Novel spintronic device can store data in four stable states

Researchers from the University of Maryland, University of California, South Dakota School of Mines and Technology, East China Normal University, KAUST and other institutes recently reported all‑van der Waals multiferroic tunnel junctions (MFTJs) that combine ferromagnetism and ferroelectricity in a single nanoscale spintronic device, enabling four non‑volatile resistance states for multibit memory operation. 

These multistate junctions are realized by vertically stacking three atomically thin crystals: two ferromagnetic electrodes and a ferroelectric tunnelling barrier, all obtained by mechanical exfoliation and then assembled into a clean, defect‑sparse heterostructure. In their prototypical structure, Fe₃GeTe₂/CuInP₂S₆/Fe₃GeTe₂, multilayer Fe₃GeTe₂ serves as the ferromagnetic electrodes, while CuInP₂S₆ (CIPS) provides a ferroelectric spacer with switchable polarization. Because the layers are coupled by van der Waals forces rather than epitaxial bonding, the stack avoids stringent lattice‑matching and chemical‑compatibility constraints that hinder oxide‑based MFTJs and is far less susceptible to interfacial defects and interdiffusion.

The device concept merges the functionalities of magnetic tunnel junctions and ferroelectric tunnel junctions into a single element: two ferromagnetic electrodes separated by an ultrathin ferroelectric barrier. In this configuration, the junction exhibits four distinct, non‑volatile resistance states instead of the two states available in conventional MTJs or FTJs, because both the relative magnetization (parallel vs. antiparallel) of the electrodes and the polarization direction (up vs. down) of the ferroelectric barrier modulate the electron tunnelling probability. 

Experimentally, the team demonstrates that by independently toggling the magnetization of the ferromagnetic layers and the polarization of the ferroelectric barrier, all four resistance states can be reliably written and read, providing a compact platform for multilevel memory in which two bits of information are stored per junction. The all‑vdW Fe₃GeTe₂/CuInP₂S₆/Fe₃GeTe₂ devices already show sizable tunnelling magnetoresistance of about 102% and tunnelling electroresistance of about 10^4%, indicating strong sensitivity of the tunnelling current to both magnetic and electric order parameters.

A key advantage of the vdW approach is the ability to tailor performance simply by swapping constituent 2D crystals while keeping the same three‑layer architecture. The researchers systematically explore this design freedom by replacing one or more of the layers. Using asymmetric Fe₃GeTe₂/Fe₅GeTe₂ electrodes with a CuInP₂S₆ barrier, they boost the tunnelling electroresistance by roughly 10³% compared to the symmetric configuration, showing that electrode asymmetry can strongly enhance the ferroelectric control of tunnelling. Introducing In₂Se₃ as a ferroelectric spacer with a smaller bandgap yields an ON‑state current density enhanced by about 10⁴%, reaching 10⁴ A cm⁻² while maintaining robust tunnel behavior, a crucial step towards fast, low‑resistance readout compatible with dense integrated circuits. Further, by employing Fe₃GaTe₂ as the ferromagnetic electrodes, the team demonstrates that similar MFTJ functionality and multistate operation persist at room temperature, an essential requirement for practical spintronic memory technologies.

The most advanced configuration the team reports combines both electrode asymmetry and a small‑bandgap ferroelectric barrier in Fe₃GeTe₂/In₂Se₃/Fe₅GeTe₂ junctions. In this architecture, they simultaneously achieve a tunnelling electroresistance of 10⁶% and an ON‑state current density of 10⁴ A cm⁻², both about two orders of magnitude higher than the best values reported for conventional oxide‑based MFTJs. Across all these variants, the constituent layers - Fe₃GeTe₂, Fe₅GeTe₂, Fe₃GaTe₂, CuInP₂S₆, and In₂Se₃ - are prepared by mechanical exfoliation (for example, using scotch tape) and vertically assembled, underscoring the practicality of vdW stacking as a fabrication route. Because van der Waals bonding does not require lattice matching, the same methodology can be extended to many other 2D ferromagnets and ferroelectrics, allowing direct tuning of tunnelling magnetoresistance, tunnelling electroresistance, resistance‑area product, and current density without altering the fundamental device layout. This combination of multistate operation, giant electroresistance, high current density, and room‑temperature functionality positions all‑vdW MFTJs as a promising platform for low‑energy, high‑performance multistate spintronic memories and for probing fundamental magnetoelectric coupling and interlayer tunnelling physics in 2D heterostructures.