Coercive field

A magnetised ferromagnetic material can be demagnetised by various processes. For example, by strong impacts on the material or by heat. However, demagnetisation also takes place in an external opposing magnetic field. The opposing field strength required for demagnetisation is called coercive field strength.

Experimental testing of the coercive field

To test this assertion experimentally, we first need to magnetise a ferromagnetic material. To do this, you can hold a ferrous material, such as a screw, between the north pole and the south pole of two permanent magnets and then carefully pull them away from the screw in both directions. This magnetises the screw and afterwards, it has a magnetically attractive effect, for example, on pins.

The magnetisation of the screw is lost if it is heated or subjected to strong vibrations (e.g. hard blows with a hammer). Another way to demagnetise the screw is to expose it to a magnetic field of opposite polarity. It must have the strength of the so-called coercive field strength. If the screw was magnetised by the head of the screw being in contact with the north pole of a permanent magnet and the tip being in contact with the south pole, the screw can be demagnetised by exposing it to a weaker and oppositely directed magnetic field. The head of the screw must be in contact with the south pole of a permanent magnet and the tip of the screw must be in contact with the north pole, i.e. exactly the opposite of the magnetisation. If magnets as strong as the magnets used for magnetisation were utilised for demagnetisation, the screw would be magnetised again, only with the poles reversed.

How do you measure the coercive field strength?

The coercive field strength (Hc) is a measure of the resistance of a magnetic material to demagnetisation. It is determined by means of hysteresis curve measurement (see Illustration 1). To do this, the material is exposed to an external magnetic field whose strength is gradually increased and then reduced again to zero. The coercive field strength corresponds to the magnetic field strength at which the magnetisation of the material falls to zero. This value can be read on the hysteresis curve as the intersection of the curve with the horizontal axis at which the magnetisation is zero (in Illustration 1, this intersection is marked with Hc).

An experimental setup for measuring the hysteresis curve typically comprises a test piece of the material to be examined, a coil for generating the magnetic field, a magnetometer for measuring the magnetisation of the test piece and a control unit that varies the external magnetic field. The test piece is placed in the coil and the control unit gradually increases the current through the coil, changes the direction of the current and reduces it again to create a cyclic magnetic field. The magnetisation of the test piece is continuously measured and correlated with the applied magnetic field to create the hysteresis curve (Illustration 2).

Illustration 2 shows an experimental setup for measuring the coercive field strength. First, a voltage U is applied, which successively increases the current I through the coil. This leads to a magnetic flux density B, which magnetises the ferromagnetic material in the coil and causes a magnetic field H that can be measured with a Hall probe. If the voltage U is now turned to zero, no more current flows through the coil and the external flux density B is zero. The remaining measured field strength Hc on the Hall probe is then the coercive field strength (cf. Illustration 1).

Difference between the coercive field strengths bH_c and jH_c

To make the magnetic flux density inside the material disappear completely, an external magnetic field of coercive field strength H_c must be applied.

A distinction is made between two different coercive field strengths:

• The coercive field strength bH_c is the coercive field strength of the flux density.
• The coercive field strength jH_c, which is referred to as the coercive field strength of the magnetisation (or the magnetic polarisation).

The following will explain it in more detail:

When a magnetised material (i.e. a magnet, for short) is exposed to a field strength of bH_c, the magnetic flux density in the magnet disappears. The magnetic flux density inside the magnet is then zero. However, this is only because the remaining magnetisation is compensated for by the external opposing field. Both fields cancel each other out inside the magnet. The material itself is therefore still magnetic. You will notice this immediately when the external opposing field is switched off again, as magnetic forces still emanate from the material.

If the external field strength is increased to jH_c, ai.e. the strength of the opposing field is increased, then the magnet is permanently demagnetised.

In Illustration 1 above, the field strength bH_c is shown as H_c. The magnetic flux density B inside the magnet is only zero as long as bH_c is present. The permanently demagnetising field strength jH_c is not shown and is greater in magnitude than bH_c. This coercive field strength of the magnetisation jH_c not only compensates for the magnetic field of the aligned atomic spins in the material, but also leads to a cancellation of the stabilisation of the spin alignment due to the exchange interaction. The magnetic field strength inside the magnet is not zero for opposing fields greater than bH_c, but has a certain value. However, it is directed in the opposite direction to the aligned atomic spins and attempts to reverse them. At jH_c, the magnetic field can overcome the exchange interaction and actually leads to the atomic spins being reversed. The material is thus demagnetised. A further increase in the magnetic field strength results in a new alignment of the atomic spins in the opposite direction. A new magnetisation can be detected, but with reversed magnetic polarisation, i.e. with reversed poles compared to the original polarity of the magnet.