Electrical Control of Metallic Heavy-Metal–Ferromagnet Interfacial States

Voltage-control effects provide an energy-efficient means of tailoring material properties, especially in highly integrated nanoscale devices. However, only insulating and semiconducting systems can be controlled so far. In metallic systems, there is no electric field due to electron screening effects and thus no such control effect exists. Here, we demonstrate that metallic systems can also be controlled electrically through ionic rather than electronic effects. In a $\mathrm{Pt}/\mathrm{Co}$ structure, the control of the metallic $\mathrm{Pt}/\mathrm{Co}$ interface can lead to unprecedented control effects on the magnetic properties of the entire structure. Consequently, the magnetization and perpendicular magnetic anisotropy of the Co layer can be independently manipulated to any desired state, the efficient spin toques can be enhanced about 3.5 times, and the switching current can be reduced about one order of magnitude. This ability to control a metallic system may be extended to control other physical phenomena.

Figure 1

STEM EELS results at different EF-controlled states. (a) Top view and typical STEM image of a $\mathrm{Pt}/\mathrm{Co}/{\mathrm{GdO}}_x$ sample. (b) The vertical field-dependent $R_H$ for samples $c$, as-deposited state, $d$, after $V_G=-5\mathrm{V}$ application, $e$, fully oxidized state, and $f$, after $V_G=-5\mathrm{V}$ and then $V_G=+5\mathrm{V}$ applications. (d)–(f) The corresponding Co and oxygen distribution from EELS line scan along the $z$ direction at the $\mathrm{Pt}/\mathrm{Co}$ interface.

Figure 2

Schematic of the voltage control of magnetism through metallic $\mathrm{Pt}/\mathrm{Co}$ interfaces. (a) The as-deposited state showing PMA without $V_G$ application. The Co layer has been partially oxidized and the $\mathrm{Pt}/\mathrm{Co}$ interface is clean. (b) Under $+V_G$ application, the ${\mathrm{O}}^{2-}$ at the $\mathrm{Co}/{\mathrm{CoO}}_x$ or $\mathrm{Co}/{\mathrm{GdO}}_x$ interface is driven into the ${\mathrm{GdO}}_x$ layer. The ${\mathrm{CoO}}_x$ is partially reduced to Co and the $\mathrm{Pt}/\mathrm{Co}$ interface keeps clean. (c) The ${\mathrm{CoO}}_x$ is finally reduced to Co and the Co layer becomes in-plane anisotropic under the $+V_G$ application. (d) After $-V_G$ application, the ${\mathrm{O}}^{2-}$ is driven back to the $\mathrm{Co}/{\mathrm{GdO}}_x$ interface and further contaminates the $\mathrm{Pt}/\mathrm{Co}$ interface through diffusion. PMA is rebuilt and the Co layer is oxidized. Because of the contamination of the $\mathrm{Pt}/\mathrm{Co}$ interface, $H_k$ becomes weaker compared to (b) even with the same other conditions. (e) The Co layer is fully oxidized by $-V_G$ application. The blue line texture in the $\mathrm{Co}/{\mathrm{CoO}}_x$ layer symbolically illustrates grain boundary. (f)–(j) The schematic of expected vertical and in-plane field-dependent $M_z$ at each state.

Figure 3

The evolution of magnetism under $V_G$ control. (a),(b) Typical $R_H$ curves under vertical (a) or in-plane fields (b). The red lines in (b) are fitting results by using the Stoner-Wohlfarth model. (c) Typical time dependences of $H_k$ and $R_H$ under $V_G$ control. (d) The measured $H_k$ as a function of $R_H$ in a typical $V_G$ control cycle. The red arrows indicate the time evolution of the magnetism from states 1 to 8. Inset: the sample geometry and the experimental configuration of the $V_G$ application.

Figure 4

The effects of reversal $V_G$ applications on the control effects. (a) $H_k$ versus $R_H$ curves when $-V_G$ is applied from $R_H=0.33\mathrm{\Omega }$ (purple), $0.37\mathrm{\Omega }$ (red), and $0.41\mathrm{\Omega }$ (violet). (b) $H_k$ versus $R_H$ curves where $+V_G$ is applied from different initial magnetic states. The gray curve is from Fig. 3 for comparison. The arrows in (a),(b) indicate the time evolution of controlled magnetic states.

Figure 5

SOT switching under $V_G$ control. (a) Typical current induced SOT switching under in-plane fields. (b) The SOT-switching curves under $V_G$ control. (c),(d) The extracted $H_k$ (c) and corresponding $I_c$ (d) as a function of $R_H$ at each magnetic state controlled by $V_G$. The arrows indicate the time evolution of control process. (e) $I_c$ is replotted as a function of $R_H{H}_k$. The solid lines are the liner fitting results.