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

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.

Fluence-dependent femtosecond quasiparticle and Eu2+ spin relaxation dynamics in EuFe2(As,P)Pogrebna1, Mertelj2, Cao3 et al. 2016Phys. Rev. BSelf Cite1100View full textAdd to dashboardCite

“…Concurrently with the Eu 2+ spin ordering [10,24] below ∼17-19 K we observe [13] in both samples appearance of another much slower relaxation component with a rise time of ∼1 ns in Eu-122 and ∼100 ps in EuP-122 (at T = 1.5 K) and the decay time beyond the experimental delay range (see Fig. 3).…”

“…On the basis of a detailed temperature and magnetic field dependence we have shown previously [13] that the nanosecond component can be associated with Eu 2+ spin demagnetization in both the AFM and FM states. The saturation of the nanosecond response at high excitation fluences can therefore be associated with a complete suppression of the Eu 2+ sublattice magnetizations due to the lattice temperature rise above the magnetic ordering temperatures.…”

“…Since the T and B dependence of this component have been already analyzed and discussed previously [13] we omit further details here, focusing on further aspects of our data not discussed elsewhere.…”

“…Here we extend our previous transient reflectivity study [13] first focusing briefly on the spin-density-wave dominated part of the phase diagram followed by a study of the superconducting phase-diagram region at varying excitation density to study a suppression of the coexistent orders on an ultrafast time scale.…”

“…Our recent transient magneto-optical spectroscopy study [13] also points towards the simple FM Eu 2+ spin order in the superconducting EuFe 2 (As 1−x P x ) 2 with a slow energy transfer between the FeAs-plane quasiparticles and Eu 2+ spins indicating a weak magnetic-dipole dominated coupling between the SC and FM order parameters.…”