Ferromagnetism is a kind of magnetism that is associated with the transition metals iron, cobalt, nickel, and some alloys or compounds containing one or more of these elements. It also occurs in gadolinium (ferromagnetic below 17 C, gadolinium is a stronger magnet at 0 K than iron), the rare earth metals samarium and neodymium, and the metalloid boron. Only iron, cobalt, and nickel are ferromagnetic in the strict sense of the word and have a strong residual magnetization. Unlike other substances, ferromagnetic materials are magnetized easily, and in strong magnetic fields, the magnetization approaches a definite limit called saturation.