Device interface magnetic properties change with different fabrication processes Kobe University achieves results: Expected to enable high-performance devices

A research group consisting of Graduate Student Naohiro Matsumoto, Assistant Professor Mitsuharu Uemoto, and Professor Tomoya Ono from the Graduate School of Engineering at Kobe University announced on September 18 that they used quantum mechanics-based theoretical calculations and supercomputers to clarify how different fabrication processes affect the magnetic properties of device interfaces in tunneling magnetoresistance devices using two-dimensional layered materials. They discovered that the magnetic properties at the interface differ between the graphene transfer process, where the NiFe ferromagnetic alloy substrate is fabricated first, and the deposition process, where the NiFe ferromagnetic alloy substrate is fabricated afterward on graphene. Theoretical calculations revealed that the strength of bonding between carbon atoms in graphene and iron atoms in the substrate influences the fabrication process dependence of interface magnetic properties. These findings are expected to be applicable to the fabrication processes of spintronic devices using two-dimensional layered materials. The results were published in the Journal of Applied Physics on September 9.

Aiming for further integration and power reduction in information devices, there is a call for the practical implementation and enhanced functionality of spintronic devices that use not only electron charge but also spin for information recognition.

Tunneling magnetoresistance devices, a type of spintronic device, sandwich an insulating material as a tunnel layer between ferromagnetic metals and control the on-off switching of current passing through this tunnel layer by switching the spin orientations of the two ferromagnetic metals to parallel or antiparallel. Currently, oxides are widely used as this tunnel layer, but sheet-like two-dimensional layered materials with superior flatness are attracting attention.

However, there are challenges that need to be overcome in order to commercialize such devices, such as the oxidation of ferromagnetic metal surfaces and degradation of two-dimensional layered materials during the junction interface fabrication process. Various interface fabrication processes have been proposed to date. For example, there are methods involving transferring two-dimensional layered materials onto ferromagnetic metal substrates and depositing ferromagnetic metals onto two-dimensional layered materials. The magnetic properties of the two-dimensional layered material/ferromagnetic metal interface are thought to be deeply related to the switching characteristics of tunneling magnetoresistance devices.

In this study, the researchers systematically investigated the relationship between interface electronic structure and magnetic properties arising from the fabrication process by using quantum mechanics-based theoretical calculations and supercomputers.

Using a model with an NiFe ferromagnetic alloy and adsorbed graphene, they examined not only the adsorption position of the graphene but also varied the composition ratio of the NiFe ferromagnetic alloy substrate and its surface.

Two-dimensional layered materials are known to stack through weak interactions called van der Waals forces. When examining the adsorption position of the graphene on the NiFe ferromagnetic alloy, they found that the graphene takes on a structure in which carbon atoms are positioned directly above the surface metal atoms.

Examining the relationship between the composition of iron and nickel atoms at the surface and the adsorption energy revealed that structures with more iron atoms in the substrate surface layer showed the largest adsorption energy, regardless of the composition ratio of iron to nickel atoms in the NiFe ferromagnetic alloy substrate.

This demonstrated that in the process of depositing NiFe ferromagnetic alloy onto graphene, a layer rich in iron atoms first forms on the graphene, followed by the formation of the NiFe ferromagnetic alloy layer on the layer formed first.

Next, when examining the surface elemental composition of the NiFe ferromagnetic alloy substrate before graphene transfer, they found that surfaces rich in nickel atoms tend to appear regardless of the elemental composition ratio of the NiFe ferromagnetic alloy substrate.

From this, they predicted that in the process of transferring graphene, the substrate surface layer between the graphene and the NiFe ferromagnetic alloy substrate would be rich in nickel atoms.

Furthermore, examining the magnetic properties of the junction interface revealed that the magnetic moment increases with higher iron atom content in the substrate surface layer. The magnetoresistance ratio, which is a figure of merit for tunneling magnetoresistance devices, is thought to be strongly influenced by the magnitude of the magnetic moment at the interface.

These results indicated the possibility of controlling the magnetic moment of the interface transition layer through the interface fabrication process.

The research group commented: "There are many two-dimensional layered materials proposed as candidates for insulating layers in tunneling magnetoresistance devices. By utilizing quantum mechanics-based calculation methods and supercomputers, we want to design high-performance devices through the elucidation of the fabrication processes and functions of devices using graphene and other two-dimensional layered materials."

Journal Information
Publication: Journal of Applied Physics
Title: Theoretical investigation of interface atomic structure of graphene on NiFe alloy substrate
DOI: 10.1063/5.0283881