A team of physicists at the University of California, Riverside has found an ingenious way to induce magnetism in graphene while also preserving graphene’s electronic properties. They have accomplished this by bringing a graphene sheet very close to a magnetic insulator – an electrical insulator with magnetic properties.
“The magnetic graphene acquires new electronic properties so that new quantum phenomena can arise. These properties can lead to new electronic devices that are more robust and multi-functional.”
The finding has the potential to increase graphene’s use in computers, as in computer chips that use electronic spin to store data.
The magnetic insulator Shi and his team used was yttrium iron garnet grown by laser molecular beam epitaxy in his lab. The researchers placed a single-layer graphene sheet on an atomically smooth layer of yttrium iron garnet. They found that yttrium iron garnet magnetized the graphene sheet. In other words, graphene simply borrows the magnetic properties from yttrium iron garnet.
(a) Magnetic hysteresis loops in perpendicular and in-plane magnetic fields. Inset is the AFM topographic image of YIG thin film surface. (b) Optical image (without top gate) and (d) schematic drawing (with top gate) of the devices after transferred to YIG/GGG substrate (false color). (c) Room temperature Raman spectra of graphene/YIG (purple), graphene/SiO2 (red), and YIG/GGG substrate only (blue).
Magnetic substances like iron tend to interfere with graphene’s electrical conduction. The researchers avoided those substances and chose yttrium iron garnet because they knew it worked as an electric insulator, which meant that it would not disrupt graphene’s electrical transport properties. By not doping the graphene sheet but simply placing it on the layer of yttrium iron garnet, they ensured that graphene’s excellent electrical transport properties remained unchanged.
In their experiments, Shi and his team exposed the graphene to an external magnetic field. They found that graphene’s Hall voltage – a voltage in the perpendicular direction to the current flow – depended linearly on the magnetization of yttrium iron garnet (a phenomenon known as the anomalous Hall effect, seen in magnetic materials like iron and cobalt). This confirmed that their graphene sheet had turned magnetic.
We demonstrate the anomalous Hall effect (AHE) in single-layer graphene exchange coupled to an atomically flat yttrium iron garnet (YIG) ferromagnetic thin film. The anomalous Hall conductance has magnitude of ∼0.09(2e2/h) at low temperatures and is measurable up to ∼300 K. Our observations indicate not only proximity-induced ferromagnetism in graphene/YIG with a large exchange interaction, but also enhanced spin-orbit coupling that is believed to be inherently weak in ideal graphene. The proximity-induced ferromagnetic order in graphene can lead to novel transport phenomena such as the quantized AHE which are potentially useful for spintronics.
SOURCES – University of California Riverside, Physical Review Letters