Magnetic proximity effect in graphene coupled to a $\mathrm{BiFe}{\mathrm{O}}_3$ nanoplate

Graphene, a very intriguing two-dimensional Dirac electronic system with high carrier mobility, is promising for spintronics. However, the long-range ferromagnetic order is always absent in pristine graphene. Here we report the fabrication and transport properties of graphene-$\mathrm{BiFe}{\mathrm{O}}_3$ heterostructures. It is found that the magnetic proximity effect results in a strong Zeeman splitting in graphene with the exchange field up to hundreds of tesla. The $\nu =0$ quantum Hall state of graphene is further transformed into a ferromagnetic state or a canted antiferromagnetic state in the presence of a perpendicular magnetic field. Our findings in graphene/$\mathrm{BiFe}{\mathrm{O}}_3$ heterostructure are therefore promising for future spintronics.

Figure 1

Fabrication of $\mathrm{BiFe}{\mathrm{O}}_3$-graphene hybrid devices. The $\mathrm{BiFe}{\mathrm{O}}_3$-graphene hybrid transistors were fabricated as following processes. (a) Monolayer graphene sheets were exfoliated by scotch tape method on Si substrate with a 285 nm $\mathrm{Si}{\mathrm{O}}_2$ layer. (b) $\mathrm{BiFe}{\mathrm{O}}_3$ nanoplates grown by hydrothermal method were transferred onto graphene by a micromanipulator. (c) Electron beam lithography (EBL) and electron beam evaporation techniques were used to fabricate Pd/Au (5 nm/80 nm) electrodes. (d) Hall-bar structure was shaped by oxygen plasma etching after another round of EBL.

Figure 2

Characterization of $\mathrm{BiFe}{\mathrm{O}}_3$/graphene heterostructure. (a) Diagram of the crystal structure of $\mathrm{BiFe}{\mathrm{O}}_3$/graphene hybrid heterostructure. Red atoms: oxygen; dark gray atoms: iron; purple atoms: bismuth; light gray atoms: carbon. (b) High resolution TEM image of a typical $\mathrm{BiFe}{\mathrm{O}}_3$ nanoplate. Scale bar: 2 nm. (c) Diagram for the device. (d) Back-gate voltage dependence of resistivity of the BFO/graphene hybrid device. Inset: Optical image of a typical BFO/graphene hybrid transistor. The scale bar is $2\mu \mathrm{m}$.

Figure 3

Low field nonlocal measurement results. (a) Diagram for graphene band structure containing Zeeman splitting. Red (green) arrow denotes spin-up (spin-down). (b) Schematics for measurement configurations 1 and 2. (c) and (d) Nonlocal resistivity measured under external magnetic field of 0 and 0.5 T, respectively, with configurations 1 (blue) and 2 (red).

Figure 4

Nonlocal measurement results with both positive and negative magnetic fields. (a) The nonlocal resistance measured with −0.5 T and measurement configuration 1 (blue curve), and with 0.5 T and measurement configuration 2 (red curve). It is clear that the $R_{\mathrm{nl}}\text{−}{V}_g$ curves exhibit similar relationship for the two measurement configurations with opposite polarity of the magnetic field. (b) The nonlocal resistance measured with 1 T and measurement configuration 1 (blue curve), with 1 T and measurement configuration 2 (red curve), and with −1 T and measurement configuration 1 (brown curve).

Figure 5

The theoretical predictions (dashed lines) and experimental results (blue triangle: $N=+1$; green triangle: $N=-1)$ of $N=\pm 1\mathrm{LL}$ positions are compared.

Figure 6

Four-terminal measurement results. (a) Longitudinal resistivity measured by four-terminal configuration under external field from 0.5 to 14 T. For clarity of linear fitting of the LL, the curves are shifted in direct proportion to the applied magnetic field. (b) ${\rho }_{xx}\sim B_{\perp }$ evolution at $\nu =0$. (c) Transverse conductivity ${\sigma }_{xy}$ and longitudinal resistivity ${\rho }_{xx}$ measured under 14 T. (d)–(f) Illustrations of the antiferromagnetic, canted antiferromagnetic, and ferromagnetic phases with spin-momentum locked helical edge states.

Figure 7

Magnetotransport nonlocal measurement results. (a) Nonlocal resistance $R_{\mathrm{nl}}$ measured under 8–13 T perpendicular external field. The reduction of $R_{\mathrm{nl}}$ emerges under 12/13 T is further evidence for FM-CAF phase transition in LL ground state. (Inset: Nonlocal measurement configuration.) (b) The $R_{\mathrm{nl}}$ peak value [highest points of the curves in panel (a)] also decreases as the field increases from 8 to 13 T. (c) Two-terminal resistance $R_L$ obtained under $B_{\perp }=13\mathrm{T}$. Part of QHE plateaus are defined as $\nu =\pm 2(\frac{h}{2e^2}\sim 12.9\mathrm{k}\mathrm{\Omega })$ and $\nu =\pm 6(\frac{h}{6e^2}\sim 4.3\mathrm{k}\mathrm{\Omega })$. (d) Evolution of $R_{L,D}$ at the Dirac point (the highest point) with increasing external field. It is very obvious that $R_{L,D}$ rapidly rises after $B_{\perp }=10\mathrm{T}$, indicating the gap opening.