Alnico
Alnico
(OP)
We have an application for a disc magnet which needs to be small and mechanically clamped on the flat faces (see attached).
It violates the guidelines for the length being 5x diameter and so may be more susceptible to demagnetizing over time.
Does anyone have experience with how much demagnetization will occur and over what time period? Or how it can be minimised if at all?
It violates the guidelines for the length being 5x diameter and so may be more susceptible to demagnetizing over time.
Does anyone have experience with how much demagnetization will occur and over what time period? Or how it can be minimised if at all?





RE: Alnico
If the magnet is part of an assembly involving magnetic steel keepers (such as pole pieces) and is magnetized while those keepers are in place, the permeance coefficient will increase and there is a chance it will not demagnetize drastically.
Can you use SmCo instead?
-Mike
RE: Alnico
The part we've been using so far is of the same geometry but was made from a large piece Alcomax 3. This was spark eroded to get the shape we wanted so it's properties are probably not ideal but it seems to work for us.
We don't mind so much if the magnet is not he strongest, but would just like its magnetism not to drift too much over time. It's a sensing application so strength needs to be enough to register in a coil 2mm away but as long as we can pick it up and the strength doesn't drift we are OK.
RE: Alnico
However, I wouldn't recommend this if it is a critical sensing application.
It won't take much to knock out more magnetism from the magnet, so you'll have to ensure that it is well protected. It's hard to estimate how much of a field would be necessary. It'll take something stronger than the earth's magnetic field but that is all I could guess.
-Mike
RE: Alnico
TTFN
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RE: Alnico
In addition, NdFeB magnets have much higher coercivities. It is much harder to demagnetize NdFeB compared to Alnico.
-Mike
RE: Alnico
TTFN
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RE: Alnico
If the application is room temperature, then the OP could also consider NdFeB.
-Mike
RE: Alnico
Thanks for the info, Mike...
TTFN
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RE: Alnico
From what I'm hearing most of the demagnetisation is immediate and as long as the long term drift isn't too bad I think we'll just get pay for the tooling and get them cast. I can't imagine our prototype spark eroded piece is 'ideal' magnetically due to the processes it has undergone since it was originally magnetized.
Elevated temperatures aren't seen by this magnet in use.
RE: Alnico
IRstuff: The very high energy product NdFeB magnets (52 MGOe and up) used in hard disk drives will tend to start losing their magnetism at temperatures above 150°F or so. They are designed to maximize their permeance coefficient, to keep them above the knee of the BH curve.
-Mike
RE: Alnico
TTFN
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RE: Alnico
The way that this is usually managed in low H alloys like Alnico 5 is to first assemble the circuit and then magnetize.
Alnico has advantages other than strength and max temp. One of those is a low temperature coef. With Neo you will get very large changes in field with changes in temp.
Your drawing says Alnico 3, is that correct? This material has no preferred orientation and is very weak. I would assume that you would use an Alnico 5 or even 8 for this. * will give lower field strength than 5, but be much more resistant to demag.
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