High-Mobility Spin-Polarized Two-Dimensional Electron Gases at $\mathrm{EuO}/{\mathrm{KTaO}}_3$ Interfaces

Two-dimensional electron gases (2DEGs) at oxide interfaces, which provide unique playgrounds for emergent phenomena, have attracted increasing attention in recent years. While most of the previous works focused on the 2DEGs at ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interfaces, here we report on a new kind of 2DEGs formed between a magnetic insulator EuO and a high-$k$ perovskite ${\mathrm{KTaO}}_3$. The 2DEGs are not only highly conducting, with a maximal Hall mobility of $111.6{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$ at 2 K, but also well spin polarized, showing strongly hysteretic magnetoresistance up to 25 K and well-defined anomalous Hall effect up to 70 K. Moreover, unambiguous correspondences between the hysteretic behaviors of 2DEGs and the EuO layer are captured, suggesting the proximity effect of the latter on the former. This is confirmed by the results of density-functional theory calculations: Through interlayer exchange, EuO drives the neighboring ${\mathrm{TaO}}_2$ layer into a ferromagnetic state. The present work opens new avenues for the exploration for high performance spin-polarized 2DEGs at oxide interfaces.

Figure 1

Crystal structure of the EuO film grown on KTO. (a) A schematic model for the epitaxial growth of EuO on (001) KTO. The EuO lattice has been rotated by 45° around the $z$ axis. (b) Reflection high-energy electron diffraction pattern recorded during the growth process of the EuO layer, clearly showing the $\mathrm{EuO}[100]⫽\mathrm{KTO}[110]$ correspondence. (c) $\theta \text{−}2\theta$ x-ray diffraction pattern specifying the well-orientated EuO layer on KTO. (d) Reciprocal space mapping of the (204) reflection of EuO, neighboring the (113) reflection of KTO. The elliptic (204) spot indicates lattice relaxation. All data shown here were obtained for the EuO film grown at a substrate temperature of $450°\mathrm{C}$.

Figure 2

Magnetic and transport properties of the $\mathrm{EuO}/\mathrm{KTO}$ heterostructures. (a) Temperature dependence of magnetization of the $\mathrm{EuO}/\mathrm{KTO}$ heterostructure measured in the field-cooling mode, showing a magnetic onset around 71 K. (b) Magnetic field-dependent magnetizations, measured at 5 K with in-plane and out-of-plane fields, respectively. The easy magnetic axis of the EuO layer lies in film plane. (c) Sheet resistance as a function of temperature. All $\mathrm{EuO}/\mathrm{KTO}$ interfaces are highly conducting, though detailed resistive behavior varies with fabrication temperature. Inset plot is a sketch for the measurements of sheet resistance and Hall resistance. (d) and (e) Carrier density and Hall mobility, respectively, as functions of temperature.

Figure 3

Magnetotransport properties of the 2DEGs. (a) Magnetoresistance of the 2DEG obtained at $450°\mathrm{C}$, measured at different temperatures with the magnetic field parallel to applied current. A constant upward shift was adopted for two successive curves for clarity. Arrows above the curves mark the sweeping direction of the magnetic field. The upper left corner shows the geometry for MR measurements. (b) A magnified view of the MR obtained at 25 K. Upper right corner shows the geometry for MR measurements. (c) Temperature dependence of the maximal MR (origin symbols), the peak position of the MR (green symbols), and the coercive force field of the EuO film (blue symbols). Solid lines are guides for the eye. (d) MR measured at 2 K with the in-plane fields that are perpendicular to the applied current. Upper right corner shows the geometry for MR measurements. (e) Anomalous Hall resistance as a function of magnetic field (left panel) and temperature (right panel) for the 2DEG obtained at $400°\mathrm{C}$. Solid line is a guide to the eye. Arrow in the left panel marks the anomaly due to hysteretic MR.

Figure 4

(a) (${\mathrm{EuO}}_6/{\mathrm{KTO}}_6$) superlattices for density-functional theory calculations. (b)-(c) Projected density of states for $A$-site Ta atoms, obtained for the ${\mathrm{TaO}}_2/\mathrm{EuO}$ (b) and $\mathrm{KO}/\mathrm{EuO}$ (c) interfaces. Positive and negative densities of states represent the spin-up and spin-down states, respectively. Numbers in the figure indicate the net magnetization in the unit of ${\mu }_B/\mathrm{Ta}$.