1. Magnetism
Experiments show that any substance can be magnetized to some degree in an external magnetic field, only the degree of magnetization differs. Based on the properties exhibited by substances in an external magnetic field, substances can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances. We call paramagnetic and diamagnetic substances weakly magnetic substances, and ferromagnetic and ferrimagnetic substances strongly magnetic substances.
2. Magnetic Materials
Soft magnetic materials: These materials can achieve the maximum magnetization intensity with the minimum external magnetic field. They are magnetic materials with low coercivity and high permeability. Soft magnetic materials are easy to magnetize and also easy to demagnetize. Examples include soft magnetic ferrites and amorphous nanocrystalline alloys.
Hard magnetic materials: Also called permanent magnet materials, these are materials that are difficult to magnetize and difficult to demagnetize once magnetized. Their main characteristic is high coercivity. Examples include rare-earth permanent magnet materials, metallic permanent magnet materials, and permanent magnet ferrites.
Functional magnetic materials: These mainly include magnetostrictive materials, magnetic recording materials, magnetoresistive materials, magnetic bubble materials, magneto-optical materials, and magnetic thin film materials.
3. Neodymium Iron Boron (NdFeB) Permanent Magnet Materials
Sintered NdFeB permanent magnet materials are produced using powder metallurgy. The smelted alloy is powdered and pressed into a compact in a magnetic field. The compact is then sintered in an inert gas or vacuum to achieve densification. To improve the coercivity of the magnet, aging heat treatment is usually required, followed by post-processing and surface treatment to obtain the finished product.
Bonded NdFeB is produced by mixing permanent magnet powder with flexible rubber or hard, lightweight plastics and other bonding materials, and directly molding it into various shapes of permanent magnet components according to user requirements.
Hot-pressed NdFeB can achieve magnetic properties similar to sintered NdFeB without adding heavy rare earth elements. It has advantages such as high density, high orientation, good corrosion resistance, high coercivity, and near-net-shape forming. However, its mechanical properties are inferior, and due to patent monopolies, the processing cost is high. 4. Remanence (Br)
This refers to the magnetic flux density exhibited by a sintered NdFeB magnet after it has been magnetized to technical saturation in a closed environment and the magnetic field has been removed. In simpler terms, it can be understood as the magnetic force of the magnet after magnetization. The unit is Tesla (T) and Gauss (Gs), where 1 Gs = 0.0001 T.
5. Coercivity (Hcb)
The strength of the reverse magnetic field required to reduce the magnetic flux density of a magnet to zero when it is magnetized in the reverse direction is called the magnetic coercivity. However, the magnetization of the magnet is not zero at this point; the applied reverse magnetic field cancels out the magnetization. If the external magnetic field is removed, the magnet still retains some magnetic properties. 1 A/m = (4π/1000)Oe, 1 Oe = (1000/4π) A/m.
6. Intrinsic Coercivity (Hcj)
The strength of the reverse magnetic field required to reduce the magnetization of a magnet to zero is called its intrinsic coercivity. Magnetic material grades are classified according to the magnitude of their intrinsic coercivity: Low coercivity (N), Medium coercivity (M), High coercivity (H), Extra-high coercivity (UH), Extremely high coercivity (EH), and Highest coercivity (TH).
7. Maximum Energy Product (BH)max
This represents the magnetic energy density established between the two magnetic poles of a magnet, i.e., the static magnetic energy per unit volume of the air gap. It is the maximum value of the product of B and H, and its magnitude directly indicates the performance level of the magnet. Under the same conditions—that is, the same size, the same number of poles, and the same magnetizing voltage—magnetic components with higher magnetic energy products (BH) will have a higher surface magnetic field. However, at the same (BH)max value, the levels of Br and Hcj have the following effects on magnetization:
Higher Br, lower Hcj: Under the same magnetizing voltage, a higher surface magnetic field can be obtained.
Low Br, higher Hcj: To obtain the same surface magnetic field, a higher magnetizing voltage is required.
8. SI and CGS Systems
These are the International System of Units (SI) and the Gaussian System of Units (CGS), much like the difference between “meter” and “mile” in length units. There are complex conversion relationships between the SI and CGS systems.
9. Curie Temperature
This is the temperature at which a magnetic material changes between ferromagnetic and paramagnetic. Below the Curie temperature, the material becomes ferromagnetic, and the magnetic field associated with the material is difficult to change. Above the Curie temperature, the material becomes paramagnetic, and the magnetic field of the magnet changes easily with changes in the surrounding magnetic field.
The Curie temperature represents the theoretical operating temperature limit of magnetic materials. The Curie temperature for neodymium iron boron (NdFeB) is approximately 320-380 degrees Celsius. The Curie temperature is related to the crystal structure formed during magnet sintering. If the temperature reaches the Curie temperature, the molecules within the magnet undergo violent movement and demagnetization, which is irreversible. After demagnetization, the magnet can be remagnetized, but the magnetic force will decrease significantly, reaching only about 50% of its original strength.
10. Operating Temperature
The maximum operating temperature of sintered NdFeB is much lower than its Curie temperature. Within the operating temperature range, the magnetic force decreases with increasing temperature, but it largely recovers upon cooling.
Relationship between operating temperature and Curie temperature: The higher the Curie temperature, the higher the operating temperature of the magnetic material, and the better its temperature stability. Adding elements such as cobalt, terbium, and dysprosium to the raw materials of sintered NdFeB can increase its Curie temperature. Therefore, high coercivity products (H, SH, etc.) commonly contain added dysprosium.
The maximum operating temperature of sintered NdFeB magnets depends on their inherent magnetic properties and the selection of their operating point. For the same sintered NdFeB magnet, the more closed the working magnetic circuit, the higher the maximum operating temperature and the more stable the magnet’s performance. Therefore, the maximum operating temperature of a magnet is not a fixed value, but varies with the degree of closure of the magnetic circuit.
11. Magnetic Field Orientation
Magnetic materials are divided into two categories: isotropic magnets and anisotropic magnets. Isotropic magnets have the same magnetic properties in all directions and can attract each other arbitrarily; anisotropic magnets have different magnetic properties in different directions, and the direction in which they obtain the best magnetic properties is called the magnet’s orientation direction.
A square sintered NdFeB magnet will have the strongest magnetic field in the orientation direction, while the magnetic field strength in the other two directions will be much weaker. If a magnetic material undergoes an orientation process during production, it is an anisotropic magnet. Sintered NdFeB magnets are generally shaped and pressed using magnetic field orientation, making them anisotropic. Therefore, the orientation direction, i.e., the direction of magnetization, needs to be determined before production. Powder magnetic field orientation is one of the key technologies for manufacturing high-performance NdFeB magnets. (Bonded NdFeB magnets can be isotropic or anisotropic.)
12. Surface Magnetism
This refers to the magnetic flux density at a point on the surface of a magnet (the surface magnetism at the center and edges of the magnet is different). It is a value measured by a gaussmeter in contact with a specific surface of the magnet, and does not represent the overall magnetic properties of the magnet.
13. Magnetic Flux
In a uniform magnetic field with magnetic flux density B, there exists a plane with area S perpendicular to the direction of the magnetic field. The product of the magnetic flux density B and the area S is called the magnetic flux through this plane, abbreviated as magnetic flux, symbol “Φ”, and the unit is Weber (Wb). Magnetic flux is a physical quantity that represents the distribution of a magnetic field. It is a scalar quantity, but it can be positive or negative, with the sign only indicating its direction. Φ = B·S. When there is an angle θ between the perpendicular planes S and B, Φ = B·S·cosθ.
14. Electroplating
Sintered NdFeB permanent magnets are produced using powder metallurgy, a highly chemically active powder material with micropores and voids. They are easily corroded and oxidized in air, requiring strict surface treatment before use. Electroplating, as a mature metal surface treatment method, is widely used.
The most common plating for NdFeB strong magnets is zinc plating and nickel plating. They differ significantly in appearance, corrosion resistance, lifespan, and price:
Polishing Difference: Nickel plating is superior to zinc plating in polishing, resulting in a brighter appearance. Products with high aesthetic requirements generally choose nickel plating, while those with less stringent aesthetic requirements, such as non-exposed magnets, are typically zinc-plated.
Corrosion Resistance Difference: Zinc is a reactive metal that reacts with acids, resulting in poor corrosion resistance. Nickel plating, after surface treatment, offers significantly higher corrosion resistance.
Service Life Difference: Due to differences in corrosion resistance, galvanized magnets have a shorter service life than nickel-plated magnets. This is mainly because the surface coating is more prone to peeling off over time, leading to magnet oxidation and affecting magnetic properties.
Hardness Difference: Nickel-plated magnets have higher hardness than galvanized magnets. During use, they can greatly reduce the risk of chipping or cracking from impacts.
Price Difference: Galvanized magnets have a significant advantage in this aspect. Prices, from lowest to highest, are: galvanized, nickel-plated, and epoxy resin.
15. Single-Sided Magnets
Magnets have two poles, but in some applications, single-sided magnets are required. Therefore, one side of the magnet is covered with an iron sheet to shield its magnetism. Such magnets are collectively called single-sided magnets. There is no such thing as a truly single-sided magnet.