Types of permanent magnet

Permanent magnets can largely be classified into the ferrite magnets, the samarium cobalt (Sm-Co) magnets, and the neodymium iron boron (Nd-Fe-B)magnets. Stronger magnetic flux density can be obtained with the latter type when the geometry is the same. The Sm-Co magnets are the most expensive, Nd-Fe-B magnets come next, and the ferrite magnets are relatively inexpensive. However, a feature of the Sm-Co magnets are that the thermal characteristics of the magnetic flux density are excellent. The characteristics of different magnets are usually shown on the websites of their respective manufacturers. Please refer to the relevant magnet manufacturer’s website for details and other references.

Magnet Residual magnetic flux density (Br:mT) Thermal coefficient of Br(%/°C) Cost Ferrite magnets 300 - 500 Approx. - 0.18% Low Neodymium iron boron magnets (Ne-Fe-B) 1,100 - 1,500 Approx. - 0.11% Normal Samarium cobalt magnets (Sm-Co) 1,000 - 1,200 Approx. - 0.03% High

<Sintered magnet>
A magnet manufactured by pressing magnetic powders into a shape and baked by intense heat. This type includes ferrite magnets, samarium cobalt magnets, and neodymium magnets. It is suitable to a standard shape such as cylinder and rectangular solid.

<Bond magnet>
A magnet manufactured by mixing magnetic powders with a base material such as rubber or plastic, and molding the mixture into a shape through a pressing or extrusion process. Though suitable to a special shape, its magnetic force is weaker than a sintered magnet because the amount of magnetic substance per unit volume is smaller.

<Others>
Other manufacturing methods include casting and forging. These are applied to the manufacturing of alloy magnets.

These properties represent the orientation of easy axis of magnetization.
A magnet made by mixing magnetic powders directly with a base material (e.g. plastic), such as a bond magnet, has no unified tendency in the easy axis of magnetization (orientation in material's crystalline organization that is easy to magnetized). Such a magnet is called isotropic magnet. An isotropic magnet can be magnetized in any orientation.
To the contrary, a magnet for which the easy axes of magnetization for magnetic powders are unified in the molding process is called anisotropic magnet. An anisotropic magnet can be provided with a highly intensive magnetic force by magnetizing it along the easy axis of magnetization. To unify the easy axes of magnetization, several methods are available, for example, using magnetic fields in the molding process and applying mechanical pressure.

Even a permanent magnet may degrade in the magnetic force due to temperature changes and aging. This phenomenon is called demagnetization. Among others, the irreversible temperature change as shown in item (2) below is critical because it causes unrecoverable magnetic force deterioration.

<(1) Reversible change>
This is a phenomenon that a magnet changes its magnetic characteristic according to the temperature coefficient inherent to the material when the temperature is changed and restores the original characteristic when the temperature is restored.

<(2) Irreversible change>
This is a phenomenon that a magnet changes its magnetic characteristic according to the demagnetization characteristic inherent to the material when the temperature is changed but cannot recover from the demagnetized status even when the temperature is restored. Some ferrite magnets have an irreversible change zone in a low temperature range and some neodymium magnets have an irreversible change zone in a high temperature range. The irreversible zone is affected by not only the material but also the permeance coefficient determined by the shape. For further details, consult each magnet manufacturer.

<(3) Aging>
All magnets, even permanent magnets, are demagnetized little by little due to the influence of thermal energy. The degree of demagnetization varies with the magnet material, permeance coefficient, and environmental temperature. Besides, physical stresses (e.g. distortion due to manufacturing) and chemical changes (including rust) contribute to the aging. Nowadays, these aging effects are controlled to a practically negligible level by adequate preventive measures including the progress of the manufacturing technologies.

A magnet exhibits different behaviors in the reversible zone and the irreversible zone.
The concept in the reversible zone is as follows. A magnet material has an inherent temperature coefficient, Kt, which determines the temperature characteristic. For example, assuming that the residual magnetic flux density at 20C Br(20) is 1200mT and Kt is -0.12%/C, this magnet exhibits the following residual magnetic flux density when placed in an environment at 60C.
When the temperature is returned to the original level, the magnet also recovers its original characteristic. Next, the irreversible change is discussed as below.

<High temperature demagnetization>
Generally a magnet has a negative temperature coefficient and its magnetic force weakens as the temperature rises. Note that, however, some rare-earth magnets including neodymium magnets present an irreversible change in a high temperature range. The irreversible demagnetization at high temperature depends on the material characteristics and permeance coefficient. Demagnetization is likely to progress as the permeance coefficient is smaller. To determine the expected demagnetization level, the demagnetization curve of the magnet material is required. This data should be acquired from the magnet manufacturer.
The temperature at which a magnet loses its 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.

<Low temperature demagnetization>
In principle, a magnet with a negative temperature coefficient provides a larger magnetic force as the temperature lowers. However, some ferrite magnets have an irreversible change zone in a low temperature range.
The irreversible demagnetization at low temperature depends on the material characteristics and permeance coefficient. Demagnetization is likely to progress as the permeance coefficient is smaller. To determine the expected demagnetization level, the demagnetization curve of the magnet material is required. This data should be acquired from the magnet manufacturer.

Calculation of magnetic flux density produced by a magnet

Figure 1 shows a convenient expression, which is useful for making a magnet with an intensity of ** mT at * mm from the magnet." When the size of the magnet, residual magnetic flux density Br at the surface, and distance from the magnet surface are entered, the magnetic flux density at that position is obtained. This expression is obtained by solving the equation for electromagnetism, assuming uniform distribution of the magnetization on the pole surface, and is considerably in agreement in most cases for the above three types of magnets. (Excel sheet for caluculating magnetic flux density)

Magnetic field analysis

Magnetic field analysis means to calculate the magnetic flux density generated by a permanent magnet or a coil in a space.It is difficult to directly solve the equation for electromagnetism for a magnet with much more complex shape than the simple one mentioned above or for a case where multiple magnetic substances such as yokes exist.
In such cases, numerical analysis techniques, such as the finite element method or the integral element method, are used. The optimal solution is obtained by a computer breaking down the magnetic substance with a complex shape as an assembly of a minute magnetic substance element.
Our company provides support for magnetic field analysis for the optimum solution to magnetic design, in accordance with the specific requirements of each customer.