Figure
shows (left) the concept of the terraced single-layer graphene formation. This
is similar to the terraced paddy fields used widely in Asia for agriculture.
(Right) Atomic force microscopy image of the terraced morphology for graphene
on strontium titanate (STO, top left) and bare STO substrate (bottom right). Courtesy:
Advanced Materials.
Researchers
from National University of Singapore developed a sensitive two-dimensional magnetic
field sensor that can potentially improve the detection of nanoscale magnetic
domains for data storage applications.
Magnetoresistance
(MR), the change in the electrical resistance of a material due to the
influence of an external magnetic field, has been widely used in magnetic
sensors, magnetic memories and hard disk drives. However, in traditional
three-dimensional (3-D) material-based magnetic sensors that use giant MR (GMR)
or tunneling MR (TMR) spin-valves, the detectable signal of the magnetic field
decays exponentially with the thickness of its sensing layer. This limits the
spatial resolution and sensitivity of the sensors. Therefore, a 2-D-based
sensor can potentially improve the detection of minuscule magnetic fields, as
the decay is limited to only one atomic layer thickness.
Graphene
is an atom-thick thin material with high mobility and high current carrying
capability. By adding a graphene layer on top of an artificial terraced substrate,
the research team led by Prof Ariando from the Department of Physics, NUS has
developed a 2-D magnetic sensor with an electrical resistance that can increase
its original value 50-fold at room temperature. This is ten folds higher than
that reported on previous single-layer graphene devices at the same conditions.
The
detection of nanoscale magnetic domains is a fundamental challenge. As magnetic
domains become smaller (nanoscale), the dimensions of the sensor need to be
reduced accordingly to maintain the high spatial resolution and signal-to-noise
ratio. However, for traditional 3-D material-based sensors, the reduction in
size will lead to thermal magnetic noise and spin-torque instability. The
recent discovery by the team paves the way for the development of 2-D magnetic
field sensors that can operate at room temperature for the detection of
nanoscale magnetic domains. This can improve the performance of scanning probe
magnetometry, biosensing, and magnetic storage applications.
Mr
Junxiong Hu, a Ph.D. student on the research team, said, "The core part of
the 2-D magnetic sensor is the terraced graphene formed by stacking graphene on
an atomically terraced substrate. The process is similar to placing a carpet on
a staircase."
Due to its
flexibility, the graphene will also replicate the staircase morphology. During
this process, topographic corrugations and charge puddles will be induced in
the terraced graphene. In the presence of a magnetic field, the current in the
terraced graphene will not travel in a straight line but is strongly distorted
by the discontinuities at the boundary of the puddles, causing a significant
change of its resistance.
Prof
Ariando said, "This technology has the potential for developing the next
generation of highly sensitive sensors for the detection of the nanoscale
magnetic domains. The single-layer graphene films used for the sensor can be
manufactured by batch production for scalability."
The
research team has filed a patent for the invention. Following this proof-of-concept
study, the researchers plan to optimize the terraced geometry further and adapt
it for large-scale production techniques. This will then scale up their
experimental outcomes leading to the manufacture of industry-size wafers for
commercial use.