  In this article, we study the energy dissipation property of time-fractional
Allen-Cahn equation. We propose a decreasing upper bound of energy that
decreases with respect to time and coincides with the original energy at $t =
0$ and as $t$ tends to $\infty$. This upper bound can also be viewed as a
nonlocal-in-time modified energy, the summation of the original energy and an
accumulation term due to the memory effect of time fractional derivative. In
particular, this indicates that the original energy indeed decays w.r.t. time
in a small neighborhood at $t=0$. We illustrate the theory mainly with the
time-fractional Allen-Cahn equation, but it could be applied to other
time-fractional phase-field models such as the Cahn-Hilliard equation.
  On the discrete level, the first-order L1 and second-order L2 schemes for
time-fractional Allen-Cahn equation have similar decreasing modified energies,
so that the stability can be established. Some numerical results are provided
to illustrate the behavior of this modified energy and to verify our
theoretical results.
