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Considering type-I superconductors (specifically the magnetisation curves), why can one assume that the magnetisation is zero when we consider a normal state (when $H>H_C$)?

EDIT: For clarification, I'm going through the first few chapters of Tilley and Tilley, and I'm trying to get my head around why they seem to assume this. Basically, I'm looking for an explanation of the Type-I magnetisation curve, as seen in this link here. This kind of graph appears quite a lot, but what I am yet to understand fully is why does a normal (non SC) state have zero average magnetisation. My guess is something to do with the 'average' nature of it?

EDIT2: The content in T+T was slightly poorly worded and confusing to the uninitiated. Thank you all for the clarification.

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  • $\begingroup$ What do you mean? Who is assuming that the magnetization is exactly zero for H>$H_c$? $\endgroup$ – Samuel Weir Sep 12 '19 at 17:13
  • $\begingroup$ I don't know. I'm going through the first few chapters of Tilley and Tilley, and I'm trying to get my head around why they seem to assume this. Basically, I'm looking for an explanation of the Type-I magnetisation curve. I will edit my question. $\endgroup$ – Brad Sep 12 '19 at 21:25
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    $\begingroup$ That generic diagram is not intended to suggest that the magnetization goes to strictly zero at H>$H_c$. It's intended to show that when the superconducting state is killed off by the high magnetic field, the magnetization gets very small because the resulting paramagnetic or diamagnetic state of the normal material will have a very small characteristic magnetization compared to that of a superconductor. $\endgroup$ – Samuel Weir Sep 12 '19 at 22:25
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I think when drawing this picture one should pay attention to the superconducting region where we have a straight line with slope $-\chi=1$ (because y-axis is $-M$), which means that $\chi=-1$, exactly the magnetic susceptibility of an ideal diamagnet$^*$. That means the applied magnetic field is completely expelled from the interior by a magnetization field induced opposite to the applied field. This is exactly the Meissner effect. For a type II superconductor this magnetization will not fall to zero abruptly, there will be the Shubnikov phase, in which vortices will form and thus the internal magnetic field will no longer be zero.

*When speaking of an ideal diamagnet what we mean is just that it behaves as an ideal diamagnet. Don’t think of it as a real diamagnet, because in a diamagnet the induced magnetization is microscopic, while in the case of a superconductor there are macroscopic shielding/suppercurrents that provide the opposite magnetization to expel the applied field from the interior.

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