Magnet Myths and Misconceptions

[minnie]

    Koala,
            If we made a hypothetical permanent magnet out of individual atoms would
they all line up or would there be a "fault" somewhere in the middle?
                         John.

[dieter]

An other aspect that I want to mention is:


there are two modes of activity of permanent magnets. One is the Seeking mode and the other one is the Jet Stream mode. A pole is always trying to connect to an opposite pole. If it can't, it will use the surrounding air to link with its own opposite pole. Which is very "unsatisfying". This way the common iron filing images are made.


This trough-the-air-link is so unsatisfying that the poles keep on seeking for a better link.


If we introduce a piece of iron now, stick it to one pole, the pole loves the permeability and seeks for opposite poles in every corner of the iron.
We stick a nail to the other end of the iron, the pole now even seeks in the entire nail, although airgaps and distance naturally lower the strength.
We stick a 2nd magnet to the iron, with the same pole. Now they share the iron, are both seeking, the nail sticks more. They do not want to be close to one another, but basicly they share the iron in their seeking.


Now we remove one magnet , turn it around and stick it to the iron with the other pole. Surprise. No seeking anymore. The nail drops off. The magnets build a rather compact path, now they're in Jet stream mode. Most of the iron became rather unmagnetical, IF both magnets have the same strength.


When you put two magnets on oneanother, NS NS, then they unite to one magnet the Bloch wall is located in the middle between them.


When you put them together NS SN, then the Bloch walls of the two remain in place.


BR

[MarkE]

I need one of them ipads for when im on the road ,this phone screen to bloody small.
I have come up with a doozy idea as to how we can see the exact field shape of a PM. This I will do right after theo's free heat generating bismuth do'hicky experiment..I am about 1100km away from my workshop ATM, but should be home by the weekend, when I will whip up a quick demo of the non generating field.
@MarkE
What are the arrows on the field lines supose to represent-the flow of what?

They indicate orientation.

[MarkE]

    Koala,
            If we made a hypothetical permanent magnet out of individual atoms would
they all line up or would there be a "fault" somewhere in the middle?
                         John.

If one could construct perfectly stacked crystals of magnetic material, then the domain walls would be lined up like paving stones.  If the material were not magnetized, then the orientation of each domain would be randomly rotated.  Magnetizing the material would progressively bring the domains into greater and greater alignment.

[TinselKoala]

    Koala,
            If we made a hypothetical permanent magnet out of individual atoms would
they all line up or would there be a "fault" somewhere in the middle?
                         John.

See MarkE's response above. With domains in the material, the "paving stones", which are much larger than individual atoms, you get something like this:

N-S|N-S|N-S|.... and so on, like a chain of tiny magnets all stuck together. Each "N-S" pair represents a single "paving stone" and the "|" symbol represents a true Bloch wall, as according to the definition, between the domains, I think. No "Bloch wall" _inside_ the individual domains, but Bloch walls between them, just as in the definition of the Bloch wall and in Synchro's photomicrograph images of randomly-aligned domains. And no special "Bloch wall" at the "equator" of a permanent magnet made up of mostly aligned domains. If there is a bulk, macroscopic "Bloch wall" anywhere, it is at the junction of _two_ permanent magnets stuck together in the usual way by mutual attraction of unlike poles. This does not represent the situation at the "waist" of an individual permanent magnet, though.

For individual atoms the magnetic dipole is formed by the spin of unpaired electrons in specific outer shells (electron orbitals). "Unpaired" because the normal pairing of opposite-spinning electrons in a single orbital causes their fields to cancel.  The "domain" is a much larger region where the atoms are aligned so that all or most of the electron spins are in the same direction, reinforcing each other and creating "mini-permanent magnets" which are the domains themselves. In an unmagnetized material these domains are themselves randomly oriented, even though within the domains the individual unpaired electron spins of the atoms concerned are aligned. To magnetize such a material one brings the domains into alignment like MarkE's paving stones, by any of a number of means. You can physically align a piece of the material to an external field, even the Earth's relatively weak field, and start aligning domains by striking the stuff with a hammer, even. Or you can supply a stronger pulsed external field and align the domains that way. In a "soft" magnetic material the domains are easy to align and easy to "unalign" again. In a "hard" material they stay aligned even when the original aligning field is removed.