Alnico 1

[pedrof]

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?

 

[MagMike]

The permeance coefficient for that magnet geometry is approximately 0.3 which means it will be operating well below the knee of the BH curve for most grades of Alnico. It will demagnetize, losing most of its magnetism very quickly when removed from the magnetizing fixture. The loss will occur within seconds of removal.

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

 

[pedrof]

Thanks, that's interesting. After the removal from the magetizing fixture and the quick loss of magnetism you describe, what is the nature of that loss and the loss thereafter? So, just picking figures here i.e. if it's 100% in the fixture, then drops to 20% once removed (not sure at what point you would class it as demagnetized), what change would there be over the next month/year?

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.

 

[MagMike]

If the magnet can be protected from stray magnetic fields, it'll remain at the (hypothetical) 20% value indefinitely (i.e. years). It won't drift as long as it is protected from corrosion, excessive heat and vibration.

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

 

[IRstuff]

So, what's done differently with hard drive magnets? I see items that are no more than about 3/16" thick, that are over 1" in diameter, and they appear to be quite strong

TTFN

FAQ731-376

 

[MagMike]

IRstuff: Hard disk drive magnets use fully sintered NdFeB and typically display straight-line behavior in the 2nd quadrant (in other words, no "knee"). They won't self-demagnetize like Alnico because of this. If they are heated up, that is a different matter...

In addition, NdFeB magnets have much higher coercivities. It is much harder to demagnetize NdFeB compared to Alnico.

-Mike

 

[IRstuff]

Ok, so would there be any reason to not use NdFeB in the OP's application?

TTFN

FAQ731-376

 

[MagMike]

NdFeB could also be an option. However, I've found most Alnico applications involve elevated temperatures, which usually rule out NdFeB. SmCo is the usual go-to material in those instances.

If the application is room temperature, then the OP could also consider NdFeB.

-Mike

 

[IRstuff]

I don't know enough to disagree, but I've occasionally found my harddrives to hot to the touch, so apparently, temperatures in the low to mid 100's °F doesn't appear to damage the magnets too much.

Thanks for the info, Mike...

TTFN

FAQ731-376

 

[pedrof]

Thought I'd better clarify, the disc is clamped mechanically so we have looked at NdFeB but it's not robust enough and is prone to shattering.
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.

 

[MagMike]

pedrof: Thanks for the clarification. I shouldn't have been so quick to assume a high temperature application. Since you mentioned shattering, I was going to suggested a compression bonded NdFeB material. This consists of NdFeB powder mixed in an epoxy binder. Energy products range around 10 MGOe. The epoxy binder allows for ductility, which will minimize the risk of shattering. As a bonus, it can be machined by conventional tooling.

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

 

[IRstuff]

Luckily, my hard drives don't get quite that hot ;-) probably only around 120-130, or so.

TTFN

FAQ731-376

 

[EdStainless]

What determines the extent of self demagnetization is the effective l/d. If this magnet is in a circuit with steel parts then the effective l is longer than the magnet.
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube