Exploring the Fragile Antiferromagnetic Superconducting Phase in ${\mathrm{CeCoIn}}_5$

${\mathrm{CeCoIn}}_5$ is a heavy fermion type-II superconductor showing clear signs of Pauli-limited superconductivity. 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 $\mathbf{c}$ axis. When the field is perpendicular to the $\mathbf{c}$ axis, antiferromagnetic order develops on the high-field side of the transition. This order remains as the field is rotated out of the basal plane, but the associated moment eventually disappears above 17°, indicating that anomalies seen with the field parallel to the $\mathbf{c}$ axis are not related to this magnetic order. We discuss the implications of this finding.

Figure 1

A sketch of the geometrical constraints imposed by the cryomagnet. ${\mathbf{k}}_i$ (${\mathbf{k}}_f$) is the incident (outgoing) neutron wave vector. For this sample geometry, absorption is minimized if the outgoing beam is nearly parallel to the sample plate. The circular arrow indicates the direction of tilt required to rotate magnetic reflections into the scattering plane.

Figure 2

The scattering plane with the cryomagnet level. The dashed lines show the two field orientations examined in this paper. The marked areas are the reciprocal space accessible with a wave vector of $1.4{\AA }^{-1}$ (the wave vector used to gather the data presented here). The open circles represent nuclear reflections and the closed points represent magnetic reflections, although the magnet had to be tilted to bring them into the scattering plane (see main text). For measurements of the nuclear reflections, the direction of the field is irrelevant, and the sample could be rotated freely to bring them into the window of accessible reciprocal space.

Figure 3

Scan of the sample rotation angle $\omega$ through the magnetic peak at ($0.56,-0.44,1.5$), rotating the sample relative to the field and the incident neutron beam, with the magnetic field close to 12° to the basal plane. The background is comprised of data taken at 10.9 T (all blades), and those blades showing no peak at 10.8 T. The horizontal line is a straight line fit to the background data, and the other line is a Gaussian fit to the peak on top of the horizontal background. The temperature of the sample was 50 mK. A monitor count of $8\times {10}^6$ represents 2 hours of counting time.

Figure 4

The field dependence of the magnetic peak ($0.56,-0.44,1.5$) with a field angle of 12°. The background is the averaged sum of data obtained from the other analyzer blades during the measurement, and so represents neighboring momentum transfer values. Corrections for blade efficiency have been made. The horizontal line is a straight line fit to the background. The vertical lines mark the transition fields from the magnetostriction measurements in Ref. 9.