[Paper Review] Intrinsic nature of the giant spin Hall conductivity of Pt

Lijun Zhu, Lujun Zhu+3 more|arXiv (Cornell University)|May 11, 2019

Magnetic properties of thin films59 references124 citations

TL;DR

This study establishes that the giant spin Hall conductivity (SHC) in platinum (Pt) originates primarily from the intrinsic spin Hall effect due to Berry curvature in its band structure, not extrinsic mechanisms. By incorporating MgO scattering centers into Pt films to tune long-range crystalline ordering, the authors demonstrate a direct correlation between SHC and lattice perfection, achieving a 100% enhancement in spin-orbit torque efficiency and identifying Pt₀.₆(MgO)₀.₄ as a highly promising material for low-power spintronic devices with high spin Hall angle, low resistivity, and strong Dzyaloshinskii-Moriya interaction.

ABSTRACT

More than a decade after the first theoretical and experimental studies of the spin Hall conductivity (SHC) of Pt, both its dominant origin and amplitude remain in dispute. Resolving these questions is of fundamental importance for advancing understanding of very strong spin-orbit effects in conducting systems and for maximizing the spin Hall effect for energy-efficient spintronics applications. Here, we report the experimental determination of the rapid variation of the intrinsic SHC of Pt with the carrier lifetime ({ au}) in the dirty-metal regime by incorporating finely dispersed MgO inter-site impurities into the Pt while maintaining the essential elements of its band structure (face-centered-cubic order). This findings conclusively validate the theoretical prediction that the SHC in Pt in the dirty-metal regime should be dominated by the intrinsic Berry curvature contribution and should decrease rapidly with shortening { au}. This also establishes the limit to which the spin Hall ratio { heta}SH of pure Pt can be increased by shortening { au}. When the spin backflow at the Pt/ferromagnet interface due to the finite interfacial spin-mixing conductance is taken into account, the amplitude of the intrinsic SHC of Pt in the clean limit is found to be at least 1.6x10^6 (h_bar/2e) {\Omega}-1 m-1, more than 3.5 times greater than the available theoretical predictions. Our work also establishes a compelling spin Hall metal Pt0.6(MgO)0.4 that combines a giant { heta}SH (0.73) with a moderate resistivity (74 {\mu}{\Omega} cm), a strong Dzyaloshinskii-Moriya interaction, easy growth, and good integration compatibility for spintronics technology.

Motivation & Objective

• To resolve the long-standing debate on the origin of the giant spin Hall conductivity (SHC) in platinum (Pt), which remains disputed between intrinsic and extrinsic mechanisms.
• To experimentally determine whether the dominant contribution to SHC in Pt arises from intrinsic band structure effects (Berry curvature) or extrinsic mechanisms such as skew scattering or side-jump.
• To engineer a new spin Hall material with enhanced spin-orbit torque efficiency, low resistivity, and compatibility with silicon-based integration for energy-efficient spintronic applications.
• To clarify the discrepancy between theoretical predictions and experimental measurements of SHC in Pt, particularly the underestimation of intrinsic SHC in first-principles calculations.

Proposed method

• Co-sputtered Pt₁₋ₓ(MgO)ₓ films with varying MgO concentration (x = 0 to 1) were grown on Si/SiO₂ substrates to systematically tune long-range crystalline ordering (LRCO) and resistivity (ρxx).
• High-resolution cross-sectional STEM, EDS mapping, and XPS were used to confirm homogeneous dispersion of MgO in Pt without clustering or oxidation of Pt atoms.
• X-ray diffraction (XRD) confirmed the absence of lattice parameter shifts with MgO concentration, indicating interstitial rather than substitutional doping of MgO in Pt.
• Damping-like spin-orbit torque (SOT) efficiency (ξDL) and spin Hall conductivity (σSH) were measured via in-plane harmonic response techniques and correlated with resistivity and LRCO.
• Deterministic magnetization switching of a Co layer was demonstrated using SOT from Pt₀.₇(MgO)₀.₃, confirming strong SOT efficiency and Dzyaloshinskii-Moriya interaction (DMI).
• Theoretical analysis compared experimental SHC values with first-principles and tight-binding calculations, revealing significant underestimation in existing models.

Experimental results

Research questions

• RQ1 What is the dominant physical origin of the giant spin Hall conductivity in Pt—internal band structure (intrinsic) or scattering-related mechanisms (extrinsic)?
• RQ2 How does the degree of long-range crystalline ordering (LRCO) in Pt films affect the spin Hall conductivity and spin-orbit torque efficiency?
• RQ3 To what extent do current theoretical models accurately predict the intrinsic spin Hall conductivity of Pt?
• RQ4 Can the spin Hall conductivity and spin-orbit torque efficiency be enhanced in Pt-based materials through controlled disorder engineering?
• RQ5 What are the key material properties of the new Pt₀.₆(MgO)₀.₄ alloy that make it suitable for low-power spintronic devices?

Key findings

• The giant spin Hall conductivity in Pt is predominantly intrinsic, arising from Berry curvature in the bulk band structure, as confirmed by the direct correlation between SHC and long-range crystalline ordering (LRCO).
• The measured intrinsic spin Hall conductivity exceeds 1×10⁶ (ℏ/2e) Ω⁻¹ m⁻¹, significantly higher than theoretical predictions, indicating that current first-principles and tight-binding models underestimate the true intrinsic SHC in Pt.
• A 100% enhancement in spin-orbit torque efficiency (ξDL) was achieved by increasing resistivity through MgO incorporation, with Pt₀.₆(MgO)₀.₄ showing ξDL = 0.30 and ρxx = 74 μΩ cm.
• The Pt₀.₆(MgO)₀.₄ alloy exhibits strong Dzyaloshinskii-Moriya interaction (DMI), enabling deterministic switching of perpendicular Co layers at a critical current density of 1.15×10⁷ A/cm².
• The material is compatible with silicon substrates and sputtering-based fabrication, making it highly suitable for integration into spintronic devices such as SOT-MRAMs and skyrmion devices.
• The results exclude significant contributions from skew scattering, side-jump, or interfacial effects, as evidenced by the scaling of ξDL with ρxx and the lack of dependence on interface structure.