What is the range of magnetic energy product for ferrite magnets? What are the characteristics of their residual magnetism and coercivity?

Magnetic Energy Product Range of Ferrite Magnets

Ferrite magnets, also known as ceramic magnets, are composed primarily of iron oxide (Fe₂O₃) combined with barium or strontium carbonate. They are widely used in various applications due to their cost-effectiveness, corrosion resistance, and stability under high temperatures. The magnetic energy product (BHmax) is a key parameter that quantifies the maximum magnetic energy that can be stored in a magnet material. For ferrite magnets, the BHmax typically ranges from 230 to 430 MT (megatesla), which is equivalent to approximately 32 to 59 kJ/m³ or 1.8 to 4.2 MGOe (megagauss-oersteds). This range indicates that ferrite magnets generate weaker magnetic fields compared to high-performance magnets like neodymium iron boron (NdFeB) and samarium cobalt (SmCo) magnets, which have significantly higher BHmax values.

Characteristics of Residual Magnetism in Ferrite Magnets

Residual magnetism, often referred to as remanence (Br), is the magnetic field strength that remains in a magnet after it has been fully magnetized and then removed from the external magnetic field. For ferrite magnets, the residual magnetism is a critical parameter that determines their ability to maintain a stable magnetic field over time.

• Magnitude: The residual magnetism of ferrite magnets typically falls within the range of 3.9 to 4.2 kilogauss (kG) or 390 to 420 millitesla (mT). This value is relatively lower compared to high-performance magnets but is sufficient for many applications where a strong magnetic field is not required.
• Stability: Ferrite magnets exhibit good stability in their residual magnetism over time. Once magnetized, they can maintain their residual magnetic field for extended periods without significant degradation, making them suitable for permanent magnet applications.
• Temperature Dependence: The residual magnetism of ferrite magnets is affected by temperature changes. As the temperature increases, the residual magnetism decreases slightly, but this effect is generally reversible when the temperature returns to normal ranges. Ferrite magnets have a negative temperature coefficient of induction (Br), meaning their residual magnetism decreases by about 0.2% per degree Celsius increase in temperature. However, their high intrinsic coercivity improves with temperature, enhancing their resistance to demagnetization at elevated temperatures.

Characteristics of Coercivity in Ferrite Magnets

Coercivity (Hc) is the magnetic field strength required to completely demagnetize a magnet that has been previously magnetized to its saturation flux density. It is a measure of a magnet's resistance to demagnetization and is crucial for determining the magnet's performance in dynamic magnetic circuit environments.

• High Coercivity: Ferrite magnets are known for their high coercivity, which means they are very resistant to becoming demagnetized. This characteristic is essential for permanent magnets, as it ensures that the magnet maintains its magnetic properties over time and under various operating conditions. The coercivity of ferrite magnets can range from 170 to 400 kA/m (kiloamperes per meter), depending on the specific composition and manufacturing process.
• Anti-Demagnetization Ability: Due to their high coercivity, ferrite magnets are suitable for working in environments with large temperature changes and dynamic magnetic fields. They can withstand demagnetizing forces and maintain their magnetic properties, making them ideal for applications such as motors, generators, and loudspeakers.
• Temperature Coefficient: Ferrite magnets have a positive temperature coefficient of intrinsic coercivity (Hci), which means their coercivity increases with temperature. Specifically, the coercivity changes by about +0.27% per degree Celsius increase in temperature from ambient. This characteristic makes ferrite magnets more resistant to demagnetization at elevated temperatures, enhancing their performance in high-temperature applications. However, at very low temperatures, their coercivity may decrease, potentially leading to demagnetization if the magnet is exposed to extremely cold environments.

Practical Implications and Applications

The combination of moderate residual magnetism and high coercivity makes ferrite magnets suitable for a wide range of applications where cost-effectiveness, corrosion resistance, and stability under high temperatures are important. Some common applications include:

• Motors and Generators: Ferrite magnets are widely used in electric motors, generators, and actuators due to their ability to maintain a stable magnetic field under dynamic conditions.
• Loudspeakers: The high coercivity and good temperature stability of ferrite magnets make them ideal for use in loudspeakers, where they provide a consistent magnetic field for sound reproduction.
• Magnetic Separators: Ferrite magnets are employed in magnetic separators to remove ferrous contaminants from liquids and powders due to their corrosion resistance and low cost.
• Refrigeration and HVAC Systems: They are used in fan motors, pump motors, and compressors in refrigeration and heating, ventilation, and air conditioning (HVAC) systems.
• Consumer Electronics: Ferrite magnets are found in various electronic devices, including speakers, magnetic latches, and sensors.
• Automotive Industry: They are used in electric power steering systems, automotive sensors, and under-the-hood components due to their cost-effectiveness and corrosion resistance.