Effects of Magnetic Order on the Superconducting Length Scales and Critical Fields in Single Crystal ${\mathrm{ErNi}}_2{B}_{2}C$

The flux line form factor in small angle neutron scattering and transport data determines the superconducting length scales and critical fields in single crystal ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$. For $H\Vert c$, the coherence length $\xi$ increases and the penetration depth $\lambda$ decreases when crossing $T_N=6.0\mathrm{}\mathrm{K}$, the Néel transition. The critical fields show corresponding anomalies near $T_N$. For $H\perp c$, the fourfold modulation of the upper critical field $H_{c2\mathrm{}}$ is strongly temperature dependent, changing sign near $T_N$, and can be modeled using the anisotropy of the sublattice magnetization.