Magnetocaloric Effect and Quantum Spin Relationship? I watched few videos and read few articles on Magnetocaloric Effect. Basically, some materials, like Gadolinium, heat up if simply placed in magnetic field.
But I can't get the explanation they are giving. They say, when you put Gadolinium into the very strong magnetic field, all the spins align parallel to the field lines. Than material looses its ability to absorb heat, because there is one degree of freedom less and temperature of material rises.
Somehow, maybe from school, I would understand it in an opposite way. If atoms are randomly oriented, than they would absorb the radiation, heat will "stay inside" and temperature would rise. If atoms can't move because they are aligned with a field, radiation would "bounce", reflect to outside and temperature would stay the same.
Where am I wrong?
 A: I think there is a subtle point you are missing. the "putting gadolinium in strong magnetic field" process is done fast, i.e. adiabatically. Consider any system of atoms with intrinsic spin. Now at any finite temperature the magnetic dipoles are randomly oriented. Now if you suddenly put it in a strong magnetic field, all the dipoles allign(not completely though, quantum mechanics prevents that) in the direction of magnetic field. We know any such allignment decreases the randomness and that is associated with decreasing entropy. In this case the magnetic entropy.
You said if atoms are randomly oriented it helps them to heat faster. No, its actually opposite. When atoms are randomly oriented their heat capacity is more thus it actually takes more heat to increase their temp. In other words it takes more energy to take a system from 300K to 301K than to take it from 100K to 101K.
Now to our case. The process we did was adiabatic. So total entropy is conserved. But magnetic entropy decreased(due to orientation). The only way out of this is to increase the heat content by increasing it's temperature.
There is another way of looking into it. When the dipoles are randomly oriented magnetic potential energy is more. When they get alligned it decreases and comes out of the crystal as heat.
