Inspite of a high magnetic moment of Fe proportional to Co, Fe is weak magnetic material but Co is strong magnetic material. Does the exchange interaction matter?
The question of strongly vs. weakly ferromagnetic materials is just a question of band filling, the size of the resulting spin moment is of no consequence. If the majority spin channel is completely filled, it is a strong magnet, else it is a weak magnet. In Fe, the majority spin is not completely filled, so it is a weak ferromagnet, in Co and Ni it is, so they are strong ferromagnets.
As a matter of fact I think that the notion of strong and weak ferromagnets based on majority band filling is pretty much a historic and outdated one. Actually the 3d occupation number in the majority band of Ni does not amount to 5. You can find this information in Stöhr & Siegmann's book on magnetism, which in this context refers to unpublished work (calculations) by R. Wu. This happens to be in line with earlier results from G. Van der Laan in the context of thin Ni films. The number of unoccupied 3d states per Ni amounts to about 1.4, which does not fit with the notion of a hard ferromagnet given the magnetic moment of about 0.6 Bohr magnetons per atom.
From experiments (XMCD) we also get more consistent resuls by assuming this higher number of 3d holes.
On the other hand, most of these unoccupied d states are outside the region of very high DOS, so that qualitatively, the 'strong ferromagnet' notion is not totally off.
Let's see if I dare to go on here with Kai on the thread... oh well, I'll make an attempt ;-)
The rationale for the naming is from the difference in susceptibility. So:
- If, like in a strong ferromagnet, the majority spin is full (well OK, NEARLY full), then the susceptibility will be small, since further spin splitting of the d-band is not possible. In other words, the magnetization does not increase (much) by an applied field. It is already as high as it gets - it is strong.
- If, like in a weak ferromagnet, the majority spin is not fully occupied, then the susceptibility will be much higher, since the further split of the spin channels caused by the field is free to cause a redistribution in occupation between the spin channels in this case. The magnetization can thus be further enhanced by a field - it is weak.
At least this is what my textbooks have taught me, but it is true that if you actually calculate or carefully measure the occupancies, then you always find complications in the crude old models.
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