Observation of large magnetoresistance switching in topological-insulator/ferromagnetic-metal heterostructure with perpendicular magnetic anisotropy

In the last decade, studies of magnetoresistance in heterostructures of topological insulator (TI) and ferromagnetic (FM) insulators have indicated the existence of induced magnetization at TI surfaces. However, the magnetic proximity effect in heterostructures of TIs and FM metals has been less explored. Here, we report a spin-valve-like magnetoresistance (MR) switching observed in a bilayer device of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and a Co/Pt multilayer ([Co/Pt]), where the [Co/Pt] is a FM metal with a perpendicular magnetic anisotropy. This MR switching happens at temperatures below 1 K and for magnetic field sweeps along all in-plane and out-of-plane directions, at values much lower than the magnetization switching fields of the top FM layer. The in-plane field sweeps at various angles also reveal a threefold symmetry of the switching fields, matching the symmetry of the TI crystal structure. We suggest that the large MR switching rises from the interplay between the magnetization of the top FM metal layer and the induced magnetization at the TI surface.

Figure 1

(a) Schematic of a ${\mathrm{Bi}}_2{\mathrm{Se}}_3$/[Co/Pt] device. $x, y, z, \varphi$, and $\theta$ are defined as shown. Typical (b) AHE and (c) anisotropic MR in [Co/Pt] Hall bar structure, measured at $T$ = 100 mK and 300 K. Anisotropic MR was measured with 1-T field rotated out of plane in the $zx$ plane as shown in the schematic. AHE result demonstrates Co/Pt magnetization switching at $H_z \sim$ 50 mT at 100 mK.

Figure 2

Spin-valve-like hysteretic MR response of TI/[Co/Pt] device at $T$ = 100 mK for magnetic field sweeps (a) along the current ($x$), (b) perpendicular to the current ($y$), and (c) out of plane ($z$). In all cases, the abrupt 20% MR change between high- and low-resistance states happens at low fields $H<$ 8 mT, much lower than the switching field of the [Co/Pt] FM layer.

Figure 3

Temperature dependence of the spin-valve-like MR response to (a) in-plane and (b) out-of-plane field sweeps. Both the high-MR state and the width of the hysteresis out of plane decrease as a function of increasing temperature, before the effect disappears after $T\sim$ 1 K. (c) MR% plotted as a function of temperature. High- and low-MR states are considered identical for in-plane and out-of-plane field sweeps due to their same resistance at a given temperature.

Figure 4

(a) MR for in-plane field sweep measurements at different angles $\varphi$ stepped by 30°. While the MR magnitude is constant, a threefold variation in $H_{\mathrm{LtoH}}$ and $H_{\mathrm{HtoL}}$ is evident. (b) $H_{\mathrm{LtoH}}$ and $H_{\mathrm{HtoL}}$ switching fields as a function of $\varphi$. The polar plots for (b) 10 0 mT (switch up) and (c) 0 10 mT (switch down) reveal threefold symmetry of the maximum switching fields that matches with the crystalline symmetry of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, hence signaling the direct correlation of the spin-valve-like MR response and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The intermediate angles are interpolated.