Magnetics Business & Technology - November/December 2020 - 7

FEATURE ARTICLE
the systematic errors is a complex and time-consuming process.
The Hall effect sensor from Paragraf solves these problems
because the active sensing component is made of atomically thin
graphene. Since the material is two-dimensional, it only senses
magnetic fields along one direction, giving a negligible planar
Hall effect. This enables the true perpendicular magnetic field
value to be obtained, allowing for higher precision mapping of the
local magnetic field.
"Using Hall effect sensors without planar effect would open the
door to a new mapping technique by mounting a stack of sensors on a rotating shaft. The compelling advantage would be
measurements of the harmonic content in accelerator magnets
almost point-like along the magnet axis", commented Stephan
Russenschuck, head of the magnetic measurement section at
CERN.
In December, Paragraf raised another £3.4m to bring its series
A round funding to a total of £16.2m, enabling the company to
accelerate the delivery of its first graphene-based electronics
products to market, transitioning the company into a commercial,
revenue-generating entity. Presently the company is providing its
sensors to lead partners in small volumes for testing and product
development efforts. Based in north of Cambridge in Somersham, the company is a spin-out from the Department of Materials
Science at the University of Cambridge.

Graphene for magnetic field measurement

The extraordinary properties of graphene have been known for
over 15 years. However, to date, the nanomaterial has not delivered on its potential in electronic device applications because of
how it is manufactured and processed. Paragraf has developed a
process which overcomes these issues.
Hall-Effect sensors are used in many applications, from measuring magnetic fields and electric currents to timing the speed of
wheels and shafts. However, existing products have a limited
temperature range, sensitivity, accuracy and magnetic field
range. The electrical conductivity, inherent thinness and flexibility
of graphene have meant that it has received significant attention
as a revolutionary material for electronics devices. However,
significant barriers to adoption in mass-market electronics remain
due to the lack of contamination-free, transfer-free, large-area
graphene.

As silicon transistors reach the limits of their performance, new
solutions are required, and graphene is one of the front runners for this paradigm shift. To date, many leading electronics
companies such as Intel, IBM, and Samsung, have collectively
invested billions in attempting to bring electronic devices made
from graphene to market.
Paragraf has developed a new method of producing graphene
that makes it suitable for electronic devices. Its patented process uses scalable processes to allow the manufacturing of
large-area, high-quality graphene, currently up to 8" diameter.
The approach uses a modified deposition method that removes
the need for the transfer processes commonly used in most
graphene synthesis methods. The graphene can be grown in a
uniform, single layer directly on a wide range of substrates. As
well as producing large-area graphene for electronics, Paragraf
has commenced electronic device production by launching its
own Hall-Effect sensor.
The lack of a planar Hall-Effect is due to the inherent thinness
of monolayer graphene, which is a single layer of carbon atoms.
This results in a negligible planar Hall-Effect, meaning false
signals are not induced. It enables only the actual perpendicular
magnetic field value to be obtained, allowing for higher precision mapping of magnetic fields. Paragraf's graphene Hall-Effect
sensor is the only Hall-Effect sensor on the market where the
planar Hall-Effect does not affect measurement accuracy, says
the company.
The Hall-Effect sensor is a specialist sensor that measures the
magnitude of a magnetic field. Its
output voltage is directly proportional to the magnetic field strength
through it. The Hall-Effect is the
production of a voltage difference,
the Hall voltage, across an electrical conductor when a mutually
perpendicular electric current and
magnetic field are applied.
One of the issues with traditional Hall-Effect sensors is the thickness of the sensing material, which causes the sensing layer to
be three-dimensional. This causes field components that are not
perpendicular to the sensing direction to also be sensed, and as
a result, false signals are produced. This is known as the planar
Hall-Effect. These false measurement signals then become
mixed with the real signals and are incorporated into the data of
the magnetic field map. As a result, this can give a false impression that there are additional magnetic poles present when there
are none. These false poles are known as pseudo-multipoles
and can significantly reduce the accuracy of the measurements.
It is possible to separate the real signals from the false signals
manually. However, this is a complicated and time-consuming
process.

Paragraf scientists performing experiments in a glovebox

In another partnership, with the National Physical Laboratory as
part of an Innovate UK funded project, the suitability of Paragraf's
sensor in high radiation environments is being investigated to
see if it might be more robust than traditional Hall-Effect sensors.
Other work is ongoing with the University of Cambridge, the NPL
and other partners to further investigate the suitability of the sensors to a wide range of commercial applications.

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Magnetics Business & Technology - November/December 2020

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