Direct Observation of Spontaneous Weak Ferromagnetism in the Superconductor ${\mathrm{ErNi}}_2{B}_{2}C$

Neutron measurements show that superconducting ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ ( $T_C=11\mathrm{}\mathrm{K}$) develops antiferromagnetic spin density wave magnetic order ( $T_N=6\mathrm{}\mathrm{K}$), which squares up with decreasing temperature yielding a series of higher-order magnetic Bragg peaks with odd harmonics. Below $T_{\mathrm{WFM}}=2.3\mathrm{}\mathrm{K}$ where magnetization indicates a net moment develops, even-order Bragg peaks develop which low field ( $\sim 3\mathrm{Oe}$) polarized beam measurements show are magnetic in origin. The data directly demonstrate the existence of a net magnetization with a periodicity of $20\mathbf{a}$, confirming the microscopic coexistence of spontaneous weak ferromagnetism with superconductivity.