Transforming a layered ferromagnet for future spintronics

Control of magnetism by electric voltage is vital for developing future, low-energy high-speed nano-electronic and spintronic devices, such as spin-orbit torque devices and spin field-effect transistors.

In a 2021 FLEET study, ultra-high-charge, doping-induced magnetic phase transition in a layered ferromagnet allows promising applications in antiferromagnetic spintronic devices.

The collaboration of Centre researchers at RMIT, UNSW, the University of Wollongong and partner organisation the High Magnetic Field Laboratory (China) demonstrated for the first time that ultra-high electron doping concentration can be induced in the layered van der Waals (vdW) metallic material Fe₅GeTe₂ (F5GT) by proton intercalation, and can further cause a transition of the magnetic ground state from ferromagnetism to antiferromagnetism.

The emergence of layered, vdW magnetic materials has expedited a growing search for novel vdW spintronic devices.

Compared to itinerant ferromagnets, antiferromagnets (AFMs) have unique advantages as building blocks of future spintronic devices. Their robustness against stray magnetic fields makes them suitable for memory devices, and some applications can operate on a lower current density.

However currently vdW itinerant antiferromagnets are still scarce.

Besides directly synthesising a vdW antiferromagnet, another possible method towards this function is to induce a magnetic phase transition in an existing vdW itinerant ferromagnet.

“We chose to work with newly-synthesised vdW itinerant ferromagnet Fe₅GeTe₂ (F5GT)” says the study’s first author, FLEET Research Fellow Dr Cheng Tan (RMIT).

“Our previous experience with this material enabled us to quickly identify and evaluate its magnetic properties, and we knew Fe₅GeTe₂ could be sensitive to local atomic arrangements and interlayer stacking configurations, meaning it would be possible to induce a phase transition in it by doping,” Cheng says.

The team first investigated the magnetic properties in Fe₅GeTe₂ nanosheets of various thicknesses by electron transport measurements.

However, the initial transport results also showed that the electron density in Fe₅GeTe₂ was high as expected, indicating that it is hard to modulate magnetism by traditional gate voltage due to the electric-screen effect in metal.

“Despite the high charge density in Fe₅GeTe₂, we knew it was worth trying to tune the material via protonic gating, as we have previously achieved in Fe₃GeTe₂ (in a 2020 paper in Physical Review Letters), because protons can easily penetrate into the interlayer and induce large charge doping, without damaging the lattice structure,” says co-author Dr Guolin Zheng (also at RMIT).

The team are seeking to build an improved form of the transistor, the switches that provide the binary backbone of modern electronics.

A solid protonic field-effect transistor (SP-FET) switches based on insertion (intercalation) of protons; this method has been demonstrated to be very powerful in tuning thick metallic materials, which are very difficult to modulate via traditional techniques because of screening effects.

By fabricating an SP-FET with Fe₅GeTe₅, the team were able to dramatically change carrier density and change the material’s magnetic ground state. Theoretical calculations confirmed the experimental results.

“All the samples show that the ferromagnetic state can be gradually supressed by increasing proton intercalation,” says Cheng. “Again, this demonstrates that our protonic gate technique is a powerful weapon in electron transport experiments, and probably in other areas well.”

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