(color online) (a) Theoretical calculation of the angular dependence of the free energy for different applied stresses parallel to the initial hard axis of the FeNi film according to Eq. (1). In this polar pattern, the energy decreases from the inner circle to the outer circle (from Fig. 2 in Ref. [45]). (b) M–H loops of FeNi (15 nm)/piezoelectric actuator for different applied voltages (from Fig. 3(c) in Ref. [45]). (c) Voltage dependence of the H_K of FeCo (10 nm)/PI heterostructures. The left and right insets show the M–H loops and the coercivity along the original EA direction at different applied voltages (from Fig. 4 in Ref. [47]). (d) Thickness-dependent remanence ratios at 0 and 110 V of the piezoelectric actuators along the original hard axis directions (from Fig. 6 in Ref. [47]).