Can magnetic energy be calculated? The relationship between surface magnetism and magnetic properties
The concept and measurement of surface magnetism
Surface magnetic field, also known as surface magnetic field, refers to the magnetic induction intensity at a certain point on the surface of a magnet, measured in Gauss Gs or Tesla T (1T=10000Gs). Magnetic field is the most easily measured parameter in daily life. When the size of the magnet is fixed, people often judge and compare the performance of the magnet by comparing the magnetic field. For some magnets with particularly large or small sizes and special shapes that are not suitable for routine measurements, measuring the magnetic field of the meter becomes very important. Regarding magnetic field measurement, the following two points are essential for everyone to know: ■ Magnetic field measurement is the value obtained when a Gaussian meter comes into contact with a certain point on the surface of a magnet. It is a reflection of the magnet itself on the measuring tool and does not represent the overall performance of the magnet. The surface magnetic field of a magnet varies at different positions. For magnets with regular shapes and non multipole magnetization, the central surface magnetic field is generally measured. The magnetic field of the watch is easily affected by the external environment. Using Gaussian meters from different manufacturers on the same magnet may result in different center magnetic field measurements; The measured surface magnetism of the same magnet may vary in different environments. The center magnetic field of the N and S poles of the same magnet is also different. From the above two points, it can be seen that the measurement of magnetic field is not objective and cannot fully reflect the performance of the magnet. It is not recommended to use it as an evaluation indicator for product transactions.
What is the relationship between surface magnetism and magnetic performance parameters (such as remanence Br, coercivity Hc, and maximum magnetic energy product (BH) max)? Can you find a mathematical calculation formula? These two questions are frequently raised by readers. The answer to the first question is yes, but in the past, some people only conducted empirical statistics. For example, for a sintered neodymium iron boron cylinder with a length to diameter ratio of 1, the residual magnetic Br is 2-3 times its surface field. However, this statement cannot strictly establish a quantitative relationship. The relationship between surface magnetism and residual magnetism. Residual magnetism refers to the magnetic induction intensity retained in a ferromagnetic material when it is magnetized to saturation by an external magnetic field and gradually reduced to zero. Its full name is residual magnetic induction intensity (Br). The remanence is determined by the characteristics of the magnet itself, and under specific conditions, the remanence of the same magnet remains constant and has a single value. The remanence to some extent determines the surface magnetism of a magnet, but it is not necessarily the same for magnets with the same remanence. The surface magnetism is also influenced by the shape, size, and magnetization method of the magnet. Two magnets with identical shape, performance, and size, the one with higher residual magnetism has stronger surface magnetism. Two magnets with different shapes, properties, or sizes cannot simply determine the magnitude of residual magnetism based on the height of the magnetic field
The surface magnetism of magnets is always lower than the residual magnetism. The residual magnetism is tested in a closed circuit state, while the surface magnetism is tested using a Gaussian meter in an open circuit state. At the same time, the magnet itself has a demagnetization field, so the maximum surface magnetism of a single magnet is much smaller than its residual magnetism. At present, the maximum remanence of sintered neodymium iron boron magnets is around 14000Gs, so we can say with certainty that the maximum surface magnetization of a single neodymium iron boron magnet cannot exceed 14000Gs. (Note that it is a "single magnet". In some magnetic components and magnet arrays, special magnetic circuit designs can be used to improve the surface magnetism of the magnet.)
Due to the extremely high magnetic anisotropy field of sintered neodymium iron boron magnets, the magnetization vectors are arranged in the easy magnetization direction. Therefore, we can consider it as a uniformly magnetized body, and use the current shell model to calculate the magnetic field generated by the magnet in space. At present, many magnetic calculation models online are based on this principle to derive calculation formulas, but there are two assumptions involved: one is that the magnet is a completely uniform magnetized body, and the other is that the demagnetization curve is completely straight. However, the actual situation is not completely true, so the calculation results may differ from the actual measurement results to some extent. In addition to remanence, the surface magnetism of a magnet is greatly affected by its shape and size, and the calculation formula for surface magnetism is different for magnets of different shapes.