Ferromagnetism and induced magnetism

Question

According to Wikipedia:

A magnetic domain is a region within a magnetic material where individual magnetic moments of the atoms are aligned with one another and point in the same direction thus resulting in uniform magnetisation in that region.

Below Curie temperature, a piece of magnetic material such as iron spontaneously divides into separate magnetic domains rather than stay in a state with magnetisation in the same direction in order to minimise its internal energy.

A large region of ferromagnetic material with a constant magnetisation throughout will create a large external magnetic field which requires a lot of magnetostatic energy.

To reduce this energy, the magnetic material can split itself into smaller domains which acts to contain the magnetic field within itself in closed loops, with small amounts of field outside the material. However, the domain structure of actual magnetic materials does not usually form by the process of large domains splitting into smaller ones. The equilibrium domain configuration simply appears when the material is cooled below Curie temperature.

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Curie temperature is the temperature above which certain materials lose their permanent magnetic properties to be replaced by induced magnetism, what does "permanent magnetic properties" mean exactly? Would spontaneous magnetism in ferromagnetic materials be one of these properties(1)? Or is it referring to the state where magnetic domains aligned with an external magnetic field are pinned in place by defects in the crystal lattice(2)?

If it refers to the second point(2), what's the difference between ferromagnetism and induced magnetism (or permanent magnets and temporary ones)? How can iron, for example, be a ferromagnet and an induced magnet? Is induced magnetism similar to paramagnetism?

I'm not too sure what exactly is meant by the second paragraph; does it mean that when all the magnetic domains are aligned in the same direction by an external magnetic field and when that external field is removed and the material is cooled below the Curie temperature, it will "lose" its external magnetic field? But then what happens when you heat it above the temperature, does it just cause the magnetic domains to return back to its initial lowest energy state or does it destroy these domains? I'm confused because this is what was mentioned on the site:

Aren't paramagnetic materials materials that don't have magnetic domains?

Answer 1

Wikipedia describes well, how iron maintains its magnetic domain structure below the Curie temperature:

Below its Curie point ... iron changes from paramagnetic to ferromagnetic: the spins of the two unpaired electrons in each atom generally align with the spins of its neighbors, creating an overall magnetic field.

If there is a question about spin, one should remember that spin and magnetic dipole of a subatomic particle are parallel (or anti-parallel) oriented and the overall field is made from the aligned individual magnetic moments of the involved electrons.

The same article explains how magnetic domains are created:

In the absence of an external source of magnetic field, the atoms get spontaneously partitioned into magnetic domains, about 10 micrometers across, such that the atoms in each domain have parallel spins, but different domains have other orientations. Thus a macroscopic piece of iron will have a nearly zero overall magnetic field.

In fact, the atoms vibrating under heat, during their cooling down are no longer disturbed and, as happens with permanent magnets, they form clusters respectively domains.

No to your questions:

Curie temperature is the temperature above which certain materials lose their permanent magnetic properties to be replaced by induced magnetism, what does "permanent magnetic properties" mean exactly?

A material with permanent magnetic properties is a material where the involved electrons with their magnetic moments are aligned in such a way that a overall magnetic field outside the material exists. A material with domains has also measurable magnetic fields near the surface, but these fields are directed in different directions and cancel each another out at bigger distances.

To make a permanent magnet, one applies a strong external field. This works for some materials at room temperature, but better on higher temperatures with a following cooling, still applying the external magnetic field. Simply more domains are alignable at higher temperatures.

Induced magnetism means the conversion of magnetizable material into a magnet.

does it mean that when all the magnetic domains are aligned in the same direction by an external magnetic field and when that external field is removed and the material is cooled below the Curie temperature, it will "lose" its external magnetic field?

To say it explicitly, if one remove an external magnetic field above the Curie temperature, the induced magnetic field gets destroyed again. This happens because of the heat vibrations. Cooling the material, still with the applied external field, the material will be a permanent magnet.

Answer 2

Q: what's the difference between ferromagnetism and induced magnetism (or permanent magnets and temporary ones)?

A: The answer lies in the B vs H curve of the material. If you google "soft magnet" and "hard magnet" you will find more details. Hard magnets can be made permanent because of what is called curve hysteresis. If you apply an external field and then take it away, some magnetism remains (also called "remanent magnetic field"). For a soft magnet, the hysteresis effect is minimal (ideally zero). So the material will magnify an applied field but the effect disappears when the applied field is removed. When you see equations assuming constant permeability (mu) they generally are assuming soft magnets.