Magnets and iron

[El Zagna]

Please forgive any abuse of technical terms in this post. It’s been a long time since my last chemistry class.

The reason that iron is attracted to magnets has to do with the imbalance of the electron shells – at least that’s what I understand from this (http://www.straightdope.com/classics/a3_339.html ). When iron combines with oxygen to form rust (ferrous oxide) that substance is not magnetic even though it is iron based because the oxygen atoms match up with the iron atoms in such a way that the electron shells are in balance. Is that right?

So, if it’s all about imbalanced electron shells, one would expect that there would be substances that are made up of none of the ferromagnetic elements but are magnetic anyway, just because when the way the atoms come together to form molecules, the outer electron shell gets left out of balance. Do such things exist?

Does this question make any sense?

[raygirvan]

I don't know the theory well enough to comment, but yes, such things exist: Heusler alloys, which are ferromagnetic, but made of compositions such as Cu2MnSn.

[Bakhesh]

I remember going to a chemistry lecture where the lecturer said that many elements show magnetism, but it is much less pronounced than the effect in iron, nickel and cobalt.

The lecture was about liquid oxygen. He put a test tube of liquid oxygen on a long pendulum, and was able to show how the tube moved when a magnet got near it. It didn't move very much, but it did move.

[alimarx]

A regular fridge magnet will attract a slow, steady stream of water coming out of the tap.

[Chronos]

Bakhesh, that's paramagnetism, not ferromagnetism. The two are similar insofar as both paramagnetic and ferromagnetic materials are attracted to a magnet. The key difference is that a ferromagnetic material will remain at least somewhat magnetized, even after you take the original magnet away.

There's also diamagnetism, where a substance will (weakly) be repelled by a magnet. All (or nearly all; I'm sure there are some oddball exceptions) substances are magnetic in some way, with most things (including water and most organic materials) being diamagnetic, some things being paramagnetic, and a very few being ferromagnetic. But since ferromagnetism is typically much stronger than the others, it's the only one you notice.

[raygirvan]

Bakhesh: true; however, this takes us into the various types of magnetism. For instance, liquid oxygen is paramagnetic, not ferromagnetic.

[El Zagna]

Paramagnetism, diamagnetism, ferromagnetism; is it fair to say that they are all based on the same physical property, i.e., the outer electron shell being out of balance with the rest?

In the above link (http://www.straightdope.com/classics/a3_339.html) it says: Take a gander at the third subshell of the M shell of iron, for example. (A shell is an electron's orbit. Electrons are rigidly organized into layers of shells, with so many electrons per shell.) What a wacky sight! We find five electrons with a positive spin and one with a negative spin. This gives the iron atom a pronounced magnetic field. So it looks like the thing that gives iron its strong magnetism is the 5 to 1 imbalance in an electron shell. So is this one of those exponentially increasing things such that a 2 to 1 imbalance will show a barely perceptible magnetism, 3 to 1 is still pretty much nothing, and you need to get to 5 to 1 before things really start happening? If so are there substances with 6 to 1 or greater imbalances?

[Mangetout]

I seem to remember something a while back about a completely nonmetallic polymer magnet; it was incredibly weak magnetism IIRC.

[Jake4]

Originally posted by Chronos
Bakhesh, that's paramagnetism, not ferromagnetism. The two are similar insofar as both paramagnetic and ferromagnetic materials are attracted to a magnet. The key difference is that a ferromagnetic material will remain at least somewhat magnetized, even after you take the original magnet away.

There's also diamagnetism, where a substance will (weakly) be repelled by a magnet. All (or nearly all; I'm sure there are some oddball exceptions) substances are magnetic in some way, with most things (including water and most organic materials) being diamagnetic, some things being paramagnetic, and a very few being ferromagnetic. But since ferromagnetism is typically much stronger than the others, it's the only one you notice.

Diamagentism and paramagnetism are opposites. Paramagnetism is brought about by unpaired electrons, diamagnetism by paired electrons. Since chemical bonds are pairs of electrons shared by atomic nuclei, virtually every material is diamagnetic. Non-bonding paired electrons (such as those in filled shells) also produce diamagnetism. But, like Chronos said, this effect is very very weak, so we don't bother to think about it much/at all.

[Mort Furd]

Originally posted by alimarx
A regular fridge magnet will attract a slow, steady stream of water coming out of the tap.

Ehh. I'll have to try this out. I seem to remember reading (in a very old physics book) that water was slightly diamagnetic and actually would be repelled by a magnetic field.

[Trinopus]

The demonstration I saw in physics class that impressed me was when the prof showed that a big permanent magnet can't pick up a piece of copper...but it can sure slow copper down! He took a spinning disk of copper...and when he put the disk betwen the poles of the (large!) horseshoe magnet, the disk slowed to a halt. The prof also took a strip of aluminum and dropped it, so that it fell, freely, between the magnet's poles. It fell slowly...then, once free of the magnet, fell at a more normal acceleration.

So my question is: this is "ordinary" magnetism, isn't it?

Trinopus

[mok]

Ferromagnetic elements include the following (more than most people realize):


Iron
Nickel
Cobalt
Gadolinium
Dysprosium ( only magnetic when it's real cold, < 85K )


I've always wanted an opportunity to mention this. :)

Ferromagnetism is a quantum effect that has a macroscopic effect which is quite visible. Here's a good page with explanations: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html

-mok

[raygirvan]

Trinopus: that's 'eddy current damping' and to do with current/field interaction rather than magnetic properties of materials. When a moving conductor cuts a magnetic field, eddy currents are induced in the conductor, which interact with the field to oppose the movement.

[raygirvan]

Mangetout: there's an article about one here (http://nebraska.statepaper.com/vnews/display.v/ART/2001/11/15/3bf45ba7e7677). Weak, as you say, and only stable at cryogenic temperatures. A Google search finds a few references to compressed buckyball sheet that's weakly magnetic (ferro, I assume) at room temperature.

[Mort Furd]

I'll be damned. The magnet does attract the water stream.

[raygirvan]

Hrrrrmmm [Marge Simpson noise]. I can't get it to work even with the big magnet from my Levitron base, and I'm a bit suspicious as to why it should attract anyway (as you say, water is diamagnetic). Maybe this is about electrostatics?

[Q.E.D.]

Originally posted by Mort Furd
I'll be damned. The magnet does attract the water stream. I doubt it's the magnetism. I have some really high-energy rare earth magnets, and I can't get the stream to flinch a bit. I'm with raygirvan on the electostatic idea.

[raygirvan]

I'm interested, though. Those who've got it to work: what precisely was the procedure?

[Mangetout]

Apparently you can get a powerful magnet (one of those neodymium jobs) to push a dimple in the surface of water. I tried the stream thing and got nothing.

[raygirvan]

Yep; Diamagnetic Water (http://www.otherpower.com/diamagh2o.html) shows the setup and a couple of other experiments that demonstrate the (very small) effect.

[Mort Furd]

Originally posted by raygirvan
I'm interested, though. Those who've got it to work: what precisely was the procedure?

Let a slow but continuous stream of water run from the faucet.
Line yourself up so that you look straight through the stream at some object on the wall of the sink. My sink has a screw holding a grid over an overflow outlet just below the faucet. I sighted on the center of the phillips head screw.
Brace yourself in this position with one hand. Be careful not to wobble.
With your free hand, bring the magnet close to the stream.
You must be careful not to wobble during this part.
Watch carefully, and you will see the position of the screwhead change ever so slightly with respect to the stream.
The motion is minute, you'll probably only see it as a change in the distortion of the screwhead.
Repeat several times, and be aware of any wobbling. It does move.

[raygirvan]

No luck yet. I tried that setup, and another: a keyring laser blu-tacked to the sink, projecting the shadow of the stream on the far wall of the kitchen.

[raygirvan]

I'm also wondering about air pressure effects. Moving liquid stream close to an object: would that give reduced air pressure in the gap, tending to draw the stream toward the object?

[DougC]

- - - Somewhat Related-? I have just ordered some high-permeability mu-metal foil (.006" thick, ~$10.50 per square foot) for use in some transformers I am making. It's a nickel/iron alloy that conducts magnetic flux ~1000 times as well as ordinary steel. Someone on the net told me that if I wanted to play with a piece of the foil and a magnet that I should put the magnet inside a sock first. The reason he gave was that the foil is so thin and sticks so hard that it can be basically impossible to removed. One has to carve little slivers of it off at a time, because the whole thing will not peel off in one piece, and then all the little slivers stick just as hard. I ordered way more than I needed; anybody know any cool science-class-type experiments I can do with a bit of this stuff? It is normally used for magnetic shielding. People also said that it will "jump" onto a strong magnet from quite a ways away, so if it's really impressive I may post a video of that...
~

[El Zagna]

Originally posted by DougC
[BI have just ordered some high-permeability mu-metal foil (.006" thick, ~$10.50 per square foot) for use in some transformers I am making. It's a nickel/iron alloy that conducts magnetic flux ~1000 times as well as ordinary steel.[/B] Ah-ha! This seems to be getting at my follow-up question. It is the 5 to 1 imbalance of the electron shells that makes ordinary iron so magnetic. So what is the electron imbalance of your mu-metal foil?

[DougC]

- - - I have no clue, really. The place I ordered it from has some technical info on it:
http://www.mushield.com/home.html
........
- I have run across info online that says that instead of being mixed evenly, after annealing the material is bubbles of {several hundred iron atoms each} scattered throughout the nickel. This structure is the key to the high flux transmission, somehow. If you disturb the molecular structure (such as by bending a thick piece of the material, or by heating any thickness hot enough to upset the annealing) the flux conduction drops way down to not much above regular steel. ..... At any rate, this material is said to have the highest magnetic flux transmission available.
~

[bbeaty]

It's not "imbalance" in electron shells.

It's ELECTRON SPIN.

In other words, all electrons are actually little bar magnets (or more like tiny electromagnets running by perpetual motion.) Imagine an electron's spin axes as having little "N" and "S" labels.

If something causes a large group of electrons to all line up with their spin axes all pointing in the same direction, then this group of electrons will form an extremely powerful permanent magnet.

When the books start talking about imbalanced electron shells, what they really mean is that an even number of neighboring electrons can cancel each other out magnetically. How? Well, hat happens when you let two bar-magnets fall together? They slam together with north poles next to south poles, and on the whole the combination is no longer a magnet (i.e. all the field lines follow circular paths through the two magnets and don't spew out into space.) Imagine what happens with two horseshoe-magnets with N's next to S's. You end up with a ring, with all the field lines going in a loop. The N and S poles cancel out. The same thing happens if you let a bunch of sphere-magnets all fall together in a glob. They cling together strongly, but the overall blob of magnets has no main N or S pole. Electrons behave like this.

If you have an odd number of electrons, then there's one electron left over which does not pair up with a neighbor and get cancelled out magnetically.

Now ferromagnetism, that's just plain weird. As I understand it, the extra electrons (the "magnets") in each atom all rotate their spin axes so the "N" of one electron is pointing at the "S" of the electron in the next atom over. They form long chains, and at the end of each chain is an extra "N" or "S" with no neighboring electrons to cancel it out. In the space along the chain the magnetic fields are fairly weak. Many parallel chains form, with all of the spins pointing in the same direction. (There are other materials where the opposite happens, where every other chain points backwards, the anti-ferromagnets.)

I've heard that the distance between atoms controls everything, and if the spacing of the atoms in an iron crystal was very different, then iron would not be ferromagnetic. ALong the same lines, if you can juggle the atomic spacing by adding dopants, you can make the effect stronger or weaker, or make it vanish entirely. Or you can take a nonmagnetic substance and mix it with another non magnetic substance and create a ferromagnetic material.

[stuyguy]

The OP sez:
When iron combines with oxygen to form rust (ferrous oxide) that substance is not magnetic ...
Are you sure about this? Won't rust dust adhere to a magnet? Isn't/wasn't basic magnetic audio recording tape made with an iron oxide coating fused to a plastic substrate?

[El Zagna]

Originally posted by stuyguy
Are you sure about this? Oh, hell no. I'm not sure about anything I've written here.

[Ring]

Ferromagnetism is not simple phenomenon as the following post might make clear. If you would like a somewhat simpler explanation do a Google search on ”Ising Model"+ferromagnetism.

The simplest details that I know come from the Hartree-Fock theory, which is the zeroth-order many-body theory of electrons in solids (or atoms). Here's how one derives the Hartree-Fock equations for the electron energies: First, form a wavefunction called a Slater determinant which guarantees that the Exclusion Principle will be satisfied (by taking linear combinations of single-particle states which change sign when particles are exchanged). Next, take the <psi/H/psi> product to evaluate the energies, where H, the Hamiltonian, involves kinetic and Coulomb potential energies. Because of the form of the Slater determinant, there are TWO types of terms with coefficient e^^2, where e is the electronic charge. One is the simple repulsion of an individual electron to all the others. The other term is called the exchange energy because it involves an integral over products like (a_up b_down -- b_up a_down),
where a and b are single-particle wavefunctions and up, down refer to spin direction. This exchange energy is the fundamental origin of the itinerant electron ferromagnetism of transition metals like Fe, Ni, Co. It is not a concocted toy model like the Ising model; it comes from fundamental QM.

[raygirvan]

Isn't/wasn't basic magnetic audio recording tape made with an iron oxide coating fused to a plastic substrate?

There are different oxides of iron: the one used in magnetic media is Fe3O4 - a.k.a. magnetite - which is ferromagnetic. Fe2O3 and FeO aren't (rust, I think, is generally hydrated FeO).

[El Zagna]

Originally posted by Ring
The simplest details that I know come from the Hartree-Fock theory, which is the zeroth-order many-body theory of electrons in solids (or atoms). Here's how one derives the Hartree-Fock equations for the electron energies: First, form a wavefunction called a Slater determinant which guarantees that the Exclusion Principle will be satisfied (by taking linear combinations of single-particle states which change sign when particles are exchanged). Next, take the <psi/H/psi> product to evaluate the energies, where H, the Hamiltonian, involves kinetic and Coulomb potential energies. Because of the form of the Slater determinant, there are TWO types of terms with coefficient e^^2, where e is the electronic charge. One is the simple repulsion of an individual electron to all the others. The other term is called the exchange energy because it involves an integral over products like (a_up b_down -- b_up a_down),
where a and b are single-particle wavefunctions and up, down refer to spin direction. This exchange energy is the fundamental origin of the itinerant electron ferromagnetism of transition metals like Fe, Ni, Co. It is not a concocted toy model like the Ising model; it comes from fundamental QM. :eek: Well, I did ask for it, didn't I? Are you sure this isn't from Chomsky-bot (http://language.home.sprynet.com/lingdex/chombot.htm)?

[alimarx]

Originally posted by alimarx
A regular fridge magnet will attract a slow, steady stream of water coming out of the tap.

Originally posted by raygirvan
I'm also wondering about air pressure effects. Moving liquid stream close to an object: would that give reduced air pressure in the gap, tending to draw the stream toward the object?

In my OP, I was recalling an experiment I read about as a child, and performed subsequently many times (life of the party, I am), most recently 2 years ago. After reading through this tread, I figured I'd try it again being a little more empirical about it. Well, wouldn't ya know, dad burn it, it doesn't work now; magnet, fingertip, Barbie doll head, the stream wouldn't budge. This is the first time I've tried this in my current residence. Maybe the mineral content, or lack thereof, has something to do with it? :confused:

[raygirvan]

If you want to see it working, use a plastic ballpoint pen or comb: rub it on your sleeve (preferably wool or synthetic fibre) to charge it up with static. Then it'll bend the stream easily.

[DougC]

- - -Isn't that static electricuty? I have done that with a balloon....
....
- Anyway, the mu-metal arrived today. I am fairly underwhelmed. A magnet sticks to it about as well as any other steel object, maybe even a bit less than other steel objects. It came with an MSDS sheet saying it is what I paid (well) for. I don't have any regular steel .006 foil to compare, but magnets stick harder to the refridgerator, for instance.... <:/
~

[DougC]

- - - ?? Hmmm......
........
- I attempted to test it the only way I could thinkof with what I had on hand: by seeing how well the roll would shield the effects of a speaker magnet from my computer monitor, compared to a piece of ordinary 1/16" 4130 steel I had around. The magnet is a typical center-pole concentrated type with the iron concentrators still attached, the ceramic element is maybe 3/8" thick and 2' across. The mu-metal is on a large cardboard roll about 10-inches dia and 15 inches wide, wrapped around about 4.75 times. The steel is a sheet perhaps seven inches by twenty inches, and 1/16" thick. The result of subjective testing was that when either the steel or the mu-foil was placed directly up against the monitor with the mangnet on the other side, both shielded the monitor from the effects of the magnet completely......
???
-