Magnetism (B.Sc.)

 Origin of the Magnetism or Magnetic Properties of Materials:

         The materials consist of atoms and atom contains electrons and nucleus. Therefore properties/behaviour of materials are governed by the properties of electron and nucleus.

        Let us consider orbital picture model of the atom. According to the orbital model, atom consists of positively charged massive nucleus around which electrons revolve in a close orbit (circular, elliptical, zigzag etc.) due to mutual attraction of nucleus and electron.

       In general the electron can have two kinds of motion namely- (1) Orbital Motion and (2) Spin Motion. Because of two types of motion, there are two types of angular momentum of the electron namely- (1) Orbital Angular Momentum (p_l) and (2) Spin angular Momentum (p_s)

        Consequently, there are two kinds of magnetic moment of the electron namely-

(1) Orbital Magnetic Moment (µ_l)      

(2) Spin Magnetic Moment (µ_S) 

Orbital magnetic moment of the revolving electron is given by-

                                                               µ_l = lµ_B

where l = 0,1,2,3,.......... and is known as orbital quantum number and  µ_B =  eh/2m = 9.27 × 10^-34 J/T is known as Bohr Magneton.

    Similarly, the spin magnetic moment of a revolving electron's non-zero and given by

                                                                µ_s = 1µ_B

      _{In order to determine magnetic moment of the atom, one has to write electronic configuration. Then consider partially or partly or incomplete sub-shell electrons as the completely filled sub-shell or close sub-shell does not contribute to the total magnetic moment of the atom because of Paul's Principle.}

    _{Whenever, electron belongs to the 's' sub-shell then its orbital magnetic moment vanishes ( as}  µ_l = lµ_B = 0 × µ_B = 0 ) while the spin magnetic moment is non-zero and equal to 1 Bohr Magneton i.e. for 's' sub-shell electron µ_l = lµ_B = 0 × µ_B = 0 

                                                                µ_s = µ_B

  Net magnetic moment of the 's' sub-shell electron is non-zero and equal to 1 Bohr Magneton.

    Whenever electron belongs to the closed sub-shell containing maximum number of electrons such as  S², p⁶, d¹⁰, f¹⁴ , then resultant orbital magnetic moment and resultant spin magnetic moment vanishes.

      Depending upon the magnitude of the magnetic moment , the materials are classified into there categories namely

(a) Non- Magnetic or Diamagnetic Materials:

     Those materials whose magnetic moment vanishes are called non-magnetic or diamagnetic materials. For a substance Zn, Cd, Hg, Pb, Sn etc.

Zinc

Zinc

                                                     

 (b) Paramagnetic Materials :

      Those materials for which magnetic moment is non-zero and small are called paramagnetic materials. Sometimes these materials  are also defined as “Those materials whose magnetic moment is 1 Bohr magneton is called paramagnetic materials”. For example  H, Li, Na, K, Rb, Cs, Ag, Cu, Au, etc.

Sodium

Gold

(c ) Feromagnetic Materials :

       All those materials whose magnetic moment is quite large or >1 Bohr magneton is called ferromagnetic materials. For examples Fe, Ni, Co, Dy, Gd, etc.

Iron

Cobalt