Anomalous superconductivity and field-induced magnetism in CeCoIn₅

In the heavy fermion superconductor CeCoIn₅ (T_c=2.3 K) the critical field is large, anisotropic, and displays hysteresis. The magnitude of the critical-field anisotropy in the a-c plane can be as large as 7 T and depends on orientation. Critical-field measurements in the (110) plane suggest two-dimensional superconductivity, whereas conventional effective-mass anisotropy is observed in the (100) plane. Two distinct field-induced magnetic phases are observed: H_a appears deep in the superconducting phase, while H_b intersects Hc₂ at T=1.4 K and extends well above T_c. These observations suggest the possible realization of a direct transition from ferromagnetism to Fulde-Ferrel-Larkin-Ovchinnikov superconductivity in CeCoIn₅.