13 FIG. 17: (Color online) Incommensurate spin structure for the helical order along the c axis with the phase (0, 0, 0.7023). FIG. 15: (Color online) structure in the phase ( h = 0 , k = 0 , l = 1). FIG. 16: (Color online) Spin structure in the phase ( h = 1 / 2 , k = 0 , l = 1 / 2). FIG. 18: (Color online) Commensurate spin structure for the helical order along the c axis. with the phase (0 , 0 , l = 5 / 7), which is proposed by Yoshimori. 2 D. Helical ordered phase along the c axis (Yoshimori); commensurate structure A. Phase with ( h = 0 , k = 0 , l = 1) E. Helical ordered phase along the a axis: ( h, 0 , 1) C. Helical ordered phase along the c axis; (0 , 0 , l ) B. Phase with ( h = 1 / 2 , k = 0 , l = 1 / 2) A typical spin structure for the phase with (0, 0, 1) is described in Fig. 15. A typical spin structure for the phase with (1/2, 0, 1/2) is described in Fig. 16. A typical spin structure with l = 0 . 71421 is commensu- rate with the lattice structure and described by Fig. 18. The magnetic unit cell along the c axis is seven times larger than the unit cell of the crystal unit cell. The value l is obtained as l = 0 . 7023 for J$_{2}$ = − 1 . 68469 from Eq. (22). A typical spin structure is incommensu- rate with the lattice structure and described by Fig. 17. When J$_{3}$ = − 2 . 0 and J$_{1}$ = − 1, h is equal to 2/3 from Eq. 23). Then a typical spin structure is described by Fig. 19.