JEE Main & Advanced Physics Magnetism Magnetic Materials

  On the basis of mutual interactions or behaviour of various materials in an external magnetic field, the materials are divided in three main categories. (1) Diamagnetic materials : Diamagnetism is the intrinsic property of every material and it is generated due to mutual interaction between the applied magnetic field and orbital motion of electrons. (2) Paramagnetic materials : In these substances the inner orbits of atoms are incomplete. The electron spins are uncoupled, consequently on applying a magnetic field the magnetic moment generated due to spin motion align in the direction of magnetic field and induces magnetic moment in its direction due to which the material gets feebly magnetised. In these materials the electron number is odd. (3) Ferromagnetic materials : In some materials, the permanent atomic magnetic moments have strong tendency to align themselves even without any external field.  These materials are called ferromagnetic materials. In every unmagnetised ferromagnetic material, the atoms form domains inside the material. Different domains, however, have different directions of magnetic moment and hence the materials remain unmagnetised. On applying an external magnetic field, these domains rotate and align in the direction of magnetic field. (4) Curie Law : The magnetic susceptibility of paramagnetic substances is inversely proportional to its absolute temperature i.e. \[\chi \propto \frac{1}{T}\]\[\Rightarrow \]\[\chi \propto \frac{C}{T}\];  where C = Curie constant, T = absolute temperature. On increasing temperature, the magnetic susceptibility of paramagnetic materials decreases and vice versa. The magnetic susceptibility of ferromagnetic substances does not change according to Curie law. (5) Curie temperature \[({{T}_{c}})\] : The temperature above which a ferromagnetic material behaves like a paramagnetic material is defined as Curie temperature \[({{T}_{c}})\]. or The minimum temperature at which a ferromagnetic substance is converted into paramagnetic substance is defined as Curie temperature. For various ferromagnetic materials its values are different, e.g. for Ni, \[{{T}_{{{C}_{Ni}}}}={{358}^{o}}C\]for Fe, \[{{T}_{{{C}_{Fe}}}}={{770}^{o}}C\] for          CO, \[{{T}_{{{C}_{CO}}}}={{1120}^{o}}C\] At this temperature the ferromagnetism of the substances suddenly vanishes. (6) Curie-weiss law : At temperatures above Curie temperature the magnetic susceptibility of ferromagnetic materials is inversely proportional to \[(T-{{T}_{c}})\] i.e. \[\chi \propto \frac{1}{T-{{T}_{c}}}\] \[\Rightarrow \] \[\chi =\frac{C}{(T-{{T}_{c}})}\] Here \[{{T}_{c}}=\] Curie temperature   \[\chi -T\] curve is shown (for Curie-Weiss Law)