Multiferroic van der Waals heterostructure ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$: Nonvolatile electrically switchable electronic and spintronic properties

Multiferroic van der Waals (vdW) heterostrucutres offer an exciting route toward high-performance nanoelectronics and spintronics device technology. Here we investigate the electronic and transport properties of multiferroic vdW heterostructures composed of a ferromagnetic ${\mathrm{FeCl}}_2$ monolayer and a ferroelectric ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ monolayer using first-principles density functional theory and quantum transport simulations. We show that ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructure can be reversibly switched from semiconducting to half-metallic behavior by electrically modulating the ferroelectric polarization states of ${\mathrm{Sc}}_2{\mathrm{CO}}_2$. Intriguingly, the half-metallic phase exhibits a type-III broken gap-band alignment, which can be beneficial for tunneling field-effect transistor applications. We perform a quantum transport simulation based on a proof-of-concept two-terminal nanodevice to demonstrate all-electric-controlled valving effects uniquely enabled by the nonvolatile ferroelectric switching of the heterostructure. These findings unravel the potential of ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ vdW heterostructures as a building block for designing the next generation of ultimately compact information processing, data storage, and spintronics devices.

Figure 1

Crystal structures of the 2D ferroelectric ${\mathrm{FeCl}}_2$, the 2D ferromagnetic ${\mathrm{Sc}}_2{\mathrm{CO}}_2$, and their 2D/2D vdW heterostructures. Top view and side view of the structures of single layers of (a) ${\mathrm{FeCl}}_2$ and (b) ${\mathrm{Sc}}_2{\mathrm{CO}}_2$. The blue line represents the unit cell. (c)–(h) ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ vdW heterostructures with different stacking configurations under various polarization states. The $P\uparrow$ and the $P\downarrow$ states are shown in (c) (e), (g) and (d), (f), (h) $\mathrm{P}\downarrow$, respectively, for $AA, BB$, and $CC$ stacking configurations. The Fe, Cl, Sc, C, and O atoms are denoted by dark cyan, green, white, gray, and red spheres, respectively.

Figure 2

Electronic band structures of (a)–(d) $AA$-stacked heterostructure, (e)–(h) $BB$-stacked heterostructure, and (i)–(l) $CC$-stacked heterostructure under $P\uparrow$ (top panel) and $P\downarrow$ (bottom panel) polarized states. The Fermi level is set to zero. Here green, blue, and red symbols denote the contributions from ${\mathrm{Sc}}_2{\mathrm{CO}}_2$, spin-up and spin-down electronic states of ${\mathrm{FeCl}}_2$, respectively.

Figure 3

Plane-averaged electrostatic potential of ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ (a) $P\uparrow$ and (f) $P\downarrow$ along the $Z$ direction; the plane-averaged differential charge density $\mathrm{\Delta }\rho$ of ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ (b) $P\uparrow$ and (g) $P\downarrow$; (c)–(e) and (h)–(j) the three-dimensional isosurface of the electron density difference (isosurface value is $0.005e{\AA }^{-3}$) of the multiferroic system with $P\uparrow$ and $P\downarrow$ states, respectively, where the blue and red areas represent electron depletion and accumulation, respectively. Band alignments for ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ with ferroelectric polarization of (k) $P\uparrow$ and (l) $P\downarrow$, respectively, of isolated layers (left panel) and that after forming heterostructure (right panel). The energies are displayed in the unit of eV. The blue horizontal dashed lines denote the work functions of the systems.

Figure 4

(a) Optimized geometries of four configurations of bilayer ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{FeCl}}_2$ or ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ vdW heterostructures; (b)–(f) electronic band structures of bilayer ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{FeCl}}_2$ or ${\mathrm{Sc}}_2{\mathrm{CO}}_2/{\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ vdW heterostructures under different polarized states, respectively.

Figure 5

Simulated transport current as a function of the applied bias for ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_{2}BB$ stacking with (a) different ferroelectric polarization states. (b) The spin-up and spin-down currents when the heterostructure is in the $P\downarrow$ state. The spin-dependent transmission spectrum of the device in (c) $P\uparrow$ and (d) $P\downarrow$ polarization states at zero bias. (e) and (f) show the schematic diagrams of a ${\mathrm{FeCl}}_2/{\mathrm{Sc}}_2{\mathrm{CO}}_2$ multiferroic all-electric-controlled device based on the nonvolatile switching of the $P\uparrow$ and the $P\downarrow$ polarization states of the ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ monolayer.