# Magnetic Properties of Solids

Magnetic Properties of Solids

Solids can be divided into different classes depending on their response to magnetic fields.

(a)    Paramagnetic (Weakly magnetic): Such materials contain permanent magnetic dipoles due to the presence of atoms, ions or molecules with unpaired electrons e.g. $O_2, Cu^{2+}, Fe^{3+}$ and  They are attracted by the magnetic field. They, however, lose their magnetism in the absence of a magnetic field.

(b)   Diamagnetic: They are weakly repelled by magnetic fields. Diamagnetism arises due to the absence of unpaired electrons e.g. $H^-, Li^+, Be^{2+} (2 \text{electron type}), O^{2-}, F^-, Na^+, Mg^{2+}(8 \text{electron type}), Ag^+, Zn^{2+}, Cd^{2+}$(18 electron type).

(c)    Ferromagnetic: It is caused by spontaneous alignment of magnetic moments in the same direction.

(d)   Ferrimagnetism: It occurs when the moments are aligned in parallel and antiparallel directions in unequal number resulting in a net moment. E.g. $Fe_3O_4$

Fe > Co > Ni > Cu

(e)   Antiferromagnetism: It occurs if the alignment of moments is in a compensatory way so as to give zero net moment e. g. MnO. .

All magnetically ordered solids (ferromagnetic and antiferromagnetic solids) transform to the paramagnetic state at some elevated temperatures. This is most probably due to the randomisation of spins.

Ferromagnetic Character

Anti-Ferromagnetic Character

Ferrimagnetic Character

Effect of Temperature: Ferromagnetic, Antiferro magnetic and Ferrimagnetic solids convert into paramagnate at a specific temperature on heating due to greater alignment of spin in any one direction on heating.

Ex: $Fe_3O_4$ at 850 K.

At Curie temperature ferromagnetism is not observed or lost.

Pyroelectricity: It is electricity produced during the heating of some polar crystals.

Piezo electricity: It is electricity produced on applying mechanical stress on polar crystals.

Ferro electricity: It is found that in some piezo electric crystals the dipoles are permanently polarized even in the absence of electric field. However, when electric field is applied, the direction of polarization is changed. It is called ferroelectricity.

Some example: $BaTiO_3$ (Barium titanate)

Rochelle Salt (Sodium Potassium tartrate), $KH_2PO_4$  (Potassium di hydrogen phosphate)

• Antiferro electricity is observed in $PbZrO_3$ (lead zirconate).

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