Figure 1. The geometric structures for cap-(9, 0)-Def [6, 6] and cap-(9, 0)-Def [5, 6] CNTs.

Figure 2 reflects the net spin distribution of ground states of Cap-(9, 0)-Def [6, 6] and Cap-(9, 0)-Def [5, 6] CNTs. From this figure, we can clearly see the source of the spin density. The spin density of open end is mainly derived from the p_z electrons of the carbon atoms at the zigzag edge, which is consistent with previous reports.^{8, 16} And our previous study on cap-(9, 0)/(10, 0) CNTs indicated that the spin density of carbon atoms at the cap is 0, as shown in Figure 2c. And the ground state of perfect C₆₀ is the closed-shell singlet state, without any net spin electron.³⁶ For the two defect structures, there exist spin density at the cap end, and the net spin electrons are mainly originated from the C adatoms. And the number of net spin electrons distributed on the C adatoms of Cap-(9, 0)-Def [6, 6] and Cap-(9, 0)-Def [5, 6] are about 1.6 and -0.3, respectively. For Cap-(9,0)-Def [6, 6] CNT, the net spin orientation of the adatoms at cap end and the carbon atoms at zigzag edge are both spin up, showing ferromagnetic coupling phenomenon. While for Cap-(9, 0)-Def [5, 6] CNT, the net spin orientation of the adatom at the cap is spin down, and the spin orientation of the carbon atoms at zigzag edges is spin up, appearing antiferromagnetic coupling. This also shows that different types of adsorbed atoms can regulate the magnetic properties of cap-CNTs.