Magnetization Process and Magnetic Measurement of Permanent Magnets

Lichun Zhan 2024/11/13

Magnetization Process

When a permanent magnet is placed in an external magnetic field, an interesting process called magnetization takes place. As the magnetic field strength increases, the magnet's spontaneous magnetization direction gradually aligns with that of the external field. Through this process, the chaotic magnetic domains inside thermally demagnetized magnets become orderly, involving the displacement of the magnetic domain wall and the rotation of the magnetic domain.

Figure 1 shows the M-H corresponding curve of the magnetization process. As magnetization M changes with the applied magnetic field H, it eventually reaches saturation.

Figure 1: M-H corresponding curve of magnetization process

Permanent magnets can be either anisotropic or isotropic. Isotropic magnets can be saturated in any direction during the magnetization process. For anisotropic magnets, however, the orientation direction must be defined first. As shown in Figure 2, this orientation direction could be along the x, y, or z axis. During magnetization, this orientation direction of magnet should be parallel to the axis of the magnetizing coil.

Figure 2: Schematic diagram of permanent magnet

During the magnetization process, the capacitor of the magnetizing machine is initially charged with DC high voltage for energy storage. Subsequently, it is discharged through a coil with extremely low resistance. The discharge pulse current can attain an extremely high peak value of tens of thousands of amperes. This current pulse generates a strong magnetic field within the magnetizing fixture, enabling the permanent magnetization of the magnet placed inside.

The direction of the N/S pole produced by the magnetizing coil is dictated by the direction of the current. This can be ascertained by using the right hand spiral rule. Figure 3 shows the unipolar axial magnetizing coil and its morphology of Mag B and B_Vector.

Figure 3: Axial magnetizing coil of single pole and the morphology of Mag B and B_Vector.

The magnetization of magnetic materials is generally classified into unsaturated magnetization (less than 95% of the energy of saturated magnetization), saturated magnetization, and supersaturated magnetization. Saturated magnetization requires a peak field approximately 1.5 to 2.5 times the intrinsic coercive force (Hcj). For instance, if the intrinsic coercive force is 20KOe, the applied peak field would be around 30 to 50kOe. Supersaturated magnetization requires a peak field around 3 times the intrinsic coercive force(Hcj). However, a higher magnetic field isn't always better. An excessively high magnetizing field can result in wasted energy and increased cost. It can also shorten the service life of the magnetizing coil. Furthermore, the reverse magnetic field generated by coil eddy current can demagnetize a saturated magnet back to an unsaturated state, particularly for magnets with low Hcj, such as AlNiCo magnets.

Figure 4 shows typical magnetization patterns for permanent magnets, which include “axially magnetized”, “diametrically magnetized”, “axially multipole magnetized”, “skewed magnetized”, “diametrically multipole magnetized”, and “radially magnetized” and etc.

Multipole magnetization often requires specially customized magnetization coils.

Figure 4: Typical magnetization patterns

Magnetic Measurement

Magnetic measurement devices typically include Gauss meter, Flux meter, Magcheck and etc.

A Gauss meter measures the static or dynamic (AC) magnetic induction intensity of an object at a specific point in space. It generates current and voltage via a Hall sensor passing through the object's magnetic field line.As shown in Figure 5, the units are usually Gs (gauss), mT (millitesla), T (tesla). Since the Gauss meter detects the magnetic induction intensity at a specific point on the magnet's surface, the detection result is significantly influenced by the operator. During the measurement, it is necessary to ensure that the magnetic field line passes perpendicularly through the probe surface.

Figure 5: Gauss Meter,Hall Effect Diagram,and Probe placement diagram within the magnetic field

A flux meter is typically equipped with a Helmholtz coil and is capable of measuring magnetic flux and magnetic moment. (As shown in Figure 6.) The magnetic flux Φ in a surface of area A that is crossed by a perpendicular magnetic field of constant intensity B is defined as Φ = B × A. The magnetic moment M can be obtained through the relationship M = KH × Φ, where KH is the Helmholtz coil constant and Φ is the magnetic flux measured by the coil. The units are usually Wb.m ,Wb.cm,Vs.m, Vs.cm.

Figure 6: Flux meter and Helmholtz coil

The Magnetic angle deviation refers to the difference between the actual magnetic field direction and a reference direction.

To measure the magnetic angle deviation precisely, a three-axis Helmholtz coil (as shown in figure 7) is required to measure magnetic flux in three directions, Øx, Øy, and Øz. The magnetic moments in three directions, Mx, My, and Mz, can be calculated according to the corresponding constants of each coil. The total magnetic moment M of the permanent magnet can be obtained by taking the square root of the sum of squares of the magnetic moments of each component. Then magnetic angle deviation α of the permanent magnet can be calculated through the relationship cosαX = Mx/M, cosαY = My/M, and cosαZ = Mz/M

(See figure 7).

Figure 7: Magnetic angle deviation diagram and three-axis Helmholtz coil Helmholtz coil

In conclusion, understanding the magnetization process and magnetic measurement of permanent magnets is crucial for various applications. Contact us to discuss your requirements with our team to achieve optimal magnetic performance for your projects.

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