A Novel Route to Order-by-Disorder in a Ferromagnet

Order-by-disorder, whereby fluctuations lift an accidental symmetry to stabilize a collection of broken symmetry states, is a recurrent and prominent theme in the field of frustrated magnetic systems with magnetic moments, or spins, subject to competing interactions. Thus far, such a phenomenon has been observed in systems where the selected state is not a true ground state of the quantum Hamiltonian. In part motivated by the ferromagnet pyrochlore oxides Lu₂V₂O₇ and Y₂V₂O₇, we present results exposing a novel route to order-by-disorder in a pyrochlore ferromagnet with spin-orbit interactions, where the selection is purely thermal, unaffected by zero-point fluctuations, and where the selected states are true ground states of the quantum Hamiltonian. We show that this selection can be explained solely from the thermal population of single-magnon bands. In certain circumstances, using both linear spin-wave theory and classical Monte Carlo simulations, one or more spontaneous rotations of the magnetization axis is found at intermediate temperature(s) within the ferromagnetic phase.