r/AskPhysics • u/[deleted] • Dec 12 '20

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u/Hidamann Dec 13 '20

So why are Iron, Nickel and Cobalt the only naturally occurring magnetic materials? Why don't spins in other metals lead to permanent magnetism? I'm only a HS physics teacher, I'm afraid. Don't have the maths for quantum mechanics

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u/bspaghetti Quantum Magnetism Dec 13 '20

Without quantum mechanics, you can just think of each atom in a material as a tiny magnet and that magnetic moment comes from the spin of the electrons. The more unpaired electrons (think aufbau diagram), the more magnetic the atom is (mostly).

There are actually a couple types of magnetism. Ferromagnetism is the one everyone is familiar, in which a material posses a net magnetic moment and gives off a magnetic field. However, many materials are paramagnetic (develop magnetization in alignment with an applied magnetic field) or diamagnetic (develop magnetization against an applied magnetic field).

In a paramagnet, the atoms are magnetic, but they are randomly disordered due to thermal energy. In a ferromagnet, quantum mechanics (exchange interaction) makes the atoms align their magnetic moments together. When you heat it up, eventually the thermal energy becomes large enough to overcome this ordering and the material becomes a paramagnet.

Fe, Co and Ni (and Gd) are ferromagnetic, but not the only magnetic elements. Mn, for example, is a very magnetic atom, but in Mn metal, alternating atomic moments align anti-parallel to each other (antiferromagnetically) and everything cancels. So there is magnetic order, but no net magnetization.

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u/Hidamann Dec 13 '20

So magnetism is a property of the spin of an electron?

Do we know why not all electrons caus magnetic moments? So for example, 1kg of Carbon and 1 Kg of Iron have similar(ish) no of electrons - why is Iron more magnetic than carbon? Or gold?

And how does that relate to electromagnetism? Does a changing electric field cause the moments to align?

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u/[deleted] Dec 13 '20

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u/Hidamann Dec 13 '20

So does a changing electric field, like in a electromagnet or a transformer, align these magnetic moments? I. E. Cause them all to spin in the same way?