(i) Ferromagnetic substances :
Substances which are attracted very strongly by a magnetic field are called ferromagnetic substances, e.g., Fe, Ni, Co and CrO ‌2 show ferromagnetism. Such substances remain permanently magnetised, once they have been magnetised. This type of magneticmoments are due to unpaired electrons in the same direction.
The ferromagnetic material, CrO‌2, is used to make magnetic tapes used for audio recording.(ii) Paramagnetic substances : Substances which are weakly attracted by the external magnetic field are called paramagnetic substances. The property thus exhibited is called paramagnetism. They aremagnetised in the same direction as that of the applied field. This property is shown by those substances whose atoms, ions or molecules contain unpaired
electrons, e.g., O‌2, Cu ‌2+ , Fe ‌3+ , etc. These substances, however, lose their magnetism in the absence of the magnetic field.
(iii) Ferrimagnetic substances : Substances which are expected to possess large magnetism on the basis of the unpaired electrons but actually have small net magnetic moment are called ferrimagnetic substances, e.g., Fe ‌3 O ‌4 , ferritesof the formula M‌2+Fe‌2O ‌4 where M = Mg, Cu, Zn, etc. Ferrimagnetism arises due to the unequal number of magnetic moments in opposite direction resulting in some net magnetic moment.(iv) Antiferromagnetic substances : Substances which are expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons but actually they possess zero net magnetic moment arecalled antiferromagnetic substances, e.g., MnO. Antiferromagnetism is due to the presence of equal number of magnetic moments in the opposite directions.(v) 13-15 group compounds : When the solid state materials are produced by combination of elements of groups 13 and 15, the compounds thus obtained are called 13-15 compounds. For example, InSb, AlP,GaAs, etc.
12-16 group compounds : Combination of elements of groups 12 and 16 yield some solid compounds which are referred to as 12-16 compounds. For example, ZnS, CdS, CdSe, HgTe, etc. In these compounds, the bonds have ionic character.