# Homework Help: Difference between paramagnetism and ferromagnetism

1. Jan 21, 2014

### Denver Dang

1. The problem statement, all variables and given/known data
I've been asked to describe the difference between these two, but I'm a but confused.

In my book, it says that paramagnetism is weaker than ferromagnetism, but I don't see why.

If I'm not mistaken, both paramagnetism and ferromagnetism arise from the fact that you align magnetic moments. I then get, that in ferromagnets the orientation of the moments are somewhat kept, whereas in the paramagnetic situation, it is destroyed when the applied magnetic field is removed.

But if it is the same thing that happens, why is ferromagnetism stronger that paramagnetism ? Or is that just me that are reading it wrong ?

2. Relevant equations

3. The attempt at a solution

2. Jan 21, 2014

### tiny-tim

Hi Denver Dang!
no, it's not the same, it's dipole-dipole interaction for paramagnetism, but exchange itneraciton for ferromagnetism

see http://en.wikipedia.org/wiki/Ferromagnetism#Exchange_interaction
According to classical electromagnetism, two nearby magnetic dipoles will tend to align in opposite directions, so their magnetic fields will oppose one another and cancel out. However, this effect is very weak, because the magnetic fields generated by individual spins are small and the resulting alignment is easily destroyed by thermal fluctuations
In a few materials, a much stronger interaction between spins arises because the change in the direction of the spin leads to a change in electrostatic repulsion between neighboring electrons, due to a particular quantum mechanical effect called the exchange interaction. At short distances, the exchange interaction is much stronger than the dipole-dipole magnetic interaction. As a result, in a few materials, the ferromagnetic ones, nearby spins tend to align in the same direction.​

3. Jan 21, 2014

### Denver Dang

But does this explain why the field created by a ferromagnet is stronger than that of the paramagnet ?

It sounds to me, that you have explained why the ferromagnetic state is upheld when turning off the magnetic field, due to the exchange interaction which is a stronger bond that the dipole-dipole interaction. But if you have a strong enough field, shouldn't you be able to align the same magnetic moments in a paramagnetic material, as in a ferromagnetic, and then getting the same strength of field ?

Again, it just sounds as if you are explaining WHY the ferromagnetic state is upheld, and why the paramagnetic changes back when the magnetic field is removed.

But I could be mistaken ?