Direct Observation of Spontaneous Weak Ferromagnetism in the Superconductor ErNi₂B₂C

Neutron measurements show that superconducting ErNi2B2C (T_C = 11 K) develops antiferromagnetic spin density wave magnetic order (T_N = 6 K), which squares up with decreasing temperature yielding a series of higher-order magnetic Bragg peaks with odd harmonics. Below TWFM = 2.3 K where magnetization indicates a net moment develops, even-order Bragg peaks develop which low field (∼3 Oe) polarized beam measurements show are magnetic in origin. The data directly demonstrate the existence of a net magnetization with a periodicity of 20a, confirming the microscopic coexistence of spontaneous weak ferromagnetism with superconductivity.