# Magnets and QM

1. Mar 19, 2005

### weio

HI

How does a magnetic field build in a natural magnet like iron from a quantum mechanical point of view? I know it has to do with electrons spin and their orbital momentum, but how exactly does it work in a magnet?

thanks

2. Mar 19, 2005

### clive

The main point of a QM theory of magnetism is the exchange interaction between neighbor spins:
$$H=-J \sum \vec{S_1}\cdot \vec{S_2}$$ (Heisenberg Hamiltonian)
where J is the exchange integral.
This energy is responsible for the spin alignment in ferromagnetic materials (because it is minimal when the two spins are parallel) and then for the magnetic field created by these materials.

The ferromagnetism can not originate from orbital magnetic moments of electrons (Miss van Leeuwen's theorem).

Hope it helps.....

3. Mar 19, 2005

### weio

Thanks a bunch!

I will look that up.

wieo