  The interplay between magneto-electricity (ME) and magneto-elasticity (MEL)
is studied in the context of voltage-controlled magnetic anisotropy (VCMA).
Strain plays more than a role of changing lattice constant but that of the
internal electric field in the heterostructure. As a prototype, FePt/MgO(001)
is visited, where the behavior of two interfaces are drastically different: one
exhibits switching the other does not. Whether an external electric field
($E_{ext}$) is present or not, we found VCMA coefficient larger than 1
pJ/V$\cdot$m, as a consequence of the rearrangement of $d$ orbitals with
$m=\pm1$ and $\pm2$ in response to an external electric field. In addition,
magneto-crystalline anisotropy (MA) is analyzed with strain taken into account,
where non-linear feature is presented only accountable by invoking second-order
MEL.
