Field-induced magnetic phases in the normal and superconducting states of ErNi₂B₂C

We present a comprehensive neutron-diffraction study of the magnetic structures of ErNi₂B₂C in the presence of a magnetic field applied along [010], [110], or [001]. In zero field, the antiferromagnetic structure is transversely polarized with Q≈0.55a^* 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 [010]. Except for this possible effect, the influences of the superconducting electrons on the magnetic structures are not directly visible. Another peculiarity is that Q rotates by a small, but clearly detectable, angle of about 0.5° away from the [100] and the field direction, when the field is applied along [110] and is approximately equal to or larger than the upper critical field.