We investigated the voltage-controlled magnetic anisotropy (VCMA) in an ultrathin Ir-doped Fe layer with a CoxFe1-x termination layer. The VCMA effect depends on the concentration of the CoxFe1-x alloy, and a large VCMA coefficient, as high as -350 fJ/Vm, was obtained with a Co-rich termination layer. First principles calculations revealed that the increased VCMA effect is due not only to the added Co atoms but also to the Fe and Ir atoms adjacent to the Co atoms. Interface engineering using CoFe termination is also effective for recovering the tunneling magnetoresistance while maintaining a high VCMA effect. The developed structure is applicable for voltage-controlled magnetoresistive devices.