HMN 2026: How Nanoengineered spintronic device can store data in four different ways

Over the past decades, electronics engineers have been trying to develop increasingly smaller devices that can store information reliably, even when they are not powered on. A promising type of non-volatile memory device is spintronics, solid-state systems that store and process information leveraging the spin (i.e., an intrinsic form of angular momentum) of electrons.

Researchers at University of Maryland and other institutes recently introduced a new spintronic device based on nanoscale structures based on materials that exhibit ferromagnetism (i.e., a permanent yet switchable magnetic order) and ferroelectricity (i.e., a permanent yet switchable electric polarization). This device, presented in a paper published in Nature Nanotechnology, can switch between four stable resistance states and could thus serve as a multistate memory.

The system that was nanoengineered by the researchers combines two different types of devices, known as magnetic tunnel junctions (MTJs) and ferroelectric tunnel junctions (FTJs). An MTJ consists of two magnetic thin films separated by an insulating thin film, while an FTJ is composed of two different metal electrode layers separated by a thin ferroelectric film. Both these types of devices have proved to be promising information storage solutions.

“When MTJs and FTJs are combined—that is, two different ferromagnetic electrodes separated by a thin layer of ferroelectrics—they form a new device named ‘multiferroic tunnel junctions (MFTJ),'” Cheng Gong, senior author of the paper, told Phys.org.

“This is a four-state device: when the magnetizations of the two electrodes are in parallel or antiparallel orientations, two distinct tunneling resistances are generated due to tunneling magnetoresistance effect (TMR); when the ferroelectric polarization is switched between two opposite directions, two distinct tunneling resistances are generated due to tunneling electroresistance effect (TER).”

Building MFTJs using atom-thin crystals

A key advantage of MFTJ devices is that they are expected to exhibit four distinguishable types of electrical resistance (i.e., states), instead of the two states exhibited by MTJs and FTJs. One can switch between these states by applying external electric and magnetic fields.

Despite their potential, fabricating MFTJs by precisely growing stacked oxide materials using a technique known as epitaxy and creating heterogeneous structures is challenging. This is mainly because the underlying materials need to fit together almost perfectly at the atomic level and need to be chemically compatible.

If atoms in neighboring layers do not line up precisely and if materials chemically react with each other, this can impact the performance of the resulting device. Moreover, tiny imperfections (i.e., defects) or the diffusion of atoms from one layer to another can also lead to a decline in performance.

“In this context, building MFTJ by means of van der Waals (vdW) layers would fundamentally mitigate the challenges, since assembling dissimilar vdW layers is not restricted by lattice constant match,” said Gong. “In short, each constituent layer of vdW-MFTJ can be freely tailored in material type, thickness, and potentially their inter-layer sliding registries and twisting angles. By saying so, you can imagine how much freedom vdW-MFTJ has in device construction, property design and performance optimization.”

The MFTJ devices created by Gong and his colleagues were constructed using electrodes based on the ferromagnetic materials Fe₃GeTe₂, Fe₅GeTe₂, or Fe₃GaTe₂ and a spacer (i.e., thin insulating barrier) based on the CuInP₂S₆ or In₂Se₃. Instead of chemically growing the layers in their device all together, the team assembled them individually by stacking very thin crystal layers on top of each other.

“The magnetic electrodes we used are based on Fe₃GeTe₂, Fe₅GeTe₂, and Fe₃GaTe₂,” said Gong. “The ferroelectric spacer we used is either CuInP₂S₆ or In₂Se₃. We used scotch tape to exfoliate each constituent layer and mechanically transfer and assemble them together vertically to form MFTJ.”

Towards the deployment of multistate spintronic memories

Gong and his colleagues were the first to experimentally demonstrate an MFTJ based solely on two-dimensional vdW crystals, atomically thin materials that are held together by weak attractions known as vdW forces. The team showed that this device exhibits the 4 predicted switchable, non-volatile types of electrical resistance.

“An MFTJ exhibits four electrical resistance states, corresponding to when the two magnetic layers are magnetized in parallel or antiparallel (tunneling magnetoresistance effect) and when the ferroelectric spacer is polarized up or down (tunneling electroresistance effect),” said Gong.

“We have now built an MFTJ using three vdW layers (two magnets and one ferroelectric) and demonstrated the four states by switching the magnetization and/or polarization of each layer. The device’s performance (e.g., ON/OFF ratio and current density) can be easily tailored by replacing any one of the three constituent layers—the unlimited freedom that only vdW materials can give in layer assembling.”

Notably, by fabricating the MFTJ using different materials, engineers could change how strongly the device switches and how much current flows through it. In the future, the team’s design and their fabrication methods could inspire the creation of highly performing spintronic devices based on MFTJs.

“Low energy consumption and high performance are always the two directions to advance electronic devices,” added Gong. “As part of our next studies, we plan to study the fundamental magnetoelectric coupling in MFTJs and its impact on new physical phenomena and device performance.”

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