Magnetic Properties of Solids

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The magnetic properties of materials are a consequences of magnetic moments associated will individual electrons. The magnetic moment of an electron is due to its orbital motion and also due to its spin around its own axis.

A moving electron may be considered to be a small current loop, generating a small magnetic field, which has a magnetic moment along the axis and the magnetic moment due to electron spin is directed along the spin axis.

fig 2.37

The fundamental magnetic movement is the Bohr Magneton (m_B), which is equal to 9.27 x 10^-24 Am².

For each electron in an atom, the spin magnetic moment is

(depending on the two possibilities of the spin). The contribution of the orbital magnetic movement is equal to m_l m_B where m_l is the magnetic quantum number of the electron.

Materials can be classified depending on their response to external magnetic field.

Diamagnetic materials

(e.g., TiO₂, NaCl, C₆H₆) are weakly repelled by external magnetic fields. This property is shown only by those substances which contain fully-filled orbitals i.e., no unpaired electrons is present.

Paramagnetic materials

(e.g., O₂, Cu²⁺, Fe³⁺) are attracted by a magnetic field but lose their magnetism in the absence of the magnetic field. This property is shown by those substances whose atoms, ions or molecules contain unpaired electrons.

Ferromagnetic substances

show permanent magnetism even when the magnetic field is removed.

Examples: FeO and CrO₂

Fe, Co, Ni show ferromagnetism even at room temperature.

CrO₂ is used to make magnetic tapes for use in cassette recorders.

The spontaneous alignment of magnetic moments due to unpaired electrons in the same direction gives rise to ferromagnetism.

Anti-ferromagnetic substances

Anti-ferromagnetic substances are expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons present in them but actually they possess zero net magnetic moment.

This is because of the alignment of dipoles is in a compensatory way so as to give a net zero dipole moment, then we get antiferromagnetic substances (MnO).

Ferrimagnetic substances

Ferrimagnetic substances are expected to possess large magnetism on the basis of the number of unpaired electrons present in them, but actually have small net magnetic moment.

This is because the moments are aligned in parallel and anti-parallel directions in unequal numbers resulting in a net dipole moment.

Example: Fe₃O₄, ferrites of formula M²⁺Fe₂O₄ where, M = Mg, Cu, Zn etc.

Effect of temperature

Ferromagnetic, Antiferromagnetic and Ferrimagnetic solids transform to the paramagnetic state at some temperature due to randomisation of spins. Ferrimagnetic Fe₃O₄ becomes paramagnetic at 850 K.

It is also observed that Ferromagnetic substances have a characteristic temperature above which no ferromagnetism is observed. This is known as Curie temperature.