Magnetic materials occupy an important position in modern life and industry. For magnetic materials, we mainly utilize their magnetism and the magnetic field they generate for work. Based on the performance requirements and working environment of different equipment, we need to select the most suitable magnetic material. To achieve this goal, we first need to understand how to measure the performance level of magnets in order to make the best choice.
This article will focus on introducing the four basic performance indicators of magnetic materials: remanence (Br), coercivity (Hcb),intrinsic coercive force (Hcj),and maximum energy product (BHmax),
Remanence (remanent magnetization,residual magnetism) is the magnetization left behind in a magnetic material (such as iron) after an external magnetic field is removed.Colloquially, when a magnet is “magnetized”, it has remanence.
Its representative symbol is Br.The unit of Remanence in the International System of Units is Tesla (T), and the unit in the CGS Gaussian Unit System is Gauss (G), 1T = 10000G.
The limit value of remanence is called saturation remanence.After applying a huge magnetic field that saturates the remanence of the magnet, remove this external magnetic field. At this point, remanence is saturation remanence.Usually, we default that the remanence labeled on magnetic material products is saturation remanence.
The remanence of a magnet is mainly determined by its microstructure, and factors such as raw materials and processing technology can affect it. For example, in neodymium magnets, the addition of lanthanum (La) and cerium (Ce) can reduce the remanence of the magnet. But due to their very cheap prices, they can reduce the cost of raw materials. Increasing the proportion of neodymium (Nd) can significantly increase the remanence of magnets, but the cost of raw materials will increase significantly. In the manufacturing process, neodymium magnets reduce contact with oxygen and also increase the remanence of the magnets.
In engineering applications, remanence is also known as residual magnetization. Electromagnets, motors, and generators don’t want to have too much residual magnetism, as residual magnetism can bring undesirable results.For example, if an electromagnet has residual magnetism, it will still have magnetism when the coil is not conductive, which will affect the normal operation of the machine.Where it is unwanted, it can be removed by degaussing.
|Materials||Maximum saturation remanence|
|Ferrite magnet||0.35 T（3,500 G）|
|Samarium cobalt magnet||1.16T（11,600G）|
|Aluminum nickel cobalt||1.28 T（12,800G）|
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of magnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in oersted(Oe) or kiloampere/meter(KA/m) units and is denoted Hc or Hcb.
Under the action of an external magnetic field, the magnetic flux density of the magnet weakens with the increase of the external magnetic field.
Hcb: Coercive force is the applied magnetic field intensity corresponding to a magnetic flux density of 0.
But at this point, the magnetization of the magnet itself is not zero. If the external magnetic field is removed, the magnet still has a certain degree of magnetism.
|Samarium cobalt magnet||3200|
|Aluminum nickel cobalt||150|
3. Intrinsic coercivity(Hcj)
When the reverse magnetic field intensity (H) reaches the Hcb point, although the magnetic induction intensity of the magnet exhibits zero, the magnetic polarization intensity(J) is not zero at this time. When the reverse magnetic field (H) reaches the Hcj point, the remanence decreases to 0, and the magnet completely loses its magnetism.
Knee point (Hk): As the reverse magnetic field intensity continues to increase, the magnetic polarization intensity (J) of the magnet decreases very slowly. But when the reverse magnetic field exceeds a certain level, the magnetic polarization intensity will rapidly decrease.
Usually, we refer to the point on the intrinsic demagnetization curve with J=0.9Br as the knee point, which corresponds to a magnetic field of Hk. When the external magnetic field is greater than Hk, irreversible huge loss of magnetism will occur.
The squareness (Q) of the intrinsic demagnetization curve is also an important attribute of the magnets, which we obtained by calculating Q=Hk/HcJ. The closer Q is to 1, the closer the intrinsic demagnetization curve is to a square, indicating better performance of the magnet.
When discussing magnetic materials, we can divide them into two categories based on their intrinsic coercivity: soft magnetic and hard magnetic materials. This classification is based on whether the material is easily magnetized and demagnetized.
① Soft magnetic materials, such as soft magnetic ferrite and silicon steel, belong to this category of materials. They are usually used to make iron cores for transformers and motors. This is a type of magnetic material with low coercivity and high permeability, which means that the maximum magnetization can be achieved by applying the minimum external magnetic field. They are easily magnetized and also easily demagnetized.
② Hard magnetic materials, also known as permanent magnetic materials, have opposite characteristics to soft magnetic materials. Neodymium magnets, samarium cobalt magnets, and other typical hard magnetic materials. They are mainly used to make magnets or increase the stable magnetic field for motors. They are difficult to magnetize and once magnetized, they are difficult to demagnetize.
Learn more about soft and hard magnetic materials:
4.Maximum energy product (BHmax)
The maximum energy product is an important figure-of-merit for the strength of a permanent magnet material. It is the maximally attainable product of magnetic flux density (B) and magnetic field intensity (H) for a material.It is often denoted (BH)max and is typically given in units of either kJ/m3 (kilojoules per cubic meter) or MGOe (mega-gauss-oersted).1 MGOe is equivalent to 7.958 kJ/m3.
(BH) max is the product of B and H at the point with the highest intensity on the B-H curve.
(BH)max can be graphically defined as the area of the largest rectangle that can drawn in the second quadrant of the B-H loop.
Although humans were familiar with naturally occurring lodestone magnets as early as the sixth century BC, it was not until the development of the KS Steel magnet in 1917 that a magnet with an energy product of close to 1 MGOe (7.96 kJ/m3) was produced. Following this, however, improvements in permanent magnet properties was very rapid. The 20th century was a period of great permanent magnet innovation which included the development of alnico, sintered ferrite, Sm-Co and finally, NdFeB magnets, four of the most important discoveries in the history of this industry. Between 1930 and the early 1980s, energy product increased by a factor of 50 and intrinsic coercivity by a factor of 100.
As of this date, sintered NdFeB magnets remain the reigning permanent magnet champion with commercial products as high as 52 MGOe (414 kJ/m3).
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