What Makes Ferromagnetic Materials Unique in Quantum Magnetism?

[RestlessMind]

I heard that the reason why we can't put our hands through a wall (or such a thing) is because of atomic electromagnetic forces pushing things away from each other (since after all, things are mostly empty space, and only seem solid because of the scale). I also heard that that is the same force that makes magnets work, except that it is amplified in ferromagnetic substances because those atoms have the ability to align their "spin", or something.

So I have some questions, excuse me, they might be a bit newbish.

1. Why can only ferromagnetic materials align their spin like that?

2. When a ferromagnetic material is magnetized, what exactly is happening to the atoms, and how is it done?

 

[JDługosz]

My understanding of why walls are solid is that the individual atoms, defined by their electron clouds, act "solid" and won't overlap. This is due to Pauli Exclusion, or more basically, the way fermions in general behave.

The reason some materials exhibit ferromagnetism (iron, nickle, cobalt; and various careful arrangements of atoms of more varied types) also has to do with Pauli Exclusion. Two electrons can overlap slightly if they have opposite spins, and two electrons of the same spin must stay farther apart. So, in this particular lattice, the atom bearing the odd electron is just so positioned that making the spins aligned will force them farther apart, and the energy levels of all bonding, levels, etc. involved are just so so that you come out ahead that way. (Things that attract each other have bonding energy from being closer together and potential energy from being farther apart; just like a stone on the earth.)

Normally, you come out ahead (way ahead) if the nearby odd electrons cancel out by pointing in opposite directions.

―John

 

[ZapperZ]

I heard that the reason why we can't put our hands through a wall (or such a thing) is because of atomic electromagnetic forces pushing things away from each other (since after all, things are mostly empty space, and only seem solid because of the scale). I also heard that that is the same force that makes magnets work, except that it is amplified in ferromagnetic substances because those atoms have the ability to align their "spin", or something.

So I have some questions, excuse me, they might be a bit newbish.

1. Why can only ferromagnetic materials align their spin like that?

2. When a ferromagnetic material is magnetized, what exactly is happening to the atoms, and how is it done?

This is now a question of quantum magnetism, which is tackled in solid state/condensed matter physics.

At the most naive level, it is something called the Heisenberg spin-coupling (J) of the atom with its nearest neighbor, next-nearest neighbor, next-next nearest neighbor, etc.. etc. At some point, when you calculate these coupling (often using some approximation because we have no ability to solve for a gazillion of these things), you end up with an energy state that favors, say, the spin aligning parallel to each other, resulting in an ferromagnetic material, or spin aligning anti-parallel to each other, resulting in an antiferromagnetic material. So these outcomes are very much dependent on the location of the spin dipoles, how they are arranged (crystal structure), etc... etc.

Zz.