High-field superconducting transition of CeCoIn5 studied by local magnetic induction measurements

To investigate the unusual normal-superconducting phase transition near the upper critical fields at low temperatures of the quasi-two-dimensional heavy fermion compound CeCoIn5, local magnetic induction measurements have been performed by using a micro Hall probe for both H parallel to c and H parallel to a. The local magnetic induction as a function of temperature displays a discontinuous jump at the superconducting phase boundary below the critical temperature T-0 approximate to 0.8 K for both field directions, indicating a change from the second- to first-order transition and confirming previously reported bulk measurements on CeCoIn5. The magnetic induction data display an unexpected undershoot behavior just below the jump down, which has never been reported in the bulk measurements. This undershoot is observed at the phase boundary between the normal state and the high-field superconducting phase, which has been attributed to the spatially inhomogeneous Fulde-Ferrell-Larkin-Ovchinnikov state. Possible origins of this unusual undershoot behavior are discussed.