When we heat a magnet, how does the magnet's magnetic field change?

The magnetic field of NdFeB magnets will become smaller when encountering high temperatures, because NdFeB magnets themselves have a transition temperature. Below this temperature, they show ferromagnetism, while above this temperature, they show no ferromagnetism (paramagnetism). This transition temperature is called the "Curie temperature". And when the temperature reach to 80℃, N35-N52 will lose all magnetism.  When the temperature reach to 220℃, N35AH will lose all magnetism. Some manufacturer choose heating method to demagnetize the magnet.

Why the magnetic field of NdFeB magnets will become smaller when encountering high temperatures?

According to Ampere's molecular current hypothesis, the reason why NdFeB magnets can show magnetism is because the electrons in the molecules in the magnet move in the same direction. That is, the electrons rotate in same direction around the nucleus at high speed, so multiple magnetic fields in the same direction are superimposed. Macroscopically, it shows the magnetism of the magnet.

When a magnet is heated in a fire, its magnetism disappears. This is because the magnet is magnetic, and the movement of free electrons inside it has a certain directionality, so the magnet is magnetic. However, after heating, the direction of movement of the internal electrons becomes chaotic and loses directionality. , so the magnetism disappears and weakens.

If you heat a NdFeB magnet, the energy gained by the electrons will disrupt the movement of the electrons. Microscopically, there will be magnetic fields in many different directions. After the vector is added, the magnet will weaken macroscopically. Similarly, when a NdFeB magnet encounters a large external force (such as a hit or collision), the molecular current hypothesis will disrupt the movement of the original electrons, and the magnet's magnetism will weaken in an instant.

Why do magnets demagnetize when heated?

The magnetic molecules inside the unmagnetized magnet (the theory of molecular magnets) are randomly arranged. After the magnetization process, the magnetic molecules will be arranged regularly. At this time, the N pole and S pole of the magnetic molecules will face the same direction, making the magnet magnetic and becoming a magnet. If the magnet is heated, the arrangement of the magnetic molecules will be disrupted, and the magnet will lose its magnetism. If the magnet is heated to the Curie temperature, the arrangement of the magnetic molecules cannot be restored by itself, and the magnet will demagnetize.