The low-temperature magnetization of high-quality samples of Mg1−xB2 (x < ~0.15) revealed a re-entrant behavior as the deficiency concentration x was increased. This uncommon result was related to the percolative nature of superconductivity in this system, which exhibited a superconducting phase, poor in Mg-deficiency, co-existing with an insulating phase, vacancy-rich. This contribution reported the magnetic response of a number of samples of this system. The results were consistent with the idea that Mg-vacancies clustered to form insulating regions. Particularly enhanced in Mg0.975B2, the effect upon the magnetic response resembled that of a columnar-like structure that percolated throughout the length of the sample, a feature that could not be detected by direct observational techniques.

Re-Entrant Magnetic Behavior and Other Oddities Related to an Intragranular Ordered Mesh of Vacancies in Magnesium-Deficient MgB2. W.A.C.Passos, P.A.Sharma, N.Hur, S.Guha, S.W.Cheong, W.A.Ortiz: Physica C, 2004, 408-410, 853-6

Figure 4

 Structure of Type-A Stacking-Fault, Seen along [100]