  We studied electric field modification of magnetic properties in a
Pt/Co/AlO$_x$ trilayer via magneto-optical Kerr microscopy. We observed the
spontaneous formation of labyrinthine magnetic domain structure due to
thermally activated domain nucleation and propagation under zero applied
magnetic field. A variation of the period of the labyrinthine structure under
electric field is observed as well as saturation magnetization and magnetic
anisotropy variations. Using an analytical formula of the stripe equilibrium
width we estimate the variation of the interfacial Dzyaloshinskii-Moriya
interaction under electric field as function of the exchange stiffness
constant.
