If an iron is exposed to an external magnetic field, the iron's domains are all aligned with that ext-B . If I strike it with a hammer, will it be demagnetized?
Comparing it with heat, I know the Tc point, but with vibrations what is the point like in heat? When would I expect the iron to be temporarily demagnetized?
How much force must I apply to move those domains apart by hammering it?
Best Answer
You don't give any sources, and some of your statements are not universal in the best case. Iron's domains are not necessarily aligned along the external magnetic field (e.g., they can be in a metastable state with low average magnetization). Furthermore, striking iron with hammer can actually be used to magnetize iron in external magnetic field (see, e.g., http://www.youtube.com/watch?v=GaD9vAuj20s , where an iron pipe is magnetized in the weak magnetic field of the Earth using gentle persuasion with a hammer:-) ) - as a result of hammering, iron domains transition to a more stable state with larger average magnetization.
EDIT (10/15/2013): I am afraid I have similar problems with the edited question. Your statement "If an iron is exposed to an external magnetic field, the iron's domains are all aligned with that ext-B" is not universally correct. For example, a permanent magnet's magnetization typically does not follow the direction of the Earth's magnetic field.
In general, there is an equilibrium (stable) state of a ferromagnetic for fixed values of temperature, pressure, external magnetic field, etc. However, the ferromagnetic is not necessarily in this stable state, it can be in a metastable state, e.g., because of domain friction. As far as I understand, what hammering does, it helps overcome domain friction. As a result, the ferromagnetic tends to approach the stable state as a result of hammering. However, this stable state can have larger or smaller magnetization, or a different direction of magnetization than that of the metastable state, depending on the external parameters (temperature, pressure, external magnetic field). Therefore, hammering can either magnetize or demagnetize the ferromagnetic, depending on the relationship between the stable and the metastable state. The following link may be useful: http://capone.mtsu.edu/phys2020/Lectures/L12-L18/L17/Ferromagnetism/ferromagnetism.html