Multiphase magnetism in Yb2Ti2O7

Quantum materials have properties that defy conventional theories of solids. Explaining these unusual properties is a frontier in physics, which promises both technological applications and fundamentally new states of matter. Yb₂Ti₂O₇ is a center of attention in this work. While it becomes ferromagnetic at very low temperature, its excitation spectrum resembles that of a quantum spin liquid. We show using neutron scattering on high-quality crystals that the unusual spectra may arise as a superposition of ferromagnetic and anti-ferromagnetic spin waves. This indicates that these disparate forms of magnetism actually coexist in Yb₂Ti₂O₇, which is consistent with near-perfect degeneracy. The unconventional properties of Yb₂Ti₂O₇ thus appear to arise from the atomic-scale interplay between two conventional types of order.