# How does Mu-metal differ on a atomic level from soft iron

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Hi,

What is the difference of mu-metal and soft iron on a atomic scale? Both soft iron and mu-metal have a high mu and low solubility but soft iron enhances a magnetic field and mu-metal absorbs it. So what is the difference?

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On an atomic level the difference is that mu metal has atoms other than iron in it.

"soft iron enhances a magnetic field and mu-metal absorbs it"

No. You can't "absorb" a magnetic field. Soft iron and mu metal do basically the same thing to magnetic fields.

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2 hours ago, swansont said:

On an atomic level the difference is that mu metal has atoms other than iron in it.

"soft iron enhances a magnetic field and mu-metal absorbs it"

No. You can't "absorb" a magnetic field. Soft iron and mu metal do basically the same thing to magnetic fields.

Mu-metal is used in shielding applications while soft iron is not. They both have a high mu and low solubility so what makes mu-metal different so that it can be used in shielding applications?

Edited by Hami Hashmi
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1 hour ago, Hami Hashmi said:

Mu-metal is used in shielding applications while soft iron is not. They both have a high mu and low solubility so what makes mu-metal different so that it can be used in shielding applications?

Other properties, most likely. Fabrication, final product. Iron tends to rust, for example. It's not that iron won't shield magnetic fields, it's that it's not the best material in some cases.

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1 hour ago, swansont said:

Other properties, most likely. Fabrication, final product. Iron tends to rust, for example. It's not that iron won't shield magnetic fields, it's that it's not the best material in some cases.

Mu metal has a higher magnetic permeability apparently... whatever that means/

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9 hours ago, Hami Hashmi said:

Mu-metal is used in shielding applications while soft iron is not.

Nope.

Soft iron is used for shielding. It's not quite as good (because it has a lower permeability), but it's cheap.

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10 hours ago, StringJunky said:

Mu metal has a higher magnetic permeability apparently... whatever that means/

Meaning it will do a better job of concentrating the field lines in it, but that also means it tends to saturate — there's a limit to the field it can shield.

Up to that point, you can do better shielding with less material. Which makes it attractive for some applications, but it's also relatively expensive (the set shown in here, with which I am familiar, was $25k each, IIRC, and that was 12-15 years ago) ##### Link to comment ##### Share on other sites 1 hour ago, swansont said: Meaning it will do a better job of concentrating the field lines in it, but that also means it tends to saturate — there's a limit to the field it can shield. Up to that point, you can do better shielding with less material. Which makes it attractive for some applications, but it's also relatively expensive (the set shown in here, with which I am familiar, was$25k each, IIRC, and that was 12-15 years ago)

Does it mean it has a higher tendency to attract the field into the body of itself?  I noticed in one description it said the electrons -  presumably the outer ones - are 'unordered'. Does that mean that they can absorb the magnetic flux until they reach a point of saturation and then become ordered, like an ordinary magnet?

It is very expensive... wow!

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There is a description here: https://www.mumetal.com/about_mumetal.php

6 hours ago, John Cuthber said:

Nope.

Soft iron is used for shielding. It's not quite as good (because it has a lower permeability), but it's cheap.

Ok but in an electromagnet the wire wrapped around the core (which is mostly soft iron) generates a magnetic field when a current is passed through it, which then is amplified by the core. But in a shield, soft iron absorbs the magnetic field instead of enhancing it. So why is that?

Just found a relevant thread

So the way I see it is that mu-metal has a low saturation which is why it is good for shielding. But how do mu-metal's atoms differ from soft iron's as to make it have low saturation?

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3 hours ago, Hami Hashmi said:

There is a description here: https://www.mumetal.com/about_mumetal.php

Ok but in an electromagnet the wire wrapped around the core (which is mostly soft iron) generates a magnetic field when a current is passed through it, which then is amplified by the core. But in a shield, soft iron absorbs the magnetic field instead of enhancing it. So why is that?

Just found a relevant thread

So the way I see it is that mu-metal has a low saturation which is why it is good for shielding. But how do mu-metal's atoms differ from soft iron's as to make it have low saturation?

Try this:

Quote

Mu Metal is a "soft" ferromagnetic material that does not retain a macroscopic internal field after the removal of an external magnetizing field. Most alloys are permalloys containing approximately 80% Nickel (Ni), 20% Iron (Fe) and small amounts of Molybdenum (Mo). MuShield high permeability magnetic shielding material is a non-oriented 80% nickel-iron-molybdenum alloy (permalloy) which offers extremely high initial permeability and maximum permeability with minimum hysteresis loss.

Ferromagnetism originates at the quantum level. Ferromagnetic elements have a lowest energy e- orbital state that aligns the e- spins parallel, thus giving an intrinsic magnetic moment to the atom. The lowest microscopic energy state of an ensemble of atoms is with these magnetic moments aligned to produce a net magnetic field. Since maintaining an external magnetic field would require energy, the lowest macroscopic energy state requires that the atoms divide into domains about 1000 atoms across and that the magnetic orientation of these domains be randomized.

In the presence of an externally applied magnetic field, the domains magnetically realign to some degree and thus generate their own external field. The primary field continues to exist, but now the net field is the sum (or superposition) of the primary and induced field. The induced field must have an alignment of opposite polarity to the primary field (just like two bar magnets must align North to South and South to North), and the superposition of the two fields results in a lower observed field, and there you have magnetic shielding.  https://www.mumetal.com/about_mumetal.php

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4 hours ago, StringJunky said:

Does it mean it has a higher tendency to attract the field into the body of itself?  I noticed in one description it said the electrons -  presumably the outer ones - are 'unordered'. Does that mean that they can absorb the magnetic flux until they reach a point of saturation and then become ordered, like an ordinary magnet?

It is very expensive... wow!

Field lines "prefer" to be in the high mu region but it's not attraction, per se. IIRC a bar of such material put in a uniform field would result in something that looks like a dipole.

The domains align when there is an external field.

4 hours ago, Hami Hashmi said:

There is a description here: https://www.mumetal.com/about_mumetal.php

Ok but in an electromagnet the wire wrapped around the core (which is mostly soft iron) generates a magnetic field when a current is passed through it, which then is amplified by the core. But in a shield, soft iron absorbs the magnetic field instead of enhancing it. So why is that?

The same thing is going on. A shield doesn't absorb the field - it doesn't disappear. The flux lines go into the shield, rather than through it. They exit the shield as well. The interior - the shielded region - has a substantially smaller field.

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11 minutes ago, swansont said:

Field lines "prefer" to be in the high mu region but it's not attraction, per se. IIRC a bar of such material put in a uniform field would result in something that looks like a dipole.

The domains align when there is an external field.

The same thing is going on. A shield doesn't absorb the field - it doesn't disappear. The flux lines go into the shield, rather than through it. They exit the shield as well. The interior - the shielded region - has a substantially smaller field.

Cheers.

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3 hours ago, swansont said:

Field lines "prefer" to be in the high mu region but it's not attraction, per se. IIRC a bar of such material put in a uniform field would result in something that looks like a dipole.

The domains align when there is an external field.

The same thing is going on. A shield doesn't absorb the field - it doesn't disappear. The flux lines go into the shield, rather than through it. They exit the shield as well. The interior - the shielded region - has a substantially smaller field.

Yeah I understand that it doesn't completely absorb the field.

But what if there was a soft iron box with a magnetic material inside, and a magnetic field was produced outside of the box, would the soft iron box enhance the magnetic field or diminish it?

Edited by Hami Hashmi
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40 minutes ago, Hami Hashmi said:

Yeah I understand that it doesn't completely absorb the field.

But what if there was a soft iron box with a magnetic material inside, and a magnetic field was produced outside of the box, would the soft iron box enhance the magnetic field or diminish it?

Enhance it where? The field will tend to be in the iron box. But both magnets will be attracted to the shield

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3 minutes ago, swansont said:

Enhance it where? The field will tend to be in the iron box. But both magnets will be attracted to the shield

Enhance it outside the box.

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11 minutes ago, Hami Hashmi said:

Enhance it outside the box.

No. The field will tend to be inside the iron.

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16 hours ago, swansont said:

No. The field will tend to be inside the iron.

Ok but if the had the same scenario but with wires wrapped around the box and a current passed through them and no external magnetic field, would the magnetic field then be enhanced outside the box?

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2 hours ago, Hami Hashmi said:

Ok but if the had the same scenario but with wires wrapped around the box and a current passed through them and no external magnetic field, would the magnetic field then be enhanced outside the box?

That's going to generate a magnetic field, which will be enhanced in the shielding material. Outside, not really. Depending on the shapes and sizes involved, you might modify the dipole pattern emanating from the box.

You use a core in a transformer to ensure a strong field is in both the primary and the secondary. It doesn't enhance anything outside.

Here's what a the fields looks like when a cylindrical set of shields is placed in a uniform vertical field. The intensity follows the color spectrum (red is the most intense field, blue is least. There is a small field present in the innermost cylinder)

IIRC the exterior field would be depicted as orange

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32 minutes ago, swansont said:

That's going to generate a magnetic field, which will be enhanced in the shielding material. Outside, not really. Depending on the shapes and sizes involved, you might modify the dipole pattern emanating from the box.

You use a core in a transformer to ensure a strong field is in both the primary and the secondary. It doesn't enhance anything outside.

Here's what a the fields looks like when a cylindrical set of shields is placed in a uniform vertical field. The intensity follows the color spectrum (red is the most intense field, blue is least. There is a small field present in the innermost cylinder)

IIRC the exterior field would be depicted as orange

Ok thanks.

So outside the box and above the box the field would be strongest. Does the box enhance the field in the red areas?

So the main difference between a soft iron electromagnet and a soft iron shielding box is that the box is hollow and the magnet is solid? Is that what causes the difference between them?

Edited by Hami Hashmi
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3 hours ago, Hami Hashmi said:

Ok thanks.

So outside the box and above the box the field would be strongest. Does the box enhance the field in the red areas?

In this case yes; the original field was uniform. Now the flux lines preferentially go through the long dimension of the shields instead of passing through inside or outside.

Quote

So the main difference between a soft iron electromagnet and a soft iron shielding box is that the box is hollow and the magnet is solid? Is that what causes the difference between them?

The electromagnet or transformer core is not a magnet itself. It concentrates the flux lines from the windings so that the north/south pole is stronger. Flux lines that might have left the windings further away are now in the core. In a shield, flux lines that might have entered the interior are instead in the shield walls.

edit:

This is a different geometry (transverse to the cylinder instead of aligned with it) but it shows field lines outside but nearby the shield bending toward it, for the lines entering, and bending away when leaving. The field inside is small.

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1 hour ago, swansont said:

In this case yes; the original field was uniform. Now the flux lines preferentially go through the long dimension of the shields instead of passing through inside or outside.

The electromagnet or transformer core is not a magnet itself. It concentrates the flux lines from the windings so that the north/south pole is stronger. Flux lines that might have left the windings further away are now in the core. In a shield, flux lines that might have entered the interior are instead in the shield walls.

edit:

This is a different geometry (transverse to the cylinder instead of aligned with it) but it shows field lines outside but nearby the shield bending toward it, for the lines entering, and bending away when leaving. The field inside is small.

Thanks for the explanation! So basically in an external field a shield will nullify the field inside it but enhance it outside around the north and south poles?

Basically, yes.

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Alright thanks for your help!

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Is there any known material or process that will not enhance the outside field but block the field inside?

bump

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