Field-induced magnetic phases in the normal and superconducting states of ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$

We present a comprehensive neutron-diffraction study of the magnetic structures of ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ in the presence of a magnetic field applied along $\left[010\right],$ $\left[110\right],$ or $\left[001\right].$ In zero field, the antiferromagnetic structure is transversely polarized with $\mathbf{Q}\approx 0.55{\mathbf{a}}^{*}$ and the moments along the b direction. At the lowest temperatures, the modulation is close to a square wave, and transitions of Q between different commensurable values are observed when varying the field. The commensurable structures are analyzed in terms of a detailed mean-field model. Experimentally, the minority domain shows no hysteresis and stays stable up to a field close to the upper critical field of superconductivity, when the field is applied along $\left[010\right].$ Except for this possible effect, the influences of the superconducting electrons on the magnetic structures are not directly visible. Another peculiarity is that $\mathbf{Q}$ rotates by a small, but clearly detectable, angle of about $0.5°$ away from the $\left[100\right]$ and the field direction, when the field is applied along $\left[110\right]$ and is approximately equal to or larger than the upper critical field.