We have successfully developed a technique to obtain atomically flat ultrathin Fe(001) layers on MgO(001). In contrast to the usual island morphology of ultrathin Fe layers grown directly on MgO(001), the insertion of an ultrathin gamma-Fe2O3 layer improves the crystalline quality and interface flatness of the ultrathin Fe layer of less than 1 nm thick. Owing to the continuous layered structure, even 0.6-nm-thick Fe layers exhibited a ferromagnetic hysteresis curve with high remanent magnetization. The technique enables flexible structural design of spintronic devices with ultrathin ferromagnetic electrodes. (C) 2013 The Japan Society of Applied Physics