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Let me see if I get it right. When an iron bar is attracted by a permanent magnet it becomes a magnet itself because all of its magnetic domains start to point in the same direction. When the iron bar is no longer attracted by the permanent magnet, it is no longer a magnet itself because its magnetic domains point in different directions again.

When iron is heated up to curie temperature and cooled down all of its magnetic domains also start to point in the same direction. ( If I am not wrong the atomic structure does not change)

So why is it permanent in the second case and not in the first ? (Correct me if I messed up something here)

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    $\begingroup$ Residual magnetic strength varies with the relative freedom of movement for the atoms. It is not uncommon in your first case for iron to remain magnetic, but when iron is heated more atoms line up easier and then when cooled tend to remain more magnetic. Introducing an electric field during the cooling will also help create a stronger magnet as it aligns even more atoms. Cooling tends to freeze them into place...thus becoming 'permanent'. $\endgroup$ – user6972 Nov 1 '14 at 4:10
  • $\begingroup$ I feel that nobody has really answered this question. $\endgroup$ – Fattie Nov 6 '14 at 16:20
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Ferromagnetic materials exhibit a long-range ordering phenomenon at the atomic level which causes the unpaired electron spins to line up parallel with each other in a region called a domain. Within the domain, the magnetic field is intense, but in a bulk sample the material will usually be unmagnetized because the many domains will themselves be randomly oriented with respect to one another. Ferromagnetism manifests itself in the fact that a small externally imposed magnetic field, say from a solenoid, can cause the magnetic domains to line up with each other and the material is said to be magnetized. The driving magnetic field will then be increased by a large factor which is usually expressed as a relative permeability for the material. There are many practical applications of ferromagnetic materials, such as the electromagnet.

Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field. This tendency to "remember their magnetic history" is called hysteresis. The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets.

Within this framework, your statement

When the iron bar is no longer attracted by the permanent magnet, it is no longer a magnet itself because its magnetic domains point in different directions again.

is not true. Some magnetization remains, depending on the material and its characteristics. It is called the hysteresis of the material.

hysterisis loop

When iron is heated up to curie temperature and cooled down all of its magnetic domains also start to point in the same direction. ( If I am not wrong the atomic structure does not change)

Note: if there exists a magnetic field to line up against, otherwise it will be random.

So why is it permanent in the second case and not in the first ? (Correct me if I messed up something here)

Permanence depends on the hysteresis characteristics of the material as it responds to an external magnetic field.

At atomic dimensions, the domain model, see figure and links, explains what happens. Domains get energy from external fields to orient in the direction of the field, and depending on the magnetic characteristics of the unpaired electron spins in the domain, can remain there, in a metastable state, until energy is supplied to move them again, (as heat or another magnetic field).

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  • $\begingroup$ "Domains get energy from external fields to orient in the direction of the field" Should not domains give up energy when they align with the external field? $\endgroup$ – Ján Lalinský Feb 7 '15 at 10:32
  • $\begingroup$ @JánLalinský I am thinking metastable as in a flat table with a lot of shallow holes. A ball will stay in the hole but can roll out given some energy, which it will give up again rolling in a different hole. I guess it depends on the collective cohesive forces domain-domain, which is what makes the constants that define the materials and the hysterisis loop. $\endgroup$ – anna v Feb 7 '15 at 10:42
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when we apply magnetic field to any ferromagnetic substance and remove it , small fraction of magnetic strength remains. for completely demagnetize it, an opposite magnetic field is needed this magnetic field is called coercivity of the material.this is different for different materials. coercivity is low for iron, so that it demagnetize easily and it is very high for the materials by which permanent magnet is formed. so greater magnetic magnetic field is needed to demagnetize permanent magnet.

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  • $\begingroup$ This answer seems to be more about how to demagnetize a magnet, rather than why a permanent magnet is permanently magnetic. $\endgroup$ – Kyle Kanos Nov 6 '14 at 15:41
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When you cool it the domains so formed align to get minimum energy and that is not the energy in which all are pointing in the same direction.

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  • $\begingroup$ There is a competition between weak long range (varies as 1/r3) magnetic dipolar interactions favoring spins in opposite direction and the strong short range electron electron interaction favoring triplet (parallel spin) over singlet states. When dipoles are strong enough as when many spins are pointing in same direction as in a domain then they are able to overcome the short range triplet interactions and domains tend to point in opposite directions. This determines how much magnetized a material is. $\endgroup$ – SAKhan Feb 7 '15 at 17:43

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