Demagnetization is the reduction or complete loss of a magnet’s magnetic properties.
How Do You Demagnetize a Permanent Magnet
Permanent magnets work based on an arrangement of micro-regions within an alloy material; these small regions are called magnetic domains, each of which acts as a microscopic magnet within the larger whole.
One step in the process of producing a permanent magnet is to magnetize the magnetic material: usually NdFeB, SmCo, AlNiCo, or ferrite. During the magnetization process, individual magnetic domains, which usually point in different directions, are aligned in the direction of the magnetic field. When almost all of the magnetic domains are aligned with the external magnetic field, the magnetic material becomes a permanent magnet with a certain magnetization direction.
When a magnet is demagnetized, its magnetic domains are no longer perfectly aligned, and the magnet is demagnetized when the alignment of the domains is disrupted, which is showed by demagnetization curve.
Image Source: Hsmagnets
Influencing Factors of the Demagnetization of a Permanent Magnet
High Temperature
One of the most common causes of demagnetization is increased temperature. Increased heat means increased atomic motion, which ultimately disrupts the alignment of the magnetic domains.
Magnets have two temperature thresholds: operating temperature and Curie temperature. These are important parameters for evaluating the performance and resistance of a magnet when subjected to heat.
Operating Temperature | Operating temperature is the highest temperature that a magnet can withstand for a long time without causing a temporary or permanent loss of magnetic properties.
Curie Temperature | Curie temperature is the temperature at which the magnet permanently loses all of its magnetizing strength. In this case, the structure is damaged beyond repair and is not re-magnetizable. Demagnetization occurs at different levels as the magnet’s temperature approaches its Curie point. The level of demagnetization varies greatly depending on the material and grade of a particular magnet and is described by the demagnetization curve for that magnet.
Standard neodymium magnets operate at 80°C. Special neodymium magnets have a higher temperature resistance. This is indicated by the letter following the magnet grade.
| Temperature Type | Max. Working Temperature | Curie Temperature |
| N | 80 °C | 310 °C |
| M | 100 °C | 340 °C |
| H | 120 °C | 340 °C |
| SH | 150 °C | 340 °C |
| UH | 180 °C | 350 °C |
| EH | 200 °C | 350 °C |
| *Note: M: Medium. H: High. SH: Super High.UH: Ultra High. EH: Extreme High | ||
If the heat reaches a value slightly above the maximum operating temperature, the magnet loses some of its magnetic field strength while remaining hot. For every 1°C rise above the operating threshold, neodymium magnets lose about 0.11% of their strength. This loss can be recovered by cooling the magnet. The final magnetic strength is unaffected by exposure time and the number of times the magnet is heated and cooled. Be careful, however, as repeated and rapid heating and cooling cycles may cause the magnet to break.
If the heat reaches a value significantly above the maximum operating temperature, the magnet will weaken permanently. Cooling it may restore some magnetization strength, but will result in a permanent loss of some of the magnetism.
If the heat exceeds the Curie temperature, the structure of the magnet is permanently altered and the loss of magnetization strength cannot be recovered. It cannot be remagnetized. It is important to note that the shape and size of the magnet and the direction of magnetization can significantly change its heat resistance.
Low Temperatures
In addition to high temperatures, very low temperatures can actually produce demagnetization. Depending on the magnetic material, this can happen in very different ways. Ferrite magnets are easily damaged, while neodymium magnets have a completely different behavior.
In fact, at a temperature of -125°C, neodymium magnets become slightly stronger. As the temperature decreases, the magnetic force will steadily decrease. At -196°C, for example when the magnet is immersed in liquid nitrogen, only about 85-90% of the magnetization strength is retained. When the temperature returns above 125°C, the magnetic force will return completely to its original value.
When ferrite magnets are exposed to temperatures below -40°C, a loss of permanent magnetization may result.
Magnetic products that differ from standard magnets are more susceptible to low temperatures. For example, magnetic tapes can be damaged at temperatures of -20°C.
Collisions and Volume Loss
The magnetic properties of permanent magnets are largely unaffected by vibration. As long as the magnet does not break, its magnetization strength will not be affected. The stronger the magnet, the greater its resistance to loss of magnetization strength. However, permanent magnets are usually very brittle, and if a part of the magnet is chipped or broken, this will inevitably result in a loss of magnetization.
Another factor resulting in the demagnetization of a permanent magnet is collision. For example, repeatedly striking a magnet with a hammer can interfere with the motion of its atoms, affecting the alignment of the magnet’s north and south poles and causing it to demagnetize.
External Magnetic Fields
Prolonged contact with or proximity to the magnetic field produced by a stronger magnet will inevitably weaken the magnetic field of another magnet. Therefore, you should avoid placing strong magnets with different orientations close to each other, and stacking permanent magnets in the same direction is the correct way to store them.
We know that direct current can be used to magnetize ferromagnetic materials, but if the same magnet is subjected to alternating current, the previously obtained structural arrangement will be destroyed by the change in frequency. If the magnet is placed in an AC current strong enough to overcome its coercivity, the magnet can be slowly weakened, eventually reducing its magnetic field to zero. This is how commercial demagnetizers work, and are commonly used to demagnetize tools and hard drives.
Rusting
Most magnets contain iron, and neodymium-iron-boron magnets are more than 60% iron, which can easily rust and oxidize when exposed to moisture and water. Because oxidation and rust change the chemical structure of the magnetic material, this results in a loss of strength. That’s why neodymium magnets usually need anti-corrosion coating for protection.
Unlike NdFeB magnets, ferrite magnets are made of about 90% iron oxide and therefore do not corrode even when immersed in water.
Secondary Magnetization of Demagnetized Permanent Magnets
If the Curie temperature has not been reached, depending on the cause of the demagnetization and the duration of the demagnetization, it is possible to restore the magnet’s magnetic properties by reintroducing the magnet to a sufficiently strong external magnetic field.
One method used to restore permanent magnets is to use a solenoid. When current is passed through the coiled metal wires of the solenoid, it may be sufficient to restore the magnetic field of the magnet.
It is important to note that the magnetic field of the second magnetization must be higher than that of the first magnetization so that the magnet that has been demagnetized can be charged to saturation and the magnetism of the permanent magnet restored.
Proper Storage Methods to Avoid Demagnetization
To prevent loss of magnetic field strength, magnets need to be stored in a dry and protected area.
Avoid temperatures below -40°C for ferrite magnets and above 80°C for standard neodymium magnets.
Do not expose them to strong electrical currents, magnetic fields, or strong vibrations.
If it is necessary to store several magnets together, stack them on top of each other and avoid placing strong magnets close to each other.
Permanent magnets are very brittle, so strong shocks may chip or crack their structure, resulting in a loss of performance.
Neodymium magnets are susceptible to oxidation and corrosion, so exposure to water and humidity should be avoided.
Image Source: Stanfordmagnets & Supermagneticshop
Positive Side of Demagnetization
Although demagnetization can have an impact on the performance of permanent magnets, it is important to mention the positive effects of demagnetization in industrial production in order to understand demagnetization more fully.
For parts with high demands of dimensional accuracy, the machining reference surface must be cleaned, because that is the benchmark for parts processing. For example, the dimensional accuracy of a part is 0.025mm, if the part is magnetic and adsorbs an iron chip, this iron chip is likely to be greater than 0.025mm, so the parts can not meet the requirements of the dimensional accuracy.
For some digital-display tools, because the reading of the tool is using electronic induction output, the related parts need to be demagnetized in the process of manufacturing, otherwise, the magnetic parts may lead to an error in the reading.
Methods of Demagnetization
AC demagnetization | For AC magnetized parts, AC demagnetization should be used, AC demagnetization method and equipment are simple, and the operation is relatively easy to master. The demagnetization can be carried out by passing method or attenuation method.
Passing Method: For batch demagnetization of small and medium-sized workpieces, it is best to put the workpieces on the demagnetization machine equipped with rails and drag boards for demagnetization. Place the workpiece on the drag plate 30cm in front of the coil. When the coil is energized, slowly pass the workpiece along the track through the coil and away from the coil. The workpiece is not de-energized until it is at least 1m away from the coil.
Attenuation Method: Because the direction of the alternating current is constantly changing, the current can be gradually attenuated to zero by automatic attenuation demagnetizer for demagnetization.
DC demagnetization | For workpieces magnetized by DC, DC demagnetization equipment is more complex than AC and requires special equipment, but the demagnetization effect is better than AC demagnetization. DC commutation attenuation and automatic demagnetization with ultra-low frequency current can be used.
Note that in DC demagnetization, it is important to ensure that the current is commutated when the power is off. The number of current attenuation should be as high as possible. This requires more than 30 times and the amplitude of each attenuation current should be as small as possible, if the amplitude of the attenuation is too large, it will not achieve the purpose of demagnetization. After DC demagnetization, if you use AC demagnetization again, you can get the best result.
Types of Demagnetizers
A demagnetizer is a kind of equipment used to eliminate the residual magnetism produced by machining. The magnetic field is generated by electromagnetic coils and interferes with the magnetic properties of the workpiece, achieving demagnetization of the workpiece. The main demagnetizers are table demagnetizers, portable demagnetizers, and frame demagnetizers.
Image Source: Braillon
Precautions
Even if the same demagnetization method and demagnetization equipment are used, the demagnetization effect is still different for different materials, shapes, and sizes. Therefore, the demagnetization of the workpiece should be measured after the demagnetization is completed, especially for workpieces with strict requirements for remanent magnetization and complex shapes.
The demagnetization effect of the workpiece can generally be measured by a magnetic field strength meter or other remanent magnetization detector, and the remanent magnetization intensity after demagnetization should be no more than 0.3mT (240A/m) or as specified in the product technical conditions.
The magnetic field strength used for demagnetization should be greater than the maximum magnetic field strength used for magnetization.
When demagnetizing radiation-magnetized parts, the workpiece should be magnetized longitudinally and then demagnetized longitudinally so that the remanent magnetism after demagnetization can be detected.
