A magneto resistor is a resistor of which its electrical resistance varies or changes as a result of presence of external magnetic field. A magnetic field is a region in space where the effect of magnetic force is felt by a body/object placed near it. Magneto resistors have a variable resistance which is subject to the magnetic field intensity. A Magneto resistor can be utilized to gauge magnetic field intensity, presence and direction. They are otherwise called magnetic dependent resistors (MDR). A magneto resistor is a subfamily of magnetic field sensors or magnetometers.
How A Magneto Resistor Works
The magneto resistor will experience change in resistance when placed in a magnetic field. The resistance of the magneto resistor increases/decreases when the strength of the magnetic field is increased/decreased I.e it is proportional to the strength of the magnetic field. This change occurs as a result of magneto resistive effect. In the absence of magnetic field, the charge carriers (electric current) in the material move in a straight path until magnetic field is applied to the material, the magnetic forces cause the mobile charge carriers to change their direction from direct path to indirect path. This increases the length of electric current path. As result of this, large amount of free electrons collides with the atoms and loses their energy in the form of heat and only a small amount of free electrons flow through the conductive path. The small amount of free electrons moving from one place to another place carries the electric current. Therefore, the resistance of the material increases with increasing magnetic field.
Magneto Resistive Effect
Magneto resistive effect is the property of some materials, which makes them to change their resistance under the presence of magnetic field. This effect was first discovered by an Irish mathematician, physicist and engineer called William Thomson, otherwise known as Lord Kelvin in 1856. This effect is noticed in semiconductors, non-magnetic metals, and magnetic metals (like ferromagnetic) materials and it is dependent on the magnetic field intensity and the angle between the direction of electrical current and the magnetic field. This effect is therefore known as (AMR).
Permalloy, an alloy comprising 81% nickel (Ni) and 19% iron (Fe) has a high anisotropic magneto resistance as well as a low magnetostriction (change in size due to magnetic fields) which makes it a preferred material for magneto resistors.
Magneto resistors are usually made of long thin films of permalloy. In order to increase the sensitivity of a permalloy magneto resistor, shorting bars of aluminium or gold are placed on the thin permallow films at an angle of 45 degrees. This forces the current to flow in a direction of 45 degrees proportional to the length of the film. This is called a barber pole configuration. A typical AMR magnetoresistive sensor is made of a combination of 4 permalloy thin film magnetoresistors, connected in a wheatstone measurement bridge. Most routine magneto resistors use the AMR impact.
Types Of Magneto Resistive Effect
Magneto resistive effects are of 4 types:
- Giant Magneto Resistance (GMR)
- Extraordinary Magneto Resistance (EMR)
- Tunnel Magneto Resistance (TMR)
- Collosal Magneto Resistance (CMR)
Giant Magneto Resistance (GMR)
Giant Magneto Resistance was discovered by Albert Fert and Peter Grunberg in 1988. This effect is observed in the ferromagnetic materials. The resistance of the ferromagnetic material is dependent on the whether the magnetization of adjacent ferromagnetic layers are aligned parallel or anti-parallel. The resistance is large for the ferromagnetic layers with anti-parallel alignment whereas the resistance is small for the ferromagnetic layers with parallel alignment.
Extraordinary Magneto Resistance (EMR)
The extraordinary magneto resistive effect was discovered in the year 2000. The magnetic field effect of Extraordinary Magneto Resistance (EMR) is much greater than Giant Magneto Resistance (GMR). The EMR effect occurs in semiconductor metal hybrid systems when a transverse magnetic field is applied. In the absence of magnetic field, the resistance of the semiconductor metal hybrid system is very small whereas in the presence of strong magnetic field, the resistance of semiconductor metal hybrid system is large.
Tunnel Magneto Resistance (TMR)
In 1975 M. Julliere discovered the Tunnel Magneto Resistance (TMR). The Tunnel Magneto Resistance (TMR) occurs in Magnetic Tunnel Junction (MTJ). Magnetic Tunnel Junction (MTJ) is a component comprises two ferromagnets separated by an insulator. The electrons will flow from one ferromagnet to another ferromagnet through the tunnel barrier or insulator. The amount of electric current (charge carriers) moving through the tunnel is dependent on the orientation of magnetizations.
If the magnetic field is applied to Magnetic Tunnel Junction in such a way that the direction of magnetizations of ferromagnet is aligned in a parallel manner, a large amount of electrons flow easily. As a result, electric current increases and resistance decreases. Likewise, if the magnetic field is applied in such as way that the direction of magnetizations of ferromagnet is aligned in an anti-parallel manner, only a small number of charges carriers (free electrons) flows through the tunnel and large amount of free electrons are blocked. As a result, electric current decreases and resistance increases.
Collosal Magneto Resistance
Colossal magnetoresistance (CMR) is a property of some materials, mostly manganese-based perovskite oxides , which enables them to dramatically change their electrical resistance in the presence of a magnetic field . It was first discovered by G.H. Jonker and J.H. Van Santen in 1950s. The magnetoresistance of conventional materials enables changes in resistance of up to 5%, but materials featuring CMR may demonstrate resistance changes by orders of magnitude. More info here.
Applications of magneto resistors
The various applications of magneto resistors include:
- Angle position sensors
- Position sensors
- Linear position sensors
- Rotary position sensors
- Hard disk drives
- Ferrous metal detection
- Magnetic field sensors
- Vehicle and traffic detection
- Electronic compass
- Magneto resistors are used in electronic compass for measuring earth’s magnetic field.
- Magneto resistors are used for measuring electric current.
- Magnetometry,measurement of magnetic field intensity and direction