Principle and method of demagnetization

What is demagnetization?

Demagnetization, also known as magnetic cleaning and degaussing, refers to the process of the magnet returning to the magnetic neutral state, also known as magnetic neutralization.
Hard magnetic materials (such as NdFeB) have two distinct characteristics: one is that they can be strongly magnetized under the action of external magnetic field, the other is hysteresis, that is, hard magnetic materials remain magnetized after removing the external magnetic field. So how to make the magnet which has been magnetized to the saturation state lose its magnetism? Today, we will understand the principle and method of demagnetization.

Classification of demagnetization methods

Demagnetization of mining industry

• (1) In this paper, the method of magnetic cleaning is used to study the influence of magnetic field on the stability of rocks.
• (2) Thermal magnetic cleaning is a process in which unstable components (interference factors) in rock magnetism are treated by heating or low temperature in paleomagnetic research, which is called thermal magnetic cleaning.
• (3) Chemical magnetic cleaning (CMC) is a process of chemical treatment of unstable components (interference factors) in the paleomagnetic study, which is called CMC.

Demagnetization of industrial

Static demagnetization

Add a magnetic field opposite to the original magnetization direction of the magnet, and the intensity of the counter magnetic field should be such that when it is removed, the magnetic induction of the magnet will become zero. The resulting magnetic neutral state is called static magnetic neutral state.
In the hysteresis curve, the red line segment in the second quadrant represents the demagnetization curve, that is, when a magnetic field opposite to the magnetization direction is applied to the magnet, its magnetic induction decreases with the increase of the reverse magnetization field strength. When the reverse magnetization field strength reaches – HC, the magnetic induction of the magnet decreases to 0, and the magnet no longer has magnetism.
The hysteresis loop is measured at room temperature. The demagnetization curve of the magnet at different working temperatures is not the same, as shown in the figure below. Therefore, the intensity of reverse magnetic field applied by demagnetization at different temperatures is different.

Dynamic demagnetization

Then, the amplitude of the alternating magnetic field is gradually reduced to 0. The resulting magnetic neutral state is called dynamic magnetic neutral state.
The principle of this method is to put the workpiece in the alternating magnetic field and demagnetize it by decreasing the hysteresis loop. As the amplitude of the alternating magnetic field gradually decreases, the track of the hysteresis loop becomes smaller and smaller. When the magnetic field gradually decays to zero, the residual magnetism in the workpiece will be close to zero. The principle of demagnetization is shown in the figure below. It can be seen that the direction and magnitude of current and magnetic field must be changed and attenuated simultaneously during demagnetization.

Alternating current demagnetization
Alternating current demagnetization can be used to demagnetize the workpiece which has been electromagnetized by alternating current.
A. By law
For the batch demagnetization of small and medium-sized workpieces, it is better to place the workpieces on the demagnetizing machine with track and carriage. During demagnetization, the workpieces should be placed on the carriage 30 cm in front of the coil. When the coil is powered on, the workpieces should slowly pass through the coil along the track and be cut off at least 1 m away from the coil. For the heavy or large workpieces that cannot be demagnetized on the demagnetizing machine, the coil can also be put on the workpieces, and the coil can be slowly passed through and far away from the workpieces when the power is on, and the power is cut off at least 1m away.

(demagnetizer, picture from Internet)
B. Attenuation method
As the direction of alternating current is changing continuously, it can be demagnetized by using the automatic attenuation demagnetizer or voltage regulator to gradually reduce the current to zero. The workpiece is placed in the coil, clamped between the two magnetized chucks of the flaw detector, or the current is reduced to zero after contacting the workpiece with the strut contact.

Direct current demagnetization
Demagnetization is carried out by changing the direction of direct current and decreasing the current through the workpiece to zero. The waveform of DC demagnetization current is shown in the figure below. In the figure, T1 is the time interval of current conduction, and T2 is the time interval of current power-off. It is necessary to ensure the current commutation during power-off. The times of current attenuation should be as many as possible (generally more than 30 times), and the amplitude of current attenuation should be as small as possible. If the amplitude of current attenuation is too large, the purpose of demagnetization can not be achieved.

Thermal demagnetization

It is a way of heating the magnetic body above Curie temperature and then cooling and demagnetizing it without external magnetic field. The sintered NdFeB can be roasted at 350 ℃ for 30 minutes to 1 hour by thermal demagnetization.
Within the working temperature, the magnetic force of the magnet will decrease with the increase of temperature, but most of the magnetic force will recover after cooling. If the temperature reaches Curie temperature, the molecules in the magnet will move violently and demagnetize, which is irreversible.
No matter which of the above three ways is used to demagnetize the magnet, the internal structure of the magnet will be permanently changed. After demagnetization, the magnetic properties of the magnet can not be restored to the previous level.

Precautions for demagnetization

(1) The magnetic field intensity of demagnetization should be greater than (at least equal to) the maximum magnetic field intensity used in magnetization.
(2) When demagnetizing the circumferentially magnetized workpiece, the workpiece should be magnetized longitudinally and then demagnetized longitudinally, so as to detect the residual magnetism after demagnetization.
(3) AC electromagnetism is demagnetized by AC, and DC electromagnetism is demagnetized by DC. After DC demagnetization, if AC demagnetization is used again, the best effect can be obtained.
(4) Attention should be paid to the demagnetization by coil passing method

• ① The workpiece should be parallel to the coil shaft and placed against the inner wall.
• ② When the L / D of the workpiece is less than or equal to 2, the extension block should be used to lengthen it before demagnetization.
• ③ Small workpieces should not be placed in the frame by bundling or stacking for demagnetization.
• ④ Ferromagnetic baskets or disks cannot be used to place the workpiece for demagnetization.
• ⑤ Ring workpiece or complex workpiece should be demagnetized by coil while rotating.
• ⑥ The workpiece should pass slowly and be 1 m away from the coil before power off.
• ⑦ Demagnetizing machine should be placed in the east-west direction, demagnetized workpiece should also be placed in the east-west direction, perpendicular to the geomagnetic field can effectively demagnetize.
• ⑧ The demagnetized workpiece should not be placed near the demagnetizing machine or magnetizing device.

Source: China Permanent Magnet Manufacturer – www.rizinia.com