2D FERROMAGNET/TOPOLOGICAL INSULATOR HETEROSTRUCTURE FOR SPIN-ORBIT TECHNIQUE

Work package:
Current-induced magnetic torques known as spin-orbit torques (SOTs) allow for the electrical manipulation of the magnetization 𝑀 in thin-film heterostructures and consequently have application in low-power magnetic memories and logic devices. Due to their high spin-orbit interaction generating spin Hall effect (bulk) and/or Rashba-Edelstein effect (interfacial), heavy metals (HM) such Pt and Ta, are reference material as SOT source, i.e. they convert a charge current density 𝐽𝐶 into a spin current density 𝐽𝑆 which is absorbed by an interfaced ferromagnet (FM) and generates a torque on its magnetization (Fig. 1). However, they present low efficiency, with critical 𝐽𝐶 injected in the HM for magnetization switching as high as 106-108 A/cm2. For reducing 𝐽𝐶 and power, promising SOT materials are topological insulators (TIs), such as BiSbTe and BiSb: these materials possess strong spin-orbit coupling and consequent conductive surface states with spin-momentum locking that enables a high spin accumulation and critical current densities one order of magnitude lower than for heavy metals. However, the reported charge-to-spin conversion efficiency varies drastically depending on the TI deposition method, crystalline and interface quality of the TI/FM heterostructures. The Nanomagnetism group at LPCNO studies TI/FM heterostructures, namely BiSb/Pt/Co/Pt, with BiSb as the TI (grown by MBE at LAAS, Toulouse), and cobalt as the ferromagnet (Fig.2(a)). We already demonstrated the SOT-driven magnetization reversal in this system with 𝐽𝐶 ≈ 106 A/cm2 (Fig.2(d)). The proposed internship will focus on the optimization of the spin transparency at TI/FM interface to enhance torque efficiency and therefore reducing critical current. We will focus on the additional buffer layer at the BiSb (and BiSbTe)/Co interface to promote the Co perpendicular anisotropy and spin transparency. Alternatively, we will use van der Waals FM, exfoliated and directly transferred on TI, in order to benefit from the weak interaction between layers of this kind of materials to avoid intermixing at TI/FM interface. The internship concerns (i) the deposition and characterization of such TI/FM heterostructures, (ii) the electrical measurement of the magnetic properties thanks to anomalous Hall effect (AHE) in Hall bars designed samples (Fig.2(b)-(c)), and (iii) the SOT measurements (Fig. 2(d))