The magnetic properties of materials are due to magnetic moment associated with individual electron. Each electron in atom has magnetic moment which originates from two sources:
- Orbital motion of electron around the nucleus.
- Spin of electron around its own axis.
Based on the behavior in external magnetic field, the substances are divided into five different categories:
- Diamagnetic Substances
- Paramagnetic substances
- Ferromagnetic substances
- Anti-ferromagnetic Substances
- Ferrimagnetic Substances
Substances which are weakly repelled by the external magnetic field are called diamagnetic substances and phenomenon is called diamagnetism. Diamagnetism arises when all the electrons are paired which cancels their magnetic moments.
Example: NaCl, Water, Benzene, TiO2 etc.
Substances which are weakly attracted by the external magnetic field and lose their magnetism in the absence of external magnetic field are called paramagnetic substances and the phenomenon is called paramagnetism. This property is shown by those substances whose atom, ions or molecules contain unpaired electrons.
Example: Cu+2, Fe+3, Cr+3, O2 etc.
Substances which are strongly attracted by the external magnetic field and show permanent magnetism even in the absence of external magnetic field are called ferromagnetic substances and the phenomenon is called ferromagnetism.
Example: Fe, Ni, Co, Gadolinium (Gd) and CrO2 etc.
[CrO2 is used to make magnetic tapes used for audio recording.]
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 unmagnetized piece of 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 magnetic field.
Each ferromagnetic substance has a characteristic temperature above which no ferromagnetism is observed. This temperature is known as curie temperature.
A substance which is expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons but possesses zero magnetic moment is called anti-ferromagnetic substance. This is because all domains are aligned in equal number in opposite directions and the net magnetic moment is zero.
Example: MnO, MnO2 , FeO, NiO, V2O3 etc.
V2O3 become paramagnetic from anti-ferromagnetic at 150 K.
Substances which are expected to possess large magnetism on the basis of the magnetic moments of domains but actually have small net magnetic moment are called ferrimagnetic substances. This is because domains are aligned in parallel and anti-parallel directions in unequal numbers resulting in magnetic moment.
Example: Fe3O4, MgFe2O4, CuFe2O4, ZnFe2O4 etc.
These substances also lose ferrimagnetism on heating and become paramagnetic. Fe3O4 become paramagnetic at 850 K.
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