Magnetic Properties of Matter

(3) Classification of magnetic material

(A) Phenomenological classification

  • Such type of classification is based on sign and magnitude of magnetic susceptibility χ
  • According to this type of classification there are three type of magnetic material
    i) Diamagnetic materials -> χ < 0
    i.e magnetic susceptibility is negative
    ii) Paramagnetic material -> χ > 0
    i.e magnetic susceptibility is positive and less then unity iii) Ferromagnetic material -> χ>> 0
    i.e magnetic susceptibility is positive and is very high
  • This approach ignores the nature of microscopic carriers of magnetism and does not consider their interaction
  • Through this approach magnetic states like anti-ferromagnetic ,ferromagnetic cannot be recognized

(B) Main Effects of external Field

  • Main effects related to the actions of external field on magnetic moments of atomic carriers are
    i) Diamagnetic effects
    ii) Paramagnetic effects
  • It was first proposed by the Ampere that the magnetic properties of a material arises due to large number of tiny current loops within the material
  • These tiny microscopic current loops are associated with the motion of electrons within the atoms and each current loop has a magnetic moment associated with it
  • In addition to the orbital motion of electron around the nucleus electron also spin or rotate about their own axis
  • Thus internal magnetic field in a material is produced by electron orbiting around the nucleus and by the spin of the electrons as shown below in the figure .This is how internal magnetism is produced in the material

    Diamagnetic effects and  paramagnetic effect

(i) Diamagnetic effects
  • Diamagnetic effects occurs in materials where magnetic field due to electronic motions i.e orbiting and spinning completely cancels each other
  • Thus for diamagnetic materials intrinsic magnetic moments of all the atoms is zero and such materials are weakly affected by the magnetic field.
  • The diamagnetic effects in material is a result of inductive action of the externally applied field on the molecular currents
  • To explain the occurrence of this effect ,we first consider the Lenz law accordingly to which, whenever there is a change in a flux in a circuit, an induced current is setup to oppose the change in flux linked by the circuit
  • Here the circuit under consideration is orbiting electrons in an atom, ions or molecules constituting the material under consideration
  • We know that moving electron are equivalent to current and when there is a current ,there is a flux
  • On application of external field ,the current changes to oppose the change in flux and this appear as a change in the frequency of the revolution
  • The change in frequency gives rise to magnetization as a result of which each atom will get additional magnetic moment ,aligned opposite to the external field causing it.
  • It is this additional magnetic moment which gives diamagnetic susceptibility a negative sign which is order of 10-5 for most diamagnetic material (e g. bismith,lead,copper,silicon,diamond etc).
  • All substances are diamagnetic ,although diamagnetism may vary frequently be masked by a stronger positive paramagnetic effect on the part of external magnetic field and as a result of internal interactions
  • Diamagnetic susceptibility is independent of temperature as effect of thermal motion is very less on electron orbits as long as it deform them

(ii) paramagnetic effect
  • Materials having non zero permanent magnetic moment may either be paramagnetic or ferromagnetic but in this section we will only discuss paramagnetic effects
  • Para magnetism occurs in material where the magnetic field produced by orbital and spin motion of the electron do not cancel each other completely
  • Materials showing paramagnetic effects in the presence of external magnetic field have permanent magnetic moment of the atoms
  • In the absence of external field paramagnetic material have magnetic moments but they are oriented randomly.
  • These moments (both due to spin and orbital motion of electron) experience an orienting effect in the presence of externally applied magnetic field
  • Due to this orienting effect material gets magnetized parallel to the external applied field resulting in positive paramagnetic susceptibility
  • This alignment of atomic magnetic moments in paramagnetic substance is opposed by the thermal motion of the atoms ,so alignment increases with the decrease in temperature and increase in strength of applied magnetic field
  • Thus there is a sharp dependence of paramagnetic susceptibility on temperature
  • For paramagnetic substance magnetic susceptibility is of the order of 10-5 to 10-3 and it is temperature dependent

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