Magnetoresistance
Encyclopedia
Magnetoresistance is the property of a material to change the value of its electrical resistance
when an external magnetic field
is applied to it. The effect was first discovered by William Thomson
(more commonly known as Lord Kelvin) in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This effect was later called ordinary magnetoresistance (OMR). More recent researchers discovered materials showing giant magnetoresistance (GMR), colossal magnetoresistance
(CMR) and magnetic tunnel effect (TMR).
. A discussion of the disc is provided by Giuliani. Initial interest in this problem began with Boltzmann in 1886, and independently was re-examined by Corbino in 1911.
In a simple model, supposing the response to the Lorentz force is the same as for an electric field, the carrier velocity v is given by:
where μ = carrier mobility. Solving for the velocity, we find:
where the reduction in mobility due to the B-field is apparent.
In a semiconductor
with a single carrier type, the magnetoresistance is proportional to (1 + (μB)2), where μ is the semiconductor mobility
(units m2·V−1·s−1 or T −1) and B is the magnetic field (units tesla
s). Indium antimonide, an example of a high mobility semiconductor, could have an electron mobility above 4 m2·V−1·s−1 at 300 K. So in a 0.25 T field, for example the magnetoresistance increase would be 100%.
To compensate for the non-linear characteristics and inability to detect the polarity of a magnetic field, a somewhat more complex structure is used for sensors. It consists of stripes of aluminum or gold placed on a thin film of permalloy
(a ferromagnetic material exhibiting the AMR effect) inclined at an angle of 45°. This structure forces the current not to flow along the “easy axes” of thin film, but at an angle of 45°. The dependence of resistance now has a permanent offset which is linear around the null point. Because of its appearance, this sensor type is called 'barber pole'.
The AMR effect is used in a wide array of sensors for measurement of Earth's magnetic field (electronic compass
), for electric current measuring (by measuring the magnetic field created around the conductor), for traffic detection and for linear position and angle sensing. The biggest AMR sensor manufacturers are Honeywell
, NXP Semiconductors, and Sensitec GmbH.
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
when an external magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
is applied to it. The effect was first discovered by William Thomson
William Thomson, 1st Baron Kelvin
William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, PRSE, was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging...
(more commonly known as Lord Kelvin) in 1856, but he was unable to lower the electrical resistance of anything by more than 5%. This effect was later called ordinary magnetoresistance (OMR). More recent researchers discovered materials showing giant magnetoresistance (GMR), colossal magnetoresistance
Colossal magnetoresistance
Colossal magnetoresistance is a property of some materials, mostly manganese-based perovskite oxides, that enables them to dramatically change their electrical resistance in the presence of a magnetic field...
(CMR) and magnetic tunnel effect (TMR).
Discovery
William Thomson (or Lord Kelvin) first discovered ordinary magnetoresistance in 1856. He experimented with pieces of iron and discovered that the resistance increases when the current is in the same direction as the magnetic force and decreases when the current is at 90° to the magnetic force. He then did the same experiment with nickel and found that it was affected in the same way but the magnitude of the effect was greater. This effect is referred to as anisotropic magnetoresistance (AMR).The Corbino disc
Figure 1 illustrates the Corbino disc. It consists of a conducting annulus with perfectly conducting rims. Without a magnetic field, the battery drives a radial current between the rims. When a magnetic field parallel to the axis of the annulus is applied, a circular component of current flows as well, due to the Lorentz forceLorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
. A discussion of the disc is provided by Giuliani. Initial interest in this problem began with Boltzmann in 1886, and independently was re-examined by Corbino in 1911.
In a simple model, supposing the response to the Lorentz force is the same as for an electric field, the carrier velocity v is given by:
where μ = carrier mobility. Solving for the velocity, we find:
where the reduction in mobility due to the B-field is apparent.
Anisotropic magnetoresistance (AMR)
AMR is the property of a material in which a dependence of electrical resistance on the angle between the direction of electric current and orientation of magnetic field is observed. The effect is attributed to a larger probability of s-d scattering of electrons in the direction of magnetic field. The net effect is that the electrical resistance has maximum value when the direction of current is parallel to the applied magnetic field. AMR up to 50% has been observed in some ferromagnetic uranium compounds.In a semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
with a single carrier type, the magnetoresistance is proportional to (1 + (μB)2), where μ is the semiconductor mobility
Electron mobility
In solid-state physics, the electron mobility characterizes how quickly an electron can move through a metal or semiconductor, when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility...
(units m2·V−1·s−1 or T −1) and B is the magnetic field (units tesla
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
s). Indium antimonide, an example of a high mobility semiconductor, could have an electron mobility above 4 m2·V−1·s−1 at 300 K. So in a 0.25 T field, for example the magnetoresistance increase would be 100%.
To compensate for the non-linear characteristics and inability to detect the polarity of a magnetic field, a somewhat more complex structure is used for sensors. It consists of stripes of aluminum or gold placed on a thin film of permalloy
Permalloy
Permalloy is a nickel-iron magnetic alloy, with about 20% iron and 80% nickel content. It is notable for its very high magnetic permeability, which makes it useful as a magnetic core material in electrical and electronic equipment, and also in magnetic shielding to block magnetic fields...
(a ferromagnetic material exhibiting the AMR effect) inclined at an angle of 45°. This structure forces the current not to flow along the “easy axes” of thin film, but at an angle of 45°. The dependence of resistance now has a permanent offset which is linear around the null point. Because of its appearance, this sensor type is called 'barber pole'.
The AMR effect is used in a wide array of sensors for measurement of Earth's magnetic field (electronic compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
), for electric current measuring (by measuring the magnetic field created around the conductor), for traffic detection and for linear position and angle sensing. The biggest AMR sensor manufacturers are Honeywell
Honeywell
Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....
, NXP Semiconductors, and Sensitec GmbH.
See also
- Giant magnetoresistance
- Colossal magnetoresistanceColossal magnetoresistanceColossal magnetoresistance is a property of some materials, mostly manganese-based perovskite oxides, that enables them to dramatically change their electrical resistance in the presence of a magnetic field...
- Magnetoresistive random access memory