# Magneto-Optical Kerr Effect Driven by Spin Accumulation on Cu, Au, and Pt
## 1 Introduction  
The optical detection of the magnetization has been possible since the discovery of the Faraday effect and the magneto-optical Kerr effect (MOKE) [1; 2]. The MOKE in particular is extensively used for ferromagnetic metals (FMs) because FMs tend to reflect light at a visible range of wavelength. The physical origin of the MOKE on FM is the net magnetization and spin-orbit coupling of FM [3]. The MOKE on non-ferromagnetic metals (NMs) has previously been studied by inducing a magnetic moment with an external magnetic field [4; 5; 6; 7; 8]. Recently, the MOKE detection on NMs, such as Cu and Au, has been achieved by injecting an ultrafast spin current from FM to NM in the FM/NM bilayers [9; 10; 11]. A spin current from FM to NM creates a spin accumulation on NM, and this spin accumulation in conjunction with the spin-orbit coupling of NM causes a rotation of the polarization of light upon reflection. The optical detection of the spin accumulation on NM enables the quantitative analysis of the spin transport as well as the spin conversion at a timescale of sub-picoseconds.  
NMs with heavy atoms, such as Pt and W, have a strong spin-orbit coupling; thus, they may produce strong MOKE signals. In addition, heavy metals can generate a spin current without the use of FM. The spin Hall effect in heavy metals converts a charge current to a transverse spin current, which leads to a spin accumulation on the surfaces of the heavy metal [12; 13; 14]. Several groups have investigated the direct detection of the spin accumulation on heavy metals driven by the spin Hall effect using the MOKE [15; 16; 17]. The authors of Refs. [15; 16] reported that the thermal signal driven by Joule heating dominates the optical detection and that the spin signal is absent. On the other hand, the authors of Ref. [17] reported the MOKE detection of spins on Pt and W. These conflicting reports are mainly due to the fact that the spin accumulation driven by the spin Hall effect tends to be small because of the limitation of the charge current density, the short spin relaxation time, and the small spin Hall angles of the heavy metals [12; 13; 14].  
In this work, I generate an intense spin current from FM to NM in the FM/NM bilayers, where FM is a [Co/Ni] multilayer and NM is a Cu, Cu/Au, or Cu/Pt layer, by injecting a pulsed light to FM. FM absorbs the energy of light and generates spin currents by the ultrafast demagnetization and the spin-dependent Seebeck effect (SDSE) [9; 18] (Figure 1). The spin current from FM to NM induces a spin accumulation on NM, whose peak can be as high as \(10^{3}\) A m\({}^{-1}\), i.e., \(10^{26}\)\(\mu_{\text{B}}\) m\({}^{-3}\), where \(\mu_{\text{B}}\) is the Bohr magneton (Figure 1). I measure transient MOKE signals on Cu, Cu/Au, and Cu/Pt structures, and find that an Au or Pt layer on top of Cu significantly affects the MOKE signal. Comparing the measured MOKE data and the calculated spin accumulation, I determine the ratio between Kerr rotation (\(\Delta\theta+i\Delta\varepsilon\)) and spin accumulation (\(\Delta M\)). The \(\Delta\theta/\Delta M\) values are \(-4\times 10^{-9}\) and \(-2.5\times 10^{-8}\) rad m A\({}^{-1}\) for Cu and Au, respectively, and the \(\Delta\varepsilon/\Delta M\) value is \(-3\times 10^{-9}\) rad m A\({}^{-1}\) for Pt at a wavelength of 784 nm.