Exploring the antiferromagnetic superconducting phase in CeCoIn$_5$

CeCoIn$_5$ is a heavy fermion type-II superconductor showing clear signs of Pauli-limited superconductivity. CeCoIn$_5$ is also very close to a magnetically ordered ground state; this can be achieved by, for instance, doping with Cd. A variety of measurements give evidence for a transition at high magnetic fields inside the superconducting state, when the field is applied either parallel to or perpendicular to the $c$ axis. In the latter case, antiferromagnetic order develops on the high-field side of the transition, with a magnetic wavevector of ($q$ $q$ 0.5), where $q$ = 0.44 reciprocal lattice units [1]. We show that this order remains as the field is rotated out of the basal plane, but that the associated moment eventually disappears above 17$^{\circ}$, indicating that anomalies seen with the field parallel to the $c$ axis are not related to this magnetic order [2]. Our measurements emphasise the fragility of this magnetic order.\\[4pt] [1] M. Kenzelmann et al., Science 321, 1652 (2008).\\[0pt] [2] E. Blackburn et al., Phys. Rev. Lett. 105, 187001 (2010).