Ferromagnetic behaviour of bismuth germanate oxides glass–ceramic materials

Highlights

• Diluted ferromagnetism was evidenced in Bi based glass ceramics (BGO).

• It is related mainly to oxygen defects in the atypical amorphous phase.

• The coercivity depends on the density of such defects.

• The magnetization depends on the overall number of O defects in GeO₄ tetrahedra.

Abstract

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.

Introduction

Amorphous materials become an important player nowadays in the fabrication of very resistant devices, being more efficient than the crystalline structures [1]. However, heterogeneous nucleation of these amorphous materials drastically changes the physical properties of the obtained glass–ceramic materials.

Bismuth germanate oxides are known as scintillator materials based on the Bi³⁺ emission [2], [3], [4]. These oxides have eulityne-type cubic structure and crystallize in the space group I-43d. The Bi₄Ge₃O₁₂ cubic structure consists of a regular arrangement of GeO₄ tetrahedra and distorted BiO₆ octahedrons [5], [6]. The heterogeneous crystallization of these oxides is obtained by controlled nucleation in which the glass precursor is thermally transformed, firstly in nano-crystalline dispersed particles in the whole glass [7]. At high temperatures, bismuth germanate oxides crystallize in a cubic structure as a dominant phase, but at lower temperatures some other crystalline phases are involved in the crystallization processes. For example, there is the intermediate phase of orthorhombic structure, Bi₂GeO₅, which is important due to its ferroelectric behaviour [8].

In the recent years, a series of oxides have been studied for their ferromagnetic behaviour at room temperature, e.g. MgO [9], Al₂O₃ [10], ZnO [10] and GaN [11]. The controversy regarding the ferromagnetic properties of transition metal (TM) doped oxides (diluted magnetism due to indirect interactions between TM ions versus magnetism due to TM based magnetic precipitates) is in case of these compounds based on just diamagnetic TM ions (as for example Bi³⁺) even more pronounced. Their magnetic behaviour is generally associated with inherent defects induced in a nanoparticule-like morphology, giving rise to local magnetic moments. Such defects might be also related to oxygen vacancies, which might have a direct influence on the photoluminescence properties of the oxides. The gradually annealing of the amorphous glasses of bismuth germanate oxides involves a critical temperature range in which the high and low temperature phases can co-exist and transform one to another. Expectedly, such a gradually transformation might tune the density of the oxygen vacancies together with the photoluminescence and ferromagnetic properties of the glass–ceramic materials, which can be finally used as case study of such inter-relations. For example, the photoluminescence of bismuth germanate oxides is based on three different aspects related to this material: GeO₄ luminescence, Bi³⁺ optical transitions and the oxygen vacancies produced during sample preparation. It is clearly that in the bismuth germanate oxides perfect crystals, the number of oxygen vacancies are expected to be as low as possible, whereas formation of nanocrystalline phases allows to tune in a controlled manner the density of both the oxygen vacancies and expected magnetic defects.

The aim of this paper is focused on the study of the ferromagnetic behaviour of the bismuth germanate oxides glass–ceramic materials (previously annealed between 600 °C and 700 °C, where nanocrystalline Bi₂GeO₅ is dominant) in connection with their photoluminescence and XRD analysis, providing information about different defects in materials.