Direct observation of unusual interfacial Dzyaloshinskii-Moriya interaction in graphene/NiFe/Ta heterostructures

Graphene/ferromagnet interface promises a plethora of new science and technology. The interfacial Dzyaloshinskii-Moriya interaction (iDMI) is essential for stabilizing chiral spin textures, which are important for future spintronic devices. Here, we report direct observation of iDMI in graphene/${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$/Ta heterostructures from nonreciprocity in spin-wave dispersion using Brillouin light-scattering technique. Linear scaling of iDMI with the inverse of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ thicknesses suggests primarily interfacial origin of iDMI. Both iDMI and spin-mixing conductance increase with the increase in defect density of graphene obtained by varying argon pressure during sputter deposition of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$. This suggests that the observed iDMI originates from defect-induced extrinsic spin-orbit coupling at the interface. The direct observation of iDMI at graphene/ferromagnet interface without perpendicular magnetic anisotropy opens a route in designing thin-film heterostructures based on two-dimensional materials for controlling chiral spin structure such as skyrmions and bubbles, and magnetic domain-wall-based storage and memory devices.

Figure 1

Magnetization hysteresis loop for sample stack (a) NiFe (10 nm)/Ta (2 nm) and (b) Gr/NiFe (10 nm)/Ta (2 nm) (deposited at 2-mTorr Ar working pressure) with magnetic field applied within the film plane. Here θ refers to the angle between two mutually perpendicular directions within the sample plane. (c) Variation of ${\mathrm{M}}_{\mathrm{eff}}$ (extracted from magnetic-field dependence using BLS) as a function of inverse of NiFe thickness in Gr/NiFe/Ta system. Red solid line is the best linear fit. (d) Raman spectra of CVD-grown graphene on Si/${\mathrm{SiO}}_2$ before (top panel) and after (bottom panel) the deposition of NiFe (10 nm)/Ta (2 nm) bilayer thin films.

Figure 2

(a) Schematic of the film stack along with BLS geometry. (b) Representative BLS spectra for DE spin waves acquired at a fixed wave vector $k=18.1\mathrm{rad}/\mu \mathrm{m}$ under oppositely oriented external applied fields $H=1$ kOe in graphene/${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ $\left(t\right)$/Ta (2 nm) sample (deposited at 10-mTorr Ar pressure) (top four panels) and reference sample NiFe (3 nm)/Ta (2 nm) (at the bottommost panel) for two counterpropagating directions of spin waves. The spectrum corresponding to a particular thickness of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ is indicated by mentioning thickness value in each panel. Solid curve is the fit using Lorentzian function.

Figure 3

(a) Plot of $\mathrm{\Delta }f$ vs $k$ for graphene/${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ ($t_{\mathrm{NiFe}}$)/Ta (2 nm) samples with various values of $t_{\mathrm{NiFe}}$. Symbols represent the experimental data points and solid lines are the fit using Eq. (2). (b) Variation of $D$ with the inverse of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ thickness. The error bar in $\mathrm{\Delta }f$ is shown by considering the error from the fitting of spectra as well as the instrumental resolution to determine the peak frequency and for $D$, errors in $M_s$ as well as $\mathrm{\Delta }f$ have been taken into account. The blue solid line is the linear fit.

Figure 4

(a) Raman spectra of CVD-grown graphene on Si/${\mathrm{SiO}}_2$ after the deposition of NiFe (10 nm)/Ta(2 nm) bilayer thin films at different Ar pressure. (b) Plot of $\mathrm{\Delta }f$ vs $k$ for graphene/NiFe (3 nm)/Ta (2 nm) samples deposited at various Ar pressure. Symbols represent the experimental data points and solid lines are the fit using Eq. (2). (c) Variation of effective damping constant with the inverse of NiFe thickness for various Ar pressure investigated from FMR measurement. (d) Plot of surface DMI constant with spin mixing conductance. Ar pressure value is mentioned on each data point.