Lots of people know that the high temperature will lead the magnet loss magnetism, but what other reasons? It is a question from one of our clients. He bought some Alnico 5 magnet from us, then he found the power became lower after he holds them for months. After the descriptions of his condition, I found the magnets he had not been fixed into his magnetic system but they are moved here and there. Because of the Hc of Alnico 5 Magnet is very low, the outer magnetic field would be easy to let the magnet loses magnetism.
So besides the temperature, what other will make the magnet loss magnetism as we know?
- oxidation, it is a killer of rare earth magnet, as all know rare earth magnet is usually coated by Ni, Ni-Cu-Ni….etc. Why they always need to be coated, the most important reason because they are very easy to be oxidized. From Portiscap’s engineer message, their motor had a record that the motor cannot work because of the issue of the magnet was oxidized, and the magnet had been no magnetism, and a small finger can make the magnet become powder, it is very dangerous if they are running in a system. So if you are using rare earth magnet, especially Neodymium magnet, please don’t destroy the coating of the magnet, or they will lose magnetism very fastly when they are oxidized.
Also if you are using Alnico magnet or FeCrCo magnet, usually they will not be oxidized into the inner material, the oxide will cover the surface, It is very similar to iron. But it will also make the magnetic force lower than that before oxidized. that because there will be it will make the oxide between magnet and object.
- outer magnetism makes the magnet loses magnetism. It is very easy to happen in low HC magnet, like Alnico magnet, FeCrCo magnet.
Material Hcb Hcj Work temp. Alnico-5 49kA/m 50kA/m 525˚C Alnico-8 119kA/m 134kA/m 535-550˚C FeCrCo35/5 50kA/m 51kA/m 400˚C FeCrCo44/4 44kA/m 45kA/m 400˚C N40 907kA/m 955kA/m 80˚C
You will find from the Table, you can find the Hc of Alnico and FeCrCo would be less than 1/10 of that of Neodymium magnet. That means Alnico magnet and FeCrCo magnet are very easy to be influenced by outer magnetic field.
- Hit, that is an interesting condition, I found if the Alnico magnets are hit, they also will lose magnetism. It is also related to its low coercive force (Hc).
Of course, the temperature is the most known reason, but usually, they will be thought a lot, and usually it will be thought about before designed your magnetic application.