Bismuth (Bi) or Bi-based heterostructures has attracted much interest as candidates for spin current generator. Recently, helicity-dependent (HD) photocurrent in Bi or Bi/Cu(or Ag) thin films was reported by using pulse laser-induced terahertz (THz) emission and transport measurement1, 2. It is believed that spin current in Bi induced by circularly-polarized laser via photo-spin conversion effect is converted into charge current via spin-charge conversion effect. However, details of photo-spin conversion in Bi have yet to be clarified because the photo-spin and the spin-charge conversion effects are observed simultaneously, thus, one cannot distinguish two effects. Here, we studied THz emission induced by spin-current generation using the photo-spin conversion effect in Bi and the laser-induced demagnetization in an adjacent ferromagnetic Co layer in Bi/Co films to separate two spin-related conversion effects and investigate the photo-spin conversion in Bi.
The samples, glass sub./Bi(tBi)/Co(5)/MgO(2)/Ta(2) (thickness is in nm), were prepared by a DC/RF magnetron sputtering. Laser-induced THz emission in Bi/Co films was investigated by THz time-domain spectroscopy (THz-TDS) with 120-fs Ti: Sapphire laser3,4. Figure 1 and Figure 2 show the Bi thickness tBi dependence of THz peak values induced by linearly-polarized laser and HD-THz peak values, respectively. Here, HD-THz is difference between signals obtained with left- and right-circularly polarized laser. We found different tBi dependence as shown in Fig. 1 and Fig. 2, which is possibly due to the difference in spin-relaxation length of two experiments. In the presentation, we will discuss the physics behind the different THz signals obtained in two experiments and quantitative consideration of the photo-spin conversion in Bi. This work is partially supported by KAKENHI (19K15430, 21H05000). K. I. acknowledges Grant-in-Aid for JSPS Fellow (22J22178) and GP-Spin at Tohoku University. S. M. acknowledges CSRN in CSIS at Tohoku University.
1 Y. Hirai et al. Phys. Rev. Appl. 14, 064015 (2020) 2 H. Hirose et al. Phys. Rev. B 103, 174437 (2021) 3 H. Idzuchi, et al., AIP Advances 11, 015321 (2021) 4 Y. Sasaki et al. Appl. Phys. Lett. 111, 102401 (2017)