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 presented a superconducting phase, poor in Mg-deficiency, coexisting with an insulating phase, vacancy-rich. This contribution reports on the magnetic response of a number of samples of this system. Results were consistent with the idea that Mg-vacancies cluster to form insulating regions. Particularly enhanced in Mg0.975B2, the effect on the magnetic response resembles that of a columnar-like structure that percolates throughout the length of the sample, a feature that could hardly be detected by direct observation 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