I know that magnetically hard materials hold magnetic fields (magnetic moment alignment) for longer, while soft metals do not. However, what, internally, causes these properties to arise?
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A ferrous metal consists of microscopic domains each of which is a tiny magnet with its poles pointing in a random direction. The random domain orientations in a bulk sample mean that the whole chunk of material exhibits no net (macroscopic) magnetic field. To permanently magnetize a sample of iron requires that the domain boundaries be shifted about so most of them are pointing in the same direction.
On a microscopic level, it requires work to move the domain boundaries around within the crystalline structure of the iron and thereby leave the sample with a net magnetic field. The amount of work required varies considerably with variations the crystalline structure of the iron, and the easiest way to change the microstructure is by introducing tiny amounts of carbon into the iron crystals i.e., making it into steel.
Pure iron contains no carbon and is mechanically soft- and this pliability reduces the amount of work required to reorient the magnetic domains in it. Steel is iron with a small amount of carbon dissolved in it and is mechanically harder than iron. Its lower pliability makes it harder to reorient the domains and it takes more work to magnetize.
Conveniently, then, the mechanical hardness of a magnetic material is well-correlated to the difficulty of magnetizing or demagnetizing it. A magnetic material that is hard to demagnetize is also mechanically hard and is called a hard magnet. A soft magnet is easily demagnetized and also mechanically soft.
Other alloying agents besides carbon (nickel, chrome, cobalt, and aluminum are examples) are commonly used to make hard magnets, and some of them are so mechanically hard that it is nearly impossible to saw or drill them without causing them to shatter to pieces.
answered Jun 11 at 16:20
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$\begingroup$ But don't domains depend on the net angular momentum of the atoms (electron spin)? Why should that have anything to do with mechanical strength? $\endgroup$ Commented Jun 13 at 14:41
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$\begingroup$ mechanical strength is derived by making atomic-level movement (dislocation travel, grain-boundary sliding, etc.) difficult, This also makes domain realignment difficult. $\endgroup$ Commented Jun 14 at 1:37
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$\begingroup$ Okay, but my question was how these properties arise, fundamentally: why is it easier to move magnrtic domains in soft materials (what defines the strength of the material)? $\endgroup$ Commented Jun 14 at 6:37
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$\begingroup$ recommend two materials science textbooks: Van Vlack or Shakelford. $\endgroup$ Commented Jun 15 at 4:50