Spin-polarized wide electron slabs in functionally graded polar oxide heterostructures

Abstract

We report on the high mobility wide electron slabs with enhanced correlation effects by tailoring the polarization effects in a functionally graded ZnMgO/ZnO heterostructures. The characteristics of three-dimensional (3D) spreading electrons are evidenced by the capacitance-voltage profiling and the quantization of 3D Fermi surface in magneto-transport measurements. Despite the weak spin-orbit interaction, such electron slabs are spin-polarized with a large zero-field spin splitting energy, which is induced by the carrier-mediated ferromagnetism. Our results suggest that the vast majority of electrons are localized at the surface magnetic moment which does not allow spin manipulations, and only in the region visited by the itinerant carriers that the ferromagnetic exchange interactions via coupling to the surface local moments contribute to the spin transport. The host ferromagnetism is likely due to the formation of Zn cation vacancies on the surface regime induced by the stabilization mechanism and strain-relaxation in ZnMgO polar ionic surface.

Figure 1. (a) Polarization field directions, (b) schematic distribution of polarization dipole and (c) distribution of resultant carriers in the linearly graded ZnMgO/ZnO heterostructure in the [0001] orientation.

P_SP and P_PB represent the spontaneous and piezoelectric polarizations, respectively. The parameter c is the lattice constant, σ_π is the polarization-induced charges on top of the surface and N_Dπ is the sheet density of positive unbalanced polarization charges.

Figure 2. (a) Cross-sectional STEM HAADF image and the corresponding higher magnification image of the boxed region; (b) (left to right) EDX chemical maps with projected Mg and Zn, monochromated low-loss EELS spectra and the extracted bandgap energy profile of ZnMgO/ZnO/ZnMgO double heterostructures (sample B).

Figure 3. (105) asymmetric RSMs of sample A (a) and sample B (b).

Figure 4. (a) The valance-band and (b) O 1s core-level x-ray photoelectron spectrum of sample A calibrated by carbon 1s (284.6 eV); (c) AFM images of (a) ZnMgO/ZnO double heterostructure (sample B).

Figure 5. The calculated band diagrams and charge profiles at zero bias and carrier depth profile derived from C-V measurements for (a) sample A and (b) sample B.

Figure 6. (a) Mobility and (b) sheet carrier concentration of ZnMgO/ZnO heterostructures (samples A and B) as well as the reference ZnO template.

Figure 7. (a) Angular-dependent normalized magnetoresistance R_xx/R₀ versus B and Hall resistivity R_xy of sample A as a function of magnetic field (z//B, θ = 0°) at 1.4 K; (b) FFT power spectrum of R_xx versus B at 1.4 K.

Figure 8. (a) The M-H loop after diamagnetic and paramagnetic subtraction for sample A and referenced ZnO template measured at 5 K; (b) zero-field cooled (ZFC) and field-cooled (FC) magnetization curves of sample A.