  Mott insulator plateaus in optical lattices are a versatile platform to study
spin physics. Using sites occupied by two bosons with an internal degree of
freedom, we realize a uniaxial single-ion anisotropy term proportional to
$(S^z)^2$, which plays an important role in stabilizing magnetism for
low-dimensional magnetic materials. Here we explore non-equilibrium spin
dynamics and observe a resonant effect in the spin anisotropy as a function of
lattice depth when exchange coupling and on-site anisotropy are similar. Our
results are supported by many-body numerical simulations and are captured by
the analytical solution of a two-site model.
