Ferromagnetic behaviour of bismuth germanate oxides glass-ceramic materials

The content of defects in Bi4Ge3O12 (BGO) glass-ceramic materials together with their ordering during crystallization induces ferromagnetic behaviours in these materials. The observed ferromagnetism has to be associated strictly with the GeO4 related defects in the atypical amorphous phase. The photoluminescence of BGO glass-ceramic materials can give useful information about the nature of defects, especially the oxygen vacancies from the GeO4 tetrahedra and Bi4Ge3O12 cubic structures. The concentration of these defects depends on annealing between 600 °C and 700 °C where orthorhombic and cubic phases coexist. The magnetization at saturation is higher in the samples dominated by the amorphous phase and the strength of the exchange interaction is higher in the samples dominated by the cubic phase, obtained over 700 °C, when the GeO4 tetrahedra are rearranged during crystallization together with their oxygen vacancies.