Which of the following materials has the highest temperature stability of its magnetic characteristics?
Why Powdered iron cores consist of tiny iron particles suspended in a nonmagnetic binder, so the many distributed air gaps keep effective permeability low but very predictable. That construction gives powdered iron better temperature stability and a higher Q than ferrite, which is why it dominates in tuned circuits and RF filters where drift matters.
Watch out Ferrite is the usual trap: it has much higher permeability, so it gets more inductance per turn, but its permeability shifts noticeably with temperature and it saturates more easily. Brass and aluminum are not magnetic materials at all; a brass slug actually lowers inductance when inserted in a VHF coil.
Iron for stability, ferrite for permeability. Powdered iron = many tiny air gaps = predictable.