Microscopic Evidence for Field-Induced Magnetism in ${\mathrm{CeCoIn}}_5$

We present NMR data in the normal and superconducting states of ${\mathrm{CeCoIn}}_5$ for fields close to $H_{c2}(0)=11.8\mathrm{T}$ in the $ab$ plane. Recent experiments identified a first-order transition from the normal to superconducting state for $H>10.5\mathrm{T}$, and a new thermodynamic phase below 290 mK within the superconducting state. We find that the Knight shifts of the In(1), In(2), and the Co are discontinuous across the first-order transition and the magnetic linewidths increase dramatically. The broadening differs for the three sites, unlike the expectation for an Abrikosov vortex lattice, and suggests the presence of static spin moments in the vortex cores. In the low-temperature and high-field phase, the broad NMR lineshapes suggest ordered local moments, rather than a long-wavelength quasiparticle spin density modulation expected for an FFLO phase.

Figure 1

NMR spectra of ${\mathrm{CeCoIn}}_5$ at 11.1 T (left) and at 11.485 T (right). The series of transitions at lower frequency are the seven transitions of the ${}^{59}\mathrm{Co}$, and the resonance at higher frequency is the $-\frac{3}{2}\leftrightarrow -\frac{5}{2}$) transition of the ${}^{115}\mathrm{In}(1)$. The light blue shaded spectra are in the normal state, the green spectra are within 20 mK of $T_c$ ($T_0$), the orange spectra are in the $A$ phase, and the purple spectra are in the $B$ phase.

Figure 2

NMR spectra of In(1) $-\frac{5}{2}\leftrightarrow -\frac{7}{2}$) and the $\mathrm{In}(2{)}_{\parallel }$ $-\frac{1}{2}\leftrightarrow -\frac{3}{2}$) transitions in ${\mathrm{CeCoIn}}_5$ at 11.1 T. Note that the In(1) transition at 118.3 MHz in the normal state shifts down in frequency discontinuously at $T_c\sim 470\mathrm{mK}$, whereas the $\mathrm{In}(2{)}_{\parallel }$ shifts up in frequency, as observed previously in lower fields [16]. The broad double-peak structure between 117 and 119.5 MHz at 50 mK is the $\mathrm{In}(2{)}_{\parallel }$ spectrum, and the solid line is a simulation as discussed in the text. Inset: the spectrum at 11.485 T, showing the broadened $\mathrm{In}(2{)}_{\parallel }$ central transition between 110 and 113 MHz.

Figure 3

(a) The $H\mathrm{\text{−}}T$ phase diagram, showing first-order (○) and second-order (●) transitions, from 17. The solid lines are guides to the eye. (b) The Knight shift of the In(1) at 11.8 T (○), 11.485 T (□), 11.1 T (▾), and 3.33 T (▴, [16]).

Figure 4

The second moments of the Co and In(1) resonances at 11.1 T (upper) and at 11.485 T (lower). The normal state data at 11.8 T is included in the lower panel.