Molecular-hybridization-induced antidamping and sizeable enhanced spin-to-charge conversion in ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\beta$-$\mathrm{W}/{\mathrm{C}}_{60}$ heterostructures

The development of power-efficient spintronic devices has been a compelling need in the post-CMOS technology era. The effective tuneability of spin-orbit coupling (SOC) in the bulk and at the interfaces of hybrid material stacks is a prerequisite for scaling down the dimensions and power consumption of these devices. In this work, we demonstrate the strong chemisorption of ${\mathrm{C}}_{60}$ (fullerene) molecules when grown on the high-SOC $\beta$-$\mathrm{W}$ layer. The parent ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$/$\beta$-$\mathrm{W}$ ($\mathrm{CFB}/\beta$-W) bilayer exhibits large spin-to-charge interconversion efficiency, which can be ascribed to the interfacial SOC observed at the ferromagnet/heavy-metal interface. Further, the adsorption of ${\mathrm{C}}_{60}$ molecules on $\beta$-$\mathrm{W}$ reduces the effective Gilbert damping by $\sim 15\mathrm{\%}$ in $\mathrm{CFB}/\beta$-$\mathrm{W}$/${\mathrm{C}}_{60}$ heterostructures. The antidamping is accompanied by a gigantic $\sim 115\mathrm{\%}$ enhancement in the spin-pumping-induced output voltage owing to molecular hybridization. The noncollinear density-functional-theory calculations confirm the long-range enhancement of the SOC of $\beta$-$\mathrm{W}$ upon the chemisorption of ${\mathrm{C}}_{60}$ molecules, which in turn can also enhance the SOC at the $\mathrm{CFB}/\beta$-$\mathrm{W}$ interface in $\mathrm{CFB}/\beta$-$\mathrm{W}$/${\mathrm{C}}_{60}$ heterostructures. The combined amplification of the bulk as well as the interfacial SOC upon molecular hybridization stabilizes the antidamping and enhanced spin-to-charge conversion, which can pave the way for the fabrication of power-efficient spintronic devices.

Figure 1

Schematics of the (a) $\mathrm{Si}$/${\mathrm{Si}\mathrm{O}}_2$/$\mathrm{CFB}/\beta$-$\mathrm{W}$ and (b) $\mathrm{Si}$/${\mathrm{Si}\mathrm{O}}_2$/$\mathrm{CFB}/\beta$-$\mathrm{W}$/${\mathrm{C}}_{60}$ heterostructures. (c) GIXRD patterns of the $\mathrm{Si}$/${\mathrm{Si}\mathrm{O}}_2$/$\mathrm{CFB}/\beta$-$\mathrm{W}$ heterostructures with various thicknesses of $\beta$-$\mathrm{W}$.

Figure 2

(a) Frequency ($f$) versus resonance field ($H_{\mathrm{res}}$) and (b) linewidth ($\mathrm{\Delta }H$) versus frequency ($f$) behavior for various heterostructures. The solid lines are the best fits to Eqs. (2) and (3).

Figure 3

(a) The extended simulation supercell for the ${\mathrm{C}}_{60}$ on $\beta$-$\mathrm{W}$(210) substrate. The left panel shows the top view of the surface supercell (along the $z$ axis), and the right panel shows the side view of the same. The larger pink balls and the smaller cyan balls represent the tungsten and carbon atoms, respectively. The yellow bonds highlight the section of the ${\mathrm{C}}_{60}$ that takes part in interface formation. The double-headed dashed black arrows quantify the diameter of the ${\mathrm{C}}_{60}$ spheres in two directions. (b),(c) The modification in the electronic structure due to chemisorption of the ${\mathrm{C}}_{60}$ molecule on the $\beta$-$\mathrm{W}$. (b) The atom projected orbital resolved partial density of states of $\beta$-$\mathrm{W}$(210), ${\mathrm{C}}_{60}$, and $\beta$-$\mathrm{W}$(210)/${\mathrm{C}}_{60}$. (c) The electron density redistribution due to chemisorption. The red and green isosurfaces depict electron density depletion and accumulation of the electron density at the interface, respectively. The curved bi-colored arrow depicts the direction of the electron transfer process.

Figure 4

The effect of chemisorption of the ${\mathrm{C}}_{60}$ molecule at the $\beta$-$\mathrm{W}$(210) surface on $E_{\mathrm{SOC}}$ of various atomic sites. The percentage change in $E_{\mathrm{SOC}}$ (i.e., $\mathrm{\Delta }{E}_{\mathrm{SOC}}$) is calculated in terms of the change in $E_{\mathrm{SOC}}$ of the bare $\beta$-$\mathrm{W}$(210) substrate. Layer 5 is the interfacial layer that interacts with the ${\mathrm{C}}_{60}$, and layer 1 is opposite to the $\beta$-$\mathrm{W}$/${\mathrm{C}}_{60}$ interface layer.

Figure 5

Plots of $V_{\mathrm{MEAS}}$, $V_{\mathrm{SYM}}$, and $V_{\mathrm{ASYM}}$ versus $H$ for CFB (7 nm)/$\beta$-$\mathrm{W}$ (5 nm)/${\mathrm{C}}_{60}$ (13 nm) (CFWC2) heterostructure with (a) $\varphi \sim {180}^{\circ }$ and (b) $\varphi \sim 0^{\circ }$ measured at 15 dBm rf power. The red curve is a Lorentzian fit with Eq. (5) to $V_{\mathrm{dc}}$ versus $H$.

Figure 6

Plots of $V_{\mathrm{SYM}}$ versus applied magnetic field with $\varphi \sim {180}^{\circ }$ for (a) CFB (7 nm)/$\beta$-$\mathrm{W}$ (2.5 nm) (CFW1) and CFB (7 nm)/$\beta$-$\mathrm{W}$ (2.5 nm)/${\mathrm{C}}_{60}$ (13 nm) (CFWC1) and (b) CFB (7 nm)/$\beta$-$\mathrm{W}$ (5 nm) (CFW2) and CFB (7 nm)/$\beta$-$\mathrm{W}$ (5 nm)/${\mathrm{C}}_{60}$ (13 nm) (CFWC2) heterostructures measured at 15 dBm rf power. (c) Power-dependent $V_{\mathrm{SYM}}$ for CFW1 and CFWC1 (the solid line is the linear fit). (d) Schematic showing the spin-to-charge conversion phenomenon in $\mathrm{CFB}/\beta$-$\mathrm{W}$/${\mathrm{C}}_{60}$ heterostructures.