Magneto Resistor

Chapter 3 - Resistor Types

What is a Magneto Resistor

Magneto resistors have a variable resistance which is dependent on the magnetic field strength. A magneto resistor can be used to measure magnetic field presence, strength, and direction. They are also known as magnetic dependent resistors (MDR). A magneto resistor is a subfamily of magnetic field sensors or magnetometers.

Magneto Resistor Definition

A magneto resistor is a resistor for which the electrical resistance changes when an external magnetic field is applied.

Magneto Resistor Characteristics

Magneto resistors make use of the magnetoresistance effect. This effect was first discovered in 1856 by William Thomson, also known as Lord Kelvin. The effect is noticed in ferromagnetic materials and is dependent on the magnetic field strength and the angle between the direction of electric current and the magnetic field. This effect is therefore known as anisotropic magnetoresistance (AMR). Other, more recently discovered magnetoresistance effects are the giant magnetoresistance effect (GMR), colossal magnetoresistance effect (CMR), and tunnel magnetoresistance effect (TMR). Because most conventional magneto resistors utilize the AMR effect, the other effects will not be discussed in this article.

Permalloy, an alloy consisting of 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) and therefore is a favorite material for magneto resistors.

Magneto resistors are often constructed of long thin films of permalloy. To increase the sensitivity of a permalloy magneto resistor, shorting bars of aluminum or gold are placed on the thin permalloy films under an angle of 45 degrees. This forces the current to flow in a direction of 45 degrees relative to the length of the film. This is called a barber pole configuration.

Visualization of the barber pattern used in magneto resistorsPermalloy film magneto resistor using barber pole pattern of shorting bars

 

Magneto Resistor Applications

A typical AMR magnetoresistive sensor is constructed of a combination of 4 permalloy thin film magneto resistors, connected in a Wheatstone measurement bridge. Applications of magneto resistors as magnetic field sensing devices include:

  • Electronic compass
  • Magnetometry, measurement of magnetic field intensity and direction
  • Position sensors
    • Angle position sensors
    • Rotary position sensors
    • Linear position sensors
  • Ferrous metal detection
    • Vehicle and traffic detection

Magneto Resistor Symbol

The following symbol is used for a magneto resistor. It is shown as a variable resistor which is dependent on magnetic flux, indicated by 'x'.

magneto resistor symbolMagneto resistor symbol (IEC standard)