Magnetic Properties Of Solids - Practice Questions & MCQ

Magnetic Properties : 
Every substance has some magnetic properties associated with it. The origin of these properties lies in the electrons. Each electron in an atom behaves like a tiny magnet. 
Its magnetic moment originates from two types of motions- 
(i) its orbital motion around the nucleus.
(ii) its spin around its own axis

Electron being a charged particle and undergoing these motions can be considered as a small loop of current which possesses a magnetic moment. Thus, each electron has a permanent spin and an orbital magnetic moment associated with it. Magnitude of this magnetic moment is very small and is measured in the unit called Bohr magneton, μ_B. It is equal to 9.27 × 10^–24 A-m².

Classification of substances

• Paramagnetism:
  These substances are attracted by the magnetic field and have unpaired electrons. They lose magnetism in the absence of a magnetic field.
  
  For example, Transition metals like Cr, Mn, Ni, Co, Fe, etc. Metal oxides like CuO, VO₂ etc.

• Diamagnetism:
  These substances are weakly repelled by the magnetic field and do not have any unpaired electron. They act as Insulators.
  
  For example, NaCl, Zn, Cd, Cu⁺, TiO₂, etc.

• Ferromagnetism:

  • These substances are attracted by the magnetic field and show permanent magnetism even in the absence of magnetic field.
    Example, Fe, Co, Ni, CrO₂ (used in audio and video tapes) etc. 

  • This arises due to spontaneous alignment of magnetic momenta in the same direction.
    

  • Above the curie temperature, there is no ferromagnetism

• Antiferromagnetism:

  • These are the substances which are expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons but actually have zero net magnetic moment. 
    Example, MnO, MnO₂, Mn₂O₃, FeO, Fe₂O₃ etc.

  • Anti-ferromagnetism is due to equal number of parallel and antiparallel magnetic momenta which leads to zero resulting magnetic moment.
    

• Ferrimagnetism:
  In ferrimagnetic substances, there are unequal number of parallel and antiparallel magnetic momenta which leads to some resulting magnetic moment. 
  
  For example, Fe₃O₄ , Ferrites

• Effect of Temperature:
  The ferromagnetic, anti-ferromagnetic and ferrimagnetic solids change into paramagnetic at a particular temperature. Example, Ferrimagnetic Fe₃O₄ on heating to 850 K becomes paramagnetic this is due to alignment of spins in one direction on heating.

• Curie Temperature:
  Each ferromagnetic substance has a characteristic temperature above which no ferromagnetism is observed this is called the curie temperature.