Competing superconducting and magnetic order parameters and field-induced magnetism in electron doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

TL;DR

This study investigates the coexistence and competition of magnetic and superconducting orders in electron-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, revealing field-induced magnetism and microscopic interactions using neutron scattering and modeling.

Contribution

It provides new insights into how magnetic and superconducting states coexist and compete in iron pnictides under magnetic fields, supported by experimental data and microscopic modeling.

Findings

Magnetic and superconducting orders coexist and compete below the transition temperature.

Magnetic field enhances magnetic scattering, roughly doubling Bragg intensity at 13.5 T.

Vortices can locally slow down and freeze spin fluctuations, leading to long-range antiferromagnetic order.

Abstract

We have studied the magnetic and superconducting properties of Ba(Fe$_{0.95}$Co$_{0.05}$)$_{2}$As$_{2}$ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5 T. We perform a microscopic modelling of the data by use of a five-band Hamiltonian relevant to iron pnictides. In the superconducting state, vortices can slow down and freeze spin fluctuations locally. When such regions couple they result in a long-range ordered antiferromagnetic phase producing the enhanced magnetic elastic scattering in agreement with experiments.