i have synthesised magnetic material Fe3O4 by co-ppt method. Before calcination, the magnetic property was showing but after calcination above 500 degree C , the magnetic property is losing. please let me know the reason and its that normal because i want the magnetic property to be retain for my catalyst purpose. thank you in advance.
I think Magnetite nanoparticles will lose their magnetism if heated above a point known as the Curie temperature. At this point, the energy being put into the magnet from the heat will permanently disrupt the magnetic domain structure of the material and reduce the memory of the magnet, turn it into a paramagnetic material.
I confirm the opinion of Pradip Singha that the calcination above 500 degree C of nanoparticle magnetic material Fe3O4 is higher than its Curie temperature so it converted to paramagnetic material. you can perform the DSC analysis for precursor to identify the calcination temperature exactly without lose the magnetic properties of nanoparticle magnetic material Fe3O4.
Thermal agitation is indeed well-known to disorientate randomly the spins and hence produce the loss of permanent magnetisation. Above a critical temperature, called Curie point as mentioned above and equal to 580°C for magnetite, the material undergoes a ferrimagnetic to paramagnetic transition. Is it then likely that, even if you remain below this critical point, most of the magnetisation is lost even if it not completely disappeared. You need to do a new magnetisation after heat treatment until you reach the saturation. Then, permanent magnetisation will be restored (but prone to disappear again at the next heating).
Alain
thanks Pradip and Alain Celzard, i know about the curie temp issue but thank you so much. however my question was more about how to get the magnetic property back after heat treatment.
@AlainCelzard, please let me know more about the method of restoring the property of the magnet.
thank you in advanced.
You need to place your sample between the poles of a very strong electromagnet so that your sample reaches saturation. Then decrease the field until the remanent magnetisation.
Alain
Fe3O4 oxidizes to γ-Fe2O3 (cubic structure) and then to α-Fe2O3 (rhombohedral s.g. R3c) under 500-900°C.
So, after calcination you have the two phase sample.
Have you noticed the color change from black to brown?
If yes, it is the sign of α-Fe2O3. Or check it by XRD.
To get Fe3O4 back, you should carry out reduction annealing.
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