Structural difference between soft and hard ferromagnets

[hokhani]

TL;DR

Why the hard ferromagnet can retain its magnetic dipole allignment while the soft one can not?

In the ferromagnetic materials not only the atoms have magnetic dipoles but also the dipoles are aligned well in different domains. However, what is the differnce berween atomic structure of a soft ferromagnet like iron and a hard ferromagnet like a bar magnet? My first guess is that the atomic dipoles might be stronger and more dense in the hard ferromagnets so that the exchange interaction between the dipoles don't let the external magnetic field affect them as easily as soft ferromagnet. Could anyone please explain exactly that? Any help is appreciated.

 

[Hardy]

From what I can tell, the classification of a soft vs hard magnetic material is more of an engineered property.

To put it simply, elements have various levels of being able to be magnetized and to hold onto that magnetization. Magnetization is caused by spinning electrons.

Materials with complete shells have electrons spinning in both directions in each shell, thus the magnetic effects cancel and are not as magnetically interactive. Most common magnetic elements have outer shells not complete, with more shells with single electrons, that create the 'uncanceled' dipole moment (FE, Ni and Co). If these material are exposed to external magnetic field, these dipoles align and persist. (Side note - interesting I am guessing its not likely to find many 'pre-magnetized' magnets in nature).

Back to the hard and soft, doing some brief searches, it appears hard magnets (like speaker magnets) are more pure forms of these elements, and would expect that crystalline structure of the material is made to optimize the magnetic effects. The soft magnetic material (like power transformers) are again the material than can be magnetized, but infused with other elements to reduce the ability to retain the magnetization (silicon steel was a term that came up). Hence, why it appears to be more engineered properties than elemental properties.

So, while the number of available dipoles is a factor, it looks like it could be how much they are able to interact is a significant factor. This input only came from a half hour review of an old material science text book, for what its worth.

 

[nasu]

The magnetization and demagnetization process is due to the motion of the domain walls. The hard/soft quality is related to the energy needed to move the walls. It is usually not a matter of the nature of atomic dipoles but of the grain structure. You can have steel as a hard or soft material depending in thermal treatment and impurity level.

 

[jeffinbath]

TL;DR Summary: Why the hard ferromagnet can retain its magnetic dipole allignment while the soft one can not?

In the ferromagnetic materials not only the atoms have magnetic dipoles but also the dipoles are aligned well in different domains. However, what is the differnce berween atomic structure of a soft ferromagnet like iron and a hard ferromagnet like a bar magnet? My first guess is that the atomic dipoles might be stronger and more dense in the hard ferromagnets so that the exchange interaction between the dipoles don't let the external magnetic field affect them as easily as soft ferromagnet. Could anyone please explain exactly that? Any help is appreciated.

 

[jeffinbath]

Inside every “permanent magnet” there is an internal demagnetizing force trying to demagnetize it. I find it useful to think of a “permanent magnet” as being like a box of matches with the match heads being little N poles that are all repelling each other but are being forced to lie next to each other by the strength of the surrounding substrate. If the magnet is heated then at a certain temperature (known as the Curie temperature) the substrate strength weakens enough to allow all these little magnetic dipoles to randomize their orientations. Even without such heating the traditional metal magnets can be weakened by being bashed about a lot.