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Basic Knowledge

Magnetic Hysteresis Curve

The magnetic hysteresis curve is the curve that shows the relationship between the magnetic field strength, and B, the magnetic flux density of the material magnetized by the magnetic field or J, the magnetization. Here, the residual magnetic flux density (Br) and coercivity (Hc) are explained by using the drawings.

1. Residual Magnetic Flux Density (Br)

When magnetizing a magnet, by increasing the electrical current of the magnetization coil gradually, making the magnetic field stronger, and magnetizing the magnet, the magnetic flux density in the magnet increases accordingly and saturates at a certain point.
Next, decreasing the magnetic field strength by reducing the electrical current from the saturation status, the magnetic flux density does not return to 0 but decreases along with a to b. The magnetic flux density remains at the value of b even if the magnetic field strength becomes 0, and the value is called the residual magnetic flux density (Br).

2. Coercivity or Coercive Force(Hc)

By reversing the electrical current’s direction and increasing the magnetic field in the other direction, the magnetic flux density reduces gradually from Point b and 0 at Point c. This magnetic field strength is called coercivity or coercive force (Hc). By increasing the opposite magnetic field more, the magnet is magnetized in the other direction and to the saturated state at Point d. By changing the magnetic field gradually, the magnetic flux density of the magnet changes according to the regular sequence of a → b → c → d → e → f → a. This curve is called the magnetic hysteresis loop.

3. Difference between Hysteresis Characteristics of Soft Ferrite and Hard Ferrite

Hard ferrite has coercivity more than soft ferrite, so it is more suitable for using as a magnet, whereas soft ferrite is more often used as a core for anti-noise measures or as magnetic heads.

4. Maximum Energy Product (BHmax)

It is one of the four characteristic values of a magnet ― residual magnetic flux density Br, coercivityHc, maximum energy product (BH) max, recoil rate μr―, and a measuring rule of the magnet’s strength. This means the maximum value of the product of the magnetic flux density (Br) and magnetic field strength (H) at a point of the second quadrant (demagnetization curve) of the hysteresis group. The maximum energyproduct increases and the magnet becomes stronger as the residual magnetic flux density and coercivity increase and the hysteresis group is hornier.

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