Curie temperature and working temperature of magnet

Do you know that magnets will permanently lose their magnetization when they exceed a certain temperature, and the maximum working temperature that different magnets can withstand is different. So, what are the temperature related indicators? How to choose the appropriate magnet based on the working temperature? Today we will answer these questions.

Curie temperature

When it comes to the relationship between temperature and magnetism, we first need to understand a concept - "Curie temperature". Do you feel very familiar with the words' Curie '? This concept does have some relationship with Madame Curie. More than 200 years ago, a famous physicist discovered a physical property of magnets in his laboratory, which is that when a magnet is heated to a certain temperature, its original magnetism disappears. This great physicist was Pierre Curie, the husband of Madame Curie. Later, people called this temperature Curie point, also known as Curie temperature (Tc) or magnetic transition point.

Definition: Curie temperature is the temperature at which a magnetic material transitions between ferromagnetic and paramagnetic substances. When the temperature is lower than the Curie temperature, the material becomes ferromagnetic, and when the temperature is higher than the Curie temperature, the material becomes paramagnetic. The height of the Curie point is related to the composition and crystal structure of the substance.

Temperature higher than Curie temperature: The internal molecules of the magnet move violently, causing the destruction of magnetic domains. A series of ferromagnetic properties related to magnetic domains, such as high permeability, hysteresis loops, magnetostriction, etc., all disappear, and the magnet undergoes irreversible demagnetization. After demagnetization, it can be magnetized again, but the magnetization voltage needs to be much higher than the voltage at the first magnetization, and the magnetic field after magnetization may not reach the original level.

Curie temperature and working temperature of permanent magnet materials

Curie temperature is of great significance in practical applications. In the selection process of magnetic materials, especially soft magnetic materials, for devices that need to maintain ferromagnetism at a specific temperature, selecting materials with appropriate Curie temperature can improve the stability and reliability of the device.

Work temperature

Work temperature (Tw) refers to the temperature range that a magnet can withstand in practical applications. Due to the different thermal stability of different substances, their working temperatures may also vary. The maximum working temperature of magnet is much lower than the Curie temperature. Within the working temperature, the magnetic force will decrease as the temperature increases, but most of it can recover after cooling.

The relationship between working temperature and Curie temperature: The higher the Curie temperature, the higher the working temperature of the magnetic material, and the better the temperature stability. Adding elements such as cobalt, terbium, and dysprosium to sintered neodymium iron boron raw materials can increase their Curie temperature, so dysprosium is commonly present in high coercivity products (H, SH,...).

The temperature resistance of the same type of magnet varies among different grades due to differences in composition and structure. Taking neodymium iron boron as an example, the maximum working temperature of different grades of magnetic steel ranges from 80 ℃ to 230 ℃.

Working temperature of sintered neodymium iron boron permanent magnets

Several factors that affect the actual working temperature of magnet:

The shape and size of magnet (i.e. aspect ratio, also known as magnetic conductivity Pc) have a significant impact on the actual maximum working temperature. Not all H-series neodymium iron boron magnets can operate without demagnetization at a temperature of 120 ℃. Some sizes of magnets may be in demagnetization at room temperature, so it is necessary to increase the coercivity level to increase the actual maximum working temperature.

The degree of closure of the magnetic circuit also affects the actual maximum working temperature of the magnet. The closer the working magnetic circuit of the same magnet, the higher the maximum working temperature of the magnet and the more stable its performance. So the maximum working temperature of a magnet is not a fixed value, but varies with the degree of closure of the magnetic circuit.