A lot of amateurs confuse powdered iron with ferrite. They're very
different materials, and each is best for particular purposes.
MicroMetals makes powdered iron cores. A number of other vendors, such
as Fair-Rite, Magnetics, Ferroxcube, Siemens, and others make ferrite
cores. Amidon sells both, with powdered iron part numbers beginning with
T and ferrites beginning with FT.

There are many types of powdered irons and many types of ferrite. It's
vital that you choose the type which is appropriate for the job. Once
you do that, the design of the inductor or transformer is usually pretty
straightforward. But if you choose the wrong material, you're likely to
end up with a very poor component.

Most or all the manufacturers I mentioned have web sites that provide a
lot of information about how to choose the right material for a
particular application.

Roy Lewallen, W7EL