Magnetic Properties

Magnetic Properties

Based on the behavior of substances when placed in the magnetic field, there are classified into five classes.

Table: Magnetic properties of solids

Properties

Description

Alignment of Magnetic Dipoles

Examples

Applications

Diamagnetic

Feebly repelled by the magnetic fields. Non-metallic elements (excepts O2, S) inert gases and species with paired electrons are diamagnetic

All paired electrons

TiO2, V2O5, NaCl, C6H6 (benzene)

Insulator

Paramagnetic

Attracted by the magnetic field due to the presence of permanent magnetic dipoles (unpaired electrons). In magnetic field, these tend to orient themselves parallel to the direction of the field and thus, produce magnetism in the substances.

 

 

 

At least one unpaired electron

, CuO

Electronic appliances

Ferromagnetic

Permanent magnetism even in the absence of magnetic field, Above a temperature called Curie temperature, there is no ferromagnetism.

Dipoles are aligned in the same direction

 

 

Fe, Ni, Co, CrO2

CrO2 is used in audio and video tapes

Antiferromagnetic

This arises when the dipole alignment is zero due to equal and opposite alignment.

 

MnO, MnO2, Mn2O, FeO, Fe2O3; NiO, Cr2O3, CoO, Co3O4,

Ferrimagnetic

This arises when there is net dipole moment

 

 

Fe3O4, ferrites

 (3) Dielectric properties: A dielectric substance is that which does not allow the electricity to pass through them but on applying the electric field, induced charges are produced on its faces. In an insulator, the electrons are strongly held by the individual atoms. When an electric field is applied polarization takes place because nuclei are attracted to one side and the electron cloud to the other side. As a result, dipoles are created. Such type of crystals shows the following properties,

(i) Piezoelectricity: In some of the crystals, the dipoles may align themselves is an ordered way so as to give some net dipole moment. When mechanical stress is applied in such crystals so as to deform them, electricity is produced due to the displacement of ions. The electricity thus produced is called piezoelectricity and the crystals are called piezoelectric crystals. Examples, Quartz, Rochelle’s salt ( sod. pot. tartarate). Piezoelectric crystals act as mechanical-electric transducer. These crystals are used as pick-ups in record players where they produce electric signals by application of pressure.

(ii) Pyroelectricity: On heating, some polar crystals produce a small electric current. The electricity thus produced is called pyroelectircity.

(iii) Ferroelectricity: In some of the piezoelectric crystals, a permanent alignment of the dipoles is always there even in the absence of the electric field, however, on applying field the direction of polarization changes. This phenomenon is called ferroelectricity and the crystals as ferroelectric crystal. Example, Potassium hydrogen phosphate , Barium titanate .

(iv) Antiferroelectricity: In some crystals, the dipoles in alternate polyhedra point up and down so that the crystals does not possess any net dipole moment. Such crystals are said to be antiferroelectric. Example, Lead zirconate . Ferroelectrics are used in the preparation of small sized capacitors of high capacitance. Pyroelectric infrared detectors are based on such substances. These can be used in transistors, telephone, computer etc.

Tips & Tricks

1. The reverse of crystallization is the melting of the solid.

2. The slower the rate of formation of crystal, the bigger is the crystal.

3. The hardness of metals increases with the number of electrons available for metallic bonding. Thus Mg is harder than sodium.   

4. Isomorphism is applied to those substances which are not only similar in    their crystalline form, but also possess an equal number of atoms  united in the similar manner. The existence of a substance in more than one crystalline form is known as polymorphism.  

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