What are the physical mechanisms of the magnetization and demagnetization processes of NdFeB Magnets?

The magnetization and demagnetization processes of NdFeB Magnets (neodymium iron boron magnets) involve complex physical mechanisms. Here is a basic explanation of both processes:
Magnetization process:
When making NdFeB magnets, the particles are first pressed together, and then the magnetic domains of the particles are aligned and set in one direction, called the magnetization direction, through the pressing and magnetization process. This is usually accomplished by applying an external magnetic field, which causes the magnetic domains inside the magnet to align, creating a strong magnetic field.
After pressing, the magnets are demagnetized before being sintered, which typically involves heating the material to extremely high temperatures but below the material's melting point in an oxygen-free environment. This step is to eliminate any residual magnetic fields that may exist within the material.
Sintering is the process of fusing together pressed particles to form a solid mass. During this process, the magnetic particles are locked into specific positions, forming a stable magnet. Finally, the magnet is quickly cooled through a process called quenching to maximize magnetic performance.
Demagnetization process:
The demagnetization process of NdFeB magnets is relatively complex and is affected by many factors, such as temperature, external magnetic field, etc. At high temperatures, the magnetocrystalline anisotropy and spontaneous magnetization of magnets decrease, which may result in weakened domain wall contrast and reduced magnetic domain width, thereby weakening the magnet's magnetization.
When the external magnetic field is opposite to the magnetization direction of the magnet, the magnetization strength of the magnet will gradually decrease. If the external magnetic field is strong enough, it can completely offset the magnetic field inside the magnet, causing the magnet to reach a demagnetized state.
Another demagnetization mechanism is remagnetization through displacement of domain walls. Under the action of the magnetic field component perpendicular to the sample plane, the domain wall energy is significantly reduced, causing the domain wall to be displaced at a coercive field much smaller than that of the bulk magnet, thus weakening the magnetization of the magnet.
Please note that the above explanation only provides the basic physical mechanism of the magnetization and demagnetization processes of NdFeB magnets. In practical applications, these processes may be affected by a variety of complex factors and need to be considered comprehensively.