smco5 disc D20x1mm having both N and S on the same face 1

[leslie2000]

We have smoc5 magnets, D20x1mm, axil magnetised. Now we inspect the magnets with a gauss meter at both centre and peripheral saying 12, 3, 6, 9 clock, and found a few of them having both N and S on the same face. Here is a photo. Is there anyone who know why this happened?

Thanks.

Sam

 

[MagMike]

No photo

If you are measuring at the peripheral edge, you are likely picking up stray flux lines, and the polarity will switch North/South

It is very difficult to magnetize SmCo so it'll have multiple poles on the face. Most likely it is magnetized axially and the stray flux lines are fooling you into thinking it is otherwise.

 

[MagBen]

probably the magnets were magnetized as a stack, not individually. The magnets at the both very ends of a stack were not magnetized perpedicularlly to the plan (not perfectly axially). plus, the L/D (=1/20) is very samll. One can imagine that the neutral section should be out of along the thickness of disc (with some radial component of magnetization).

 

[IRstuff]

What are the magnets intended to do? Typical kitchen magnets are expected to be only used against steel, so the polarity is irrelevant. In fact, the standard flexible kitchen magnet has alternating poles; if you take two of them back to back, you can slide them across each other and feel the cogging as the poles line attract and oppose.

TTFN
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[leslie2000]

Here is the photo links.

this phenomenon happened only on a spot over a surface, do you think it is possible that there is a small crack on the magnets?

Thanks

 

[leslie2000]

sorry I don't know how to add a link. Here it is again.

http://files.engineering.com/getfil...0fb9b3f95e9&file=Magnet_Problems_Redacted.pdf

 

[MagMike]

A small crack would not likely cause a polarity change. It might weaken the field a tiny bit, but certainly wouldn't change the polarity.

I once came across a situation where a magnet supplier made SmCo magnets with powder particles that were too large. The magnets didn't sinter properly and there were significant regions that were 'dead' magnetically. It's a longshot, but maybe you have a similar situation. If you have access to green magview paper, you could easily see the presence of any reverse domains on the surface.

 

[dgallup]

What is "green magview paper"? I googled it but did not find anything relevant.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MagMike]

dgallup:

http://www.mcmaster.com/#magnetic-pole-viewing-paper/=uc1e1m

 

[MagBen]

The results almost proved my assumption: miss alignment of axial magnetization. for such big a D/L ratio, when the miss alignemnet is more than Tan^-1(1/20)= 2.86 degree, the disc would have two poles. to further confirm, check the followings:
1. only two poles at one plane
2. one pole must be located on the edge and reside only a small portion of the plane. however, if the the angle of miss alignment is 45 degree, half of plane is N, the other half is S.
3. the polarity and area are opposite to the other side of the disc

 

[MagBen]

"Green magview paper"? it is an interesting name. I have one in my wallet, but never had a name of it.
It is nothing more than Fe, or Fe oxides soft magnetic powder whihc can be moved between the "paper" (plastic). one can easily make such a paper in minutes to view the magnetic pattern (neutral section, poles, and flux distribution etc)

 

[MagMike]

Green magview paper has been around for years, although under many different names. I commonly see it called 'magvue' too. 3M used to produce 1 meter wide rolls but no longer. Arnold used to sell their own version of it but it was gray colored.

The paper contains nickel flakes dispersed in suspension in individual cells. The flakes can rotate such that the plate of the flake is parallel to the magnetic field. If the flakes are directly over the N/S dividing line the white color of the flakes produces a white line demarcating the pole boundary.

The paper is very useful for confirming multi-pole magnetizations.

 

[MagBen]

yes, Ni flakes should also work. I guess the green color comes from the Ni flakes, and 3M made it
Arnold made gray and brown versions. gray is from Fe3O4 or Fe, brown from Fe2O3. I worked for Arnold and made myself in couple of occasions temporarialy for operators to check the effect of de-magnetization: if not fully demagnetized, one will be able to observe the movement of the powder in the paper.

 

[IRstuff]

So, I looked up the McMaster item, and thought it worthwhile to buy one, and the order confirmation showed up about 10:14 AM PST this morning, and !!!!!! they shipped a delivery confirmation at 2:29 PM. WTF, how'd they do that?

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[cermag]

I see you're using one of our Gauss meters. perhaps you want to send me a couple of samples and i will check it out?

 

[IRstuff]

The mag viewers work pretty nicely. Resolves better than about 0.05 inches, the field reversals on kitchen magnets were clearly visible. Interestingly, there are a couple of leaky areas on the portable laptop-format HDs, so probably not a good idea to put them and your credit cards in the same pocket.

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[MagBen]

I have put it with my credit cards in my wallet for years, no accident occurred. Note this "paper" itself is not a permanet magnet, just like a key ring, probably ok to keep in oyur pocket.