Magnetic properties of devicelike cobalt/2D materials interfaces

We have studied the magnetism of a cobalt ultrathin film deposited on different two-dimensional (2D) materials, namely graphene, h-BN, and ${\mathrm{WSe}}_2$ by the Brillouin light scattering technique. The studied samples are prepared by a pick-up method of large flakes deposited on ${\mathrm{SiO}}_2$ and the subsequent physical vapor deposition of metal layers, in a similar way to what is done to make spintronic devices out of such materials. Compared to the reference layer $(\mathrm{Co}/{\mathrm{SiO}}_2)$, the perpendicular magnetic anisotropy is enhanced in the Co/2D systems, although less than what could be expected on single crystal samples. This result is quantitatively discussed by comparison with ab initio calculations in the case of the Co/graphene interface. We also measure an increase of the magnetic damping and a small Dzyaloshinskii-Moriya interaction in such samples which are discussed with respect to the recent literature.

Figure 1

$100\times 100\mu {\mathrm{m}}^2$ optical microscope images of the three different samples. In all cases, the chosen flakes have been deposited on ${\mathrm{SiO}}_2$ at the center of converging gold electrodes (seen in yellow at the border of the images). The whole structure have been covered with 1.5 nm of Co and 4 nm of Au. (a) Graphene flakes. (b) h-BN flakes. (c) ${\mathrm{WSe}}_2$ flakes.

Figure 2

BLS spectra of the reference Co sample and the (a) Co/Gr, (b) Co/h-BN and (c) $\mathrm{Co}/{\mathrm{WSe}}_2$ samples, for an applied magnetic field ${\mu }_0\mathrm{H}=0.3\mathrm{T}$ and a wave number $k=11.8\mu {\mathrm{m}}^{-1}$.

Figure 3

Variation of the frequency as function of the applied magnetic field for (a) Co/Graphene (b) Co/h-BN, (c) $\mathrm{Co}/{\mathrm{WSe}}_2$, and (d) the reference Co film. Symbols are the experimental data and continuous lines represent the theoretical fit. Error bars are less than the symbol size.

Figure 4

FWHM of the BLS lines as function of the applied magnetic field for the reference sample $\mathrm{Co}/{\mathrm{SiO}}_2$ (squares), Co/Graphene (disks), Co/h-BN (triangles) and $\mathrm{Co}/{\mathrm{WSe}}_2$ (diamonds).

Figure 5

BLS spectra for (a) Co/Gr, (b) Co/h-BN and (c) $\mathrm{Co}/{\mathrm{WSe}}_2$ systems. They were recorded for a wave number $k=20\mu {\mathrm{m}}^{-1}$ under applied magnetic fields H = 0.2 T and $-0.2\mathrm{T}$ to evidence frequency difference between Stokes and anti-Stokes lines.

Figure 6

$\text{MCA}=E_{\parallel }-E_{\perp }$ for the surface layer according to the distance between graphene and cobalt. In blue, the total MCA whose the limit is the free cobalt surface value in blue dashed line. The $d$ orbitals contributions are shown in black, red and green, for respectively, the average between $d_{x^2-y^2}$ and $d_{xy}$ orbitals, the average between $d_{xz}$ and $d_{yz}$ orbitals, and the $d_{z^2}$ orbital.