| Term |
Description |
| Magnetic field |
Vector field of forces existing around a magnet or current. In Japan, different terms may be used depending on the science field, such as "magnetic field" in physics and "magnetic field" in engineering. These represent the same concept. The measurement unit for the intensity of magnetic field H is "ampere per meter [A/m]" in the SI system or "oersted [Oe]" in the cgs system. |
| Magnetization |
Phenomenon that a magnetic body takes on magnetism due to the effect of a magnetic field. An action to make a magnet material take on magnetism is expressed by a verb "magnetize". |
| Demagnetization |
An action to eliminate any magnetism is called demagnetization. To achieve demagnetization, a simple demagnetizer may be used. The principle of this device is to apply an alternate current magnetic field to the target and reduce the magnetism by gradually decreasing the alternate current magnetic field. For example, the degauss function used for a CRT (cathode ray tube) display demagnetizes the metal parts behind the screen such as the aperture grille. |
| Magnetic materials |
A substance that is magnetized under the influence of a magnetic field is called magnetic material. Among them, substances that can be magnetized more intensively are called ferromagnetic material. Iron, magnetite used for permanent magnets are typical ferromagnetic materials. A substance that cannot be magnetized under the influence of a magnetic field is called non-magnetic material. Typical non-magnetic materials include paper and plastic as well as some metals such as gold, silver, copper, aluminum, and magnesium.
To determine whether an object is a magnetic material or a non-magnetic material, bringing a magnet near it is one of the effective ways. If the magnet draws the object, the object is a magnetic material. Otherwise, it is a non-magnetic material. Some ferromagnetic materials keep the magnetism once magnetized, even after the magnetic field is removed. They are called hard magnetic material. On the contrary, others lose the magnetism as soon as the external magnetic field is removed. They are called soft magnetic material. Typical hard magnetic materials are ferrite and other materials used for permanent magnets. For the soft magnetic material, soft iron and permalloy are typical materials. The soft magnetic materials are used for transformer cores, magnetic yokes, and magnetic shields. |
| Magnetic line of force |
A magnetic field is a vector field with magnitude and direction. The directional components of the vector can be represented by virtual lines. These are the magnetic lines of force. The magnetic lines of force go out from the N pole of a magnet and return to the S pole. They never cross with each other. A smaller interval between these lines indicates a stronger magnetic force.
The magnetic force direction at a certain point can be checked visually by placing a compass at that point. Another way is to scatter iron sands around the magnet. This will indicate how the magnetic field deploys including rough directionality. |
| Magnetic flux density |
The magnetic flux represents a set of magnetic flux lines which are a virtual representation of a direction component of a magnetic field. The amount of magnetic flux per unit area is called magnetic flux density. It is used to measure the magnetic force (this is also a vector quantity: it has directionality in addition to intensity) at a certain point in a magnetic field. The measurement unit of magnetic flux density B is tesla [T] in the SI system or gauss [G] in the cgs system. |
| Hysteresis curve |
A curve indicating the relationship between an external magnetic field (H) and the magnetization (J) and magnetic flux density (B) of a material is called hysteresis curve (magnetization curve).Two types of hysteresis curves are available. One is for indicating the relationship between the magnetization and magnetic field of a material. The other is for indicating the relationship between the magnetic flux density and magnetic field. In a hysteresis curve chart, the 2nd quadrant is particularly called demagnetization curve, which is a representative curve indicating the material characteristics of a magnet. The magnetic flux density is a total value consisting of the magnetization of a magnet and any external magnetic field. It is represented by the following formula.
where, B is magnetic flux density, H is magnetic field, J is magnetization, and µ0 is magnetic permeability of vacuum. |
| Maximum energy product |
One of the parameters representing the magnetic intensity of a magnet. In a demagnetization curve (2nd quadrant of hysteresis curve), the area (B x H) of a square formed by an operating point on the B-H curve and the origin point as the opposite angle is called energy product. The maximum value of it is called maximum energy product. |
| Residual magnetic flux density |
When a material is exposed to such a maximum level of magnetic field as "any further intensity of magnetic field will not effect more magnetization" (saturated magnetization) and then completely released from the magnetic field, the material holds a certain level of magnetic flux density. This is called residual magnetic flux density.
The residual magnetic flux density (Br) is dependent on the material. It is one of the parameters to measure the magnetic intensity inherent to a material. Because it is not affected by the shape of a material, it is often used in a simulation. |
| Surface magnetic flux density |
This represents the magnetic flux density on an extreme surface of a magnet. Though it uses the term "surface", the actual value is taken at a point slightly distant from the very surface because a measurement sensor has a certain thickness.
This property is affected by the shape of a magnet and the measurement point cannot be limited. Therefore, it is not used for simulation in general. It is useful when comparing magnets in a test. |
| Coercive force |
For a material magnetized by the saturation magnetic field, the magnetic force can be reduced to zero by applying a magnetic field in the reverse direction of the magnetization. This reverse magnetic force is called coercive force. The coercive force is represented by Hcj (or jHc) if expressed in magnetic field or by Hcb (or bHc) if expressed in magnetic flux density.
The coercive force indicates how a magnet maintains its magnetic force as suggested by the name. Along with the residual magnetic flux density, it is an important parameter indicating the characteristics of a mixture material. Even if the residual magnetic flux density is large, a material with a small coercive force is not considered to be useful because it significantly depends on the shape and temperature. |
| Permeance coefficient |
This is a parameter determining the behavior of a magnet. It depends on the shape.
A magnet also has a magnetic field inside. At the same time, it always generates a magnetic field in the reverse direction (demagnetizing field). The demagnetizing field is a factor weakening the magnet. It depends on the shape. The permeance coefficient is defined by the magnetic flux density for the above demagnetizing field. In general, as the distance in the magnetization direction (thickness) decreases relative to the surface area of a pole, the demagnetizing field becomes larger, the permeance coefficient becomes smaller, and the magnetic force weakens.
A line graph indicating the permeance coefficient according to the B-H curve is called permeance line. It is a clue to examine the behavior of a magnet. As the permeance coefficient becomes smaller, the inclination of the permeance line is reduced and the line approaches a characteristic inflexion point of the B-H curve. This means that the effect to the temperature characteristics is increased.
For further details, consult each magnet manufacturer. |
| Curie temperature |
The temperature at which a magnet changes its crystal structure and loses the magnetic force completely due to temperature increase is called Curie temperature. Once reaching the Curie temperature, the magnet cannot restore the magnetic force even after the temperature is reduced to an ordinary level. The guidelines of Curie temperatures and usable (restorable) temperatures for typical magnet materials are indicated below.
| Material of magnet |
Curie temperature |
Usable temperature |
| Al-Ni-Co |
Approx. 850℃ |
400 to 450℃ |
| Ferrite |
Approx. 450℃ |
200 to 250℃ |
| Neodymium |
Approx. 300℃ |
80 to 150℃ |
| Samarium, cobalt |
Approx. 730℃ |
200 to 250℃ |
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