Electronic voltage control of magnetic anisotropy at room temperature in high-$\kappa$ ${\mathrm{SrTiO}}_3/\mathrm{Co}/\mathrm{Pt}$ trilayer

To improve power efficiency and endurance of magnetic memory technologies, a voltage-controlled mechanism is desirable. The voltage control of magnetic anisotropy (VCMA) effect in MgO stacks is a promising option, however, its strength is too low for memory applications. Replacing the standard MgO layer by an oxide with a higher permittivity $\kappa$ may help improve the VCMA strength. We demonstrate a VCMA effect up to $\xi =75$ fJ/Vm at room temperature in a Co∖Pt bilayer grown on atomic layer deposited (ALD) high-$\kappa {\mathrm{SrTiO}}_3$ (STO). After treating the STO surface with isopropanol, a thin ${\mathrm{CoO}}_x$ interfacial layer is observed, enabling VCMA. Upon cooling down from room temperature to 200 K, the VCMA effect strength increases by a factor of two. This increase is incompatible with the expected Arrhenius temperature dependence for an ionic effect and thus we argue that the observed VCMA effect is electronic. Electronic VCMA is desirable for adequate memory endurance, and hence the approach proposed here has great potential for applications.

Figure 1

(a) Transmission electron microscopy (TEM) image of a sample on the Co thickness wedge location with $t_{\mathrm{Co}}$ = 0.8 nm. The layer thickness in nm is indicated between parentheses. (b) Energy-dispersive x-ray spectroscopy (EDS) image shows the Co/Pt interdiffusion.

Figure 2

(a) The out-of-plane (OOP) magnetic hysteresis loop measured with the anomalous Hall effect for both samples. (b) The magnetic moment $M_{S}V$ and (c) the effective anisotropy energy $K_{\mathrm{eff}}{t}_{\mathrm{Co}}$ vs cobalt thickness $t_{\mathrm{Co}}$ for $8\times 8$ ${\mathrm{mm}}^2$ samples cleaned in IPA and samples with ${\mathrm{Ar}}^{+}$ ion milling $+$ 350 ${}^{\circ }\mathrm{C}$ UHV anneal.

Figure 3

The VCMA effect in STO∖Co(Pt)∖Pt for IPA cleaned samples with $t_{\mathrm{Co}}$ = 0.66 nm with ${\theta }_H$ = ${82}^{\circ }$. (a) AHE measurement as a function of the in-plane field. The upper left inset shows the sequence of gate voltages. (b) The calculated in-plane component of the magnetic moment. The dependence of (c) $B_{K0}$ and (d) $B_{K2}$ on $V_{\mathrm{Gate}}$ across STO. (e) The dependence of $B_{K0}$ on $V_{\mathrm{Gate}}$ when $B_{K2}$ is held constant at $B_{K2}(V_{\mathrm{Gate}}$ = 0V). (f) The amplitude of the VCMA effect (with $B_{K2}$ fixed) in samples with $t_{\mathrm{Co}}$ = 0.66 nm. The arrow indicates the sample shown in (a)–(e). The sign of VCMA is defined as positive when accumulation of electrons in the ferromagnetic layer leads to an increase in the anisotropy energy.

Figure 4

The VCMA effect in STO∖CoPt∖Pt at cryogenic temperatures. (a) The AHE curves at different temperatures for ${\theta }_H$ = ${82}^{\circ }$. (b) The temperature dependence of the coercivity $H_c$, the zeroth and second order anisotropy fields $B_{K0,K2}$. (c) The temperature dependence of the magnetization $M_S$. (d) The amplitude of the VCMA effect $\xi$ and the voltage control of $B_{K0}$ as a function of temperature. The trend line of the permittivity $\kappa$ is added for comparison. The discrepancy between ${\mathrm{VCB}}_{K0}$ and $\xi$ is due to the temperature dependence of the magnetization $M$. (e) The capacitance of a square capacitor as a function of temperature, and corresponding permittivity $\kappa$. (f) Temperature dependence of the mobility of oxygen vacancies $\mu V_{\text{O}x}$ from Ref. [33] (dots) and Arrhenius extrapolation using Eq. (4) (line).