Upper critical field of a two-band SrFe$_{2-x}$Ni$_x$As$_2$ superconductor

The upper critical field of an iron-based SrFe$_{2-x}$Ni$_x$As$_2$ superconductor single crystal is measured for the first time in longitudinal and transverse magnetic fields up to $16$ T in the basal ab plane and to it along the $c$ axis. The $H_{c2}(0)$ values are $18$ and $25$ T for $H\|c$ and $H\|ab$, respectively. The anisotropy of the upper critical field $\gamma(T)=H_{c2}^{ab}/H_{c2}^c$ decreases monotonically to $1.4$ with decreasing temperature. The temperature dependence of $H_{c2}$ is not described fully by the Werthamer–Helfand–Hohenberg model taking into account the influence of orbital and spin paramagnetic effects. However, an effective two-band model reproduces well the measured temperature dependence. This indicates the two-band nature of this superconductor, where one of the bands is almost isotropic. According to the two-band model approximation, the contribution from the anisotropic band dominates near $T_c$, whereas the main contribution at low temperatures comes from the isotropic band with a lower diffusion coefficient.