3D topological insulators (TIs) such as Bi2Se3 can be utilized to switch ferromagnets via spin-orbit torque (SOT). Here we compare magnetization switching of NiFe from MBE-grown bulk-insulating Bi2Se3 on novel BiInSe/In2Se3 buffer layers, where surface states dominate transport up to RT, to that of conventional bulk-conducting Bi2Se3, and found significant reduction in critical current density and enhancement in SOT efficiency. We further fabricate vdW heterostructures with exfoliated 2D ferromagnet Fe3GeTe2 with perpendicular anisotropy (PMA), and demonstrate magnetization switching with record low critical current density and a large SOT efficiency. DFT calculations reveal weak interlayer interactions lead to a weakened interfacial dipole, suppressing proximity induced magnetic moment on Bi2Se3, preserving its spin texture. Our results highlight the clear advantage of bulk-insulating TIs and all-vdW heterostructures in enhancing SOT efficiency and minimize critical current density, an important step towards realizing next generation low-power non-volatile memory and spintronic devices.
In a topological insulator (TI), spin-charge conversion (SCC) is expected to occur at the surface through the spin-momentum locking of the metallic surface states. MBE-grown TIs however are oftentimes unintentionally doped resulting in bulk states that can convert spin to charge through the inverse spin hall effect. In this work, we investigate the SCC mechanisms of the TI Bi2Se3 using time-resolved THz emission spectroscopy. Here, we demonstrate THz emission in NiFe/Bi2Se3 heterostructures using bulk-insulating Bi2Se3 and bulk-conducting Bi2Se3. We also present results comparing NiFe/Bi2Se3 to W/NiFe/Pt, a heavy metal spintronic THz emitter.
Current-generated spin in topological insulators (TIs) has been shown to efficiently switch FM magnetization via spin-orbit torque (SOT) with much lower critical currently densities. However, TI bulk are often degenerately doped and can shunt current from the surface states. Here we demonstrate SOT switching from bulk-insulating Bi2Se3, obtained by growth on BiInSe/In2Se3 buffer layers by MBE, with significantly reduced critical current density than conventional “bulk-conducting” Bi2Se3. We further grew epitaxial In2Se3 tunnel barriers on Bi2Se3, and demonstrate its spin sensitivity, towards further minimize current shunting through the FM metal and overall power consumption for magnetization switching.
The spin mixing conductance is an important figure of merit for spin transport across an interface. This is a particularly important number for Spin Orbit Torque Magnetic Random Access Devices, where spin generated in one layer is used to provide the spin torque needed to flip the magnetization in an adjacent layer. Here the spins are generated in either an topological insulator (TI) or an heavy metal (HM). The overall efficiency of such a device depends on both the charge to spin conversion in the spin generation layer and the spin mixing conductance of the interface.
Topological insulators (TIs) exhibit topologically protected metallic surface states populated by massless Dirac fermions with spin-momentum locking – the carrier spin lies in-plane, locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization. Here we show direct electrical detection of this bias current induced spin polarization as a voltage measured on a ferromagnetic (FM) metal tunnel barrier surface contact [1]. The voltage measured at this contact is proportional to the projection of the TI spin polarization onto this axis, and similar data are obtained for two different FM contact structures, Fe/Al2O3 and Co/MgO/graphene. From measurements of the carrier type and sign of the spin voltage for n-Bi2Se3 and p-Sb2Te3, we show that transport measurements can be used to determine the chirality of the spin texture [2]. The chirality inverts as one crosses the Dirac point, so that the carrier spin-momentum locking follows a left-hand rule (clockwise chirality) when the Fermi level is above the Dirac point, and right-hand rule below (counter-clockwise chirality). These results demonstrate simple and direct electrical access to the TI Dirac surface state spin system, provide clear evidence for the spin-momentum locking and bias current-induced spin polarization, and enable utilization of these remarkable properties for future technological applications.
[1] C. H. Li, O. M. J. van ‘t Erve, J. T. Robinson, Y. Liu, L. Li , and B. T. Jonker, Nature Nanotech. 9, 218 (2014).
[2] C. H. Li, O. M. J. van ‘t Erve, Y. Y. Li, L. Li and B. T. Jonker, under review.
Conference Committee Involvement (11)
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