报告题目：BiSb topological insulator with colossal spin Hall effect for ultralow power spin-orbit-torque switching
报告人：Prof. Pham Nam Hai Department of Electrical and Electronic Engineering, Tokyo Institute of Technology
报告时间：2018年11月5日(星期一)下午3:00
报告地点：2号楼303A室
内容简介：
Magnetoresistive random access memory (MRAM) is an emerging non-volatile memory technology with no idling power consumption, un-limited endurance, and fast read/write time. Commercial MRAM uses spin-transfer-torque (STT) switching method to write data to magnetic tunnel junctions (MTJs). In this method, the generated spin current is limited by the spin-polarization of the reference layer. This limits the amount of spin current generated by a given charge current and makes it difficult to reduce the writing current. Recently, spin-orbit-torque (SOT) switching using the giant spin Hall effect (SHE) in heavy metals and topological insulators (TIs) has attracted much attention as an alternating writing method for MRAM. In SOT switching, a spin Hall layer is in contact with the recording magnetic layer. A charge current flowing in the spin Hall layer can generate a pure spin current that exerts a spin torque on the recording layer. The spin current generated by this way is characterized by the spin Hall angle qSH. SOT switching may be more effective than STT switching if qSH > 1. Recently, huge spin Hall angles qSH were observed in Bi-based topological insulator (TIs) thin films, such as Bi2Se3 [1]. Because of their insulating nature, however, the conductivity s of those TIs is limited to ~ 104W-1m-1, which is much lower than that of typical metallic ferromagnets (~6′105W-1m-1) used in MRAM. For application to MRAM, spin Hall materials must have both large qSH and high s. These conditions are not satisfied simultaneously in heavy metals (such as Pt, Ta, W) with high s but small qSH (~ 0.1), and in wide-gap TIs with large qSH but low s.
Here, we focus on the Bi1-xSbx alloy as a candidate for such a spin Hall material. Although Bi1-xSbx has the same rhombohedral crystal structure as that of other well-known TIs such as Bi2Se3 and Bi2Te3, the carrier mobility of Bi1-xSbx (~104 cm2V-1s-1) is much higher than that of Bi2Se3 and Bi2Te3 (~ 102 cm2V-1s-1). Therefore, the conductivity of bulk Bi1-xSbx (~105 Ω-1m-1) is one order of magnitude higher than that of Bi2Se3 and Bi2Te3. By optimizing the growth condition, we were able to grow single crystalline Bi1-xSbx thin films on GaAs(111)A substrates by molecular beam epitaxy, despite the large and changing lattice mismatch between Bi1-xSbx and GaAs(111) with changing the Sb concentration. For thick enough thin films, their conductivity approaches those of bulk values, indicating the high crystal quality. From the temperature dependence of their electrical conductivity, we confirmed the existence of metallic surface states of Bi1-xSbx inside and even outside of the bulk TI region due to quantum confinement [2]. Furthermore, we were able to grow high quality BiSb(012) layer on top of a MnGa(001) layer on GaAs(001) substrates. Using this MnGa/BiSb bilayer, we have demonstrated colossal SHE of BiSb with qSH ~ 52 and sSH ~ 1.3′107?/2e W-1m-1 at room temperature. We show that BiSb thin films can generate a colossal spin-orbit field of 2.3 kOe/(MA/cm2) and a critical switching current density as low as 1.5 MA/cm2 in the BiSb/MnGa bilayers [3]. Furthermore, we found that the colossal SHE in BiSb is entirely governed by the topological surface states [4].
References：
[1] A. H. Mellnik et al., Nature 511 (2014) 13534.
[2] Y. Ueda, N. H. D. Khang, K. Yao, P. N. Hai. Appl. Phys. Lett. 110 (2017) 062401.
[3] N. H. D. Khang, Y. Ueda, P. N. Hai, Nat. Mater. 17 (2018) 808.
[4] T. Shirokura, K. Yao, Y. Ueda, P. N. Hai, arXiv:1810.10840 [cond-mat.mes-hall]
报告人简介：Pham Nam Hai is an associate professor at Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, and a visiting associate professor at Center for Spintronics Research Network (CSRN), the University of Tokyo. He received B.E., M.E. and Ph.D. degrees in electronic engineering from the University of Tokyo, Japan, in 2004, 2006, and 2009, respectively. His research interests include ferromagnetic nanoclusters, ferromagnetic semiconductors, ferromagnet/semiconductor hybrid systems, topological insulators, and their applications to spintronic devices. In 2012, he proposed the concept of Fe-doped narrow gap ferromagnetic semiconductors. Since then, he has realized and led the development of (In,Fe)As, (Ga,Fe)Sb, and (In,Fe)Sb ferromagnetic semiconductor with unpreceded high Curie temperature. Recently, his group has demonstrated the conductive BiSb topological insulator for the next generation spin-orbit-torque MRAM. His works are honored by the SSDM Young Researcher Award, the Marubun Award, and the Ando Incentive Prize.