Experiments demonstrate that any substance, when placed in an external magnetic field, can be magnetized to some extent—though the degree of magnetization varies. Based on the characteristics exhibited by substances within an external magnetic field, they can be classified into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances. We collectively refer to paramagnetic and diamagnetic substances as "weakly magnetic materials," while ferromagnetic and ferrimagnetic substances are termed "strongly magnetic materials."
Soft Magnetic Materials: These are magnetic materials characterized by low coercivity and high permeability, capable of achieving maximum magnetization intensity with minimal application of an external magnetic field. Soft magnetic materials are easily magnetized and, conversely, easily demagnetized. Examples include soft ferrites and amorphous/nanocrystalline alloys.
Hard Magnetic Materials: Also known as permanent magnet materials, these refer to substances that are difficult to magnetize and, once magnetized, are equally difficult to demagnetize. Their primary characteristic is high coercivity. This category encompasses rare-earth permanent magnet materials, metallic permanent magnet materials, and permanent magnet ferrites.
Functional Magnetic Materials: This category primarily includes magnetostrictive materials, magnetic recording materials, magnetoresistive materials, magnetic bubble materials, magneto-optical materials, and magnetic thin-film materials, among others.
Sintered NdFeB permanent magnets are produced using a powder metallurgy process. Following smelting, the alloy is pulverized and pressed into a compact (green body) within a magnetic field. This compact is then sintered in an inert gas atmosphere or under vacuum to achieve densification. To enhance the magnet's coercivity, an aging heat treatment is typically performed; finally, the finished product is obtained after subsequent post-processing and surface treatments.
Bonded NdFeB is produced by mixing permanent magnet powder with a binding agent—such as highly flexible rubber or rigid, lightweight plastics—and directly molding the mixture into various permanent magnet components according to user specifications.
Hot-pressed NdFeB can achieve magnetic properties comparable to those of sintered NdFeB without the addition of heavy rare-earth elements. It offers several advantages, including high density, high orientation, excellent corrosion resistance, high coercivity, and near-net-shape formability. However, it suffers from relatively poor mechanical properties, and due to patent monopolies, its processing costs remain comparatively high. 4. Residual Induction (Br)
This refers to the magnetic induction density exhibited by a sintered NdFeB magnet after it has been magnetized to technical saturation within a closed-circuit environment and the external magnetic field is subsequently removed. Simply put, it can be understood as the magnetic strength of the magnet following magnetization. The units are Tesla (T) and Gauss (Gs), where 1 Gs = 0.0001 T.
When a magnet is subjected to a reverse magnetic field, the value of the reverse field strength required to reduce its magnetic induction density to zero is termed the magnetic induction coercivity. However, at this point, the magnet's intrinsic magnetization is not zero; rather, the applied reverse field merely counteracts the magnet's intrinsic magnetization. Consequently, if the external reverse field were removed at this stage, the magnet would still retain a certain degree of magnetic properties. 1 A/m = (4π/1000) Oe; 1 Oe = (1000/4π) A/m.
The strength of the reverse magnetic field required to reduce a magnet's intrinsic magnetization to zero is termed the intrinsic coercivity. Magnetic material grades are classified based on the magnitude of their intrinsic coercivity: Low Coercivity (N), Medium Coercivity (M), High Coercivity (H), Extra-High Coercivity (UH), Ultra-High Coercivity (EH), and Supreme Coercivity (TH).
This represents the magnetic energy density established within the space between a magnet's two poles—specifically, the magnetostatic energy per unit volume of the air gap. It corresponds to the maximum value of the product of B (magnetic induction) and H (magnetic field strength), and its magnitude serves as a direct indicator of the magnet's overall performance level. Under identical conditions—specifically, identical dimensions, pole counts, and magnetization voltages—magnetic components with a higher energy product will yield a higher surface magnetic field strength. However, when comparing magnets with the *same* (BH)max value, the relative levels of Br and Hcj have the following implications regarding magnetization:
High Br, Low Hcj: Under the same magnetization voltage, a higher surface magnetic field strength can be achieved.
Low Br, High Hcj: To achieve the same surface magnetic field strength, a higher magnetization voltage is required.
These refer to the International System of Units and the Gaussian System of Units, respectively—much like the difference between the units of length "meter" and "li." A certain complex set of conversion relationships exists between the SI and CGS systems.
This is the specific temperature at which a magnetic material transitions between a ferromagnetic state and a paramagnetic state. Below the Curie temperature, the substance acts as a ferromagnet; in this state, the intrinsic magnetic field associated with the material is highly resistant to external change. When the temperature rises above the Curie temperature, the substance becomes a paramagnet; in this state, the magnet's magnetic field changes readily in response to changes in the surrounding external magnetic field.
The Curie temperature represents the theoretical upper limit for a magnetic material's operating temperature. For Neodymium-Iron-Boron (NdFeB) magnets, the Curie temperature typically ranges from approximately 320°C to 380°C. The specific level of the Curie point is directly related to the crystal structure formed during the sintering process of the magnet. If the temperature reaches the Curie temperature, the internal molecules of the magnet undergo violent thermal agitation, resulting in demagnetization—a process that is irreversible. While a demagnetized magnet can subsequently be remagnetized, its magnetic strength will be significantly diminished, typically recovering to only about 50% of its original capacity.
The maximum operating temperature for sintered NdFeB magnets is significantly lower than their Curie temperature. Within this operating range, an increase in temperature causes a decrease in magnetic strength; however, upon cooling, the majority of this magnetic strength is recovered.
Relationship between Operating Temperature and Curie Temperature: Generally, the higher the Curie temperature, the higher the relative operating temperature limit for the magnetic material, and the greater its thermal stability. The addition of elements such as cobalt (Co), terbium (Tb), and dysprosium (Dy) to the raw materials of sintered NdFeB can effectively raise its Curie temperature; consequently, high-coercivity products (designated as H, SH, etc.) commonly contain added dysprosium.
The maximum usable temperature of a sintered NdFeB magnet depends on its inherent magnetic properties as well as the specific operating point selected within its magnetic circuit. For a given sintered NdFeB magnet, the more "closed" (or complete) the magnetic circuit in which it operates, the higher its maximum usable temperature becomes, and the more stable its performance remains. Therefore, the maximum usable temperature of a magnet is not a fixed, absolute value, but rather a variable that changes according to the degree of closure of the magnetic circuit in which it is deployed.
Magnetic materials are broadly classified into two categories: isotropic magnets and anisotropic magnets. Isotropic magnets exhibit identical magnetic properties in every direction and can be attracted to one another in any orientation. Anisotropic magnets, conversely, possess magnetic properties that vary depending on the direction; the specific direction in which the magnet achieves its optimal magnetic performance is referred to as its "orientation direction."
For a square sintered NdFeB magnet, the magnetic field strength is maximized exclusively along its orientation direction, while the field strengths in the other two directions are significantly lower. If a magnetic material undergoes an orientation process during its manufacturing—typically involving magnetic field orientation during the molding and pressing stages for sintered NdFeB—it is classified as an anisotropic magnet. Consequently, the orientation direction (which corresponds to the intended direction of future magnetization) must be established prior to production. Magnetic field orientation of the powder is one of the key technologies essential for manufacturing high-performance NdFeB magnets. (Note: Bonded NdFeB magnets can be either isotropic or anisotropic.)
This term refers to the magnetic induction intensity (magnetic flux density) at a specific point on the surface of a magnet (the surface flux density typically differs between the center and the edges of the magnet). It is a value measured by placing a Gaussmeter in direct contact with a particular surface of the magnet; it does not represent the overall magnetic properties of the magnet as a whole.
Consider a uniform magnetic field with a magnetic induction intensity of *B*, containing a planar surface of area *S* positioned perpendicular to the direction of the magnetic field. The product of the magnetic induction intensity *B* and the area *S* is defined as the magnetic flux passing through this plane (often simply referred to as "magnetic flux"). It is denoted by the symbol "Φ" and is measured in units of Webers (Wb). Magnetic flux is a physical quantity used to characterize the distribution of a magnetic field; although it is a scalar quantity, it can possess a positive or negative sign, where the sign merely indicates the direction of the flux. The formula is Φ = B·S; however, if the surface *S* is oriented at an angle *θ* relative to the plane perpendicular to *B*, the formula becomes Φ = B·S·cosθ.
Sintered NdFeB permanent magnets are manufactured using powder metallurgy techniques. As a powder-based material, it is highly chemically active and contains microscopic pores and voids within its structure. Consequently, it is susceptible to corrosion and oxidation when exposed to air. To mitigate these issues, the material must undergo rigorous surface treatment prior to use; electroplating, being a well-established method for treating metal surfaces, is widely employed for this purpose. The most commonly used surface coatings for powerful Neodymium-Iron-Boron (NdFeB) magnets are zinc plating and nickel plating. These two options exhibit distinct differences in terms of appearance, corrosion resistance, service life, and price.
**Differences in Polishability:** Nickel plating offers superior polishability compared to zinc plating, resulting in a brighter and more lustrous appearance. Applications with high aesthetic requirements typically opt for nickel plating, whereas magnets that are concealed—and thus have lower aesthetic requirements—are generally zinc-plated.
**Differences in Corrosion Resistance:** Zinc is a chemically active metal that reacts with acids; consequently, it offers relatively poor corrosion resistance. Nickel plating, on the other hand, provides a surface treatment that significantly enhances the magnet's resistance to corrosion.
**Differences in Service Life:** Due to the disparity in corrosion resistance, the service life of zinc-plated magnets is shorter than that of nickel-plated magnets. This is primarily manifested over time, as the zinc coating tends to peel off, exposing the underlying magnet to oxidation and thereby compromising its magnetic properties.
**Differences in Hardness:** Nickel plating is harder than zinc plating. During use, this superior hardness helps to significantly mitigate damage caused by impacts or collisions, thereby preventing issues such as chipped corners or fragmentation in the powerful NdFeB magnets.
**Differences in Price:** In this regard, zinc plating holds a distinct advantage. When ranked from lowest to highest cost, the pricing order is typically: zinc plating, nickel plating, epoxy resin coating, and so on.
All magnets inherently possess two poles. However, certain operational contexts require a magnet that exhibits magnetic polarity on only one side. To achieve this, a metal plate (typically iron) is used to encase one side of the magnet; this shielding effect suppresses the magnetic field on the covered side. Magnets configured in this manner are collectively referred to as "single-sided magnets." It is important to note that, in a strictly physical sense, a truly "single-sided magnet" does not exist.
1.1 How are Permanent Neodymium Magnets Produced
1.2 How to Clearly Describe a Magnet Procurement Requirement
1.3 The Grades and Magnetic Properties of Sintered Neodymium Magnets
1.4 The Common Processing Methods for Neodymium Magnets
1.5 Curie Temperature and Operating Temperature of Magnets
1.6 Orientation and Magnetization of NdFeB Magnets
2.1 Surface Magnetic Field Remanence and Magnetic Flux
2.2 How to Interpret Hysteresis Loops (Magnetization Curves, Demagnetization Curves, and Intrinsic Curves)
2.3 A Detailed Explanation of Remanence, Coercivity, Intrinsic Coercivity, and Maximum Energy Product via Hysteresis Loops
2.4 Surface Magnetism and Magnetic Flux
2.5 Conversion of Magnetic Units between the SI and CGS Systems
2.6 15 Explanations of Concepts Related to Magnetic Materials (Basic Edition
2.7 Key Terms Every Magnetics Professional Must Know
2.8 The Working Principle of the Hall Element
2.9 Principles and Methods of Demagnetization
2.10 Halbach Array
2.11 Annular Halbach Magnet Array
2.12 What is a magnetic moment What is the relationship between magnetic moment and magnetic flux
2.13 Magnetism and Magnetic Moment
2.14 What are the operating point and Pc value of a permanent magnet How is the Pc value calculated
2.15 Magnetic Moment and Magnetic Flux; Magnetic Moment and Remanence
2.16 Demagnetization Curve Squareness (Q) and Knee Point (Hk)
2.17 Temperature Stability of Permanent Magnets Temperature Coefficient of Residual Induction, Temperature Coefficient of Coercivity, Reversible Temperature Coefficient
2.18 Magnetic Field Strength H and Magnetic Induction B; Magnetization M and Magnetic Polarization J
2.19 What is a magnetic domain
2.20 Eddy Current Loss in Permanent Magnet Materials
2.21 Shape Anisotropy, Demagnetizing Field, and Demagnetizing Factor of Permanent Magnets
2.22 Common Magnetic Circuit Structures
2.23 Do magnets with identical performance and volume exert the same suction force
2.24 Calculation of Surface Magnetism
2.25 Finite Element Analysis (FEA) Simulation of Permanent Magnet Fields
2.26 How to measure the surface Gauss using a gaussmeter
2.27 Magnetic Attraction and Magnetic Devices
3.1 Classification of Magnetism and Magnetic Materials
