Magnetization switching of a metallic nanomagnet via current-induced surface spin-polarization of an underlying topological insulator

Abstract

We consider a thermally stable, metallic nanoscale ferromagnet (FM) subject to spin-polarized current injection and exchange coupling from the spin-helically locked surface states of a topological insulator (TI) to evaluate possible non-volatile memory applications. We consider parallel transport in the TI and the metallic FM, and focus on the efficiency of magnetization switching as a function of transport between the TI and the FM. Transport is modeled as diffusive in the TI beneath the FM, consistent with the mobility in the TI at room temperature, and in the FM, which essentially serves as a constant potential region albeit spin-dependent except in the low conductivity, diffusive limit. Thus, it can be captured by drift-diffusion simulation, which allows for ready interpretation of the results. We calculate switching time and energy consumed per write operation using self-consistent transport, spin-transfer-torque (STT), and magnetization dynamics calculations. Calculated switching energies and times compare favorably to conventional spin-torque memory schemes for substantial interlayer conductivity. Nevertheless, we find that shunting of current from the TI to a metallic nanomagnet can substantially limit efficiency. Exacerbating the problem, STT from the TI effectively increases the TI resistivity. We show that for optimum performance, the sheet resistivity of the FM layer should be comparable to or larger than that of the TI surface layer. Thus, the effective conductivity of the FM layer becomes a critical design consideration for TI-based non-volatile memory.

Detection of current induced spin polarization in epitaxial Bi2Te3 thin film

“…[31] [5], and make the substitution s calculate the voltage drop between the two contacts situated at a distance L as:…”

“…The three dimensional (3D) topological insulators (TIs) having insulating bulk and Dirac-type two dimensional (2D) surface states (SSs) with spin-momentum locking have potential for spintronics [1][2][3][4][5][6]. The dispersion relation of the SS guarantees that any charge current flow within these states will induce a non-zero spin accumulation on the 2D surface of a 3D TI.…”

“…The dispersion relation of the SS guarantees that any charge current flow within these states will induce a non-zero spin accumulation on the 2D surface of a 3D TI. This current induced spin polarization of the SS, controllable by the magnitude and the direction of the current, can be used to torque a ferromagnet (FM) [4,5]. In recent experiments [7][8][9][10][11][12][13][14][15][16][17][18][19], the spin accumulation on the surface of 3D TIs Bi 2 Se 3 , (Bi x Sb 1−x ) 2 Te 3 , Bi 1.5 Sb 0.5 Te 1.7 Se 1.3 , BiSbTeSe 2 , Bi 2 Te 2 Se and Sb 2 Te 3 , mostly grown by molecular beam epitaxy (MBE) or exfoliated, were electrically measured by the voltage probed with FM contact, where the voltage depends on the projection of SS spin polarization onto the FM magnetization direction.…”

Voltage-controlled low-energy switching of nanomagnets through Ruderman-Kittel-Kasuya-Yosida interactions for magnetoelectric device applications

“…However, non-uniform carrier concentrations are beyond our current switching simulation capabilities. In any case, these energies are much less than expected for currentinduced switching of conventional STT-RAM memory bits, which are estimated to remain above 0.1 pJ for the foreseeable future [22], or for switching easy-plane magnets on the surface of topological insulator, estimated to be on the scale of 10-100fJ [10]. Moreover, these switching energies are comparable or smaller than perhaps even end-of-the-roadmap CMOS logic gates because of the lower gate voltages and more compact logic gates, while still allowing for nonvolatile operation.…”

“…Topological insulators (TIs) have spin-momentum locked surface states [5][6][7] and, thus, may provide still better injection efficiencies within a spin-Hall geometry [8] for power efficient switching [9]. However, some of this advantage may be lost for nanoscale magnets due to the limited length of the transport path beneath the magnet, and, particularly for TIs, current shunting to a metallic magnet [10]. Voltage-aided or induced switching of nanomagnets also is being considered to reduce or eliminate the current requirement for still more power-efficient switching [11].…”