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I came across an article that dimagnetism is one of the proof so that a material can act as superconductor. So what is dimagnetism.

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    $\begingroup$ With the correct spelling it is easier to search: diamagnetism. And look especially for Meissner effect. $\endgroup$ – Pieter Jul 22 '19 at 14:05
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DiAmagnetism is the natural desire of all substances to expel external magnetic fields. However, this tends to be an extremely weak effect, so much so that in most materials it's invisible. In fact, even magnets are diamagnetic, but their internal magnetic moments overwhelm this counteracting effect.

The effect is noticeable in any electrical conductor, like a sheet of copper. When the copper is placed in an external magnetic field, a current is induced in it. This is (likely) what is creating the electricity you're using to read this. Lenz's law tells us that that induced current will always be in a direction that creates a magnetic field that counteracts the external one. This can be used to create a magnetic brake, like the one on my bicycle trainer.

In a normal conductor like copper, there is internal resistance that causes the electrical current to dissipate as heat. But there is no such internal resistance in a superconductor. When they are placed in a field, a current begins to flow at the surface, and like the copper case, it directly opposes the external field. With nothing to stop it, the field builds up until it exactly matches the external field, expelling it from the superconductor and making it perfectly diamagnetic.

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    $\begingroup$ Not correct, it does not explain why the field is expelled when cooled below $T_c$. $\endgroup$ – Pieter Jul 22 '19 at 20:55

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