Tailorable multiferroic tunnel junctions from all-van der Waals multilayer stacking

Multiferroic tunnel junctions (MFTJs) represent a class of multistate, non-volatile spintronic devices, in which electron tunnelling can be manipulated by switching long-range lattice and spin orders. In contrast to conventional oxide-based MFTJs, MFTJs constructed from two-dimensional van der Waals (vdW) crystals promise minimal defect concentration in the constituents and at interfaces, which may allow for probing intrinsic tunnelling physics and the development of high-performance devices. Here we construct Fe₃GeTe₂/CuInP₂S₆/Fe₃GeTe₂ all-vdW MFTJs by assembling multilayer flakes of ferromagnetic Fe₃GeTe₂ electrodes and a ferroelectric CuInP₂S₆ spacer. These MFTJs exhibit four non-volatile resistance states featuring sizable tunnelling magnetoresistance of ∼10²% and tunnelling electroresistance of ∼10⁴%. To tune the properties of the vdW MFTJ, we make use of the flexibility in material choice offered by vdW heterostructure devices; we use Fe₃GeTe₂/Fe₅GeTe₂ asymmetric electrodes to boost the tunnelling electroresistance by 10³%, we integrate In₂Se₃ as a ferroelectric with a smaller bandgap to enhance the ON-state current density by 10⁴% to 10⁴ A cm^-2 and we use Fe₃GaTe₂ electrodes to demonstrate room temperature operation. Furthermore, when we combine the asymmetric ferromagnetic electrodes with the small-bandgap ferroelectric spacer to construct Fe₃GeTe₂/In₂Se₃/Fe₅GeTe₂ MFTJs, we simultaneously realized tunnelling electroresistance of 10⁶% and an ON-state current density of 10⁴ A cm^-2, both two orders of magnitude higher than the highest values achieved with conventional oxide-based MFTJs. In the future, our all-vdW MFTJs with the tailorability of all functional layers may make it possible to investigate fundamental aspects of interlayer tunnelling and enable the design of functional magnetoelectric nanodevices.