Permanent Magnet Shielding

Shielding...yes, it is the effect. Actually, you have to "bring" the magnetic field into a "core" that will let very few fringe lines outside it. Permalloy and any other metal with high "miu" would do.

Forget the mu-Metal because that is for electro static shielding and not for magnetic (unless you have precious energy to be wasted which I am not convinced about anyway) as the first reader suggested.

Basically for magnetic shielding all you need is a normal iron based sheet metal. The thickness of it will depend on the goodness of the material and the strength of the magnetic field.

If you're unconvinced wait until some of the other brains kick in this discussion however, whatever they may say this is still the basic principle you have to remember regardless.

Actually mu-metal is used for magnetic shielding. Many audio transformers have their cases made from it for shielding. Weight for weight mu-metal is far more effective than iron, however it is very expensive. In crude terms for effective magnetic shielding you need to complete a magnetic circuit around the object, shielding on one side only is not very effective.

Sometimes stray magnetic fields can be reduced by fitting a second permanent magnet in the equipment.

If you say so.

Mu-metal is everywhere, primarily in test equipment where price doesn't matter as much as quality.

But I still haven't come across using it in strong intense magnetic fields especially where weight is not a factor.

Bear in mind the relative magnetic emission of a transformer wrt an open pole magnet and he hasn't even mentioned the type of magnetic field he has in his way that he wants to suppress.

"If you say so?"

You've the whole Internet at your fingertips and it's "If you say so?"

Look it up yourself.

muMetal is everywhere? What planet are you from where this is true? muMetal is very expensive and used only where necessary.

Used as electrostatic shielding? As a bonus perhaps. muMetal was formulated *specifically* for its high magnetic permeability. Even its name strongly hints at this? Look it up. Oh that's right: you don't have access to the Internet.

The 'mu' in 'muMetal' refers to 'u,' the Greek letter used to represent *magnetic permeability.* That muMetal also happens to be a good electrostatic shielding material has to do with its being, like most metals, a good conductor of electricity but, as an electrostatic shielding material, aluminum is better, copper better yet and silver best of all. None of best electrostatic shielding materials are magnetic, btw, and are, gram for gram, cheaper than muMetal, as a rule

Everywhere? Not quite.

"Relative magnetic emission of a transformer?"

Relative? Relative to what?

Magnetic "emission"? Magnetic fields are not "emitted." Electromagnetic waves are "emitted."

Types of magnetic field? Static vs time-varying you mean? There aren't different "types" of magnetic fields. Even 'static' and 'time-varying' are inaccurate descriptors; they address what the field does, not what it is.

Give it up, mate. You've no bluddy idea what you're talking about.

Did you just wakeup or your missus told you off?

Six bloody years later you should ask the guy who raised this Q if he sorted his problem out yet. The first time I used Mu-metal as a shield on the the tube of my Oscilloscope I build, exactly 30 years ago as we speak (mid '84). It was quite expensive at the time and never used it since. Further, magnetic fields to get emitted or propagated (have u seen a Permanent Magnet yet?) and the simplest way to suppress them is iron (with v. low carbon if the field is alternating) and is cheaper than mu-metal. So, take it easy, mate (is it in down under?)

"Did you just wakeup or your missus told you off?"

Speak of the devil. How did you know?

Sorry, mate. I was in a really, really pissy mood yesterday and I should've just logged-off rather than venting like that. No excuse.

Loool the sixth-sense you know

Lead sheet i think will do.

Lead will have virtually no effect on static magnetic fields - perhaps you misread the question?

You may need to resort to a canceling force, or reverse polarity field.

Hi

Mumetal is very good but only if not strained after final anneal at high temperature in hydrogen atmosphere and if not subject to stress.

As the deformation behaviour is like lead you will have difficulties to ensure and maintain good quality.

Simplest and cheapest approach: make your box from mild steel, the lowest carbon content you can get.

These steels are used in deep drawing (automobile parts) so ask there or at a repair shop to get some.

If this is not good enough then make two layers with intermediate nonmagnetic layer.

RHABE

I wasn't sure if you join this discussion but since you here then all is well.

'Lead sheet will do'

Unfortunately I'd like this to be a lead free device.

the total thickness of the device is the main restraint as this is only 12mm.

the magnetic material being 4mm, the whole magnetic assembly is only 6mm, this means the size of a cancelling magnet could be hard when down to 1mm or so.

I've been advised the cancellation field rather than shielding is the correct way to proceed. I'm lucky to be able to FE to model the field so will hopefully be able to come up with something.

Due to the size limitations, how about some kind of coating you can apply to the inside of the case? Maybe a metal-based paint, or iron oxide powder (jeweler's rouge) mixed with an epoxy? If applied with a spray thinly enough, you could have several layers.

You might check with 3M, they specialize in coatings.

Hi,

rouge (Fe2O3) is pretty bad, magnetite (Fe3O4) is better but not really good,

flat flakes of soft iron (lowest possible carbon content) is best, but try to use only a very few % of epoxi or other glue.

Best is iron sheet (the purer - carbon free - the better).

Better is still multilayer carbon-free iron or steel sheet with intermediate nonmagnetic (conducting or isolating) layers.

RHABE

I'd also not make the external case out of anything that would attract the magnet.

Ideally I need to know a material that will shield a magnetic field but not attract it?

Unfortunately, anything practical that cancels the field externally will have an effect on the internal field; inevitably, that will generate forces on the magnet.

Theoretically, it is possible to create a situation where the net effect on the original magnet is small - for example by cancelling the attraction due to ferromagnetic screening using strategically placed permanent magnets, or by placing the screening symmetrically around the magnet such that the forces on the two poles are opposed - but this will still distort the internal fields due to the original magnet. You should also be aware that the magneto-mechanical interactions of such arrangements are intrinsically unstable, so all magnets will need to be adequately constrained mechanically.

As an aside, a superconductor would provide perfect screening, and would repel the internal magnet - but unfortunately that is not generally a practical solution.

BTW, you don't say why you want to minimise the external magnetic field. It is worth bearing in mind that high-μ materials will themselves interact strongly with external magnets; if the objective is to minimise forces on other magnets in the vicinity, the only solution I can think of is strategic-placement of permanent magnets.

One of the most impressive examples I've seen of magnetic-field confinement is found in the 'voice-coil' actuators used to move the read/write heads in hard-disk drives.

Here you have two or more very powerful rare-earth magnets separated by a small gap through which the voice coil moves. The entire assembly is immediately adjacent to the platters (these days most of which spin at 7200 RPM and greater), yet without wiping the data nor inducing eddy currents into the platters themselves, which would tend to induce drag and load the spindle motor.

The magnetic 'circuit' is very compact, yet manages to confine the field entirely within the VC assembly. I'm no metallurgist, but my guess is that the material used is some form of AlNiCo or similar. The material has to have a high permeability else it will saturate and 'leak' field.

A study of one of these assemblies may prove instructive. Whatever material is used for the magnetic circuit, it must have sufficient cross-section at a given permeability and field strength to not saturate. If it does saturate, all bets are off.

hi,

the intent is to have no stray magnetic field that would inadvertantly wipe magnetic storage or interfere with CRT's or similar.

thanks

I don't know what you have but

In that case you can use the Mu-metal can be considered but still it depends on the magnetic source; how strong it is and...it seems you might have more problems than you dare to admit for some reason.

You might want to change the magnetic design.

Use a smaller magnet with proper magnetic conducting material (iron powder, ferrite) from electronics suppliers to conduct the magnetic lines where you need them and minimize the leaks.

If you have a large magnetic conductor with respect to your magnet and that the working gap is small, there will be no need to shield.

Regards.

Permanent-Magnets are Screened by making the

Magnetic-Path [Circuit] complete; No-Reluctance in

the path.

For this purpose "Keeprs" are specially fabricated;

eg for a Shoe-Horn type a Straight-Iron-bar which has

its width = to the length of cross-section;

thickness = width of cross-section & length to fit horn's

front & for a straight bar magnet a Shoe-Hornis required.

|||======S |||

||| ||| < Keeper

|||======N|||

Shoe-Horn Magnet with Straight-Bar Keeper

S ||||||||||||||||||||||

||| ||||

||| |||| < keeper

||| ||||

||| ||||

N |||||||||||||||||||||

Straight-Bar Magnet with Shoe-Horn Keeper

And as mentioned in #3 <.. by fitting a second permanent magnet ..> .

Actually in storage a pair of Shoe-Horns is kept together .

|||======S N=====

||| |||

|||======N S=====

Shoe-Horn Magnet -- to -- Shoe-Horn Magnet

Both strongly attract each other & extremely low leakage is faced.

#4 & #8

Lead screens X-Rays; you may see the lead-foil / sheet

loaded Aprons worne by Radio-graphers & X-Rooms' doors.

#6 <..canceling force, or reverse polarity field ..>

cannot be kept together due to high repulsion force between two

kept like:

<--| |--->

|||======S S=====

||| |||

|||======N N=====

.

#9 Only Magnetic material can screen Magnetic field.

and they are attracted by magnets

#13

< ... or interfere with CRT's or similar ...>

CRTs are too sensitive to not only magnetic fields & Magnets but also to Magnetic-materials;

how much annealed those are to minimise the the residual magnetism.

Actually if you turn TV while running you can notice changes in colours on corners.

Usually we change place of TV after switching it off & Degaussing Coil corrects the Earth Magnetic-Field's effect on switch on.

By the way why are you keeping magnet close to CRT?

Regards.

Regards evrey body !

Sorry that I tried to make images with text but gone out of its format.

Will post again with hand drawings.

Regards

"By the way why are you keeping magnet close to CRT?"

It's more a 'just in case' kind of idea, The unit I've put together has already wiped a colleagues hard drive, I was thinking of other possible porblems to illustrate.

the field isn't particulary high. around 0.1T at about 2mm from the case over some areas which i presume was enough to penetrate the hard drive.

thanks for all your input