Coexistence of ferromagnetism and superconductivity in iron based pnictides: a time resolved magnetooptical study

TL;DR

This study uses time-resolved magneto-optical techniques to investigate the coexistence of ferromagnetism and superconductivity in iron-based pnictides, revealing weak coupling and ferromagnetic domain structures.

Contribution

It provides direct optical evidence that the coupling between ferromagnetism and superconductivity is weak and identifies ferromagnetic domain structures in these materials.

Findings

Weak energy transfer from electrons to spins indicates weak coupling.

Coexistence occurs without short-lengthscale ferromagnetic modulation.

Presence of two distinct ferromagnetic domains.

Abstract

Ferromagnetism and superconductivity are antagonistic phenomena. Their coexistence implies either a modulated ferromagnetic order parameter on a lengthscale shorter than the superconducting coherence length or a weak exchange coupling between the itinerant superconducting electrons and the localized ordered spins. In some iron based pnictide superconductors the coexistence of ferromagnetism and superconductivity has been clearly demonstrated. The nature of the coexistence, however, remains elusive since no clear understanding of the spin structure in the superconducting state has been reached and the reports on the coupling strength are controversial. We show, by a direct optical pump-probe experiment, that the coupling is weak, since the transfer of the excess energy from the itinerant electrons to ordered localized spins is much slower than the electron-phonon relaxation, implying the coexistence without the short-lengthscale ferromagnetic order parameter modulation. Remarkably, the polarization analysis of the coherently excited spin wave response points towards a simple ferromagnetic ordering of spins with two distinct types of ferromagnetic domains.