Precision magnetization data are given for high-purity niobium metal samples (resistance ratio $\frac{R_{300}}{{R_{4.2}}^N}=1600\mathrm{}\pm 400$) over the range from 1.1°K to $T_c=9.25(\pm .01)°$K. The thermodynamic critical-field curve, which is parabolic to 1%, is that to be expected for an intermediate-coupling superconductor. The magnetization curves are qualitatively like those associated with a type-II superconductor with $\kappa \sim 1$, but the detailed shape does not follow any theory. In particular, the magnetization increases linearly with a slope of 23 G/Oe from 1.005 $H_{c1\mathrm{}}$ to 1.025 $H_{c1\mathrm{}}$, where a sharp break occurs to a smaller slope. The magnetization data at $H_{c1\mathrm{}}$ and in the region of $H_{c2\mathrm{}}$ are consistent with published specific-heat data. The magnitudes of $H_{c2\mathrm{}}$ and $H_c$ are consistent with measured values of the penetration depth $\lambda$. Preliminary data are reported for $H_{c3\mathrm{}}$ as obtained from both resistive and ac susceptibility measurements.

• Received 7 March 1966

DOI:https://doi.org/10.1103/PhysRev.149.231