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.
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