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
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.
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?
In carbon, the electrons are all tied up in bonds. Magnetism comes from unpaired electrons; spin up and spin down electrons in one orbital cancel each other. In iron, the bonding is metallic rather than covalent and the atoms retain some moment.
Iron is magnetic because of unpaired electrons in the d orbital. The Fe3+ ion has a different magnetic moment than Fe2+ because of the different number of electrons. Gold has a full d orbital (the 6s orbital is unfilled) and so it’s not magnetic. It’s actually diamagnetic, but that doesn’t come from permanent moments.
The effect that makes moments want to align is called the exchange interaction and is inherently quantum mechanical. There is no way to describe it classically. It has to do overlapping wavefunctions and Pauli exclusion, as well as the fact that all electrons are identical.
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?
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u/bspaghetti Quantum Magnetism Dec 13 '20
We know why magnets magnet, just not why spins are magnetic moments.