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I was reading and I saw that the steel becomes permanent magnet whereas iron becomes a temporary magnet after magnetization during magnetic induction.

Why does soft iron make temporary magnets and steel permanent one?

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I think that whether an object is a soft magnet or a hard magnet has more to do with its microstructure than what material it is made of. Under a strong magnetic field, the magnetic domains of both a soft magnet and a hard magnet orient themselves along the direction of the magnetic field. But for a hard magnet the movement and rotation of the magnetic domains is hindered to that they remain largely aligned even when the magnetic field is removed. What hinders the movement and rotation of magnetic domains in a hard magnet? Microstructural features such as grain boundaries and local defects. In contrast, the magnetic domains in a soft magnet can move and rotate relatively freely, so when the magnetic field is removed the domains all revert to a fairly randomized state.

For the case of soft iron versus steel, soft iron has a very low carbon content so that it is relatively pure iron with no carbon-related defects which can act to pin and hinder the movement of magnetic domains. Steel, on the other hand, has a higher carbon content so there is more potential for pinning action.

Of course, there are many, many different types of steel and so this picture is a bit simplified. There is, for example, an iron-silicon steel alloy which apparently makes a very good soft magnet, presumably because the silicon atoms don't act as effective pinning sites.

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protected by Emilio Pisanty May 24 '18 at 22:59

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