title: 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.
authors.affiliation: 1] Complex Matter Dept., Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia [2] Jožef Stefan International Postgraduate School, Jamova 39, SI-1000 Ljubljana, Slovenia.
authors.forename: A
authors.initials: A
authors.lastname: Pogrebna
authors.affiliation: Complex Matter Dept., Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia.
authors.forename: T
authors.initials: T
authors.lastname: Mertelj
authors.affiliation: 1] Complex Matter Dept., Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia [2] Institute of Physics, Bijenička 46, HR-10000 Zagreb, Croatia.
authors.forename: N
authors.initials: N
authors.lastname: Vujičić
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.
authors.forename: G
authors.initials: G
authors.lastname: Cao
authors.affiliation: Department of Physics, Zhejiang University, Hangzhou 310027, People's Republic of China.
authors.forename: Z A
authors.initials: ZA
authors.lastname: Xu
authors.affiliation: 1] Complex Matter Dept., Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia [2] CENN Nanocenter, Jamova 39, SI-1000 Ljubljana, Slovenia.
authors.forename: D
authors.initials: D
authors.lastname: Mihailovic