MadSci Network: Physics

Re: How does heat affect the paramagnetism of iron.

Date: Thu Oct 5 06:51:26 2006
Posted By: David and John Free, Post-doc/Fellow, MFA, MFA
Area of science: Physics
ID: 1148881213.Ph

Wow, Kevin, what a neat experiment!

I do not feel qualified to answer your question in any beyond the most 
general terms, but until you find someone better, here is where to start:-

Iron (and certain other alloys - even some containing ONLY all-
nonmagnetic elements like Al, B and P and C - are magnetic because of 
their molecular arrangement (crystal structure and microstructure). In 
the case of iron it has unfilled inner electron-shells and the result is 
each atom has a net electron-spin (instead of them cancelling out in 
pairs).  The result is at LOW temperatures the clusters of iron molecules 
join up in loops and chains (domains). The overall lump of iron is NOT a 
permanent magnet as these domain-fields are closed on themselves.

But subject to an external field they will line up as though like little 
magnets - hence the permeability (increased B field) inside the iron. 
With "soft" (pure annealed) iron when we REMOVE the external field the 
iron goes back into its merely internally-magnetised state. But if it 
is "hard" (impure, alloyed with C or Cr)the domains (to some extent - 
look up coercivity in Wikipedia) RETAIN their configuration (alignments) 
and you have a not-very-permanent magnet.

But drop it and the clang (sound waves) disturbs the domains who long to 
return to their relaxed, short-circuited, state. Or heat it up and at the 
Curie temperature the electon-state energy is enough for the domains to 
disappear. This is because they are DUE TO the unbalanced electron-spin 
within each molecule (atom - but it is probably an alloy and 
crystalline!) all the electrons MUST be in the CORRECT electron energy-
shells for the iron to be magnetic. But as the temperature is raised the 
electron-states get "pumped up" to higher energy-levels (energy state) - 
see Boltzmann temperature kT in Wikipedia.

Now here is an experiment you really SHOULD do, Kevin.  Get a foot of 
iron wire (bare garden wire should work). Hang it vertically and pass an 
electric current through it. Not too much of you will fuse it!  When is 
is bright red hot SLOWLY reduce the current and WATCH carefully. SUDDENLY 
at various temperatures (colours) the iron wil REGLOW HOTTER for maybe a 
second. This will happer two or three times as you reduce the current 
eventually reaching zero!

What is happening is the iron atom groups are ALWAYS seeking to arrange 
themselves in a stable state. But THAT depends on the TEMPERATURE because 
each electron has an energy fixed by Boltzman's kT. So the iron crystal 
structure (molecular arrangement) has SEVERAL stable configurations - 
each stable is SOME temperature range!

With each configuration there is either a giving OUT of heat or a taking 
in. The ones that glow brighter are the ones that GIVE OUT heat as the 
change takes place (Have you done Le Chatelier's principle in Chemistry 

So the magnetism of iron DEPENDS on its crystal structure (molecular 
arrangement) so EACH of those various stable-states of iron have 
DIFFERING permeability, coercivity etc etc.

The hotter the iron the more FREE the electons are moving farther and 
faster independent of the atom cores. A cloud of electrons would indeed 
be repelled by a magnet (paramagnetic).

By the time the iron is WHITE hot its electons have enough energy 
actually to ESCAPE the iron (see Thermionic emission in Wikipedia), but 
with "nowhere to go" the positive charge left behind attracts them back, 
mostly; quite unlike what happens in a vacuum tube or thyratron.

Kevin, I will tell you a sectret. The REAL fun is to FIND OUT for 
yourself by thinking up experiments! That is what I used to do and by the 
time I got to college I knew it all first hand already - all except the 
maths (which NOW had a MEANING instead of just being the abstract symbols 
it was to the other students.)

So happy experimenting: let me know what you discover!


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