Magnetization Process

Reversible Losses  

 

These are losses that are recovered when the magnet returns to its original temperature.  

Reversible losses cannot be eliminated by magnet stabilization. Reversible losses are 

described by the Reversible Temperature Coefficient, -%Br/°C, shown in the table 

below. These losses vary for different magnet materials and are not always linear as the 

temperature increases. For example, a NdFeB magnet with a -.11 reversible loss will 

have 11% less magnetic flux at 120°C than at 20°C. 

 

 

Material 

Tc of Br

Tc of Hc

Neodymium  

-0.11  

-0.60  

Samarium cobalt 

-0.03  

-0.30  

Alnico  

-0.02  

+0.01  

Ferrite  

-0.18  

+0.30  

 

                                         

Reversible temperature coefficients of Br and Hc 

  

 

Irreversible Losses 

 

These losses are defined as a partial demagnetization of the magnet from exposure to 

high or low temperatures or other demagnetizing influences. These losses are only 

recoverable by remagnetization and are not recovered when the temperature returns to 

its original value. This occurs when the magnets are used at temperatures higher than 

the specified “maximum operating temperature” or when the operating point of the 

magnet falls below the "knee" of the demagnetization curve.  

 

Metallurgic changes  

 

Metallurgic changes occur when magnets are exposed to extremely high temperatures 

that are usually as high as the initial heat treatment when they were manufactured. This 

is called the magnet’s Curie temperature. When a metallurgic change occurs, the 

magnetic properties are not recoverable to the previous state even after 

remagnetization. At the Curie temperature, magnetic domains lose their “locked” 

positions and regain a random orientation in the material. The table below shows Curie 

temperatures for various materials.   

 

 

Material  

Tc 

Tmax 

Neodymium  

310 

150  

Samarium cobalt  750 

300  

Alnico  

860 

540  

Ferrite  

460 

300  

 

                                    Curie (Tc) and max working temperatures for different materials in °C  

 

 

 

 

 

 

 

 

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Reluctance  

 

These changes occur when a magnet is subjected to permeance changes such as 

changes in the air gap dimensions during operation. These changes will change the 

reluctance of the circuit and may cause the magnet's operating point to fall below the 

knee of the curve, causing partial and/or irreversible losses. The extent of these losses 

depend on the material properties and the extent of the permeance change. Stabilization 

may be achieved by pre-exposure of the magnet to the expected reluctance changes.  

 

Adverse and Stray Fields 

 

External magnetic fields in repulsion modes will produce a demagnetizing effect on 

permanent magnets. Alnico, with a coercive force of only 650 Oe, will encounter 

magnetic losses in the presence of any magnetic repelling force, including similar 

magnets. Applications involving Ferrite magnets with a coercive force of about 4 kOe 

should be carefully evaluated in order to assess the effect of external magnetic fields. 

NdFeB magnets will partially or completely demagnetize Ferrite magnets when they are 

placed too close or are touching each other. NdFeB and Samarium Cobalt magnets with 

coercive forces exceeding 15 kOe are rarely affected by repelling forces.  

 

Radiation 

 

It is usually recommended that magnets with high Hci values in objects which are 

exposed for radiation be operated at high permeance coefficients, and that they be 

shielded from direct heavy particle irradiation. Stabilization can be achieved by pre-

exposure to expected radiation levels.  

 

Shock, Stress and Vibration 

 

Below destructive limits, these effects are very minor on modern magnet materials. 

However, rigid magnet materials are brittle in nature and can be easily damaged or 

chipped by improper handling. Samarium Cobalt in particular is a fragile material and 

special handling precautions must be taken to avoid damage. Thermal shock when 

Ferrite and Samarium Cobalt magnets are exposed to high temperature gradients can 

cause fractures within the material and should be avoided. Even though Samarium 

Cobalt magnets do not corrode, and thus do not require plating, they are sometimes 

plated to provide them better resistance to forces that would cause structural damage.

 

 

 

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Magnet Assembly 

 

 

 

Because all magnetic materials are brittle and, in many cases, magnetized prior to 

assembly, care must be taken when using magnets in assemblies that require further 

processing. For example, when molding sensor components which use magnets that 

have already been magnetized, many types of magnets will demagnetize and/or crack 

during the molding operation. Here are some common magnet assembly processes:    

 

Gluing Magnets 

  

Magnets can be glued to metal rotors, housings, or onto shafts using structural acrylic, 

cyanoacrylate, and single or two part adhesives which are rated at temperatures up to 

250°C. Many adhesives available today have fast cure times to avoid the need for 

fixturing the magnets in place while the bond cures. Adhesives with higher temperature 

ratings normally require oven curing, and fixturing of the magnets to hold them in place. 

If magnet assemblies are to be used in a vacuum, potential outgassing of the adhesives 

should be considered. The most common adhesives used when gluing magnets to other 

materials are Loctite Henkel (Hysol brand), Eccobond and Bondmaster by National 

Starch, 3M Scotchweld, and several other epoxy brands.  

 

Mechanical Fastening  

 

When a number of magnets must be assembled, especially when the magnets must be 

placed in repelling positions, it is very important to consider safety issues. Modern 

magnetic materials such as Samarium Cobalt and Neodymium are extremely powerful, 

and when in repulsion they can behave as projectiles if adhesives were to break down. It 

is recommend that when magnets are used in fast rotating applications, or under other 

high stresses, mechanical fastening and/or encapsulating with non magnetic metals 

should be considered as part of the assembly design.  

 

Potting  

 

Assemblies containing multiple magnets, such as in rotors, should be potted to fill gaps 

or to cover entire arrays of magnets. Potting compounds cure to hard and durable 

finishes, and are available to resist a variety of environments, such as elevated 

temperatures, water flow turbulence, etc. When cured, the potting compounds are 

sometimes machined to provide accurate finished parts. Recently, many companies 

have been using glass fiber tape instead of stainless steel shells for encapsulating 

magnets on high speed rotors. The advantage is an easier application that can be done 

in-house and they are somewhat less expensive than stainless steel shells that must be 

ground to very tight tolerances. The disadvantage to the tape is that larger air gaps, for 

example between the rotor and stator, are required. 

 

Welding  

 

Assemblies which are required to be hermetically sealed can be welded using either 

laser welding (which is not affected by the presence of magnetic fields) or TIG welding 

(using appropriate shunting elements to reduce the effect of magnetic fields on the weld 

arc). Special care should be taken when welding magnetic assemblies so that heat 

dissipation of the weld does not cause irreversible magnetic losses.  

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