The effect of Nd³⁺ impurities on the magneto-optical properties of TeO₂–P₂O₅–ZnO–LiNbO₃ tellurite glass

Abstract

A study of the magnetic and magneto-optical properties of tellurite glass doped with Nd³⁺ is discussed in this communication. These properties were determined by measuring the magnetic susceptibility and the Faraday effect. The addition of Nd³⁺ results in deterioration of the magneto-optical properties and changes in the structure of defects in the glass.

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1. Introduction

Developing new glasslike matrices to be used in modern optoelectronic and photonic devices is possible, among other things, owing to studies investigating the optical properties of glasses doped with transition metals and rare-earth elements, which include lanthanides.¹ As a result of being doped with tri-positive lanthanide ions, the glasses obtain new specific properties, which result from their electronic structure in the form of the 4f partially filled electronic shells. As the ionic radii of lanthanides are larger than those of transition metals, their oxides form a glasslike matrix, which is particularly significant for selecting the appropriate concentration of impurities so as to prevent the crystallization of glass. Under the influence of active lanthanide ions, the glasses change the optical properties of the medium, including the optical transparency, colour, absorption coefficient, refractive index, etc.^2–4 The Nd³⁺ ions used in laser amplifier systems constitute a potential activator for improving the magneto-optical properties in glasses.^5–7

Glasses showing strong magneto-optical properties and containing paramagnetic ions of rare earth elements in their structure^8,9 are interesting due to the possibility of using them in optoelectronics. They can be used as the foundation for building magnetic field sensors or optical isolators used in laser systems for coupling semiconductor lasers with optical fibers, in order to eliminate the light reflected from the optical fiber front.

On account of their very interesting physical and chemical properties,^10–14 including a high refractive index exceeding a value of 2, special tellurite glasses constitute a very promising material for building magneto-optical devices.⁸ The basic unit building the structure of a tellurite glass is the trigonal bipyramid of TeO₄.¹⁵ Taking into account the spatial random network theory of glass, tellurite glasses can be treated as coordinate systems of atoms. The glass forming Te⁴⁺ cations are connected by O²⁻ anion-bridge bonds, i.e. so called Te–O–Te oxygen bridges. A distinctive feature of tellurite glasses is the ease of addition of rare earth elements such as La³⁺, Er³⁺, Nd³⁺ and Gd³⁺, which permits the introduction of new energy levels and improves their optical properties. Interesting results involving the improvement of the magneto-optical properties in tellurite glasses by doping them with lanthanum La³⁺ have been presented in papers.^10,16 An attempt to obtain similar results for a different tellurite glass matrix with the use of Nd³⁺ ions has been made in this paper. The usefulness of this study results from the fact that the glasses doped with rare earth elements have become competitive in their application, in comparison to other materials, due to their low price and the simplicity of the processes of manufacturing and moulding.

2. Experimental

The synthesis of the tellurite glasses was carried out using chemically pure raw materials: TeO₂, P₂O₅, ZnO, LiNbO_3, Nd₂O₃. The glasses were melted in a platinum crucible and put into a furnace at a temperature of 850 °C. Fused 50 gram sets were poured off into a brass mould heated to a temperature of 350 °C. The obtained glasses were annealed for two hours at a temperature of 320–340 °C. In order to prepare the samples for magnetic and magneto-optical measurements, the glass was subjected to appropriate mechanical working through cutting, grinding and polishing. The chemical compositions of the examined tellurite S1 and S4 glasses are given in Table 1.

Table 1 Composition of the examined glasses

Name of sample	Composition of sample [% mol]
S1	TeO2(51)–P2O5(9)–ZnO(15)–LiNbO3(25)
S4	TeO2(51)–P2O5(9)–ZnO(15)–LiNbO3(25) + 10 000 [ppm] Nd^3+

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The assessment of the magnetic properties of the examined materials was based on determining the dominating type of magnetic ordering through measuring the constant DC field magnetic susceptibility. The test stand was equipped with a Lake Shorew VSM 7301 vibration magnetometer. Measurements were taken at room temperature in a external magnetic field with a maximum induction of up to 2 T.

The magneto-optical properties were determined with the use of the Faraday effect and by calculating the Verdet constant using the value of the angle through which the plane of polarized light is rotated. The survey was carried out on a measuring position built for the specific purpose of measuring the Faraday effect. Measurements of the angle of light polarization were carried out for light wavelengths within the range of 450 nm to 650 nm in the longitudinal magnetic field of 0.06 T induction. More information on the applied measuring technique can be found in Ref. 17.

3. Results and discussion

The results of measuring the magnetic susceptibility in the examined glasses are presented in Fig. 1.

Fig. 1 Magnetization versus magnetic field for the glasses S1 and S4.

The obtained magnetic susceptibility values for the S1 glass matrix are positive and very low, which is evidence for a paramagnetic type of magnetic ordering in the examined material. Doping the S1 tellurite glass with Nd³⁺ neodymium ions results in a significant rise in the value of the magnetic susceptibility, particularly at low magnetic field values. The doped S4 glass retains its paramagnetic nature as the predominating type of magnetic ordering. Similar changes in the rise of the value of the paramagnetic susceptibility in tellurite glasses under the influence of doping with rare earth elements were observed for La³⁺ lanthanum ion impurities.¹⁶

Values of the Verdet constant for individual glasses, calculated on the basis of the Faraday effect measurements, are presented in Fig. 2 and 3.

Fig. 2 The dependence of the Verdet constant on wavelength under a constant magnetic field of 0.06 T induction for the S1 glass.

Fig. 3 The dependence of the Verdet constant on wavelength under a constant magnetic field of 0.06 T induction for the S4 glass.

The values of the Verdet constant obtained in the measured tellurite glasses are lower in comparison with oxide glasses containing elements with diamagnetic properties, such as lead or bismuth^17–20 and tellurite glasses doped with lanthanum¹⁶ ions.

Doping the pure matrix of the S1 tellurite glass with Nd³⁺ neodymium ions did not result in the expected improvement in the magneto-optical properties of the examined glass, and consequently, in an increase of the Verdet constant value for the S4 glass, as was the case in the tellurite glasses doped with lanthanum oxide.^10,16 In spite of the increased value of the magnetic susceptibility due to the introduction of Nd³⁺ neodymium ions into the glass matrix, a substantial drop of the Verdet constant value was observed, as is shown in Fig. 2 and 3. Using the applied method for measuring the Faraday effect, the magneto-optical properties are determined on the basis of the changing value of the angle through which the plane of polarized light is rotated when passing through the material. Rotation of the plane of polarization of light is caused by changes in the electronic structure of the material induced by an external magnetic field, and the influence of electron spin is dependent on the polarization of light. Changes in the distribution of electrons and in the structure of volume defects in the glass can have a significant influence on the magneto-optical properties. Introducing neodymium ions into the glass matrix resulted in their increased density from 4.6203 g cm⁻³ for the S1 sample to 4.6422 g cm⁻³ for the S4 sample containing neodymium impurities.

On the basis of earlier structural studies of tellurite glasses using positron annihilation lifetime spectroscopy (PALS),^16,21 changes in defect structures under the influence of adding rare earth ions to a pure glass matrix have been revealed. Comparing the results involving the changes in the glasses doped with lanthanum and neodymium, it is possible to say that their nature is the same, but in the case of Nd³⁺ ion impurities, the observed changes are significantly larger. Thus, the results of the PALS studies²¹ revealed over a 50 percent decrease in the number of positron trapping centres (intensity component I₂) under the influence of doping with Nd³⁺ ions, which is shown in Fig. 4. At the same time, the accompanied changes in the electron density distribution (parameter τ_b) and sizes of free-volume defects capturing positrons (parameter τ_av) are much smaller, which is shown by the results in Fig. 5 and 6. Finally, these changes under the influence of doping with Nd³⁺ ions led to a significant decrease in the concentration of positron traps defined by the trapping rate κ_d, as shown in Fig. 7.

Fig. 4 Intensity component I₂ for the glasses doped with lanthanum¹⁶ and neodymium.²¹

Fig. 5 Parameter τ_b for the glasses doped with lanthanum¹⁶ and neodymium.²¹

Fig. 6 Parameter τ_av for the glasses doped with lanthanum¹⁶ and neodymium.²¹

Fig. 7 Parameter κ_d for the glasses doped with lanthanum¹⁶ and neodymium.²¹

The structure of tellurite glass containing Nb₂O₅ was extensively studied with the use of neutron and X-ray diffraction by Hoppe et al.²²

The mechanism of rare earth doping in the studied tellurite glasses is of vital importance in view of the positive modification feedback in their physical properties. One can assume one of the hypothetical mechanisms of incorporating rare earth ions into the trigonal pyramids of the structural unit [TeO₃₊₁]/[TeO₃] or their incorporation into the LiNbO₃ structure. The influence of Nd³⁺ ions on the magneto-optical properties in amorphous materials is based on the assumption that Nd³⁺ ions are not incorporated in LiNbO₃ in the glass structure. One can assume, by accepting previous PALS results,^16,21 that the effect of rare earth ions is revealed in terms of the occupancy of positron trapping extended defects available in the glassy network possessing effective free volumes, like in the case of chalcogenide glasses.²² Under the incorporation of rare earth ions in the matrix of tellurite glass composed of trigonal [TeO₃₊₁]/[TeO₃] pyramids, some of the existing free-volume voids are eliminated as potential positron traps, resulting in gradually reduced I₂ intensity (Fig. 4) and positron trapping rate in defects κ_d (Fig. 6). This process is enhanced because of the higher electronegativity of the preferential oxygen-type environment for positron traps in oxide glasses compared to those in glassy chalcogenides,²³ where additional Ga or In co-doping is applied to employ the charge-compensation conditions needed for embedded rare earth ions.^24,25

At a very low magnetic susceptibility, over 10³ times lower than in the case of glasses doped with lanthanum ions,¹⁶ the observed much higher structural changes of tellurite glasses doped with neodymium ions can have a significant impact on the deterioration of the magneto-optical properties.

4. Conclusions

The influence of neodymium ions on the magneto-optical properties is of a completely different nature than the one observed for doping tellurite glasses with lanthanum oxide, where a significant increase in the value of the Verdet constant was observed.^10,16 Doping the examined tellurite glasses with neodymium ions in order to increase their magneto-optical properties did not bring expected results. Introducing neodymium ions characterised by a ferromagnetic influence into a tellurite glass matrix showing poor paramagnetic properties, could result in a reorganization of the magnetic domains in the examined material. This, in connection with the observed high changes in structure, influenced the deterioration of the magneto-optical properties. The obtained results may determine important technological indications for the development of new glasses doped with rare earth elements with good magneto-optical properties, for use in optoelectronic devices.

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