How to select a magnet

Although magnets are used widely around us, we do not get to directly see them often because they are usually used inside devices. Since we do not actually handle them often, many customers are not sure how to select a magnet. Please refer to the below information when selecting a magnet.

1Selecting a magnet based on the type of material

The major types of magnets are neodymium magnets, ferrite magnets, samarium cobalt magnets and alnico magnets. Each type has different characteristics. Please check the differences in properties in order to select a suitable magnet for your usage.

φ10mmX10mm	Magnet	Element symbol	Name of element	Advantages	Disadvantages	Usage
	Neodymium	NdFeB	Neodymium Iron Boron	This is the strongest magnet that currently exists. It has high mechanical strength. Recently it is high in demand. We can make these magnets even in small lots	It rusts easily so must be surface treated. Usually it is nickel plated. The price of rare earthes, the raw material used, is soaring.	Hard disk, MRI, hybrid automobiles
	Ferrite	Fe2O3 BaCO3 or SrCO3	Ferric oxide barium carbonate or strontium carbonate	This low cost raw material is suitable for use in high volume production. Anisotropic ferrite is practical because it has relatively good adsorptive power.	It chips easily like china. Not suitable for prototypes in small lots because a metal mold may be necessary.	Speakers, monitors
	Samarium cobalt	SmCo	Samarium Cobalt	It has excellent coercivity and is suitable for use in high temperature environments.	It is brittle and chips easily. The price of rare earthes, the raw material, is soaring.	Motors
	Alnico	AlNiCo	Aluminum Nickel Cobalt	It has excellent mechanical strength and is suitable for use in high temperature environments.	Recently the demand is low and in most cases metal molds are required because they are manufactured through a casting process. It demagnetizes easily even when exposed to repelling magnetic fields.	Precision machines Horse shoe (U) shaped compass
	Rubber magnet sheets	CM	Chlorinated Polyethylene	It is made from a mixture of resin and ferrite magnet powder, is flexible and can easily be machined to your desired shape.	It is made from a mixture of resin and ferrite magnet powder, is flexible and can easily be machined to your desired shape.	Learner driver decals Protection for elevators etc

2Selecting a magnet based on the size and shape

The properties of the magnet will differ greatly according to the size and shape (round, Rings, square, segmented). Take into consideration whether the magnet will fit in the object in which you wish to place it, whether the magnet is too thick or too thin, and then select the size which would be easiest to handle. For round neodymium magnets, we have a wide range of sizes from φ1mm to overφ100mm in diameter.

3Selecting a magnet based on the adsorptive power

Adsorption power is the weight in kilograms(Kilogram-force 1 kgf = 9.80665 N)when the magnet is attached to an iron plate (thicker than the magnet itself) and pulled perpendicularly in the direction of magnetization. The property value shown as "Adsorptive power Kg" is the value when the property is utilized to its maximum. It is a reference value and not a guaranteed value. The adsorptive power (Kgf) is given in the list of sizes. Please use these values to help you select a suitable magnet.

The adsorptive power and magnetic flux density are not proportional to each other. The adsorptive power increases if the installation area is large.

4Selecting a magnet based on the magnetic flux density (gauss, millitesla)

The magnetic flux density (gauss, millitesla) is given in the list of sizes as a reference value. The magnetic flux density refers to the magnetic flux (number of lines of magnetic field) per unit area. The SI unit is called the tesla (T) and the CGS unit (Mx/cm2) is called the gauss (G).
The surface magnetic flux density, a property that is shown after machining and completion of the product, can be either a value measured by a device such as the gauss meter or a predicted value based on calculations. There is no set standard in the magnet industry for measuring surface magnetic flux density since the measuring devices, environmental conditions for measuring and calculation formulas differ according to the manufacturer.

The magnetic flux density and adsorptive power are not proportional to each other. The thicker (taller) the magnet in the direction of magnetization, the higher the magnetic flux density.

The larger the maximum energy product, both the adsorptive power and magnetic flux density will be stronger.

5Selecting a magnet based on temperature of usage environment

The magnetism of a magnet repeatedly changes from strong to weak and vice versa according to the environmental temperature. If the temperature rises by even just one degrees, the magnet weakens and if the temperature rises by one degrees it will become stronger. Therefore when choosing a magnet suitable for your environmental temperature, it helps to know the coercivity of the material. The temperature coefficient and maximum operating temperature can be calculated from the coercivity. The maximum operating temperature is referred to as the heat resistant temperature.
Usually the magnetism will completely return to its original state after cooling. However, it will become irreversible if the temperature exceeds the heat resistance temperature and the magnetism will not return to its original state even at normal temperature as the magnetism will have been lost in proportion to the amount that was exceeded. Remagnetizing a magnet with a reduction of magnetism due to heat can recover the magnetism to almost its original state. Also, the heat resistance temperature is not the Curie temperature at which the magnetism is lost totally. If the temperature exceeds the Curie temperature, the magnet will completely lose its magnetism and become just a plain stone.

The heat resistance will differ according to the shape of the magnet even if the material has the same properties because the heat permeability will differ according to the size (shape and thickness) of the magnet. The heat resistance will be higher if the magnet is thicker in the direction of magnetization and lower if it is thinner. The heat resistance temperature that is stated on the specification is a reference value and not a guaranteed value.

6Selecting a magnet based on rust resistance

Neodymium magnets rust easily so they are usually coated with nickel plating to prevent rust. However using them outdoors and close to water will still cause corrosion.
The best surface treatment for rust resistance is our epoxy Hi-DEN coating.

	Magfine	Company[S]	Other companies	Magfine	Company[T]	Magfine
Duration	HDCcoating epoxy MF304	Normal epoxy	Anti-rust undercoat Zn for automobiles Zn	HDC coating polyimide MF305	Normal epoxy	NiCuNi 3 layer nickel
Before start of test
After 72 hrs
After 312 hrs
After 504 hrs

Salt spray test:37-39℃ 5%NaCl PH6.5-7.0 1.5ml/Hr
PCT: 120℃, 2atm, 100%RH, 12Hr.
For details of Hi-DEN Coat click here

7Select by characteristics

Please select magnets with magnetic characteristics suitable for the operating environment.