Which of the following materials has the highest temperature stability of its magnetic characteristics?

Among the magnetic materials used for RF cores, powdered iron is well known for having excellent temperature stability. This means its magnetic characteristics, and therefore the inductance of the coil wrapped around it, remain relatively constant as the physical operating temperature changes. Ferrite, while having higher permeability, is notably sensitive to temperature variations, making it less stable than powdered iron. Brass and aluminum are non-magnetic and are typically used as tuning slugs to decrease inductance, rather than acting as high-stability magnetic cores.