Bulk Rashba-Type Spin Splitting in Non-Centrosymmetric Artificial Superlattices

Abstract

Spin current, converted from charge current via spin Hall or Rashba effects, can transfer its angular momentum to local moments in a ferromagnetic layer. In this regard, the high charge-to-spin conversion efficiency is required for magnetization manipulation for developing future memory or logic devices including magnetic random-access memory. Here, the bulk Rashba-type charge-to-spin conversion is demonstrated in an artificial superlattice without centrosymmetry. The charge-to-spin conversion in [Pt/Co/W] superlattice with sub-nm scale thickness shows strong W thickness dependence. When the W thickness becomes 0.6 nm, the observed field-like torque efficiency is about 0.6, which is an order larger than other metallic heterostructures. First-principles calculation suggests that such large field-like torque arises from bulk-type Rashba effect due to the vertically broken inversion symmetry inherent from W layers. The result implies that the spin splitting in a band of such an ABC-type artificial SL can be an additional degree of freedom for the large charge-to-spin conversion.

Keywords: artificial superlattice; bulk Rashba-type spin splitting; charge-to-spin conversion; spin current.

Figure 1

Rashba‐split band in an ABC‐type SL. a) Schematic image of the Rashba‐split band. b) Artificial SL with non‐centrosymmetric layer structure based on Pt, Co, and W. c) HAADF‐STEM image of the [Pt/Co/W (0.6 nm)] superlattice and d) enlarged image of the red‐squared area in (c). e) The energy‐dispersive spectroscopy (EDS) mapping image of the superlattice. f) the chemical profiles of Co, Pt, and W obtained from the EDS mapping. The arrows link the mapping image and the profiles with each element's information.

Figure 2

Charge‐to‐spin conversion in the [Pt/Co/W]‐SL. a) Optical image of the Hall device and measurement circuit image. b) The first harmonic (1w) signal with magnetization up (M_up) and down (M_down) for B//x or y‐axis. Inset shows the full range curve of the Hall voltage. The second harmonic Hall voltages with c) B//x and d) B//y. e) Effective fields and f) charge‐to‐spin conversion efficiency in terms of t _w.

Figure 3

Spin Hall conductivity of the [Pt/Co/W]‐SL. a) Reported ${\xi }_{\mathrm{CS}}^{\mathrm{DL}(\mathrm{FL})}$ values as function of resistivity ρ _xx with various film stacks. b) Effective field in terms of t _Pt. c) Ratio between ${\xi }_{\mathrm{CS}}^{\mathrm{DL}}$ and ${\xi }_{\mathrm{CS}}^{\mathrm{FL}}$, and d) ${\sigma }_{\mathrm{SH}}^{\mathrm{DL}(\mathrm{FL})}$and e) ρ _xx values of the [Pt/Co/W]‐SL as a function of N.

Figure 4

Band structure of the SLs. a,b) Schematic of unit cells used to calculate the electronic structure for the Pt/Co(111) and Pt/Co/W(111) SLs. c–g) Band structure for different in‐plane magnetization directions along +x and −x axes, plotted using respectively red and blue lines in the upper part. The lower part shows the zoom of band structure as indicated by the dashed rectangles.

Figure 5

Contribution of the W orbital to the Rashba‐type splitting. a) Evaluated Δv _F,↑↓ values in terms of t _W. b) XMCD and integrated XMCD spectra of the [Pt/Co (0.6)/W(0.6)]‐SL. c) Orbital‐to‐spin moment ratio in terms of t _W. Differential charge density Δρ and partial density of states (PDOS) of d) [Pt/Co]‐ and e) [Pt/Co/W(2ML)]‐SLs. For charge density, the red (blue) distribution represents the charge accumulation (depletion). For PDOS, the d orbital states with magnetic quantum number |m| = 0, 1, 2 are shown in black, blue, and red, respectively.