The substances which are weakly repelled by magnetic field are called `text(diamagnetic)` substances.

`text(Eg :) TiO_2, NaCl`, benzene etc.

Diamagnetism arises when all the electrons are paired.

The substances which are weakly attracted by magnetic field are called `text(paramagnetic substances)`. Atoms, ions or molecules with unpaired electrons exhibit paramagnetism. Paramagnetic substances lose their magnetism in the absence of magnetic field.

`text(Eg :) TiO, VO_2 ,CuO` etc.

The substances which are strongly attracted by magnetic field are called ferromagnetic substances. Ferromagnetic substances show permanent magnetism even in the absence of magnetic field.

`text(Eg : Iron, cobalt, nickel,)` `CrO_2` etc.

Ferromagnetism arises due to spontaneous alignment of magnetic moments in the same direction. Anti ferromagnetism arises due to the alignment of magnetic moments in opposite direction in a compensatory manner and resulting in a zero magnetic moment.

`text(Eg : MnO, MnO_2, Mn_2O_3.`

Ferrimagnetism arises due to alignment of magnetic moments in opposite directions resulting in a net magnetic moment.

`text(Eg :) Fe_3O_4, M^(2+)Fe_2O_4`; (`M = Mg, Cu, Zn` etc.)

Ferromagnetic and ferrimagnetic substances change into paramagnetic substances at higher temperatures due to randomisation of spins.

Electrons in a dielectric are tightly held by individual atoms.

Due to shift in charges, dipoles are created which result into polarisation.

The dipoles may align in an ordered manner such that there is some resultant dipole moment in the crystals.

The dipoles may align in such a way that the dipole moments cancel each other and resultant dipole moment is zero.

There may be no dipoles in the crystal but only ions are present.

Depending upon the alignment of the dipoles, the properties of crystals are :

(i) Piezoelectricity (ii) Ferroelectricity (iii) Anti-ferroelectricity (iv) Pyroelectricity

(i) `text(Piezoelectricity)` :

The crystals in which the dipoles may align in an ordered way having some dipole are referred to as piezoelectrics and such crystals exhibit piezoelectricity. Piezoelectric crystals act as mechanical-electrical transducers. When these crystals are subjected to a pressure or mechanical stress, electricity is produced. These crystals are used as pickups in record players by the application of pressure.


(ii) `text(Ferroelectricity)` :

The solids, in which dipoles are spontaneously aligned in a particular direction, even in the absence of electric field, are called ferroelectric substances and the phenomenon is called ferroelectricity. The direction of polarisation in these substances can be changed by applying electric field. The examples are barium titanate `(Ba TiO_3),` sodium potassium tartrate (Rochelle salt) and potassium dihydrogen phosphate `(KH_2PO_4)`.


(iii) `text(Anti-ferroelectricity)` :

If the alternate dipoles are in opposite direction, then the net dipole moment will be zero and the crystal is called anti-ferroelectric and the phenomenon is antiferroelectricity.

(iv) `text(Pyroelectricity)` :

Some of the polar crystals when heated produce small electric current. This phenomenon is called pyroelectricity.

� Super conducting materials are those which offer no resistance to the passage of electricity. Electrical resistance decreases with decrease in temperature and becomes almost zero near the absolute zero.

� In this state, the materials become diamagnetic and are repelled by the magnets.

� Most metals become super conducting at very low temperatures (`2`-`5` `K`).

� The highest temperature at which superconductivity was known is `23` `K` in alloys of niobium(e.g. `Nb_3 Ge`).

� Some organic compounds also become super conducting at temperatures below `5` `K`.

� Some complex metal oxides passes super conductivity at higher temperatures. Superconductors are used in electronics, building magnets, power transmission, levitation transportation, (trains which move in air without rails).