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My dad has a HUGE magnet on his workshop.

I love magnets, and when I saw it, I asked him what it was for.

His reply was: "I don't know why, but inox steel bolts don't get attracted to it, so I use it to identify them."

Thus I got curious, why a magnet don't attract inox steel bolts? Steel, even if a inox variation is still mostly iron, no?

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The are various crystal forms that iron and steel can adopt, the common ones being ferritic, martensitic and austenitic. The ferritic and martensitic forms are ferromagnetic (or just magnetic in everyday terms) while the austenitic form is not. So it isn't simply that iron is magnetic and steel isn't, it is specifically austenitic iron and steel that isn't magnetic. However things may not be as simple as this since lumps of steel may well contain grains of more than one crystal type, so they may be partially magnetic.

Now you're going to ask me why the austenitic crystal form isn't magnetic, and I don't think there is a good answer for that. Ferromagnetism is actually quite a subtle effect dependant on exactly how the electrons in the iron atoms interact. It's due to a quantum mechanical phenomenon called the exchange interaction. In the ferritic and martensitic crystals this effect is large enough to make the spins line up and generate macroscopically ordered domains. In the austenitic crystal it isn't. I don't know of an intuitive way to explain why.

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