Magnetism Concept Page - 15

Definition

Paramagnetic, diamagnetic and ferromagnetic materials

Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed.Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field and are repelled by the applied magnetic field. It has a negative and small value of magnetic susceptibility.Ferromagnetic materials have large magnetic susceptibility and are characterized by non-linear relation between magnetic field (B$B$) and magnetic intensity (H$H$).They show the property of hysteresis.

Definition

Meissner effect for superconductors

When a material makes the transition from the normal to superconducting state, it actively excludes magnetic fields from its interior; this is called the Meissner effect.
This constraint to zero magnetic field inside a superconductor is distinct from the perfect diamagnetism which would arise from its zero electrical resistance. It implies that if we tried to magnetize a superconductor, current loops would be generated to exactly cancel the imposed field (Lenz's law). But if the material already had a steady magnetic field through it when it was cooled trough the superconducting transition, the magnetic field would remain. If there were no change in the applied magnetic field, there would be no generated voltage (Faraday's law) to drive currents. Hence, the active exclusion of magnetic field is an effect distinct from just zero resistance. 

Definition

Critical temperature for material to become superconductor

The temperature below which a material becomes a superconductor is called critical temperature (Tc).
Bell laboratory has developed super conductor at a temp of 40 K.by using oxide of a compound of lanthanum, barium and copper.
Once the current is set up in a super conductor, it continues to flow for a very long time without decrease in strength.

Definition

Diamagnetic materials

Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field. It has a negative and small value of magnetic susceptibility.

Definition

Paramagnetic materials

Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.

Definition

Ferromagnetic materials

Ferromagnetic materials have large magnetic susceptibility and are characterised by non linear relation between magnetic field (B$B$) and magnetic intensity (H$H$).They show the property of hysteresis.

Law

Curie's Law

Curie's Law : For paramagnetic materials, M=CB0T$M=C{\displaystyle \frac{B_0}{T}}$ where C$C$ is the Curie's constant.
The Curie constant is a material-dependent property relating a material's temperature to its magnetic susceptibility.
The Curie constant, when expressed in SI units, is given by
C=μ0μ2B3kBNg2J(J+1)${\displaystyle C=\frac{{\mu }_0{\mu }_B^2}{3k_B}N{g}^{2}J(J+1)}$
where N${\displaystyle N}$  is the number of magnetic atoms (or molecules) per unit volume, g${\displaystyle g}$  is the Land g-factor, μB(9.27400915e−24J/TorAm2)${\displaystyle {\mu }_B}(9.27400915e-24J/TorAm2)$ is the Bohr magneton, J${\displaystyle J}$  is the angular momentum quantum number and kB${\displaystyle k_B}$  is Boltzmann's constant.

Definition

Curie temperature

The temperature of transition from ferromagnetic to paramagnetism is called the Curie temperature Tc$T_c$.

Definition

Hysterisis

In ferromagnetic materials, for a given value of H, B is not unique but depends on previous history of the sample. This phenomenon is called hysterisis.

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