Magnetoresistance: One More Thing Graphene Does Differently
Context:
Researchers in the U.K., led by Nobel laureate Andre Geim, have discovered another property of graphene— a single-atom-thick layer of carbon atoms bonded in a honeycomb pattern — that further distinguishes this ‘wonder’ material.
Points to Ponder:
- At ambient temperature, graphene, a single-atom-thick layer of carbon atoms linked in a honeycomb arrangement, exhibits an abnormal giant magnetoresistance (GMR).
- GMR is a conductor property in which the electrical resistance is influenced by magnetic fields in nearby materials. Hard disc drives, magnetoresistive RAM, biosensors, automotive sensors, and medical imagers all make advantage of this characteristic.
- The researchers led by Nobel laureate Andre Geim found that a graphene-based device would not need to be cooled to a very low temperature to sense magnetic fields, making it more efficient and cost-effective compared to conventional devices.
- In a GMR-based device, a conductor is sandwiched between two ferromagnetic materials (metals attracted to magnets, such as iron). When the materials are magnetized in the same direction, the electrical resistance in the conductor is low. When the directions are opposite to each other, the resistance increases. This is known as GMR.
- The graphene-based device’s magnetoresistance was “almost 100 times higher than that observed in other known semimetals in this magnetic field range.”
- The effect is caused by the way electrons in the conductor scatter off electrons in ferromagnets, depending on the orientation of the latter’s spin, which is altered by the magnetic field direction.
- Traditional GMR devices are chilled to low temperatures to suppress the kinetic energy of their constituent particles, preventing them from deflecting electrons passing by. The inhibition was unneeded in graphene, according to the researchers.
- Under a 0.1 tesla field, the magnetoresistance of monolayer graphene held between two layers of boron nitride increased by 110% in their study. In typical metals, the magnetoresistance increases by less than 1% under these conditions.
- The presence of a ‘neutral’ plasma and the mobility of electrons were attributed to this by the researchers. Normally, plasma is a gas of charged particles, but in this experiment, “plasma consists of equal numbers of thermally excited electrons and holes.”
- A ‘hole’ is a location where an electron should be but isn’t, causing it to behave as if it is positively charged. The graphene had been ‘adjusted’ by the researchers to contain the same number of electrons as holes. “As a result, the total charge of this plasma is zero,” which is beneficial because it prevents an effect from interfering with GMR.
- The scientists employed an “extremely clean” setup using graphene with “zero defects.” The electrons in the neutral plasma were also not scattered by atomic lattice vibrations. At normal temperatures, the electrons in the material displayed “anomalously high” mobility.
- While a graphene-based GMR device cannot replace conventional devices, it may be employed in unique applications that need magnetic-field sensing in harsh environments. For example, the device is more resilient at high temperatures, which may make it helpful in areas where conventional devices fail.
