Which of the following substances are attracted very strongly by a magnetic field?

Ferromagnetic

Ferromagnetism: A few substances like iron, cobalt, nickel, gadolinium and CrO₂ are attracted very strongly by a magnetic field. Such substances are called ferromagnetic substances. Besides strong attractions, these substances can be permanently magnetised. In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet. In an unmagnetised piece of a ferromagnetic substance the domains are randomly oriented, and their magnetic moments get cancelled. When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field and a strong magnetic effect is produced. This ordering of domains persists even when the magnetic field is removed, and the ferromagnetic substance becomes a permanent magnet.