Physicists from the National University of Singapore have invented a concept that can induce and directly quantify spin splitting in two-dimensional materials. By using this concept, they experimentally achieved large tunability and a high degree of spin polarization in graphene. This research result has the potential to promote the development of the field of two-dimensional spin electronics and be applied to low-power electronics.

Researchers from the National University of Singapore demonstrated that strong spin polarization occurs in graphene on the ferromagnetic insulating oxide Tm3Fe5O12 (TmIG), with spin splitting energies as high as hundreds of meV. The spin polarization phenomenon observed in graphene with large and tunable spin splitting energy brings great hope for the application of two-dimensional spintronics in the field of low-power electronics. Source: National University of Singapore

A major challenge facing modern electronics, especially devices such as personal computers and smartphones, is the generation of heat energy when an electric current passes through a material, causing the material to increase in temperature.

One potential solution is to use spin instead of charge in logic circuits. Since Joule heating is reduced or eliminated, these circuits could in principle offer low power consumption and ultra-fast speeds. This gave rise to the emerging field of spintronics.

Graphene is an ideal 2D material for spintronics because of its long spin diffusion length and long spin lifetime, even at room temperature. Although graphene itself does not have spin polarity, placing it near a magnetic material can induce it to exhibit spin-splitting behavior. However, two major challenges currently exist. One is the lack of a direct method to determine the spin splitting energy, and the other is that the spin characteristics and tunability of graphene are limited.

Breakthrough in graphene spintronics

A research team led by Professor Arido from the Department of Physics at the National University of Singapore has proposed an innovative concept to directly quantify the spin splitting energy in magnetic graphene using the Landau sector shift. The Landau sector shift is the shift in the intercept when plotting a linear fit of oscillation frequency versus charge carrier, which is caused by the splitting of energy levels of charged particles in a magnetic field. It can be used to study the fundamental properties of matter.

Diagram showing the diffusion of spin-polarized electrons in a graphene layer placed on the ferromagnetic insulating oxide Tm3Fe5O12 (TmIG). The strong exchange interaction between graphene and TmIG leads to significant spin splitting in the graphene ribbon structure. This spin splitting in turn leads to large differences in charge carrier densities, whose spin directions are labeled "spin up" (↑) and "spin down" (↓). This difference in carrier density results in the generation of spin-polarized current. Source: Advanced Materials

Furthermore, the induced spin splitting energy can be tuned over a wide range by a technique called field cooling. The high spin polarization observed in graphene, coupled with the tunability of its spin splitting energy, offers a promising avenue for the development of two-dimensional spintronics for low-power electronic devices.

The findings were recently published in the journal Advanced Materials.

Experimental verification and theoretical support

The researchers conducted a series of experiments to validate their method. They created the magnetic graphene structure by first stacking a single layer of graphene on the magnetic insulating oxide Tm3Fe5O12 (TmIG). This unique structure allowed them to directly quantify the spin splitting energy value of 132meV in magnetic graphene using the Landau sector shift.

To further confirm the direct relationship between Landau sector shift and spin splitting energy, the researchers conducted field cooling experiments to tune the degree of spin splitting in graphene. They also applied X-ray magnetic circular dichroism (X-ray magnetic circular dichroism) technology at the Singapore Synchrotron Light Source to reveal the origin of spin polarization.

"Our work resolves a long-standing controversy in two-dimensional spintronics by proposing the concept of using the Landau sector shift to directly quantify spin splitting in magnetic materials," said Dr. Wu Junxiong, the first author of the research paper and a senior researcher at the Department of Physics at the National University of Singapore.

To further support their experimental findings, the researchers collaborated with a theoretical team led by Professor Qiao Zhenhua from the University of Science and Technology of China to calculate the spin splitting energy using first principles. The theoretical results obtained are consistent with the experimental data. In addition, they used machine learning to fit experimental data based on a phenomenological model to gain a deeper understanding of the tunability of spin splitting energy by field cooling.

Professor Ariando said: "Our work develops a powerful and unique pathway to generate, detect and manipulate electron spins in atomically thin materials. It also demonstrates the practical application of artificial intelligence in materials science. With the rapid development and great interest in the field of stacking-induced magnetism in 2D magnets and atomically thin van der Waals heterostructures, we believe our results can be generalized to a variety of other 2D magnetic systems."

Building on this proof-of-concept study, the team plans to explore manipulating spin currents at room temperature. Their goal is to apply their research results to the development of two-dimensional spin logic circuits and magnetic memory/sensing devices. The ability to effectively regulate the spin polarization of currents lays the foundation for realizing all-electric spin field-effect transistors, ushering in a new era of low-power and ultra-high-speed electronic devices.

Compiled source: ScitechDaily