Apparent vanishing of (magnetic) potential energy

[stockzahn]

Dear all,

I cannot figure out my mistake with this thought experiment:

Consider a ferromagnetic material (e.g.iron) in a magnetic field - or just any iron anywhere, since the range of magnetic fields is infinite. Between the ferromagnetic material and (permanent) magnets potential energy is stored. By heating and/or a metallurgic treatment, the iron is transformed into an austenitic steel and therefore loosing its ferromagnetic properties. What happened to the potential energy - it seems to have vanished?

Does the metallurgic treatment require more energy for the rearrangement of the molecule lattice if performed in a strong magnetic field? If this is the case, what happens with this additional energy, if the magnet is removed during the steel is austenitic and then becomes ferritic again (e.g. by cooling down)?

Unfortunately I couldn't find an explanation in the web nor in my books.

Thanks in advance,
stockzahn

 

[mjc123]

There is an exchange between magnetic and thermal energy. A magnetised ferromagnetic material has a minimum potential energy arrangement of its spins relative to the magnetic field. If it goes through a transition to a paramagnetic state, it goes to a state of higher potential energy but also higher entropy. To do this requires energy, e.g. by heating it through the Curie temperature. (Note that potential energy increases; you seem to imply that it decreases.)
This is the basis of the technique of adiabatic demagnetisation for attaining ultra-low temperatures. A sample is magnetised under isothermal conditions - the heat released by going to the low-energy state is conducted away. Then it is allowed to demagnetise under adiabatic conditions - the energy required to go to the higher energy state comes from the thermal energy of the sample - its temperature drops. Repeat and repeat.

 

[stockzahn]

Thanks for the fast response - "magnetic refrigeration" seems to be the "keyword" to find information.

stockzahn