Efficiently gate-tunable ferromagnetism in ferromagnetic semiconductor-Dirac semimetal p-n heterojunctions

Abstract

We use molecular beam epitaxy to develop a gate tunable p-n heterojunction that interfaces a canonical Dirac semimetal, Cd$_3$As$_2$, and a ferromagnetic semiconductor, In$_{1-x}$Mn$_x$As, with perpendicular magnetic anisotropy. Measurements of the anomalous Hall effect in top-gated Cd$_3$As$_2$/In$_{1-x}$Mn$_x$As devices show that the ferromagnetic Curie temperature ($T_\mathrm{C}$) can be efficiently tuned using a modest gate voltage of $\sim 10$ V, corresponding to a sensitivity to electric field ($E$) of $ΔT_{\mathrm{C}}/ΔE \sim 10$ K/MV/cm). The voltage tuning of $T_\mathrm{C}$ saturates near the charge neutrality point of Cd$_3$As$_2$ and vanishes at positive gate voltage in appropriately designed heterostructures. This non-monotonic behavior cannot be explained solely by hole-mediated ferromagnetism in the In$_{1-x}$Mn$_x$As alone, suggesting an interaction between the Dirac semimetal and the ferromagnetic semiconductor. Our results identify Cd$_3$As$_2$/In$_{1-x}$Mn$_x$As heterojunctions as a potentially attractive platform for studying emergent phenomena arising from the interplay between broken symmetry, topology, and magnetism in a topological semimetal.

Figures (4)

• Figure 1: Proposed Heterostructure: (a) Schematic of heterostructure grown. (b) Cross-sectional annular bright field scanning transmission electron microscopy (ABF-STEM) image of a $\mathrm{Cd_{3}As_{2}}$/$\mathrm{In_{1-x}Mn_{x}As}$ heterostructure. The vertical dark regions marked by red arrows in the $\mathrm{In_{1-x}Mn_{x}As}$ layer are indicative of grain boundaries. (c)-(d) Orbital-projected band structures for (c) a conventional unit cell of $\mathrm{Cd_{3}As_{2}}$ and (d) a conventional unit cell of InAs. (e) Unfolded band structure of the $\mathrm{Cd_{3}As_{2}}$/InAs heterostructure, projected onto the $k$-point path of the conventional unit cell of InAs.
• Figure 2: Band Diagrams : (a-c) Self-consistent solutions for charge density, electric field, and voltage plotted as a function of sample depth. (d-f) Depletion region band alignment cartoons showing the effect of varying $\mathrm{In_{1-x}Mn_{x}As}$ thickness. Note as thickness of $\mathrm{In_{1-x}Mn_{x}As}$ decreases, the presence of holes is reduced, allowing the presence of holes to be easily controlled by tuning the chemical potential via a gate voltage.
• Figure 3: Magnetotransport in control $\mathrm{In_{1-x}Mn_{x}As}$ and $\mathrm{Cd_{3}As_{2}}$ films and in $\mathrm{Cd_{3}As_{2}}$/$\mathrm{In_{1-x}Mn_{x}As}$ heterostructures: Upper panels show the magnetic field dependence of the 2D resistivity ( $\rho_{xx}$, left axis) and Hall resistivity ($\rho_{xy}$, right axis) in (a) S1 ($25$ nm thick $\mathrm{In_{1-x}Mn_{x}As}$ film),(b) S2 ($12$ nm thick $\mathrm{In_{1-x}Mn_{x}As}$ film), and (c) S3 ($25$ nm thick Cd$_3$As$_2$ film). The slope of $\rho_{xy}$ at high field ($\mu_0 H > 0.5$T) indicates p-type carriers in S1 and S2, but n-type carriers in S3. Lower panels show measurements of the magnetic field dependence of $\rho_{xx}$ (left axis) and $\rho_{xy}$ (right axis) in (d) S4 ($25$ nm Cd$_3$As$_2$/25 nm $\mathrm{In_{1-x}Mn_{x}As}$ heterostructure, (e) S5 (25 nm Cd$_3$As$_2$/17 nm $\mathrm{In_{1-x}Mn_{x}As}$ heterostructure, and (f) S6 (25 nm Cd$_3$As$_2$/12 nm $\mathrm{In_{1-x}Mn_{x}As}$/GaSb/GaAs).
• Figure 4: Gate-voltage and temperature dependence of Hall effect: (a) Optical micrograph of a lithographically fabricated device. The scale bar is $100$ µ m. (b) Longitudinal resistivity ($\rho_{xx}$) as a function of gate-voltage ($V_g$) at $T=2$ K. (c) and (d) Hall resistance ($\rho_{xy}$) in devices S5-F and S6-F, respectively, as a function of a magnetic field perpendicular to the sample plane, showing a gate-tunable AHE. The curves have been shifted vertically for clarity. (e) Arrott plot of S5-F with $V_g$ fixed at $-5$ V. (f) $T_C$ as a function of $V_g$ offset to the $V_{CNP}$. Error bars note the measurements are taken at 1 K intervals.