Permanent magnetic materials have two distinct characteristics: one is that they can be strongly magnetized under the action of external magnetic field, the other is hysteresis, that is, they still remain magnetized after removing the external magnetic field. The relationship between the change of external magnetic field and the change of permanent magnet magnetism can be described by two curves, namely hysteresis loop (B-H curve) and intrinsic demagnetization curve (J-H curve).
Hysteresis Loop
When the magnetic field changes in the order of Hs → Hc → o → – Hc → – Hs → – Hc → o → Hc → Hs, the corresponding magnetic induction B changes along the closed curve s-hcs’HCS, which is called B~H curve (hysteresis loop).
1. Initial magnetization curve
The origin 0 in the figure indicates that the hard magnetic material is in the magnetic neutral state before magnetization, that is, B = H = 0. When the magnetic field H increases from zero, the magnetic induction B increases slowly, then B increases rapidly with H, and then B increases slowly. When h increases to HS, B reaches the saturation value BS. This dotted line curve is called the initial magnetization curve.
2. Hysteresis
When the magnetic field gradually decreases from HS to zero, the magnetic induction B does not return to the “0” point along the initial magnetization curve, but decreases along another new curve ab. comparing lines oa and ab, we can see that h decreases and b decreases correspondingly, but the change of B lags behind the change of H. this phenomenon is called hysteresis. The obvious characteristic of hysteresis is that when h = 0, B is not zero, but retains remanence br.
3. Demagnetization curve
When the reverse magnetic field gradually changes from O to -Hc, the magnetic induction B decreases to 0, which indicates that in order to eliminate the remanence, the reverse magnetic field must be applied. HC is called coercivity, which reflects the ability of the magnetic material to maintain the remanence state, and the line BC is called demagnetization curve.
Intrinsic demagnetization curve
When the change of external magnetic field H causes the change of magnetic induction B, the magnetic polarization J also changes. We can use the J~H curve to describe the relationship between them. It is the curve reflecting the intrinsic magnetic properties of permanent magnet materials, which is called intrinsic demagnetization curve, referred to as intrinsic curve. When the magnetic polarization J on the intrinsic demagnetization curve is 0, the corresponding magnetic field strength is called intrinsic coercivity Hcj.
1. Knee point Hk
From the figure, it is not difficult to find that when the anti external magnetic field increases, the magnetic polarization of the magnet decreases very slowly, but when the external magnetic field is greater than a certain value, the magnetic polarization will decrease rapidly. Generally, we call the point Ji = 0.9Br on the demagnetization curve as the bending point or knee point of the demagnetization curve,and the corresponding magnetic field is HK, also known as the knee coercivity. When the external magnetic field is greater than HK, the performance of the magnet will be irreversibly lost, which is also the reason why the value of HK is concerned.
2. Demagnetization curve squareness Q
We use the ratio of HK and Hcj (Hk/Hcj) to express the squareness Q of the demagnetization curve. The value range of Q is between 0 and 1. The closer q is to 1, the closer the demagnetization curve is to square. Generally, we think that the products with squareness Q>0.9 are qualified products.
3. Demagnetization curves at different temperatures
Generally speaking, permanent magnet material manufacturers will provide demagnetization curves of various brands of products at different temperatures, as shown in the figure below. It seems complicated, but the essence is to present multiple demagnetization curves and intrinsic curves on one graph.
