Origin of unexpected weak Gilbert damping in the LSMO/Pt bilayer system

TL;DR

This paper explains why LSMO thin films show higher Gilbert damping than LSMO/Pt bilayers by analyzing intrinsic and extrinsic effects, spin Hall angles, and layer-specific spin transport, offering insights for designing low-loss spintronic devices.

Contribution

It provides a first-principles and semiclassical analysis revealing the extrinsic origin of high damping and the role of spin Hall effects in LSMO/Pt heterostructures, challenging previous expectations.

Findings

Intrinsic damping of LSMO is approximately 1.4×10⁻³.

Large spin Hall angle in LSMO enhances spin-current conversion.

Pt layer reduces effective damping by shunting charge current.

Abstract

This study presents a first-principles and semiclassical analysis of the puzzling observation that a La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) thin film exhibits larger Gilbert damping than an LSMO/Pt bilayer, contrary to conventional spin-pumping expectations. Density functional theory with Wannier interpolation yields an intrinsic damping of $\alpha_{\mathrm{int}}^{\mathrm{LSMO}}\!\approx\!1.4\times10^{-3}$, supporting an extrinsic origin of the high experimental value. Guided by the self-induced inverse spin Hall effect (ISHE) demonstrated in LSMO [Gupta et al., Phys.Rev. B 109, 014437 (2024)], we argue that the large spin Hall angle $|\theta_{\mathrm{SH}}|\simeq 0.093$ and low longitudinal conductivity of LSMO enable an efficient conversion of spin current to charge current boosting the effective damping. In the LSMO/Pt heterostructure the Pt cap shunts the charge current, raising $\sigma_{xx}$ and reducing the interfacial $|\theta_{\mathrm{SH}}|$ to~0.007. A Valet-Fert analysis for layer-resolved ab-initio spin accumulation gives the Pt spin-diffusion length and a non-negligible antidamping SOT coefficient, qualitatively accounting for the observed damping reduction under current bias. The seemingly anomalous damping hierarchy is thus reconciled without invoking additional interfacial mechanisms. The distinct length scales governing spin-pumping normalization, namely, the short absorption depth relevant to self-pumping in a single LSMO film versus the full magnetic thickness applicable to an LSMO/Pt bilayer, are crucial in this context. This observation suggests a practical design strategy: by simultaneously tuning the spin Hall-to-longitudinal conductivity ratio and the spin-diffusion length, one can engineer heterostructures with minimized magnetic losses for spin-orbitronics applications.