Squareness of NdFeB Magnets and Its Impact on Magnet Performance

The squareness of an NdFeB magnet refers to the shape of its demagnetization curve. In an ideal situation, the demagnetization curve of a permanent magnet is a straight line parallel to the B-axis (magnetic induction axis) until a certain critical point. The closer the demagnetization curve is to a perfect rectangle (high squareness), the better the magnetic stability of the magnet under external magnetic fields.

Mathematically, the squareness factor (S) can be expressed as the ratio of the remanence (Br) to the coercivity (Hcj) in the second quadrant of the hysteresis loop. A higher value of S indicates better squareness.

Impact on Magnet Performance

(1) Magnetic Stability

Magnets with high squareness have better resistance to demagnetization. When exposed to external magnetic fields, such as those generated by other magnets or electrical currents, they can maintain their magnetic properties more effectively. For example, in a motor application, a highly squareness NdFeB magnet can ensure stable torque output even when the motor is subjected to variable magnetic fields during operation.

In magnetic sensors, a magnet with good squareness helps to provide a more accurate and stable magnetic field signal, enabling more precise detection of magnetic field changes.

(2) Energy Product

The energy product (BH)max of a magnet is related to its square - ness. A more rectangular demagnetization curve usually means a higher energy product. This is because a high - squareness magnet can make better use of the magnetic energy stored in it. In applications such as magnetic separators, a high energy product magnet can provide stronger magnetic forces to separate magnetic materials more efficiently.

3. Requirements for Squareness in Different Application Fields

(1) Electrical Motors and Generators

In high - performance electric motors (such as those used in electric vehicles and industrial servo motors), a high squareness of NdFeB magnets is required. The magnets need to maintain a stable magnetic field during the high - speed rotation of the rotor and under the influence of the armature magnetic field. A squareness factor of usually above 0.9 is preferred to ensure efficient energy conversion and stable torque output.

In generators, a similar requirement exists. The magnets should provide a stable magnetic field to ensure the stable generation of electricity, especially in variable speed generators where the magnetic field stability is crucial for the quality of power output.

(2) Magnetic Resonance Imaging (MRI)

In MRI equipment, the magnets need to have extremely high square - ness. The magnetic field generated by the NdFeB magnets must be highly stable and uniform. A squareness factor close to 1 is required to provide accurate and clear imaging results. Any deviation in the magnetic field due to poor squareness can lead to image distortion and inaccurate diagnosis.

(3) Consumer Electronics

In applications such as hard disk drives and mobile phone vibration motors, a relatively good squareness is needed. For hard disk drives, the magnets help to position the read-write head accurately. A squareness factor of around 0.8-0.9 is usually sufficient to meet the requirements of data storage and retrieval. In mobile phone vibration motors, the magnets with appropriate squareness can ensure stable vibration performance.

(4) Magnetic Separators

In magnetic separation equipment used in the mining and recycling industries, the squareness of NdFeB magnets affects the separation efficiency. A squareness factor of about 0.85-0.95 is often required, depending on the specific separation requirements. Higher squareness magnets can generate stronger magnetic gradients, enabling more efficient separation of magnetic minerals or metallic particles from non - magnetic substances.

The squareness of a NdFeB magnet can be improved through several methods:

1. Optimization of Manufacturing Processes

Powder Preparation:

The quality of the raw powder used to manufacture NdFeB magnets has a significant impact on their properties. By using more refined powder production techniques, such as jet milling, the particle size and shape distribution of the powder can be better controlled. For example, a more uniform particle size helps in achieving a more homogeneous microstructure during the sintering process, which is beneficial for improving the squareness of the demagnetization curve.

The purity of the powder is also crucial. High purity NdFeB powder with fewer impurities can lead to better magnetic properties. Impurities can disrupt the alignment of magnetic domains and deteriorate the squareness.

Sintering and Annealing:

Sintering is a key step in the production of NdFeB magnets. Appropriate sintering temperature, time, and atmosphere can influence the microstructure and magnetic properties. By precisely controlling the sintering parameters, the density and grain structure of the magnet can be optimized. For instance, a well-controlled sintering process can result in a more compact and uniform microstructure, reducing the probability of magnetic domain wall movement and thus improving the squareness.

Annealing after sintering is another important process. Annealing in a specific temperature range and magnetic field environment can help to relieve internal stresses in the magnet and improve the alignment of magnetic domains. This process can enhance the magnetic stability and squareness of the magnet.

2. Magnetic Field Alignment

During the manufacturing process, applying a stronger and more uniform magnetic field during the orientation of magnetic powder can improve the alignment of magnetic domains. A better-aligned domain structure is more likely to exhibit a higher squareness of the demagnetization curve. For example, in the production of anisotropic NdFeB magnets, a high-intensity pulsed magnetic field can be used to align the magnetic powder particles along the direction of the magnetic field more effectively, resulting in a magnet with improved squareness.

3. Coating and Surface Treatment

The coating on NdFeB magnets not only provides corrosion protection

but can also have an impact on the magnetic properties. Some coatings can help to reduce the influence of external factors on the magnetic domains near the surface. For example, a thin layer of a non-magnetic and insulating coating can prevent the penetration of external magnetic fields or electric currents that might otherwise cause demagnetization and affect the squareness. Additionally, surface treatment methods such as chemical polishing can improve the surface quality and reduce surface defects, which is beneficial for the overall magnetic performance and squareness of the magnet.