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