# Magneto-Optical Kerr Effect Driven by Spin Accumulation on Cu, Au, and Pt
## 2 Materials and Methods  
The sample stacks consist of sap/Pt (10 nm)/FM (3.8 nm)/Cu (100 nm)/cap, where sap is a sapphire substrate with a (0001) orientation, FM is a ferromagnetic layer of [Co (0.2 nm)/Ni (0.4 nm)]\({}_{\times 6}\)/Co (0.2 nm), cap is a capping layer of MgO (3 nm), Au (10 nm), or Pt (2 nm). The 10 nm-thick Pt layer serves as a buffer layer for [Co/Ni]. For the perpendicular magnetic anisotropy of the Co-based multilayers, such as [Co/Pt], [Co/Pd], and [Co/Ni], a [111] textured buffer layer, such as Au, Pt, and Pd, is required with a sufficient thickness because the magnetic properties of the Co-based multilayers depend on the texture [19; 20]. Without the Pt buffer layer, the [Co/Ni] multilayer has an in-plane magnetic anisotropy. The [Co/Ni] thickness is chosen to be larger than the spin diffusion length of it and to have a full remnant magnetization. It is known that a thickness of a few tens of nm of the Co-based multilayer leads to a low remnant magnetization [20]. The Cu layer is selected for NM because of its large spin diffusion length. The Cu thickness more than 100 nm is required to suppress the MOKE signal from FM when measured on the Cu side of the samples. The MOKE signal through the Cu layer attenuates with \(\exp(-4\pi\kappa d/\lambda)\), where \(\kappa\) is the imaginary part of the refractive index of Cu, \(d\) is the thickness of Cu, and \(\lambda\) is the wavelength of light. With \(\kappa\) of 5.6, \(d\) of 100 nm, and \(\lambda\) of 784 nm, the attenuation is around \(10^{-4}\). When the MOKE is measured on the Cu side, the demagnetization signal dominates over the spin accumulation signal at the Cu thickness of less than 60 nm [18]. From the extrapolation of the demagnetization data of the Cu thickness 10-60 nm, the demagnetization signal through the Cu 100 nm is estimated to be \(<\)0.1 \(\upmu\)rad [18]. The MgO layer protects the Cu layer from oxidation. The additional 1 nm-thick Ta  
Figure 1: Schematic of the experiment. The sample structure is Pt/FM (ferromagnetic metal)/Cu/cap, where cap is Au or Pt. The pump energy drives the ultrafast demagnetization and the spin-dependent Seebeck effect on FM. The ultrafast demagnetization generates spins at the bulk of FM (\(g_{\text{s}}\)), and the spin-dependent Seebeck effect generates spin at the FM/NM (non-ferromagnetic metal) interfaces (\(G_{\text{s}}\)). When the FM thickness is small, the spin generation at the Pt/FM interface is the negative value of that at the FM/Cu interface. The generated spins on FM diffuse to adjacent layers and induce a spin accumulation on Cu and cap (blue arrows). The spin accumulation on Cu and cap rotates the polarization of the probe upon reflection. layer, which fully oxidizes after exposure to air, is deposited on top of MgO in order to secure the passivation of MgO. Adding a thin Ta layer on top of MgO is a simple method for improving the long-term stability of the passivation of MgO, and the MgO/Ta capping layer is often used when MgO is the only oxide target inside the sputtering chamber [21]. The high uniformity of Ta on top of MgO has been verified through tunneling electron microscopy in the literature [21]. I select the Ta thickness of 1 nm by comparing the static MOKE signal of the Co (3 nm)/MgO (3 nm)/Ta (0, 1, 2 nm) structure. Ta 1 nm does not reduce the static MOKE signal, but Ta 2 nm reduces the static MOKE signal by 20%. When the Ta thickness is too thick, it does not fully oxidize, and a non-oxidized Ta layer reduces the MOKE signal. The Au and Pt layers are inert in the air, thus, the passivation layer is not necessary. Considering the spin diffusion length of Pt of a few nm, I choose the Pt thickness of 2 nm. All samples are prepared through magnetron sputtering at a base pressure of \(<\)5 \(\times\) 10\({}^{-8}\) Torr. I measure the demagnetization of FM and the spin accumulation on NM using a time-resolved polar MOKE.