What are the methods of demagnetizing a magnet?

What are the uses of demagnetization?

Assuming that a person does not want to play a prank, it is worth considering why anyone would want to deliberately change the purpose of a magnet. it is worth noting that sometimes other objects, such as machine components or other metal parts of devices, are accidentally magnetized. the presence of a magnetic field can disrupt their functioning, which in some cases can result in, for example, an unwanted triggering of an alarm or too rapid wear.

sometimes a magnet may simply be too strong, attracting also what it should leave alone. some magnets may also accumulate unwanted elements, like iron filings. their presence sometimes affects the functioning of a machine, for example, resulting in faster wear of certain parts. for these reasons, demagnetization sometimes becomes a necessity.

Proven ways to demagnetize a magnet

Every magnet gradually loses its abilities over time. in the case of some models, this process can take many years, so waiting for the natural reduction of effectiveness sometimes is not enough. in such a case, it is worth trying one of these methods:

• Impact – subjecting to shocks and damage from blows with a heavy tool significantly weakens the magnet, the key role is played here by sufficiently large vibration;

• Heating – every type of magnet has a so-called Curie point, which is the maximum temperature to which it can be heated, after exceeding it, it loses its properties;

• Exposure to another magnet – this method works particularly well when combining a stronger and a weaker magnet, proper positioning or rubbing the stronger one should effectively demagnetize the weaker one.

How to demagnetize neodymium magnets?

In the case of neodymium magnets, although their Curie temperature is relatively low, they rarely demagnetize under normal usage conditions. this is because neodymium magnets are made of very strong magnetic materials, and their magnetic orientation is maintained by internal magnetic forces, ensuring their longevity and stability in harsh working conditions. however, neodymium magnets should be protected from large temperature changes and strong magnetic fields, which can affect their magnetic properties.

to summarize, if you want to demagnetize a magnet, there are various ways, depending on the material it is made of. you can demagnetize it by applying an opposing magnetic field of a sufficiently high value. an opposing field occurs when you bring two magnets close to each other with the same poles, so that they repel each other. for each magnetic material, a different value of this field, known as the coercivity HcJ, will be required.
another method is using the magnet's operating point (Jd), which is set for a magnet depending on its shape and the value of coercivity. once removed from the magnetizer, each magnet naturally demagnetizes itself to some extent by its own magnetic field.

another way to demagnetize a magnet is by applying an opposing magnetic field with a value greater than the coercivity HcB, but less than the coercivity HcJ. there is also the theoretical possibility of completely demagnetizing a magnet with a fading, alternating magnetic field, but in practice, there are only demagnetizers with weak fields that can demagnetize elements made of steel possessing very weak residual magnetization.

partial demagnetization of a magnet can also be achieved if we bring the magnet's operating line below the so-called 'knee' of the second quadrant of the magnetic hysteresis loop. we can do this by heating the magnet above its operating temperature, but below the Curie temperature.

remember, any magnet can be remagnetized, however, the field needed for magnetization is much greater than the coercivity value. it is also worth remembering that alnico magnets, due to their extremely low coercivity, can be significantly demagnetized by impacts or hits, especially if they are very flat.

What is coercivity HcJ?

Coercivity HcJ is the value of the magnetic field needed to completely neutralize the internal magnetization in a magnetic material. in other words, it is the value of the magnetic field we need to apply to demagnetize a magnet to zero.

coercivity HcJ is one of the most important parameters characterizing a magnetic material. it depends on the type of material and its magnetic properties. the higher the coercivity HcJ value, the harder it is to demagnetize a magnetic material.

the coercivity HcJ value is often used in the production of magnets because it allows determining the minimum force that must be applied to change the polarization of the magnet or to completely demagnetize it. in the case of neodymium magnets, the coercivity HcJ value can reach up to 30,000 Oersteds.

what is coercivity HcB?

coercivity HcB is the value of the magnetic field required to neutralize the residual magnetization in a magnetic material when it is exposed to the nullification of the magnetic field. it is one of the coercive values, alongside coercivity HcJ, which refers to the magnetic field needed to completely demagnetize the material.

in practice, coercivity HcB is used for materials that have bidirectional magnetization, such as steel. when a magnetic material is exposed to a decrease or reversal of the magnetic field, coercivity HcB determines the value of the magnetic field needed to reverse the magnetic polarization of the material and neutralize the remaining magnetization. the value of coercivity HcB depends on the type and quality of the magnetic material.

Differences between coercivity HcB and coercivity HcJ.

Coercivity HcB (coercivity) and coercivity HcJ (intrinsic coercivity) are two parameters characterizing a magnetic material. both parameters determine the value of the magnetic field required to completely demagnetize the material.

the difference between them lies in the method of measurement. coercivity HcB is determined by measuring the magnetic field value needed to demagnetize the material to the level of residual magnetization (only residual traces of the magnetic field remain). this measurement is performed by continuously decreasing the magnetic field and simultaneously measuring the magnetic induction until the residual value is reached.

coercivity HcJ, on the other hand, defines the magnetic field value needed to demagnetize the material when its intrinsic magnetization is zero. it is the minimum value that can be achieved to completely demagnetize the material. measuring coercivity HcJ is more complicated and requires performing measurements of the magnetic hysteresis curve for the material.

in short, coercivity HcB determines the minimum magnetic field needed to demagnetize the material to the residual level, while coercivity HcJ determines the minimum magnetic field needed to completely demagnetize the material when its intrinsic magnetization is zero.