[1] Univ Tsukuba, Inst Appl Phys, Tsukuba, Ibaraki 3058573, Japan
[2] Univ Tsukuba, Special Res Project Nanosci, Tsukuba, Ibaraki 3058573, Japan
[3] Natl Inst Mat Sci, Nanomat Lab, Tsukuba, Ibaraki 3050044, Japan
[4] Tohoku Univ, Mat Res Inst, Sendai, Miyagi 9808577, Japan