Large nonvolatile control of interfacial magnetic anisotropy in CoPt by a ferroelectric ZnO-based tunneling barrier

The electric control of magnetic anisotropy has important applications for nonvolatile memory and information processing. By first-principles calculations, we show a large nonvolatile control of magnetic anisotropy in the ferromagnetic/ferroelectric CoPt/ZnO interface. Using the switched electric polarization of ZnO, the density-of-states and magnetic anisotropy at the CoPt surface show a large change. Due to a strong Co/Pt orbitals hybridization and a large spin-orbit coupling, a large control of magnetic anisotropy was found. We experimentally measured the change of effective anisotropy by tunneling resistance measurements in CoPt/Mg-doped ZnO/Co junctions. Additionally, we corroborate the origin of the control of magnetic anisotropy by observations on tunneling anisotropic magnetoresistance.

Figure 1

(a) A schematic of the MgZnO magnetic tunnel junction (MTJ) and the electric-field cooling (EFC) procedure. (b) The reversal of MgZnO electric polarization ($P$) after EFC changes the surface charge at the metal interfaces of the MTJ. The charge difference is negative (positive) at the CoPt surface after $+\mathrm{EFC}$ ($-\mathrm{EFC}$). Correspondingly, the surface MAE of CoPt is perpendicular (in-plane) for $P^{+}$ ($P^{-}$) states. (c) Schematics of the MTJ potential profile at $P^{\pm }$ states.

Figure 2

(a),(b) The model of CoPt/ZnO used for the first-principles calculations, in the (a) $P^{+}$ and (b) $P^{-}$ states. (c)–(f) The projected density of states (PDOS) the (c),(d) Co and (e),(f) Pt-1 atoms in the (c),(e) $P^{+}$ and (d),(f) $P^{-}$ states.

Figure 3

The layer-resolved values of (a) the change of number of electrons ($\mathrm{\Delta }n$), (b) the spin momentum ($m_s$), its change by $P$ ($\mathrm{\Delta }{m}_s$), and anisotropy ($\delta m_s$), (c) the orbital momentum ($m_o$), its change by $P$ ($\mathrm{\Delta }{m}_o$), and anisotropy ($\delta m_o$).

Figure 4

The experimental effect of $P$ on MAE. (a) The $M\text{−}H$ curves in the in-plane $H_{\parallel }$ and out-of-plane $H_{\perp }$ directions of the as-deposited state ($P^{+}$) of blanket films. The CoPt layer shows a small in-plane total MAE, whereas the Co layer has a dominant in-plane shape MAE. (b),(c) The $R\text{−}H$ curves of a fabricated junction in the in-plane and perpendicular directions (b) after $+\mathrm{EFC}$ at $P^{+}$ state, and (c) after $-\mathrm{EFC}$ at $P^{-}$ state. The $R\text{−}H$ curves indicate that the surface MAE of CoPt changed from the out-of-plane to in-plane direction by $P$ modulation. The measurements in (a) and (b),(c) were done at 5 K and 2 K, respectively.

Figure 5

The effect of $P$ modulation on tunneling anisotropic magnetoresistance (TAMR). (a),(b) Samples of the dependence of TAMR on applied field angle ($\varphi$) at different bias voltages, (a) after $+\mathrm{EFC}$ at $P^{+}$ state, and (b) after $-\mathrm{EFC}$ at $P^{-}$ state. The curves are vertically shifted uniformly for clarity. (c) The experimental bias dependence of TAMR. (d) The energy dependence of TAMR from the first-principles calculations. A qualitative agreement is found between the experimental and calculation results.