Principles and Methods of Demagnetization

Hard magnetic materials (such as NdFeB) have two notable characteristics:
1.They can be strongly magnetized under the influence of an external magnetic field;
2.They exhibit hysteresis, meaning they retain their magnetized state even after the external magnetic field is removed.
So, how can a magnet that has been charged to a state of technical saturation lose its magnetism? Today, we will explore the principles and methods of demagnetization.
Demagnetization, also known as magnetic cleaning or degaussing, refers to the process of restoring a magnet to a magnetically neutral state, also known as magnetic neutralization. In industrial processing, there are 3 methods of demagnetization.

1. Static Demagnetization

A magnetic field opposite to the original magnetization direction is applied to the magnet. The strength of this counter-field should ensure that when it is removed, the magnetic induction of the magnetic material becomes 0. The resulting magnetically neutral state is known as the static magnetically neutral state.

In the hysteresis loop, the red segment in the second quadrant represents the demagnetization curve. When a magnetic field opposite to the magnetization direction is applied, the magnetic induction decreases as the strength of the counter-field increases. When this counter-field strength reaches -Hc, the magnetic induction of the magnet drops to zero, and the magnet no longer has magnetism.
The hysteresis loop is measured at room temperature. When the magnet operates at different temperatures, the demagnetization curve varies, as shown in the figure below. Therefore, the strength of the reverse magnetic field applied during demagnetization varies with temperature.

2. Dynamic Demagnetization

An alternating magnetic field strong enough is applied to the magnetic material and then gradually reduced to 0. The resulting magnetically neutral state is known as the dynamic magnetically neutral state.
This method places the workpiece in an alternating magnetic field, using the decaying hysteresis loop for demagnetization. As the amplitude of the alternating magnetic field gradually decays, the trajectory of the hysteresis loop becomes smaller. When the magnetic field decays to 0, the residual remanence in the workpiece is reduced to nearly zero, as illustrated below. The changes in the direction and magnitude of the current and magnetic field during demagnetization must be synchronized.

（1）AC Demagnetization
For workpieces magnetized with AC, AC demagnetization can be done using either the pass-through method or the attenuation method.
A.Pass-through Method
For batch demagnetization of medium to small workpieces, it’s best to place the workpiece on a demagnetizing machine with a track and sled. Place the workpiece on the sled 30 cm in front of the coil, and when the coil is energized, slowly move the workpiece along the track through the coil and at least 1 meter away from the coil before turning off the power. For heavy or large workpieces that cannot be placed on the demagnetizing machine, place the coil over the workpiece and slowly move the coil past and at least 1 meter away from the workpiece before turning off the power.
B.Attenuation Method
Since AC continuously changes direction, an automatic attenuation demagnetizer or a voltage regulator can gradually reduce the current to zero for demagnetization. Place the workpiece inside the coil, between the two magnetizing clamps of a flaw detector, or use a rod contact to touch the workpiece, then gradually reduce the current to zero for demagnetization.
（2）DC Demagnetization
By continuously changing the direction of DC while gradually reducing the current through the workpiece to 0, demagnetization can be achieved. The waveform of DC demagnetization current is shown below, where T1 is the interval of current conduction and T2 is the interval of current cut-off. Ensure that the current direction changes during the cut-off. The number of current attenuations should be as many as possible (generally more than 30 times), and the amplitude of each attenuation should be as small as possible. If the amplitude is too large, demagnetization will not be achieved. 

3. Thermal Demagnetization

This method involves heating the magnetic material above its Curie temperature, then cooling it without an external magnetic field. For NdFeB, thermal demagnetization involves baking at temperatures above 350℃ for 30 minutes to 1 hour.
Within the working temperature range, the magnetic force of the magnet decreases with an increase in temperature but mostly recovers after cooling. If the temperature reaches the Curie temperature, the molecules inside the magnet move vigorously, causing irreversible demagnetization.

Regardless of which of the 3 methods is used for demagnetization, the internal structure of the magnet will undergo permanent changes. After demagnetization, the magnetic properties of the magnet cannot be restored to their previous levels even if re-magnetized.