Physicists from the National University of Singapore have developed an innovative concept to induce and directly measure spin splitting in two-dimensional materials. Using this concept, they experimentally achieved large tunability and a high degree of spin polarization in graphene. This achievement could potentially advance the field of two-dimensional (2D) spintronics with applications in low-power electronics.

Joule heating is a serious problem in modern electronics, especially devices such as personal computers and smartphones. This is an effect that occurs when an electric current passing through a material produces thermal energy, which subsequently increases the temperature of the material.

A potential solution involves using rotation instead of load in logic circuits. These circuits can in principle offer low power consumption and ultra-high speed by reducing or eliminating Joule heating. This led to a new direction of spintronics.

Graphene is an ideal 2D material for spintronics due to its long spin diffusion length and long spin lifetime even at room temperature. Although graphene is not inherently spin-polarized, it can cause spin-splitting by placing it near magnetic materials. But there are two main problems. Direct methods to determine the spin splitting energy and constrain the spin properties and tunability of graphene are lacking.

Breakthrough in graphene spintronics

A research team led by Professor Ariando from the NUS Department of Physics has developed an innovative concept to directly measure spin splitting energy in magnetic graphene using Landau fan shear. The displacement of the Landau fan refers to the displacement of the intersection point during the formation of linear harmonics of the oscillation frequency with charge carriers, which occurs due to the splitting of the energy levels of charged particles in a magnetic field. It can be used to study the fundamental properties of matter.

Additionally, the induced spin splitting energy can be tuned over a wide range using a technique called field cooling. The high spin polarization observed in graphene, combined with the tunability of the spin splitting energy, offers a promising avenue for the development of 2D spintronics for low-power electronics.

The findings were recently published in the journal Advanced Materials.

Experimental validation and theoretical support

The researchers conducted a series of experiments to validate their approach. They started by creating a magnetic graphene structure by stacking single-layer graphene on top of the magnetic insulator oxide Tm.₃Fe₅HE₁₂(TmIG). This unique structure allowed them to use the drift of the Landau fan to directly measure the spin splitting energy value of 132 meV in magnetic graphene.

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

Dr Dweller, senior research fellow in the NUS Department of Physics and lead author of the research paper. Junxiong HU said: “Our work resolves long-standing debates in 2D spintronics by developing a concept that uses Landau fan replacement for direct measurement of spin splitting in magnetic materials.”

To further validate their experimental findings, the researchers collaborated with a theoretical group led by Professor Zhenhua Qiao from the University of Science and Technology of China to calculate the spin splitting energy using first principles calculations.

The theoretical results obtained were consistent with the experimental data. Moreover, they also used machine learning to fit their experimental data into a phenomenological model; This provides a deeper understanding of the tunability of spin splitting energy by space cooling.

Professor Ariando said: “Our work develops a reliable and unique way to generate, detect and manipulate electron spin in atomically thin materials. It also demonstrates the practical use of artificial intelligence in materials science. Welded 2D magnets and stacking in atomically thin van der Waals heterostructures “Due to the rapid development and significant interest in the field of magnetism, we believe that our results can be extended to a variety of other 2D magnetic systems.”

Building on this proof-of-concept study, the research team plans to study return current manipulation at room temperature. Their goal is to apply their findings to the development of two-dimensional spin-logic circuits and magnetic memory/sensor devices. The ability to effectively control the spin polarization of current forms the basis for the realization of all-electric spin field effect transistors and ushers in a new era of low-power and ultrafast electronics.