Hysteresis Curve and Intrinsic Demagnetization Curve (B-H Curve and J-H Curve)

Permanent magnet materials have two notable features, one is that they can be strongly magnetized under the action of an external magnetic field, and the other is hysteresis, that is, the material remains magnetized after the external magnetic field is removed. The relationship between the change of the external magnetic field and the magnetic change of the permanent magnet can be described by two curves, namely the hysteresis loop (B-H curve) and the intrinsic demagnetization curve (J-H curve).

Magnetic induction B: When a ferromagnetic or ferrimagnetic substance is magnetized in a magnetic field, in addition to the magnetic field in the substance, because the magnetic moments of the atoms of the ferromagnetic substance turn to the direction of the external magnetic field, it generates an additional magnetic field M inside the substance. The sum of the external magnetic field and the additional magnetic field at this time is called the magnetic induction B. B=μ0(H+M)
Magnetic polarization: The magnetic dipole moment of the magnetic medium per unit volume, also known as the intrinsic magnetic induction, the symbol is Bi or J. J=μ0M

hysteresis curve
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). Hysteresis Curve and Intrinsic Demagnetization Curve (B-H Curve and J-H Curve)

1. Initial magnetization curve

The origin 0 in the figure means that the hard magnetic substance is in a magnetically neutral state before magnetization, that is, B=H=0. When the magnetic field H increases from zero, the magnetic induction B increases slowly, as shown by the line segment oa, followed by B With the rapid increase of H, as shown in ab, the growth of B tends to slow down, and when H increases to Hs, B reaches the saturation value Bs. This red curve is called the initial magnetization curve.

2. Hysteresis

When the magnetic field gradually decreases from Hs to zero, the magnetic induction intensity B does not return to the “0” point along the initial magnetization curve, but decreases along another new curve sr. Comparing the line segments os and sr, it can be seen that H decreases B correspondingly It also decreases, but the change of B lags behind the change of H. This phenomenon is called hysteresis. The obvious feature of hysteresis is that when H=0, B is not zero, but the remanence Br is retained.

3. Demagnetization curve

When the magnetic field reverses gradually from O to -Hc, the magnetic induction intensity B drops to 0, indicating that to eliminate the remanence, a reverse magnetic field must be applied. Hc is called the coercive force, and its magnitude reflects the ability of the magnetic material to maintain the remanence state. capacity, the purple line is called the demagnetization curve.

Intrinsic demagnetization curve
When the change of the external magnetic field H causes the magnetic induction B of the magnet to change, the magnetic polarization J also changes. The J~H curve can be used to describe the relationship between the two. It is a curve that reflects the intrinsic magnetic properties of the permanent magnet material, called is the intrinsic demagnetization curve, referred to as the intrinsic curve. When the magnetic polarization J on the intrinsic demagnetization curve is 0, the corresponding magnetic field strength is called the intrinsic coercive force Hcj.

Usually we see the magnet demagnetization curve provided by the magnetic material manufacturer, which is the situation of J~H curve and B~H curve in the second quadrant of the coordinate system.

1. Knee point Hk

It is not difficult to find from the figure that when the reverse external magnetic field increases continuously, the magnetic polarization of the magnet decreases very slowly, but when the external magnetic field is greater than a certain value, the magnetic polarization decreases rapidly. Usually we will demagnetize the curve The point above Ji=0.9Br is called the bending point or knee point of the demagnetization curve, and the corresponding magnetic field at this point is Hk, also known as the coercive force of the knee point (knee). When the external magnetic field is greater than Hk, the performance of the magnet will be irreversibly lost, which is why the value of Hk is concerned.

2. Demagnetization curve squareness Q

We use the ratio of Hk to HcJ (Hk/HcJ) to represent 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 a square. Usually we think that square Products with a degree of Q>0.9 are considered 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 operating temperatures. It seems complicated, but the essence is to put multiple demagnetization curves and intrinsic curves on one graph.

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