Introduction
The essence of magnetism in all materials originates from the atomic level, where atoms, composed of nuclei and electrons, exhibit magnetic moments. These moments are a result of the electrons’ orbital motion around the nucleus, which creates an orbital magnetic moment, and the intrinsic spin of the electrons, which produces a spin magnetic moment. It is the magnetic moments of electrons that are the primary source of magnetism in materials.
One critical aspect of magnetic materials is their behavior under varying temperatures. High temperatures can alter the magnetic domain structure of a material, leading to the loss of its magnetism. This characteristic sets a maximum operating temperature (also known as working temperature) for each type of magnetic material, understanding which is crucial to prevent demagnetization due to excessive heat during use.
Curie Temperature
The concept of the Curie temperature is fundamental to understanding the relationship between temperature and magnetism. Discovered over two centuries ago by Pierre Curie, a renowned physicist and the husband of Marie Curie, this physical property of magnets reveals that magnets lose their magnetism when heated to a certain temperature. This critical temperature is known as the Curie point (or Curie temperature, Tc), signifying the temperature at which a ferromagnetic or ferrimagnetic material transitions to a paramagnetic state.
Definition: The Curie temperature is the temperature at which magnetic materials transition between ferromagnetic/ferrimagnetic states and paramagnetic states.
Ferromagnetic materials exhibit strong magnetism once magnetized. However, as temperature increases, the intensified thermal motion of the metal lattice affects the orderly arrangement of the magnetic domains’ moments. When the temperature reaches a point that disrupts this arrangement, the magnetic domains disintegrate, the average magnetic moment drops to zero, and the material’s ferromagnetic properties (such as high magnetic permeability, hysteresis, and magnetostriction) disappear, transitioning the material to a paramagnetic state with corresponding magnetic permeability. The temperature at which this transformation occurs is the Curie temperature.
Numerical Values: The Curie temperature represents the theoretical limit of a magnetic material’s operating temperature, determined by the material’s chemical composition and crystal structure. For example, iron has a Curie temperature of approximately 770°C, while cobalt’s Curie temperature is around 1131°C.
Beyond the Curie Temperature: Exceeding the Curie temperature causes intense internal molecular motion within the magnet, leading to irreversible demagnetization. A demagnetized magnet can be remagnetized, but its magnetic strength will significantly decrease, typically to around 50% of its original value.
Operating Temperature
In reality, the maximum operating temperature of most magnetic materials is much lower than their Curie temperature. Within the operating temperature range, an increase in temperature will cause a decrease in magnetic strength, but most of this loss is recoverable upon cooling, as long as the temperature does not exceed the Curie temperature.
Operating temperature vs Curie temperature: The relationship between the operating temperature and the Curie temperature is such that a higher Curie temperature generally indicates a higher potential operating temperature and better temperature stability. The addition of elements such as cobalt, dysprosium, and terbium to sintered neodymium-iron-boron (NdFeB) materials can increase their Curie temperature, making high coercivity products (H, SH, etc.) commonly contain dysprosium.
Maximum Usage Temperature of a Magnet: This depends on the magnet’s intrinsic magnetic properties and the chosen operating point. For any given magnet, the more closed the working magnetic circuit, the higher the maximum usage temperature and the more stable the magnet’s performance. Thus, a magnet’s maximum usage temperature is not a fixed value but varies with the degree of closure of the magnetic circuit.
| Curie temperature and maximum working temperature of various magnetic materials | ||
| Materials | Curie Temperature (℃） | Maximum Operating Temperature ℃） |
|---|---|---|
| Iron Chrome Cobalt (FeCrCo) Magnets | 680 | 400 |
| Alnico Magnets | 750-850 | 450-550 |
| Ceramic Magnets (also known as Ferrite magnets) | >450 | 250 |
| SmCo5 Magnets | 700-750 | 250 |
| Sm2Co17 Magnets | 800-850 | 300-350 |
| Bonded Neodymium Magnets | 310-340 | 100-150 |
| Curie Temperature and Maximum Operating Temperature of Sintered Neodymium Magnets | ||
| Type | Curie Temperature (℃） | Maximum Operating Temperature ℃） |
|---|---|---|
| N | 310 | 80 |
| M | 340 | 100 |
| H | 340 | 120 |
| SH | 340 | 150 |
| UH | 350 | 180 |
| EH | 350 | 200 |
Conclusion
Understanding the Curie temperature and operating temperatures is crucial for maximizing the performance and durability of magnetic materials. These temperatures mark the limits within which magnets maintain their magnetic properties, a vital consideration for applications ranging from everyday devices to advanced industrial equipment. Enhancing a magnet’s thermal stability, through the incorporation of specific elements, extends its utility in high-temperature environments.
For a deeper insight into how thermal stability affects permanent magnets, “Temperature Stability in Permanent Magnets: Key Coefficients Explained” is a highly recommended read. This article sheds light on the thermal coefficients critical to magnet performance and aids in selecting the right materials for specific applications, ensuring optimal functionality and longevity.