When designing an application for permanent magnets it is essential to consider the temperature range that the magnet or magnets will be expected to work in. Yet, with so much information available (some of it incorrect) it is possible to associate the wrong magnetic property with the type of magnetic material you are considering, particularly where thermal stability is concerned; the result being disappointing magnetic performance.
All magnetic materials experience a change in flux density as their temperature increases and decreases from an ambient temperature. For example, most magnets with the exception of ferrite will actually display an increase in strength as the temperature drops and all permanent magnets (including ferrite) will lose a percentage of their performance for every degree increase. Whether this lost performance is recovered on cooling depends on the materials maximum operating temperature and the temperature the magnet is exposed to.
|Grade||Ceramic 8||Alnico 5||1-5||2-17||Bonded||Sintered|
|(BH)max (MGOe) ||3.8||5.5||20||26||10||43|
|% / Deg C||-0.18%||-0.02%||-0.045%||-0.035%||-0.105%||-0.12%|
|% / Deg C||+0.4%||-0.015||-0.3||-0.3||-0.4||-0.6|
There are three types of performance loss experienced by magnets when exposed to elevated temperatures.
Reversible loss occurs for every degree rise in temperature the magnet experiences above ambient, up to its maximum operating temperature. As the magnet cools, the performance returns to the previous level.
When a magnet is heated above its maximum operating temperature but below its Curie temperature, it will experience irreversible losses in performance. This means if the magnet is then cooled, its performance will be weaker than it was before it was heated. A magnet that has experienced irreversible loss could theoretically be remagnetised back to its original strength, but this is not a cost effective process. Irreversible loss is a result of the elevated temperature reversing the magnetization of single individual magnetic domains. This means that irreversible loss happens just once; if the same thermal cycle is repeated no additional loss will occur as each individual domain can only be reversed once after it is magnetised.
Permanent loss of magnetic performance is experienced when a magnet is heated above its Curie temperature. At this point the structure of the magnetic domains change and become self-keepering, resulting in permanent magnetic damage which cannot be repaired by remagnetisation.
The Curie temperature of permanent magnetic materials is often quoted on datasheets, but when taken in isolation this is often the least useful thermal characteristic when designing an application as no design should function close to these extreme high temperatures. Therefore, other parameters such as maximum operating temperature should be considered
The degree change in performance for a neodymium magnet depends on its shape and the design of the circuit within which it used, e.g. whether it is in ‘free space’ or whether it is connected to a steel surface. Small, thin magnets will generally be more susceptible than magnets greater in volume to rising temperatures. That considered, all neodymium magnets will lose a certain amount of performance for every degree rise in temperature even if the temperature is below their maximum operating temperature. In fact, depending on size, shape, grade and how it is used, a neodymium magnet will lose 0.08%-0.12% of its magnetic strength for every degree Celsius rise in temperature.
Up to 150 degrees Celsius neodymium magnets are considered to have the best magnetic performance of all permanent magnetic materials, but when elevated to temperatures above 150 degrees Celsius their magnetic strength will be reduced below that of a magnet of the same size magnet made from samarium cobalt material. Neodymium magnets maintain their magnetic stability in very low temperatures; only at -138 degrees Celsius will their magnetic structure become affected. At this point, a neodymium magnet’s direction of magnetism will alter, resulting in a loss of performance between 10 and 20%.
Standard grade neodymium magnets have a maximum operating temperature of 80 degrees Celsius. When heated above this, they will experience irrecoverable losses in performance. High temperature grades of neodymium magnets with higher maximum operating temperatures are available and these are identified by a suffix after the name of the standard grade.
Samarium cobalt magnets are not as strong as neodymium magnets at room temperature but have a better temperature coefficient for both remanence (Br) and resistance to demagnetisation (Hci) than neodymium magnets. For example, once the temperature exceeds 150 degrees Celisus, samarium cobalt magnets outperform neodymium magnets and standard grades of samarium cobalt (Sm2Co17) magnets will not suffer irrecoverable losses until the temperature exceeds 350 degrees Celsius.
Alnico magnets are characterised by their high remanence but low coercivity, which means that they are second only to neodymium magnets in terms of magnetic strength, but are significantly more susceptible to demagnetisation by external magnetic fields and physical shock, although not by elevated temperature. In fact, of all permanent magnetic materials, alnico magnets have the greatest thermal stability only losing a fraction (0.02%) of their performance for every degree Celsius rise in temperature above ambient. Alnico magnets also have the highest maximum operating temperature of all the permanent magnet family, not suffering irreversible losses in performance until the temperature reaches 525 degrees C (alnico5).
Unique among permanent magnets, ferrite magnets actually become more resistant to demagnetisation as their temperature increases. Conversely, their strength decreases as their temperature rises, albeit at a lower rate. These characteristics make them particularly popular for high temperature applications such as electric motors and generators. The intrinsic coercivity of a ferrite magnet (resistance to demagnetisation) increases by 0.4% per degree rise in temperature, while their magnetic strength decreases by 0.2% for each degree Celsius increase. Ferrite magnets can be used in temperatures up to 180 degrees Celsius before they will begin to experience irreversible losses in performance.