Weak ferromagnetic order in the superconducting ${\mathrm{ErNi}}_2^{11}{\mathrm{B}}_2\mathrm{C}$

${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ is the first material in which a microscopic coexistence between weak ferromagnetism (WFM) and superconductivity (SC) was confirmed by neutron diffraction technique. In the present paper, we report the detailed WFM structure and its ordering process. In the WFM phase, weak but clear peaks appear at $(n/20\mathrm{}00)$ with $n=\mathrm{even}.$ The detailed analysis revealed that one from 20 Er magnetic moments contributes to the WFM ordering and such spins form ferromagnetic sheets that appear with a periodicity of $10a\left(\sim 35\mathrm{\AA }\right).\mathrm{}$