(4 nm)/Ir(4 nm)/Co(tCo)/AlOx(2 nm) heterostructures on SiO2 substrates with Co thicknesses from 1 to 3 nm.
• Used p-polarized 532 nm laser in BLS to excite Damon-Eshbach surface spin waves with in-plane wave vector kx = 0.0167 nm⁻¹.
• Measured frequency difference (Δf) between Stokes and anti-Stokes peaks to extract iDMI energy density using Δf = 2γD / (πMs kx).
• Performed angle-dependent Δf measurements (θα from -90° to +90°) to confirm iDMI sign via sinusoidal fitting to Δf(θα) = Δf₀ sinθα.
• Extracted saturation magnetization (Ms) and effective anisotropy (Keff) from Keff × tCo vs. tCo linear fits.
• Determined Gilbert damping constant α from linewidth (FWHM) of BLS resonance peaks, with α extrapolated to tCo⁻¹ = 0 to obtain bulk Co damping (αbulk).

Experimental results

Research questions

• RQ1 What is the sign and magnitude of the interfacial Dzyaloshinskii-Moriya interaction (iDMI) at the Ir/Co interface, and how does it compare to Pt/Co?
• RQ2 How does strong spin-orbit coupling in Ir influence perpendicular magnetic anisotropy (PMA) and saturation magnetization in Co?
• RQ3 To what extent does spin pumping at the Ir/Co interface enhance the Gilbert damping constant?
• RQ4 Are the contributions of spin-orbit coupling to iDMI, PMA, and spin pumping interdependent or distinct?

Key findings

• The iDMI energy density at the Ir/Co interface is 0.7 mJ/m², significantly smaller than the 1.3 mJ/m² in Pt/Co/AlOx and 1.7 mJ/m² in Ta/Pt/Co/AlOx.
• The sign of iDMI at Ir/Co is the same as in Pt/Co systems, contradicting previous reports that claimed opposite sign.
• The saturation magnetization (Ms) and effective perpendicular magnetic anisotropy (Keff) are significantly enhanced due to strong spin-orbit coupling and proximity effects.
• The Gilbert damping constant increases from 0.012 (bulk Co) to 0.024 for 1.5 nm Co due to spin pumping at the Ir/Co interface.
• The tCo⁻¹ dependence of α confirms spin pumping is the dominant source of damping enhancement, with αbulk ≈ 0.012 matching bulk Co value (0.011).
• Two-magnon scattering and iDMI-induced damping contributions were ruled out, confirming spin pumping as the primary cause of linewidth broadening.