What determines the class of ferromagnet?

Question

I believe that iron, nickel, and cobalt are all 3d ferromagnets. As I understood, those 3 are the only ferromagnetic elements, so why do we specify 3d? Here is an example of a 4f ferromagnet: https://doi.org/10.1016/j.intermet.2015.10.020 It seems to me that it's likely that the classification is based on which orbital the electrons contributing most to the ferromagnetic behavior, but I'm not very sure about this and it's been difficult trying to find results with a search engine since 3d is taken to mean 3 Dimensional. Any sources about how this classification is done would be greatly appreciated.

Edit: I've now found an example of a 4d ferromagnet https://doi.org/10.1063/1.361618

Answer 1

These aren't really classes of materials — just descriptions of which atomic orbitals are involved in the magnetic properties. These are typically determined mainly by open electron shells. Consider the electron configuration in iron: 1s$^2$2s$^2$2p$^6$3s$^2$3p$^6$3d$^6$4s$^2$ = [Ar] 3d$^6$ 4s$^2$. Note that all shells, except for 3d, are filled. Filled shells have zero net total angular momentum $\mathbf{J}=\mathbf{L}+\mathbf{S}$, and do not produce a net magnetic moment. If we're interested in understanding the magnetic properties of iron, we can, to good approximation, focus on electrons in the open 3d shell and label the system accordingly. This is why iron, nickel and cobalt are called 3d ferromagnets. For some non-elemental systems like SrRuO$_3$ you need to check the electron configurations of the ions present in the system. In this case, I think it's the Ru ions contributing 4d orbitals that cause the magnetism.

Note that this kind of shorthand isn't unique to ferromagnets — c.f. "3d block", "4d transition metal oxides", "3d-4f complexes" etc. It is useful because often systems share some properties if their electron configurations are similar - essentially the same reason that the periodic table is a useful construction.