MAGNETIC PROPERTIES OF

CRYOGENIC MATERIALS

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Topics• Magnetic Properties• Paramagnetism• Diamagnetism• Superconductivity• Meissner Effect• Change in Properties

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Magnetic Properties

• The magnetic moment of a system measures the strength and the direction of its magnetism. The term itself usually refers to the magnetic dipole moment.

• Anything that is magnetic, like a bar magnet, or a loop of electric current can have their own magnetic moments. 

• There are many different magnetic properties: Paramagnetic, Diamagnetic, Ferromagnetic, Ferromagnetic, etc...

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Paramagnetism• Paramagnetism is a form of magnetism whereby certain

materials are attracted by an externally applied magnetic field.• Have a positive magnetic susceptibility.• Paramagnetic material does not retain the magnetic properties

when the external field is removed.• Even in the presence of the field there is only a small induced

magnetization because only a small fraction of the spins will be oriented by the field.

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Diamagnetism

•  The property of an object or material that causes it to create a magnetic field in opposition to an externally applied magnetic field.

• A diamagnet is not a permanent magnet.• Diammagnetic materials get repelled by magnetic

field.• Eg: Zinc

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Superconductivity• The simultaneous disappearance of all electric resistance

and the appearance of perfect diamagnetism.• Appears only at low temperature.• Transition temperature – The temperature at which

superconductivity occurs in the absence of magnetic field.• Superconductivity can be destroyed by increasing the

magnetic field to critical field.

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Superconductivity…..(cndn)

• Type 1 superconductors have single value of critical field.• Type 2 super conductors have ‘Lower & Upper Critical

fields.• Type 1 materials obey Silsbee hypothesis which

highlight the relevance of the term critical current.

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Meissner Effect• Proposed by Meissner and Ochsenfeld in 1933.• Experiment is done in a monocrystal of tin. • The magnetic flux density in a super conducting bulk

material is always zero.• Magnetic field is expelled when the material become a

superconductor.• Basis of the frictionless bearing and superconducting

motor.

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Meissner Effect.. (cndn)

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Change in Properties• Specific Heat increase abruptly.• Thermoelectric Effects (Thomson, Seeback & Peltier) vanishes.• Thermal conductivity of pure metals decreases abruptly and for

some alloys (Eg. Pb-Bi), it increases.• Electric Resistance decreases to Zero abruptly for Type 1 and for

Type 2, change occurs in 1 K temperature range.• Magnetic Perimiability suddenly bcames Zero for Type 1 and for

Type 2, Meissner effect is incomplete for magnetic fields greater than Lower Critical Fields.

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Thank You..

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