# Impurity-induced magnetic droplet in unconventional superconductors near magnetic instability: Application to Nd doped $\mathbf{CeCoIn_5}$

## TL;DR

This paper demonstrates that magnetic impurities in unconventional d-wave superconductors near magnetic instability can induce localized SDW droplets, affecting neutron scattering signals and magnetic properties.

## Contribution

It introduces a phenomenological and microscopic model showing how impurities induce SDW droplets in Pauli limited d-wave superconductors near SDW instability.

## Key findings

- Impurities create SDW droplets without long-range order.
- Magnetic field suppresses SDW droplets and stabilizes long-range SDW.
- Results align with recent neutron scattering and thermal conductivity data.

## Abstract

Beside the spin density wave (SDW) order inside the superconducting phase in $\mathrm{CeCoIn_5}$ at high magnetic fields, recent neutron scattering measurements have found Bragg peaks in $5\%$ Nd doped $\mathrm{CeCoIn_5}$ at low fields. The intensity of Bragg peaks in low fields is suppressed by increasing field. Based on the phenomenological and microscopic modeling, we show that for the Pauli limited $d$-wave superconductors in the vicinity of SDW instability relevant for $\mathrm{CeCoIn_5}$, magnetic impurities locally induce droplets of SDW order. Because of the strong anisotropy in the momentum space in the spin fluctuations guaranteed by the $d$-wave pairing symmetry, sharp peaks in spin structure factor at $\mathbf{Q}$s are produced by the impurities, even when the droplets of SDW do not order. At zero field, the Nd impurity spins are along the $c$ axis due to the coupling to the conduction electrons with an easy $c$ axis, besides their own crystal field effect. The in-plane magnetic field cants the impurity moments toward the $ab$ plane, which suppress the droplets of SDW order. At high fields, the long-range SDW inside the superconducting phase is stabilized as a consequence of magnon condensation. Our results are consistent with the recent neutron scattering and thermal conductivity measurements.