Ferromagnetism triggered by band tripling in ruthenate Sr4Ru3 O10

Itinerant ferromagnetism is understood in terms of a quasiparticle picture with renormalized many-body effects. While the ferromagnetic ground state is destabilized by thermal and quantum fluctuations leading to exotic states such as unconventional superconductivity, how the quasiparticles evolve across the ferromagnetic transition is a target of intensive debate. Here, we present a type of ferromagnetic transition that is accompanied by a drastic reconstruction of quasiparticle spectrum in a layered ferromagnetic ruthenate, Sr4Ru3O10. Angle-resolved photoemission spectroscopy uncovered that the three-dimensional coherent states below the ferromagnetic transition temperature (TC) turn into two-dimensional incoherent electronic states slightly above TC characterized by the disappearance of trilayer band splitting, ferromagnetic exchange splitting, and long-lived quasiparticles. Our findings suggest that the electronic coherence strongly modifies the fermiology and magnetic order, pointing to an intriguing coupling between quasiparticles and magnetic properties.