Curie temperature, reversible process

[olle3770]

Hello guys, first post in this forum :)

Magnetism has always been complicated to understand for me, and I would like some help regarding Curie temperatures.

We have just bought a new measurement equipment to out lab which shall be temperature calibrated with curiepoints. We have three alloys, (Isatherm, Ni-alloy and Trafoperm).

My questions are:

* Is it possible to use the same pieces of alloy multiple times for calibration?
* Is the process reversible? i.e is the pieces as magnetic as before when cooled again?
* Is it possible to affect the Curie temperature in any way and how accurate is it?

Appreciate all help, need to explain to the chemistry guys :)

//Olle, Sweden

 

[DrDu]

1. I don't see good reasons why you shouldn't use the same piece several times. However, I am not an experimentalist.
For example, I could imagine that there are some changes in crystal structure upon tempering which may affect the Curie point.
2. The Ferromagnetic-Paramagnetic transition is fully reversible. However, this does not mean that the pieces will be "as magnetic as before". If the material is permanently magnetized below the Curie point, this magnetization will be lost if the material is first heated and then cooled down again without an external magnetic field. However, when measuring the Curie point, you usually doing measurements in the paramagnetic region (>Tc) so there is no problem here.
3. I don't know much about it's accuracy. There are little ways to influence the Curie point. The most probable ones in the alloys you are using are formation of separate phases or formation of new crystal structures. I have no idea whether this is relevant for the alloys you are using.

 

[olle3770]

Thanks for the answers DrDu!

I did just receive the results from the tests when we tried heating the same pieces three times over the Curie temperatures. The isatherm and ni-alloy seemed to have a bit changed magnetic characteristics between the tests but the curie temperature remained at the same temperature. The Trafoperm did however change curie temperature, first one was about 747 the second one 748 but the third one was 767, all numbers in degrees celsius.

 

[M Quack]

The magnetization of the material will most likely change when you go through the Curie transition several times, as DrDu has explained. The Curie temperature is more difficult to change. For one and the same material, it can be influenced by an external magnetic field, hydrostatic pressure, or - if there is a magneto-elastic distortion associated with the phase transition - by uniaxial strain. In all these cases you need a lot of external force for a relatively small change in transition temperature. Another possibility, again as DrDu pointed out, is that you somehow modify the material. In the case of an alloy, heating ("annealing") it can change details of the composition via phase separation, etc.