Meissner effect distinguishes type I and II

The Meissner effect (expulsion of flux, $B = 0$ inside, $M = -H$) behaves differently in the two classes of superconductors. Plotting magnetization $-M$ against applied field $H$ separates them clearly.

Type-I (soft) superconductors

• Show a complete Meissner effect right up to the critical field $H_c$: $-M$ rises linearly with $H$ (perfect diamagnetism).
• At $H = H_c$ the flux enters all at once, $M$ drops sharply to zero and the material becomes normal.
• $H_c$ is low (about 0.01 to 0.2 T). Examples: Pb, Hg, Sn, Al.

Type-II (hard) superconductors

• Show a complete Meissner effect only up to the lower critical field $H_{c1}$.
• Between $H_{c1}$ and $H_{c2}$ flux partly enters as quantized vortices, so the Meissner effect is incomplete (mixed or vortex state). $-M$ falls gradually.
• At the upper critical field $H_{c2}$, $M = 0$ and the material is normal. $H_{c2}$ can be tens of tesla. Examples: NbTi, Nb$_3$Sn, YBCO.

 -M                         -M
  |    /|                    |   /\
  |   / |                    |  /  \
  |  /  |                    | /    \_
  | /   |                    |/       \__
  |/    |____ H              +----|------|--- H
  0    Hc                    0   Hc1    Hc2
   Type-I                     Type-II

So by measuring how $M$ varies with $H$, a sharp drop identifies type-I and a gradual fall over a range identifies type-II.