We studied the cause that field dependences of magnetization of \(\hbox {MgB}_2\) single crystal are similar to that of ideal type II superconductor. Here, we present the cause of the behavior and main defects of \(\hbox {MgB}_2\) single crystal. Boron-deficient stacking faults (SFs) are dominant defects of \(\hbox {MgB}_2\) single crystal, which are a planar defect. SF is confirmed in a single grain of air-cooled 5 wt.% (Fe, Ti) particle-doped \(\hbox {MgB}_2\) . It was expected that they would show similar flux-pinning behavior with that of grain boundaries (GBs) because the two have nature of planar defect. We inspected first flux-pinning effects of increased GB by through of water-quenching of 5 wt.% (Fe, Ti) particle-doped \(\hbox {MgB}_2\) superconductor. The width of GB was confirmed less than 1 nm by TEM. However, the distribution of atoms at neighborhoods of GB was a little distorted from regular array of \(\hbox {MgB}_2\) owing to empty space of GB. On the other hand, width of SF was approximately less than 0.6 nm and it has almost no distortion of atoms around the neighborhoods. It was understood that planar defects in superconductor have flux-pinning effect as well as flux-penetrating effect. However, SF of \(\hbox {MgB}_2\) does not have flux-pinning effect, but have flux-penetrating effect because of their narrow width and almost no distortion from regular array of the neighborhood atoms around SF, which would cause that field dependences of magnetization of \(\hbox {MgB}_2\) single crystal are similar to that of ideal type II superconductor.
