Being able to control magnetism in materials using only an applied voltage and so make magnetoelectric devices such as a nonvolatile transistors will be important for spintronics applications in the future. A team of researchers in the US has now made an important step forward towards this goal by fabricating a device containing the “wonder material” graphene, which is a 2D carbon sheet with very high mobility, interfaced with magnetoelectric chromium oxide. The device is expected to operate with very low power and have a nonvolatile on-off current
ratio and electrically controllable spin polarization both at and above room temperature.
Spintronics is a technology that exploits the spin of an electron as well as its charge. The spin can either be "up" or "down", and this property could be used to store and process information in devices that should be smaller and more efficient than conventional electronic devices, which rely on just storing and switching charge. This is because information processing and storage using spins might be done using very little energy.
For such devices to be competitive with silicon technology, however, researchers need to find a way to flip spins by applying electric fields, rather than magnetic fields or using current driven switching (the latter requires much more energy). Materials in which electron spin can be controlled using electric fields alone are known as magnetoelectrics, but they rarely work at room temperature and are often difficult to make.
A team of researchers led by Peter Dowben at the University of Nebraska-Lincoln has now found that an external voltage can be used to control the magnetic properties of few-layer graphene interfaced with chromium oxide. Although this oxide in antiferromagnetic in the bulk and has no net magnetization, its  surface does have a well-defined magnetic moment whose direction may be controllably reversed using an applied electric field. What is more, the surface spin polarization of chromium oxide remains even if it is buried under an overlayer. This means that it can be exploited to induce spin polarization in an adjoining graphene layer thanks to the proximity effect, or exchange coupling.
Scanning probe microscopy and Raman spectroscopy experiments by the team reveal that charges transfer between the two materials and that the graphene becomes p-doped. They also found a 150 meV shift in the material’s Fermi level (the energy level that determines which levels are occupied or partially occupied and those that are unoccupied by electrons).
The researchers confirmed these results using density-functional theory calculations, which also showed that, while the charge transfer is relatively small, the induced spin polarization is extremely high in the vicinity of graphene’s Fermi level. This implies that we can expect a large electrically controllable spin current in this system, say the researchers
Graphene in its natural state has no magnetic moment and only weak spin-orbit coupling. “To develop spintronics devices from this material, previous research focused on introducing magnetic moments in the carbon sheet though defect engineering, which concomitantly comprises the charge mobility of the sample,” explains Dowben. “But there are alternative ways to do this.
“Our approach is different in that it is based on interfacial interactions with a neighbouring functional dielectric, which induces high spin polarization in the graphene while allowing it to maintain its high charge mobility,” he tells nanotechweb.org.
The team, reporting its work in Appl. Phys. Lett. 111 182402, includes researchers from the University of Nebraska-Lincoln, the University of Buffalo, the University of Science and Technology of China, Hefei, and the University of Nebraska at Omaha.
On May 11, the Nature Publishing Group released Nature Publishing Index 2010 China, remarking “a dramatic rise in the quality of research being published by China”. University of Science and Technology of China is ranked 3rd of TOP 10 Institutions in Index 2010 China.
This article came from News Center of USTC.