Spin‐Orbit Torque Switching of a Nearly Compensated Ferrimagnet by Topological Surface States

International audiencecharge-spin conversion and ii) the speed of SOT switching. Generally, SOT in a magnetic layer originates from the spin current injection from the adjacent layer with strong spin-orbit coupling (SOC). The charge-spin conversion efficiency is vital and can be quantified as the θ = / SHE s 3D e 3D J J (dimensionless) or q J J t θ = = / / ICS s 3D e 2D SHE s , where s 3D J represents the 3D spin current density; e 3D J and e 2D J represent the 3D and 2D electric (charge) current density, respectively; and t s represents the effective SOC thickness. In the conventional SOC materials such as HMs, in principle, the θ SHE should be much less than 1, which limits their potential applications in the ultralow power magnetization manipulation. [4] In topological insulators (TIs), SOC from topologically protected surface states, where the spin and orbital angular momenta are locked (spin-momentum locking [5-7]), gives rise to a very large θ SHE [8-12] (or q ICS) and the resulting ultralow switching current density [13-15] at low temperature. Recently, several works have reported the room-temperature SOT switching by TIs, [16-19] which opens the door for the applications of topological insulators. However, there are fundamental limitations of FMs: the low switching speed (≈ns) and the stray-field interaction, which limit the operation speed and the density of magnetic memory, respectively. Antiferromagnets (AFMs) can afford the THz ultrafast spin dynamics; [20] however, AFMs produce zero stray field and zero spin polarization because of the opposite coupled spin lattices from the same element, which makes it difficult to detect the antiferromagnetic order efficiently. [21] Ferrimagnets have two antiferromagnetically coupled spin sublattices, and the contribution of each spin sublattice to their properties can be tuned by the composition or temperature. At the magnetic compensation point, ferrimagnets show similar properties of AFMs, such as ultrafast spin dynamics, [22,23] while the detection is still feasible because of the different responses to the optical or electrical excitations from two spin sublattices. [23-25] Here, we combine TIs [Bi 2 Se 3 and (BiSb) 2 Te 3 ] with nearly compensated ferrimagnets [Gd x (FeCo) 1−x ], and investigate the room-temperature SOT in TI/Gd x (FeCo) 1−x systems. By changing the composition of Gd x (FeCo) 1−x , we can tune the net magnetic moment and the dominated spin sublattice (CoFe-rich and Gd-rich). The robust room-temperature SOT switching Utilizing spin-orbit torque (SOT) to switch a magnetic moment provides a promising route for low-power-dissipation spintronic devices. Here, the SOT switching of a nearly compensated ferrimagnet Gd x (FeCo) 1−x by the topological insulator [Bi 2 Se 3 and (BiSb) 2 Te 3 ] is investigated at room temperature. The switching current density of (BiSb) 2 Te 3 (1.20 × 10 5 A cm −2) is more than one order of magnitude smaller than that in conventional heavy-metal-based structures, which indicates the ultrahigh efficiency of charge-spin conversion (>1) in topological surface states. By tuning the net magnetic moment of Gd x (FeCo) 1−x via changing the composition, the SOT efficiency has a significant enhancement (6.5 times) near the magnetic compensation point, and at the same time the switching speed can be as fast as several picoseconds. Combining the topological surface states and the nearly compensated ferrimagnets provides a promising route for practical energy-efficient and high-speed spintronic devices. Topological Spintronics Spintronic devices have been considered as one of the promising candidates for the next-generation memory and logic devices, and spin-orbit torque (SOT) provides an efficient way to manipulate the magnetic moment by electrical method with ultralow power dissipation and ultrafast operating speed. [1-3] Beyond previous studies of SOT switching based on conventional heavy metal/ferromagnet (HM/FM) heterostructures, two crucial issues need to be resolved: improving: i) the efficiency o