Optical Stabilization of Fluctuating High Temperature Ferromagnetism in YTiO$_3$

Abstract: In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic, and chemical constraints. Here, we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO$3$, a material that exhibits only partial orbital polarization, an unsaturated low-temperature magnetic moment, and a suppressed Curie temperature, $T_c$ = 27 K. The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to $T{neq} >$ 80 K, nearly three times the thermodynamic transition temperature. First-principles and model calculations of the nonlinear phonon-orbital-spin coupling reveal that these effects originate from dynamical changes to the orbital polarization and the makeup of the lowest quasi-degenerate Ti $t_{2g}$ levels. Notably, light-induced high temperature ferromagnetism in YTiO$_3$ is found to be metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.