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Permanent magnet materials, also known as hard magnetic materials, are distinguished by their high anisotropic field, elevated coercive force, significant hysteresis loop area, and the substantial magnetizing field required to achieve saturation magnetization. Even after the removal of the external magnetic field, these materials can retain strong magnetism for an extended period.
Types of permanent magnet materials
1.Ferrite
Ferrite is a non-metallic magnetic material, also known as magnetic ceramics. When we take apart a traditional radio, the speaker magnet inside is made of ferrite.
Ferrite has relatively low magnetic properties. Currently, its maximum magnetic energy product (a parameter used to measure magnet performance) is only slightly higher than 4MGOe. Nevertheless, the material’s most significant advantage is its affordability. As such, it continues to find extensive use in many different fields.
Ferrite is a ceramic, so its processing performance is similar to that of ceramics. Ferrite magnets are molded and sintered. If they need to be processed, they can only be simply ground. Since it is difficult to machine, most ferrite products have simple shapes and relatively large dimensional tolerances. The block-shaped products are in better condition and can be ground. Ring-shaped machines generally can only grind two planes. Other dimensional tolerances are given as a percentage of the nominal size.
Ferrite is widely used and cheap, so many manufacturers have ready-made rings, squares and other products of conventional shapes and sizes to choose from. Since ferrite is made of ceramic, there is basically no corrosion problem. The finished product does not require surface treatment such as electroplating or painting.
2.Rubber magnet
Rubber magnets are a type of ferrite magnet that consist of bonded ferrite magnetic powder mixed with synthetic rubber. They are manufactured through various processes such as extrusion molding, calendering molding, and injection molding. These magnets are characterized by their flexibility, elasticity, and ability to be twisted. They can be fabricated into a variety of shapes including strips, rolls, sheets, blocks, rings, and other complex forms.
Its magnetic energy product of 0.60 ~ 1.50 MGOe rubber magnetic material applications: refrigerators, message notice racks, fasteners to fix the object in the metal body to be used as advertising and other fasteners. It can also be used for toys, teaching instruments, switches and sensors of the magnetic sheet.
Rubber magnets are mainly used in micro motors, refrigerators, disinfection cabinets, kitchen cabinets, toys, stationery, advertising and other industries.
3.Samarium cobalt
The main components of samarium cobalt magnet are samarium and cobalt. Since the two materials themselves are expensive, samarium cobalt magnet is also the most expensive among several magnets. At present, the magnetic energy product of samarium cobalt magnet can reach 30MGOe, or even higher.
In addition, samarium cobalt magnets have high coercivity, high temperature resistance, and can be used in high temperature conditions up to 350 degrees Celsius, so they can’t be replaced in many applications. Samarium cobalt magnets belong to powder metallurgy products. Generally manufacturers sinter samarium cobalt magnets into square blanks according to the size and shape needed for the finished product, and then use diamond blades to cut them into finished size. As samarium cobalt is electrically conductive, it can be processed by wire cutting.
Theoretically, samarium cobalt can be cut into shapes that wire cutting can cut into if magnetization and larger sizes are not considered. Samarium Cobalt magnets have very good corrosion resistance and generally do not require anti-corrosion plating or painting. In addition, samarium cobalt magnets are very brittle, so it is difficult to process small sizes or thin-walled products.
4.Neodymium iron boron (NdFeB)
NdFeB is a widely used and rapidly developing magnet product. It has only been more than 20 years since NdFeB was invented and is now widely used. Because of its high magnetic properties and easy processing, the price is not very high, so the application field is expanding rapidly.
At present, the magnetic energy area of commercialized NdFeB can reach 50MGOe, which is 10 times that of ferrite. NdFeB also belongs to powder metallurgy products, and the processing method is similar to samarium cobalt.
The maximum operating temperature of NdFeB is around 180 degrees Celsius. For harsh environment applications, it is generally recommended not to exceed 140 degrees Celsius. NdFeB is very susceptible to corrosion. Therefore, finished products are mostly plated or coated.
Conventional surface treatments for NdFeB include: nickel plating， zinc plating, aluminum plating, electrophoresis and so on. If it works in a closed environment, it can also be phosphatized. Due to the high magnetic properties of NdFeB, it is used in many applications to replace other magnetic materials to reduce the size of the product. If ferrite magnets were used in the manufacture of cell phone parts, today’s cell phones would be no smaller than half a brick.
Both magnets, samarium cobalt magnets and neodymium iron boron magnets, have better machinability. Therefore, the dimensional tolerance of the products is much better than that of ferrite. In general, the size tolerance can be (+/-)0.05mm.
5.Aluminum-nickel-cobalt（AlNiCo）
Alnico magnets have two processes: casting and sintering. In China, casted Alnico magnets are more common. Alnico magnets have a magnetic energy product of up to 9 MGOe. Their best feature is that they are heat resistant and can still work at temperatures up to 550 degrees Celsius. However, Alnico tends to demagnetize in reverse magnetic fields. If the same poles of two Alnico are pushed together, the magnetic field of one of the magnets will be demagnetized or reversed. Therefore, it is not suitable for operation in a reverse magnetic field (e.g., electric motors).
Alnico is so hard that it can be ground and wire cut, but at a higher cost. The general finished product is either well ground or unground. Alnico is more widely used in the sensor field.
Main Properties of Permanent Magnetic Materials
1.residual magnetic field density
After the permanent magnet material is magnetized to saturation in the external magnetic field, when the external magnetic field is zero, the magnetic induction strength value of the permanent magnet material is directly related to the magnetic density of the air gap in the motor. The higher the value of the magnetic induction strength, the higher the air gap density of the motor will become; the torque constant, the inverse potential coefficient and other main indicators of the motor will reach the optimum value; the electrical and magnetic load of the motor may be the most reasonable relationship between the values, and the motor efficiency can reach the best.
2.coercive force
It refers to the reverse magnetic field strength of permanent magnet material in the case of saturation magnetization, when the residual magnetic induction strength drops to zero. This index is related to the antidemagnetization ability of the motor, overload multiplier and air gap magnetic density and other indicators. The larger the coercivity, the stronger the demagnetization resistance of the motor; the larger the overload multiplier and the stronger the adaptability to the strong demagnetization of the dynamic operating environment. At the same time, the air gap magnetization of the motor will also be improved.
3.Maximum magnetic energy product
It refers to the maximum value of the magnetic field energy provided by the permanent magnet material to the external magnetic circuit. This index is directly related to the amount of permanent magnet material in the motor: the larger the maximum magnetic energy level is, the larger the magnetic field energy the permanent magnet material can provide to the external magnetic circuit, which means that less permanent magnet material is used in the motor under the same power condition.
4.Temperature coefficient
Temperature is one of the main factors affecting the magnetic properties of permanent magnetic materials. When the temperature changes every 1 degree Celsius, the percentage of reversible change in magnetic properties is called the temperature coefficient of magnetic materials. Temperature coefficient can be divided into remanent magnetic induction temperature coefficient and coercivity temperature coefficient. This index has a large impact on the stability of the performance of the motor: the higher the temperature coefficient, the greater the change in the index when the motor runs from cold to hot. It directly limits the use of the motor temperature range and indirectly affects the power volume ratio of the motor.
5.intrinsic coercivity
This is the value of the magnetic field strength when the residual magnetizing strength (M) drops to zero. The value of magnetic induction coercivity at B=0 on the demagnetization curve only indicates that the permanent magnet is not able to provide energy to the external magnetic circuit at this time, it does not mean that the permanent magnet does not have its own energy. However, the endowed coercivity value when M=0 indicates that the permanent magnet is truly demagnetized and has no magnetic energy storage. Although the intrinsic coercivity is not directly related to the working point of the motor, it is the real coercivity of the permanent magnet material, which represents that the permanent magnet material has the magnetic field energy and the ability to resist demagnetization. The magnitude of the intrinsic coercivity is closely related to the temperature stability of the PM material. The higher the intrinsic coercivity, the higher the working temperature of the permanent magnet material can be.
