Hyperfine fields and magnetic structure in the B phase of CeCoIn5

TL;DR

This paper re-analyzes NMR spectra in CeCoIn5's B-phase, revealing incommensurate antiferromagnetic order and the importance of hyperfine coupling symmetry in understanding magnetic structure.

Contribution

It provides a detailed analysis of hyperfine interactions and magnetic order in CeCoIn5, clarifying the magnetic structure and its relation to superconductivity.

Findings

NMR spectra are consistent with incommensurate antiferromagnetic order.

The hyperfine coupling symmetry affects the interpretation of magnetic moments.

The B phase may be an intrinsic modulated superconducting and magnetic state.

Abstract

We re-analyze Nuclear Magnetic Resonance (NMR) spectra observed at low temperatures and high magnetic fields in the field-induced B-phase of CeCoIn5. The NMR spectra are consistent with incommensurate antiferromagnetic order of the Ce magnetic moments. However, we find that the spectra of the In(2) sites depend critically on the direction of the ordered moments, the ordering wavevector and the symmetry of the hyperfine coupling to the Ce spins. Assuming isotropic hyperfine coupling, the NMR spectra observed for H||[100] are consistent with magnetic order with wavevector Q=pi((1+delta)/a,1/a,1/c) and Ce moments ordered antiferromagnetically along the [100] direction in real space. If the hyperfine coupling has dipolar symmetry, then the NMR spectra require Ce moments along the [001] direction. The dipolar scenario is also consistent with recent neutron scattering measurements that find an ordered moment of 0.15 mu_B along [001] and Q_n=pi((1+delta)/a,(1+delta)/a,1/c) with incommensuration delta = 0.12 for field H||[1-10]. Using these parameters, we find that a hyperfine field with dipolar contribution is consistent with findings from both experiments. We speculate that the B phase of CeCoIn5 represents an intrinsic phase of modulated superconductivity and antiferromagnetism that can only emerge in a highly clean system.