  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.
