Do Permanent Magnets ever Lose Their Magnetism or Become Weaker Over Time?

Actually permanent magnets are not permanent. A Permanent Magnet is a material which has ability to resist demagnetization, including filed demagnetization and thermal demagnetization. The ability is characterized by a physical parameter called coercivity.

In regard to field demagnetization, if a demagnetizing field or reverse field is smaller than a permanent magnet’s coercivity, the permanent magnet will keep the same magnetic flux (the demagnetizing process is linear and reversible when the demagnetizing field is lower than a threshold value) or will lose some flux (the demagnetizing process is nonlinear and irreversible when the demagnetizing field is higher than a threshold value) when the demagnetizing field removed. If a demagnetizing field excesses a permanent magnet’s coercivity, the permanent magnet will be demagnetized and remagnetized reversely.

Neodymium iron boron, NdFeB magnet, N42SH demagnetization curve, rare earth permanent magnet

In regard to thermal demagnetization, for a same material type, a permanent magnet with higher coercivity and/or higher Curie temperature usually has higher ability to resist magnetic flux loss when the working temperature increases. If the temperature excess a permanent magnet’s maximum working temperature, the magnetic flux loss will excess 5% and the decrease becomes faster subsequently. If the temperature is higher than a permanent magnet’s Curie temperature, the magnet will be totally demagnetized.

As results, to choose a permanent magnet for a motor or a device/machine, it is necessary to take the motor’s demagnetizing/reverse field and maximum working temperature into consideration so as to determine the permanent magnet’s coercivity.

Finally, it should also be mentioned that some permanent magnets, especially sintered NdFeB magnets, are vulnerable to corrosion and/or oxidation. If they suffer from corrosion/oxidation over time, their microchemistry and microstructure will change so that they will lose some magnetism even become invalid. In order to prevent from corrosion/oxidation, these permanent magnets will choose suitable coatings according to their working environments.

2 Replies to “Do Permanent Magnets ever Lose Their Magnetism or Become Weaker Over Time?”

  1. Dear Gentlemen,
    Excelent lecture or class on PM or similars.I learned coercitivity is fundamental as Courie also does.But something is concerning me, alternating forces,or loads,repulsive atractive I imagine, would cause loss of mag capacity in some time, this is a black day to me, I have been professor of FEA, linear and nonliear, all disciplines, this is the day where I say I never run an anisotropic solid body in PC,I assume rareearth sintered as Neodymium, is brittle because of anysotropy atworing temp.Always we have an special day. Fatigue is another concern, if number of cycles is known besides al sress normal values,probably is not relevant, is good to eliminate from unknowns.Sincerely Thankyou,I imagine Perendev bankrupted,ignoring the cause of PM failures,KJ said losses of 1% guaranteed, alternating forces or loads are serious respect to ave life time?
    With my best acknowledgements and regards
    Johann wegmann

    1. Dear Johann,

      You are right, alternating field does have influences on magnetism. On one hand, the alternating field acts as a reverse/demagnetizing field when it is at repulsive state. On the other hand, the alternating field also induces eddy current, subsequently it increases the temperature. These two effects both will cause magnetism loss. In fact, 5% loss is acceptable for most motor customers, 3% loss is higher requirement for some customers.

      For more information, please refer to “sintered neodymium iron boron (NdFeB) magnets” and “typical physical and chemical properties of some permanent magnets”.

      Best regards,
      HGT Co.,Ltd/Technical Support Team

Leave a Reply

Your email address will not be published. Required fields are marked *