Emergence of spin-orbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi

Abstract

Ferromagnetic-metal surface state derived from topological polarization and its spin-orbitronics functionalities in FeSi.

A Noble‐Metal‐Free Spintronic System with Proximity‐Enhanced Ferromagnetic Topological Surface State of FeSi above Room Temperature

“…Figure 1d displays T dependence of sheet magnetization M sheet in all the heterostructures. As previously reported, [19] T C is lower than room temperature in MgO/FeSi heterostructure (T C ≈ 230 K), and there is no ferromagnetic ordering in Si/FeSi heterostructure. In contrast, T C rises significantly to over 400 K in CaF 2 -or BaF 2 -capped FeSi.…”

“…The intrinsic critical current density passing through the metallic surface can be roughly estimated to be 3.4 × 10 11 A m −2 under assumptions that: i) the surface conductance corresponds to $$\Delta \sigma _{xx}^{{\rm{sheet}}} = \sigma _{xx}^{{\rm{sheet}}}(Ba{F_2}/FeSi) - \sigma _{xx}^{{\rm{sheet}}}(Si/FeSi)$$ and ii) the thickness of surface conduction layer is the same as that of surface ferromagnetic layer (0.35 nm). [ 19 ] After each pulse injection, anomalous Hall resistivity was measured with passing a low d.c. current of 0.05 mA, which corresponds to a much lower current density 1.7 × 10 9 A m −2 in the 3 nm‐thick film. The temperature rise due to the Joule heating during the input of current pulses ( I pulse = 10 mA) was estimated to be 30 K, which would assist the magnetization switching through the reduction of magnetic anisotropy effect.…”

“…Spin-orbit coupling at surfaces and interfaces has produced novel phases of matter and emergent phenomena [1,2] as recently highlighted by topological insulators [3,4] and interfacial confined to a thickness of 0.35 nm. In particular, as a consequence of its bulk-band topology with nearly quantized Zak phase, [19,[23][24][25][26] surface charges are asymmetrically distributed with respect to the surface atomic layer (Figure 1a), causing outof-plane potential gradient and hence Rashba-type spin splitting of 35 meV at the Fermi energy. [19] Despite the strong Rashba SOC, the magnetization switching through SOT has been realized only at cryogenic temperatures due to the low ferromagnetic transition temperature (T C ≈ 200 K), [19] which undermines the potential of FeSi as a spintronic material.…”

“…Recent discovery of spin–orbit coupled surface state of FeSi and its spintronic functionality showed an ideal example that may overcome the above technological issues. [ 19 ] While FeSi is a well‐known strongly correlated insulator, [ 20 ] its surface state was revealed to constitute metallic and ferromagnetic bands [ 19,21,22 ] confined to a thickness of 0.35 nm. In particular, as a consequence of its bulk‐band topology with nearly quantized Zak phase, [ 19,23–26 ] surface charges are asymmetrically distributed with respect to the surface atomic layer ( Figure a), causing out‐of‐plane potential gradient and hence Rashba‐type spin splitting of 35 meV at the Fermi energy.…”

“…Recent discovery of spin-orbit coupled surface state of FeSi and its spintronic functionality showed an ideal example that may overcome the above technological issues. [19] While FeSi is a well-known strongly correlated insulator, [20] its surface state was revealed to constitute metallic and ferromagnetic bands [19,21,22] Strongly spin-orbit coupled states at metal interfaces, topological insulators, and 2D materials enable efficient electric control of spin states, offering great potential for spintronics. However, there are still materials challenges to overcome, including the integration into advanced silicon electronics and the scarce resources of constituent heavy elements of those materials.…”