There are many different types of permanent magnets, but nowadays we generally consider using following four major types, NdFeB, SmCo, Alnico and Ferrite. Most of the permanent magnets are classed as anisotropic, that is a preferred magnetic axis determined during mechanical pressing process and so the magnets can only be magnetized in that predetermined direction. Isotropic magnets may be magnetized in any direction, but are generally lower in performance than the anisotropic magnets.
When you decide to choose a magnet material, you should take following six factors at least into consideration:
. Magnetic Flux
. Magnetic Stability
. Operating Temperature
. Cost
. Corrosion Degree Likely to Be Encountered
. Difficulty in Machining
Magnetic Flux
It is the most convenient method of comparing the magnetic performance of different types and grades of permanent magnets to consider their maximum energy product (BH)max. This is the point where the magnet will provide most energy for the minimum volume.
It is frequently necessary to know what flux density will be on the pole face of a magnet. It should depend on residual induction (Br), magnet shape/geometry and magnetization direction. Please refer to magnetic properties(NdFeB, SmCo) for you to compare magnetic performance for NdFeB and SmCo.
Magnetic Stability
The ability of a permanent magnet to support an external magnetic field results from small magnetic domains "locked" in position by crystal anisotropy within the magnet material. Once established by initial magnetization, these positions are held until acted upon by forces exceeding those that lock the domains. Factors affecting magnetic stability include time, adverse fields, radiation, shock & vibration, shape & geometry, temperature and so on.
Time
The effect of time on magnets is negligible and averages a loss of less than 1 x 10-5 per year at 20 ºC. Over 100,000 hours, these losses range from essentially zero for SmCo to less than 3% for Alnico 5 at low permeance coefficients.
Adverse Fields
External magnetic fields in repulsion modes will produce a demagnetizing effect on permanent magnets. Rare Earth magnets with intrinsic coercivity exceeding 15 kOe are difficult to affect in this manner. However, Ferrite and especially Alnico 5 will encounter magnetic losses in the presence of any magnetic repelling force, including similar magnets.
Radiation
Rare Earth magnets are commonly used in charged particle beam deflection applications, and it is recommended that magnets with a higher Hcj be used in radiation environments, because tests have shown SmCo and especially Sm2Co17 withstand radiation 2 to 40 times better than NdFeB materials.
Shock & Vibration
Unlike previous magnets, shock and vibration has very minor effect on modern magnets. However SmCo are brittle and easy to damage or chip by improper handling. SmCo and Ferrite can fracture when they are exposed to a thermal shock with high temperature gradients.
Shape & Geometry
Shape & Geometry has an effect on performance and therefore their stability, because it determines the working point along the demagnetization curve. Magnets used in a closed magnetic circuit or with a longer length, perform better and are magnetically more stable.
Operating Temperature
Temperature is the most important factor to be considered to keep magnetic stability. Temperature effects fall into three categories: reversible losses, irreversible but recoverable losses, and irreversible and unrecoverable losses.
Reversible Losses
These are losses that are recovered when the magnets return to their original temperature. Reversible losses can not be eliminated by magnet stabilization. Reversible losses are described by the reversible temperature coefficient, α(Br) or β(Hcj) which is expressed as %/ ºC.
Irreversible but Recoverable Losses- Maximum Operating Temperature
These losses are defined as partial demagnetization of the magnets from exposure to high temperatures. These losses are only recoverable by remagnetization, and are not recovered when the temperature returns to its original value.
Irreversible and Unrecoverable Losses-Curie Temperature
Each material has a maximum temperature-Curie temperature where metallurgical changes occur within the magnet structure and where the individual magnetic domains break down. Once these losses occur they can not be recovered by remagnetization.
Because these factors especially temperature and adverse fields have significant effect on magnetic stability, sometimes it’s necessary to temperature stabilize magnets before usage of magnets .
Cost
Normally shape, geometry, tolerances and quantity will influence the prices of individual magnets but the most significant effect is the cost of the basic raw material.
Sometimes tooling is necessary for new and big sizes of magnets and for series production and fixtures required for machining and magnetization.
Corrosion Degree Likely to Be Encountered
The corrosion resistance differs between four major magnets. SmCo, Alnico and Ferrite have good corrosion resistance and do not need to be coated against corrosion. Although NdFeB owns many advantages over other magnets, it’s sensible to corrosion. Usually NdFeB has to be coated with phosphate, passivation, Ni, Zn, Epoxy, Ni-Cu-Ni and Ni-Sn.
Difficulty in Machining
All magnets need machining, which may considerably influence the magnet cost. Because the physical properties differ between four major magnets, it should be considered whether it is practical to machine too big, too small, too thin or too complicated magnets, especially brittle SmCo.
Following is the detailed comparison between four types of modern magnets for your reference:
Magnet Type |
Reversible Temp. Coefficient 20~150 ºC, α(Br) |
Reversible Temp. Coefficient 20~150 ºC, β(Hcj) |
Maximum Operating Temp. |
Curie Temp. |
Comparitive Cost |
Corrosion Resistance before Coating |
Difficulty in Machining |
NdFeB |
-0.09~-0.12 %/ºC |
-0.45~-0.7 %/ºC |
80~230 ºC |
310~370 ºC |
High (Grade N, M, H & SH);
Very High (Grade UH, EH & AH) |
Poor |
Low |
SmCo5 |
-0.04 %/ºC |
-0.20 %/ºC |
250 ºC |
750 ºC |
Very High |
Excellent |
Fair |
Sm2Co17 |
-0.03 %/ºC |
-0.20 %/ºC |
350 ºC |
850 ºC |
High |
Excellent |
High |
Alnico |
-0.02 %/ºC |
0.03 %/ºC |
550 ºC |
850 ºC |
Medium |
Fair |
High |
Ferrite |
-0.2 %/ºC |
0.30 %/ºC |
250 ºC |
450 ºC |
Low |
Excellent |
Very High |
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