Uncovering the Meissner Effect: Clarifications Needed

[ananthu]

I need clarifications on Meissner effect.


1. According to this theory, when any diamagnetic substance is cooled below its critical temperature, it becomes a superconductor and it expels magnetic flux from inside. Is it applicable only to a diamagnetic substance or ferromagnetic substances like iron also?

2. Expulsion of magnetic field means 'removal of the mag flux from inside the material to the outside'. Here a point which I could not understand is that since the permeability of a diamagnetic substance is almost negligible, already no magnetic flux could have penetrated into it. Then what does it mean that it 'expels' the magnetic flux present inside it?

3. Can a 'magnetic levitation experiment' be conducted by placing a magnet on a super-cooled ferromagnetic material such as iron or steel?

 

[DrDu]

I need clarifications on Meissner effect.


1. According to this theory, when any diamagnetic substance is cooled below its critical temperature, it becomes a superconductor and it expels magnetic flux from inside. Is it applicable only to a diamagnetic substance or ferromagnetic substances like iron also?

2. Expulsion of magnetic field means 'removal of the mag flux from inside the material to the outside'. Here a point which I could not understand is that since the permeability of a diamagnetic substance is almost negligible, already no magnetic flux could have penetrated into it. Then what does it mean that it 'expels' the magnetic flux present inside it?

3. Can a 'magnetic levitation experiment' be conducted by placing a magnet on a super-cooled ferromagnetic material such as iron or steel?

ad 1: The vast majority of diamagnetic substances doesn't get superconducting on cooling. For substances to get superconducting, they have to be metallic although not all metals get superconducting. Also paramagnetic metals can get superconducting. However, ferromagnetic metals usually don't get superconducting, although there are some exceptions.
ad 2: The permeability is not negligible, it is nearly 1 for most diamagnetic substances.
ad 3: No, there is a theorem showing that this is not possible:
http://en.wikipedia.org/wiki/Earnshaw's_theorem
However, it is possible to levitate ordinary diamagnetic materials in an inhomogeneous magnetic field. You can buy ready kits containing magnets and graphite to demonstrate this effect. With stronger fields you can even levitate frogs:
http://en.wikipedia.org/wiki/Magnetic_levitation

 

[ananthu]

Thank you DrDu. What I want to know is whether magnetic flux will pass through a diamagnetic metal as it passes with ease in a ferro or para magnetic material, since the permeability of a dia is too low when compared to a ferro. Because in many illustrations the first diagram shows penetration of magnetic flux through the specimen placed in the magnetic filed under normal temp, and the second figure shows the expulsion of the same from the specimen while it it is cooled to become a superconductor.

Also I what to know what mechanism is involved in the above expulsion process. Your explanations on these lines will be appreciated.

 

[DrDu]

In a normal diamagnet, the flux inside the specimen is somewhat lower (of the order of $\mu_r-1$, depending e.g. on the geometry of the sample) than the external field.
I a superconductor all of the field will be expelled due to surface currents whose magnetic field compensates the external one.