Squared-spin-modulated versus spiral-like magnetic structures in ${\mathrm{GdNi}}_2{\mathrm{B}}_2\mathrm{C}:$  A ${}^{155}\mathrm{Gd}$ Mössbauer-effect investigation

${}^{155}\mathrm{Gd}$ Mössbauer spectroscopy has been used to study the magnetic properties of ${\mathrm{GdNi}}_2{\mathrm{B}}_2\mathrm{C}.$ It was shown that below $T_R\cong 14\mathrm{K}$ this compound orders in a bunched spiral-like magnetic structure with gadolinium magnetic moments rotating within the $(\mathbf{b},\mathbf{c})$ plane. Between $T=14\mathrm{}\mathrm{K}$ and the Néel temperature of $T_N\cong 20\mathrm{K}$ the magnetic structure is of sine-modulated type with gadolinium moments perpendicular to the tetragonal c axis. The observation of high-order harmonics indicates, however, a partial squaring of the sine modulation.