Engineers Have Achieved a New High Level of Electron Doping in a Multilayer Ferromagnet

Engineers have achieved a new high level of electron doping in a multilayer ferromagnet, resulting in a magnetic phase transition that holds great potential for future electronics. Controlling magnetism with electric voltage is critical for creating low-energy, high-speed nano-electronic and spintronic devices such as spin-orbit torque devices and spin field-effect transistors in the future.

International cooperation led by RMIT has achieved record-high electron doping in a multilayer ferromagnet, producing a magnetic phase shift with great potential for future electronics, according to a paper released this week. Electric voltage control of magnetism (or spin orientations) is critical for creating future low-energy high-speed nano-electronic and spintronic devices, such as spin-orbit torque devices and spin field-effect transistors. A multilayer ferromagnet with an ultra-high-charge, doping-induced magnetic phase transition has intriguing applications in antiferromagnetic spintronic devices.

Researchers from RMIT, UNSW, the University of Wollongong, and FLEET partner High Magnetic Field Laboratory (China) have demonstrated for the first time that proton intercalation can induce ultra-high electron doping concentrations (above 1021 cm-3) in the layered van der Waals (vdW) metallic material Fe5GeTe2, and can further cause a transition of the magnetic ground state from ferromagnetism to antiferromagnetism.

Magnetic Phase Transition with Transforming a Layered Ferromagnet for Future Electronics

Tuning Magnetism in the vdW Ferromagnet Fe5GeTe2 (F5GT)

The introduction of multilayer, vdW magnetic materials has accelerated the hunt for new vdW spintronic devices. Contrasted with nomad ferromagnets, antiferromagnets (AFMs) enjoy special benefits as building squares of such future spintronic gadgets.

They are excellent for memory devices due to their resistance to stray magnetic fields, and AFM-based spin-orbit torque devices require less current density than ferromagnets.

However, vdW itinerant antiferromagnets are still in short supply. Inducing a magnetic phase transition in an existing vdW itinerant ferromagnet is another approach for achieving this function, in addition to directly synthesizing a vdW antiferromagnet.

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

“Our previous experience on Fe3GeTe2 (Nature Communication 2018) enabled us to quickly identify and evaluate the material’s magnetic properties, and some studies indicate Fe5GeTe2 is 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 researchers used electron transport experiments to study the magnetic characteristics of Fe5GeTe2 nanosheets of varied thicknesses.

However, preliminary transport studies reveal that the electron density in Fe5GeTe2 is high, as predicted, indicating that due to the electric-screen effect in metal, conventional gate-voltage is difficult to regulate magnetic:

“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 (Physical Review Letters 2020), 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).

Fabricating the Solid Protonic Field-Effect Transistor (SP-FET)

The team, like many beyond-CMOS classical-computing researchers, is attempting to develop a better form of the transistor, the switches that supply the binary backbone of contemporary electronics. The insertion (intercalation) of protons switches a solid protonic field-effect transistor (SP-FET).

The SP-FET, unlike typical proton FETs (which switch by dipping liquid and are regarded as potential prospects for bridging traditional electronics and biological systems), is solid and therefore appropriate for application in actual devices.

The SP-FET has been shown to be extremely effective in tuning thick metallic materials (i.e., it can produce high charge doping levels) that are difficult to control using standard dielectric or ion liquid gating approaches (because of electric screening effect in metal).

The team was able to drastically alter the carrier density in Fe5GeTe2 and change its magnetic ground state by building a solid protonic field-effect transistor (SP-FET) with Fe5GeTe2. The experimental results were validated by a density functional theory calculation.

“All of the samples demonstrate that increased proton intercalation gradually suppresses the ferromagnetic state, and some samples show no hysteresis loops, indicating a shift in the magnetic ground state. The theoretical calculations agree with the experimental data,” says Cheng.

“The success of realizing an AFM phase in metallic vdW ferromagnet Fe5GeTe2 nanosheets constitutes an important step towards vdW antiferromagnetic devices and heterostructures that operate at high temperatures,” says co-author A/Prof Lan Wang (also at RMIT).

“This yet again illustrates that our protonic gate method is a potent weapon in electron transport investigations, as well as other fields.”