Dear Dr. Darren F. Kavanagh,

permanent magnets do weaken over time, but depending on the affection on it, it will retain it’s magnetism essentially forever. If you store permanent magnets correct, neodymium magnets will likely lose less than 1% of their flux density over 100 years.

For more details, please see the source at: https://magnetpartner.com/blog/magnets-weaken-time

And the following notes/papers:

-Why Magnets Lose Strength: The Effects of Volume Loss, Geometry, Elevated Temperature,...

By Dura Magnetic, Inc.

Available at: https://www.duramag.com/techtalk/tech-briefs/why-magnets-lose-strength-the-effects-of-volume-loss-geometry-elevated-temperature-and-demagnetization-from-external-fields/

-Does a magnet weaken over time?

By supermagnete,

Available at: https://www.supermagnete.de/eng/faq/Does-a-magnet-weaken-over-time

-Temperature effects on permanent magnets

By Magnet Expert

Available at: https://www.magnetexpert.com/technical-advice-i685/temperature-effects-on-magnets-i683

-Current progress and future challenges in rare-earth-free permanent magnets

Jun Cui, Matthew Kramer, Lin Zhou, Fei Liu, Alexander Gabay, George Hadjipanayis, Balamurugan Balasubramanian, David Sellmyer

Acta Materialia, Volume 158, Pages 118-137 (2018)

Available at: https://www.sciencedirect.com/science/article/pii/S1359645418305858

-Magnetic Behavior of Sintered NdFeB Magnets on a Long-Term Timescale

Minna Haavisto , Sampo Tuominen , Timo Santa-Nokki, Harri Kankaanpää, Martti Paju,and Pekka Ruuskanen

Advances in Materials Science and Engineering, Article ID 760584 (2014)

Available at: https://www.hindawi.com/journals/amse/2014/760584/

-Practical Aspects of Modern and Future Permanent Magnets

R.W. McCallum, L.H. Lewis, R. Skomski, M.J. Kramer, I.E. Anderson

Annual Review of Materials Research, Vol. 44:451-477 (2014)

Available at: https://www.annualreviews.org/doi/full/10.1146/annurev-matsci-070813-113457#abstractSection

Enjoy reading and best regards, Pierluigi Traverso

See for example

https://www.pdfdrive.com/fundamentals-of-magnetism-magnetic-materials-d36269974.html

or

Handbook of magnetism and advanced magnetic materials

https://www.pdfdrive.com/handbook-of-magnetism-and-advanced-magnetic-materials-e158397254.html

Dear D. F. Kavanagh,

I would like to elaborate he details of neodymium magnets.

Metal was developed in year 1984 by General motors and Sumitomo, magnetic in nature and this is the strongest type of most permanent magnet available commercially. There is very little damage effect on this magnet, if they are not overheated or physically damaged, neodymium magnets will lose less than 1% of their strength over 10 years. Generally, this magnet loses a fraction of their performance every 100 years if maintained within their optimum working conditions. If I talk about the dangerous nature of this magnet, small magnets can pose a serious choking hazard, in case multiple magnets are swallowed, they can attach to each other through intestinal walls causing serious injuries and even death. It is advisable to keep neodymium magnets away from anyone with a pacemaker. The neodymium magnets are quite prone to corrosion and as they are very hard, dysprosium makes them brittle.

Neodymium magnets are brittle – and can easily shatter and break. Neodymium magnets are brittle and will peel, chip, crack or shatter if allowed to slam together, even from just a few inches apart Grade- N52 is the highest grade of currently available this magnet. The grade refers to the temperature rating of the magnet.. Though,another stronger magnet is the Somarium Cobalt, compared to the neodymium, SmCo provides you with the unique capabilities like higher energy density at the elevated temperatures.

All magnetic materials experience a change in flux density as their temperature increases and decreases from an ambient temperature. 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.

Temperature affects neodymium magnets:

**All neodymium magnets can 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 (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) in performance.

Just a example, 0.250” OD x 0.250” Long Neodymium magnet, oriented and magnetized through the length, has a magnetic length to pole diameter of L/D = 1. This is a very good L/D ratio and the advertised upper operating temperature may be able to be used as the application’s maximum operating temperature. However, a magnet that is 0.250” OD x 0.125” long has an L/D = 0.5 and this would indicate that the advertised maximum temperature of the magnet must be adjusted downward or a higher heat grade selected. Typically L/D ratios over 0.7 are appropriate for the recommended operating temperatures advertised for Neo, but it is best to consult the vendor and execute thermal testing.

Refer this website for further study:

1. www.magnetexpert.com(temperature affect on Nd Magnet)

2. https://www.duramag.com/techtalk/tech-briefs/why-magnets-lose-strength-the-effects-of-volume-loss-geometry-elevated-temperature-and-demagnetization-from-external-fields/

Hope content is helpful for you.

Ashish

Estimated Darren F. Kavanagh , I would recommend the following literature:

• Book Rapidly solidified neodymium-iron-boron permanent magnets

  );

• Article Magnetization and aging of sintered NdFeB magnets

  ;

• Article Effects of the grain size on domain structure and thermal st...

• Article Irreversible losses of magnetisation in FeNdB type magnets

• Article Rare-earth magnets: Properties, processing and applications

• Article The physical metallurgy and processing of sintered rare eart...

• Article Corrosion behaviour of sintered Nd–Fe–B magnets

• Article Corrosion mechanisms of sintered Nd–Fe–B magnets in the pres...

• Article The effect of the magnetic field on the corrosion behavior o...

• Article Corrosion behavior of Nd-Fe-B/α-Fe nanocomposite magnets

• Article Corrosion studies on highly textured Nd–Fe–B sintered magnets

• Article The influence of Co on the corrosion resistance of sintered ...

• Article The corrosion behaviour of uncoated bonded NdFeB magnets i...

• Article Magnetism and Magnetic Materials

There is articles exploring thermal stability and corrosion resistance on both sintered and bonded Nd-based permanent magnets. The thermal degradation of the magnetic properties, as well as corrosion effects on both sintered and bonded permanent magnets are well stabilished on the literature.

Articles exploring alloying elements addition on Nd-Fe-B sintered magnets are a good choice as well.

Best regards.