Speaker Magnet, Motor Magnet, Flexible Magnet, Fridge Magnet

Permanent magnets are used in the following major groups: acoustic transducers, motors and generators, magneto mechanical devices, and magnetic field and imaging systems. You will find permanent magnets in many products, such as televisions, telephones, computers, audio systems and automobiles.

The permanent magnet family consists, in general terms, of non-rare earth permanent magnets and rare earth magnets. The non-rare earth magnets include Alnico (Aluminum-Nickel-Cobalt) magnets and Ceramic (Strontium and Barium Ferrite) magnets. Rare earth magnets include Sm-Co (Samarium-Cobalt) magnets and Nd-Fe-B (Neodymium-Iron-Boron) magnets.

Although non-rare earth magnets are used in the majority of these applications due to their economic cost, rare earth permanent magnets have many distinguishing characteristics, such as a large Maximum Energy Product, (one performance index for permanent magnets). Dozens of magnetic materials which contain rare earth have been developed recently. Two major families of rare earth permanent magnets, Sm-Co magnets and Nd-Fe-B magnets, have been widely used in a variety of applications. Each family has its own advantages and disadvantages.

How to Choose Permanent Magnet Materials

Each permanent magnet material discussed above has its own pros and cons. How to choose the right one for your particular application is a challenge to any user. A balance between cost and performance must be considered in selecting of permanent magnet material. In Reference Information section of this page, several reference books are listed. These books may be helpful to you in designing and selecting permanent magnet materials. Following is a Comparison Table to help you select a right permanent magnet material for your applications.

Permanent Magnet Material Comparison Table
Note: The data listed in the table are for reference only

Anisotropic Magnet: A magnet having a preferred direction of magnetic orientation, so that the magnetic characteristics are optimum in that direction.

Coercive force, Hc: The demagnetizing force, measured in Oersted, necessary to reduce observed induction, B to zero after the magnet has previously been brought to saturation.

Curie temperature: The temperature at which the parallel alignment of elementary magnetic moments completely disappears, and the materials is no longer able to hold magnetization.

Flux: The condition existing in a medium subjected to a magnetizing force. This quantity is characterized by the fact that an electromotive force is induced in a conductor surrounding the flux at any time the flux changes in magnitude. The unit of flux in the GCS system is Maxwell. One Maxwell equals one volt x seconds.

Gauss, Gs: A unit of magnetic flux density in the GCS system; the lines of magnetic flux per square inch. 1 Gauss equals 0.0001 Tesla in the SI system.

Hysteresis Loop: A closed curve obtained for a material by plotting corresponding values off magnetic induction, B (on the abscissa), against magnetizing force, H (on the ordinate).

Induction, B: The magnetic flux per unit area of a section normal to the direction of flux. The unit of induction is Gauss in the GCS system

Intrinsic Coercive Force, Hci: An intrinsic ability of a material to resist demagnetization. Its value is measured in Oersted and corresponds to zero intrinsic induction in the material after saturation. Permanent magnets with high intrinsic coercive force are referred as "Hard" permanent magnets, which usually associated with high temperature stability.

Irreversible Loss: Defined as the partial demagnetization of a magnet caused by external fields or other factors. These losses are only recoverable by remagnetization. Magnets can be stabilized to prevent the variation of performance caused by irreversible losses.

Isotropic Magnets: A magnet material whose magnetic properties are the same in any direction, and which can therefore be magnetized in any direction without loss of magnetic characteristics.

Magnetic Flex: The total magnetic induction over a given area.

Magnetizing Force: the magnetomotive force per unit length at any point in a magnetic circuit. The unit of the magnetizing force is Oersted in the GCS system

Maximum Energy Product, (BH)max.: There is a point at the Hysteresis Loop at which the product of magnetizing force H and induction B reaches a maximum. The maximum value is called the Maximum Energy Product. At this point, the volume of magnet material required to project a given energy into its surrounding is a minimum. This parameter is generally used to describe how "strong" this permanent magnet material is. Its unit is Gauss Oersted. One MGOe means 1,000,000 Gauss Oersted.

Oersted, Oe: A unit of magnetizing force in GCS system. 1 Oersted equals 79.58 A/m in SI system.

Permeability, Recoil: The Average slope of the minor hysteresis loop.

Polymer-Bonding: Magnet powders are mixed with a polymer carrier matrix, such as epoxy. The magnets are formed in a certain shape, when the carrier is solidified.

Rare Earths: A family of elements with an atomic number from 57 to 71 plus 21 and 39. They are lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium, and yttrium.

Remenance, Bd: The magnetic induction which remains in a magnetic circuit after the removal of an applied magnetizing force. If there is an air gap in the circuit, the remenance will be less than the residual induction, Br.

Reversible Temperature Coefficient: A measure of the reversible changes in flux caused by temperature variations.

Residual Induction, Br: A value of induction at the point at Hysteresis Loop, at which Hysteresis loop crosses the B axis at zero magnetizing force. The Br represents the maximum magnetic flux density output of this material without an external magnetic field.

Saturation: A condition under which induction of a ferromagnetic material has reach its maximum value with the increase of applied magnetizing force. All elementary magnetic moments have become oriented in one direction at the saturation status.

Sintering: The bonding of powder compacts by the application of heat to enable one or more of several mechanisms of atom movement into the particle contact interfaces to occur; the mechanisms are: viscous flow, liquid phase solution-precipitation, surface diffusion, bulk diffusion, and evaporation-condensation. Densification is a usual result of sintering.

Surface Coatings: Unlike Samarium Cobalt, Alnico and ceramic materials, which are corrosion resistant, Neodymium Iron Boron magnets are susceptible to corrosion. Base upon of magnets' applications, following coatings can be chosen to apply on surfaces of Neodymium Iron Boron magnets.