Magneto Resistor – Symbol, Working, Types & Characteristics

john November 9, 2017 3 Comments

Back in 1856, William Thomson a Scots-Irish mathematical physicist and engineer while experimenting with iron pieces discovered a phenomenon called magneto-resistance. This effect was later on used to build a special type of variable resistor known as magneto-resistor.

In this article you shall get a brief about a magneto – resistor: its working principle, characteristics and applications.

So let’s delve in!

What is a Magneto Resistor?

It was found by Sir William Thomson that, when the iron pieces were kept in magnetic feild, their electrical resistance somehow changed when the direction of magnetic feild with respect to the current flow through them changed. This effect was found when he experimented with nickel too. Thus a new effect or phenomenon was introduced to the world and it was named magneto-resistance. This effect was then further studied and a new type of variable resistor was formed. This resistor would have its electrical resistance vary with the magnetic field strength around it. The current through this resistor also changes with change in magnetic force applied to it.

As you already know that magnetic field is a vector quantity meaning, it us specified in both direction and magnitude, just as a current.

Thus, we can define a magneto resistor in one line as:

A special kind of variable resistor, whose electrical resistance depends on the external magnetic force applied to it.”

Magneto Resistor Symbol

Schematically, in circuit diagram the magneto resistance is represented by the symbol shown below. The arrow through the resistor symbol signifies a variable resistor, while “x” below it denotes that the variable resistor used is “magneto resistor”

The working of the magneto resistor is quite easy; it is based on the effect already mentioned: the magneto resistance.

Magneto Resistor – Working Principle

To understand the working principle of a magneto resistor, let us first brush up our concept that relates the direction of current and that of magnetic force.   The magnetic field strength is highest, when the current is in same direction as that of the magnetic force, while weakest when it is 900 to the magnetic force.

So how does this effect resistance of the material? The answer is simple. What is current? Current is nothing but flow of free electrons. When a material is placed in absence of any magnetic force, these electrons move in an orderly fashion, mostly in straight lines.

As soon as it is subjected to magnetic force, the free electrons get excited and start moving in an indirect motion creating collision among them. These collisions restrict the flow of free electrons such that only few can flow freely. This means the flow of current is restricted, that means the electrical resistance has increased with increase in magnetic field strength.

Thus, to put in short terms, the resistance of a magneto resistor increases with increase in magnetic field strength and decreases with a decrease in magnetic field strength.

According to the types of magneto-resistance effect, magneto resistor are also categorised into three groups. Lets discuss about them in brief.

Magneto Resistor – Types

#1. Based on Giant Magneto-resistance:-

Discovered by two scientists Albert Fert and Peter Grunberg, this effect is commonly observed in ferromagnetic materials. The resistance here depends on whether the adjacent ferromagnetic layers are in parallel or anti-parallel. If they are in parallel alignment then the resistance is low, while the resistance is high when they have an anti-parallel alignment.

#2. Based on Extra ordinary Magneto-resistance:-

Discovered in 2000, this effect occurs in semiconductor hybrid systems under transverse magnetic field. The resistance of the semiconductor hybrid system increases under the transverse magnetic field. It is low in the absence of this magnetic field. This effect is greater than the Giant magneto resistance effect.

#3. Based on Tunnel Magneto-resistance:-

Discovered by M. Julliere in 1975, this effect is true to its name. The magnitude of the current that flows through a “tunnel”, which is nothing but an insulator, that connects two ferromagnetic electrodes, here depends on the direction of magnetization. When the magnetization of the two ferromagnetic electrodes is parallel to each other, then the current through the “tunnel” is high. Similarly an anti-parallel alignment of magnetization of the two ferromagnetic electrodes decreased the flow of current.

Characteristics of Magneto Resistor

The characteristic curve of the magneto resistor is shown in figure below. Here we see that the sensitivity of the magneto resistor varies with the magnetic field strength.

From the curve we see that when magnetic strength is zero, the sensitivity is zero. Also, as the magnetic field strength increases, the magneto resistor started showing an increase in resistance. However, we see that the curve starts bending down after a certain point, C. This is due to saturation of the magnetic field. Beyond this if the magnetic field is increased, the resistance starts to decrease.

Now that we have covered the working and characteristic of a magneto resistor, let us now discuss some of the common uses and applications of the same.

Magneto Resistor – Uses and Applications

Owing to their dependency on the magnetic field, these magneto resistors find use in magnetic field sensing devices. These include, electromagnetic compass, magnetometers which measure magnetic field intensity and direction, position sensors and ferrous metal detection. Position sensors can be angle , rotary or linear position sensors. They also find use in bio sensors and hard disk drives.

With this we come to the conclusion of the article, let us rewind what we have discussed in this short article.

Magneto Resistors: A Quick Recap

• Magneto-resistors: variable resistor whose resistance depends on magnetic field strength.
•  Direction if current = direction of magnetic force, then magnetic field intensity is high. This means the resistance is high.
• Resistance of magneto-resistor is directly proportional to the strength of magnetic field.
• This effect is called magneto resistance, discovered by Sir William Thomson
• Three types of magneto resistance effect: Giant magneto resistance, Extraordinary magneto resistance and tunnel magneto resistance.
• The characteristic curve shows the dependency of resistance on the magnetic field strength.
• Most prominent uses include those involving magnetic field sensors, like electromagnetic compass, magnetometers etc. Also used in bio sensors and hard disk drives.