Rotational tuning of $H_{c2\mathrm{}}$ anomalies in ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}:$ Angular-dependent superzone gap formation and its effect on the superconducting ground state

${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ is a member of the $R{\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ family of magnetic superconductors, with $T_c\approx 11$ K and $T_N\approx 6$ K. For magnetic fields applied along the magnetically hard c axis, the upper critical field $H_{c2\mathrm{}}$ manifests a clear, sharp local maximum at $T_N.$ For magnetic fields applied within the basal plane, features in $H_{c2\mathrm{}}$ are less distinct and there are field induced changes in the local magnetic ordering. In order to address the relationship between $H_{c2\mathrm{}}$ and local moment order, anisotropic magnetic and transport measurements on single crystals of ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}\mathrm{}$ will be presented. In particular, detailed $H-T$ phase diagrams for different orientations of the applied field, as well as possible evidence of a superzone gap formation at the antiferromagnetic phase transition and its effect on anisotropic upper critical field, will be discussed. The anomaly in $H_{c2\mathrm{}}$ as well as sharp features in the normal-state resistivity can be tuned by changing the orientation of the applied field. This is consistent with tuning in and out of the 0.55a* magnetically ordered, superzone gapped phase.