Stabilizing fluctuating high-temperature ferromagnetism in YTiO₃ by optically driving the lattice

In complex oxides, the coupling between spin, orbital, and lattice degrees of freedom leads to competing ground states and, often, large fluctuations of the order parameter up to high temperatures. The Mott insulating rare-earth titanates provide a prime example of such physics, where structural distortions dictate the t_2g orbital order and the low-temperature magnetism. Here, we explore the effect of selective optical phonon excitation in ferromagnetic YTiO₃ which is known to exhibit magnetic fluctuations well in excess of T_c (= 27 K). We discover an ultrafast, phonon-dependent magnetization change below T_c induced by the pump and a corresponding modified ferromagnetic onset temperature. The strongest effect is found when driving the 9 THz B_2u mode, for which the enhanced magnetization at low temperatures saturates at the ideal spin-½ limit and non-equilibrium ferromagnetism persists up to ~100 K, more than 3 equilibrium T_c. Our findings are explained with a coupled spin-orbital model, in which the optically driven lattice vibrations modify the occupied t_2g orbital states and reduce competing antiferromagnetic fluctuations. These results highlight the ability to optically engineer crystal structures to realize non-equilibrium functional properties in complex oxides.