1. Introduction

Magneto-optical (MO) glass refers to a class of magneto-optical materials with paramagnetism or diamagnetism. MO glass is an isotropy material without magnetic saturation. Because of its excellent MO performance, good transmittance in the visible and near-infrared, and ability to be made into a variety of complex optical components and large-size products and pulled into optical fibers, it has been used in MO devices such as MO modulators and MO isolators for aviation, military, guidance, satellite control, etc. [1–4] These materials are also known as Faraday materials, with the Faraday effect being defined by the rotation angle (θ=VBl) of a linearly polarized light beam under the influence of an external magnetic field (B) along a known optical path (l) [5–7]. The MO property of the material is typically described by the Verdet constant (V), which is generally proportional to the concentration of paramagnetic ions (such as some transition metals and rare earth (RE) ions) [5,8]. Therefore, the Verdet constant can be increased by introducing ions with high paramagnetism and high magnetic moment.

Among RE ions, Tb³⁺ ions exhibit strong magnetic susceptibilities (J = 6, g = 1.46) and moments (µ_eff= 9.5–9.72) [7,9–12], making it widely studied in the MO glass. However, Tb ions exist in two valence states(+3, + 4), and the presence of Tb⁴⁺ ions is detrimental to MO performance [13]. The magnetic dipole transition and electric dipole transition of the 4f-5d electron of Tb³⁺ ions are the main sources of their MO effect. Due to the unfilled 4f electron layer of RE elements, unpaired 4f electrons generate uncompensated magnetic moments, leading to strong magnetism. Currently, Tb³⁺ ions in MO glass are mainly introduced through Tb₄O₇, so not only does the raw material but also the oxidation reaction of Tb³⁺ to Tb⁴⁺ during the melting process can bring Tb⁴⁺ ions into the glass. Therefore, how to increase the ratio of the Tb³⁺/Tb⁴⁺ in the glass is an urgent issue that needs to be addressed. Yin et al. discovered that by substituting B₂O₃ for SiO₂, the ratio of Tb³⁺/Tb⁴⁺ increases, resulting in improved MO performance [8]. Imaizumi et al. found that the Mn²⁺ (the new magnetic couplings) can improve the MO performance [14].

Considering the oxidizing properties of Ti²⁺, an appropriate amount of TiO can suppress the oxidation of Tb³⁺ to Tb⁴⁺. Therefore, TiO as a glass matrix can adjust the MO performance. In this work, the TiO, which has good reducibility, was introduced to suppress the oxidation of the Tb³⁺ ions. A series of Tb₂O₃-doped TiO-B₂O₃-Al₂O₃-Na₂O glasses were prepared using the melting-quenching method. By introducing TiO, the mechanism of network structure and valence state changes in the glass were studied. Finally, a high concentration of 40 mol% Tb₂O₃ was successfully introduced in the glass, resulting in a Verdet constant as high as 117 rad/(T·m) at 650 nm.

2. Experimental

The experiment involved preparing a series of MO glasses using the melt-quenching method, and the compositions of the glasses are shown in Table 1. The raw materials comprised Tb₄O₇ (99.999%, Aladdin), H₃BO₃ (99.999%, Aladdin), Al₂O₃ (99.999%, Aladdin), TiO (99.999%, Aladdin), and Na₂CO₃ (99.999%, Aladdin). According to the pre-designed composition, the ingredients were thoroughly mixed for approximately 30 min and then transferred to alumina crucibles for melting at 1450 °C for 90 min in a high temperature lifting furnace. The glass was rapidly poured onto a preheated copper plate to prevent crystallization. Subsequently, the glass disks were transferred to an annealing furnace at 550 °C and naturally cooled to room temperature to eliminate the internal stress. The prepared MO glasses were then polished and characterized (Supplement 1, Fig. S1).

Table 1. Compositions of the MO glasses

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The glass structure is characterized by Fourier transform infrared spectroscopy (FTIR, INVENIO S) and Raman spectroscopy. For infrared spectroscopy testing, the glass samples are ground into powder and then mixed with dried spectroscopic-grade KBr powder in a ratio of 1:100 and pressed into disks at 10 MPa using a hydraulic press. Raman spectra were recorded on a Raman spectrometer (LABHRev-UV). The excitation wavelength is 532 nm. For the Differential Scanning Calorimetry (STA449F1), 30 mg powder samples were used to test from 25 °C to 1000°C. The density of the glass is measured using the Archimedes’ principle method. XRD (D8 Discover, Bruker, Karlsruhe, Germany) measurements of the powder sample within the 2θ range of 5°-70°. The samples were measured by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with Mg-Kα as the X-ray source. The binding energies derived from the XPS measurements were calibrated concerning the C1s peak. The prepared glass samples were subjected to optical transmittance measurements using an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer (UH4150, Hitachi, Japan). The photoluminescence (PL) spectra were recorded using an assembled optical system (QM/TM/NIR, PTI). Faraday rotation angles was recorded by commercial measurement setup (FD-FZ-C, Fudan Tianxin, Shanghai) under different magnetic field intensities. The light source is a 650 nm semiconductor laser.

3. Results and discussion

3.1 Structural analysis

The FTIR spectra of borate glass in Fig. 1(a) and 1(b) show the main absorption peaks between 400 and 1600 cm^-1. The spectra can be divided into three regions: (1) 1200–1600 cm^-1 corresponds to the asymmetric stretching vibration of the B-O bond in triangular [BO₃] units. (2) 950–1200 cm^-1 corresponds to the asymmetric stretching vibration of the B-O bond in tetrahedral [BO₄] units. (3) 500–950 cm^-1 corresponds to the vibration of Al-O and Ti-O bonds in [AlO₄] and [TiO₄] units, respectively [15–20]. The detailed distribution of corresponding peaks can be found in Supplement 1, Table S1. As shown in XRD patterns (Fig. S2), no crystal precipitated in all the prepared glasses, confirming the amorphous nature of these glasses.

 

Fig. 1. FTIR spectra of MO glasses with different concentrations of (a) TiO and (b) Tb₂O₃. (c) peak fitting of Raman spectra in 600-1100 cm^-1. Quantitative analysis of [TiO₄]/[TiO₆] and [BO₃]/[BO₄] area fraction ratio with different concentrations of (d) TiO and (e) Tb₂O₃.

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As shown in Fig. 1(a), the absorption peak of the [BO₄] units between 950 and 1200 cm^-1 weakened. This is due to the reduction of free oxygen atoms in the network structure as B₂O₃ is replaced by TiO, leading to the transformation from [BO₄] units to [BO₃] units. Additionally, the peak at 680 cm^-1 for [TiO₆] is enhanced, indicating the transformation from [TiO₄] units to [TiO₆] units with the increase of TiO[18,21]. Moreover, the absorption peak of the [BO₄] unit in this range weakens with the increase of Tb₂O₃ as shown in Fig. 1(b). Since Tb ions do not participate in forming the glass network structure, it will deteriorate the structure with the increase of Tb₂O₃. The peak at 680 cm^-1 for [TiO₆] also increased due to the transformation from [TiO₄] units to [TiO₆] units.

Raman spectra can recognize the Ti-O vibration more clearly. As shown in Supplement 1, Fig. S3, the peaks in samples are mainly located in three bands: 350 cm^-1, 600-850 cm^-1, and 800-1000 cm^-1. The corresponding peak assignments can be found in Table S2. For the 20T0Ti sample, there is no peak between 600 and 850 cm^-1 due to the lack of TiO doping in the glass [22]. As shown in Fig. 1(c), peak fitting was conducted on the Raman spectra between 600 and 1100 cm^-1 to facilitate a more precise quantitative analysis. As shown in Fig. 1(d) and 1(e), the area ratio of [TiO₄] to [TiO₆] continuously decreases with the increase of TiO. The peak of [TiO₄] at 820 cm^-1 can be considered as the former of the glass network, while the peak of [TiO₆] at 718 cm^-1 can be considered as the modifier of the glass network. This implies a transformation from [TiO₄] to [TiO₆] structural units, resulting in a decrease in the glass-forming ability (Fig. S4). The increase of the area ratio of [BO₃] to [BO₄] indicates that [BO₄] is transforming into [BO₃] structural unit. With the increase of Tb₂O₃, the area ratio of [TiO₄] to [TiO₆] continuously decreases, indicating a transformation from [TiO₄] to [TiO₆]. Furthermore, the area ratio of [BO₃] to [BO₄] increases, indicating a transformation from [BO₄] to [BO₃]. It is confirmed that the increase of Tb₂O₃ makes the structure of the glass network worse.

3.2 Valance states of Ti and Tb in MO glass

The glass contains a mixture of variable valence elements Ti and Tb. Since the MO property mainly arises from the Tb³⁺ ions, it is essential to understand the mechanism of Tb valence variation. Through XPS spectra of the Tb 3d and Ti 2p orbitals, the valence states of Tb and Ti can be analyzed.

Figure 2(a) shows the XPS fitting peak of the Tb element, in which the peaks can be divided into two parts. The binding energies at 1239, 1263, and 1274 eV correspond to Tb³⁺, while the binding energies at 1241, 1243, 1251, 1276, and 1278 eV correspond to Tb⁴⁺ ions [3,23]. As shown in Fig. 2(b), the binding energies at 463, 458, and 457 eV correspond to the +4, + 3, and +2 valence states of Ti ions, respectively [24–26]. For the 20T0Ti glass, no peak in the 450-470 cm^-1 can be seen. The XPS peak positions and their corresponding Ti and Tb ions valences are summarized in Supplement 1, Tables S3 and S4.

 

Fig. 2. XPS fitting peak of the (a) Tb element and (b) Ti element. The Calculated valence state of Ti and Tb ions, as well as Ti and Tb ions area fraction with different concentration of (c) TiO and (d) Tb₂O₃.

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As shown in Fig. 2(c), when the Tb₂O₃ concentration is fixed, the calculated valence state distributions of Tb ions decrease first and then increase with the increase in TiO concentration, and the calculated valence state distributions of Ti are the opposite. This is because more electrons enter the glass network structure with the increase of TiO, leading to simultaneous processes of Ti²⁺→Ti³⁺→Ti⁴⁺ and Tb⁴⁺→Tb³⁺. However, there is a critical point when the TiO concentration reaches 10 mol% (20T10Ti). The two processes mentioned above are inhibited when the TiO concentration increases to 15 mol%. Therefore, it is concluded that under the same Tb₂O₃ concentration, the ratio of Tb³⁺/Tb⁴⁺ increases first and then decreases with the increase of TiO. As shown in Fig. 2(d), when the TiO concentration is fixed, the valence state distribution of Ti and Tb ions increases with the increase in Tb₂O₃ concentration. The optical basicity of Tb₂O₃ and B₂O₃ are 0.954 and 0.425, respectively. With the increase of Tb₂O₃ concentration, the optical basicity of the glass increases. The high optical basicity of Tb₂O₃ makes it easy for Tb³⁺ to lose electrons and oxidize to Tb⁴⁺. As a result, the proportion of Ti²⁺ ions and Tb³⁺ ions decrease.

3.3 Optical properties

The transmittance and absorption spectra of the polished MO glasses are shown in Fig. 3(a) and 3(b). The overall average transmittance of the glass exceeds 70%, and the maximum is more than 86.6%. A prominent absorption peak is observed at 483 nm, mainly attributed to the Tb³⁺: ⁷F₆→⁵D₄ transition. Adding TiO and Tb₂O₃ to the glass reduces the glass transmittance, thereby lowering its laser damage threshold. As shown in Supplement 1, Fig. S5, the band gap (E_g) of the glass decreases with the increase of TiO or Tb₂O₃ concentration, resulting in the gradual shift of the UV absorption edge towards longer wavelengths.

 

Fig. 3. (a) Transmission and (b) absorption spectra of MO glasses. (c) Excitation and (d) emission spectra of MO glasses. (e) Energy diagram with the electronic transition process of Tb³⁺ ions.

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As shown in Fig. 3(c), the excitation peaks of MO glass doped with different concentrations of Ti and Tb are mainly located at 305, 317, 339, 351, 369, and 377 nm between 300 and 400 nm, corresponding to the transitions of Tb³⁺ ions from ground state ⁷F₆ to excited states ⁷H₆, ⁵H₇, ⁵L_6,7,8, ⁵L₉+ ⁵G₄, ⁵L₁₀+ ⁵G₅, and ⁵D₃+ ⁵G₆, respectively. Due to the shielding effect of the 5s²5p⁶ shell on the 4f electrons of Tb³⁺ ions, the excitation band remains unchanged. The maximum intensity of the excitation peak is at 377 nm [27–30].

As shown in Fig. 3(d), the emission spectra of the glasses show four emission peaks at 489, 544, 585, and 621 nm under 377 nm excitation, originating from the Tb³⁺: ⁵D₄→⁷F_J (J = 6, 5, 4, 3) transitions. Among these emissions, the highest emission is at 544 nm. With the increase of TiO, the decreased emission intensity indicates the increase in the concentration of Tb³⁺ ions. Concentration quenching is typically the result of nonradiative energy transfer between Tb³⁺ ions. As the concentration of Tb₂O₃ increases, the emission intensity further quenches to almost no luminescence. To provide a clear description of the electronic transition process in Tb³⁺ ions, a schematic of the electronic transition process for Tb³⁺ ions is presented in Fig. 3(e).

3.4 MO property of glass

The MO property is an essential parameter for measuring the performance of MO glass. The Verdet constant is commonly used to represent the MO property, expressed as θ=VBl. This work verifies the relationship between the valence state and MO property by comparing the effective concentration of Tb ions (N_Tb), the concentration of Tb³⁺ in MO glasses (C_Tb³⁺), and the Verdet constant of MO glasses at 650 nm.

3.4.1 Tb³⁺ concentration in MO glass

In Tb-doped paramagnetic glasses, the effective concentration of Tb ion (N_Tb) can effectively predict the MO properties [10]. The N_Tb refers to successfully incorporating Tb into the glass structure, as simulated with Formula S3 in Supplement 1. As shown in Fig. 4(a), N_Tb increases continuously with the increase of TiO. As shown in Fig. 5(a), the variation of the Verdet constant first increases and then decreases because the TiO leads to an initial increase followed by a decrease in the Tb³⁺/Tb⁴⁺ ratio. Therefore, the N_Tb cannot effectively predict the MO properties in this system. We introduced the concept of the concentration of Tb³⁺ (C_Tb³⁺) in MO glasses and simulated it with Formula S4 in Supplement 1 [5], Comparing Formula S3 and S4, it can be observed that using N_Tb alone does not accurately reflect the valence state distribution of Tb ions. N_Tb calculates the total concentration of Tb ions entering the glass network. The formula for C_Tb³⁺ includes the proportion of the Tb³⁺ in the total Tb ions (P_Tb³⁺), enabling the determination of the quantity of Tb³⁺ ions entering the glass network. When considering the existence of multiple valence states, it is evident that P_Tb³⁺ significantly increases with the increase in TiO concentration in the MO glass. As shown in Fig. 4(c) and 4(d), the variation trendency of C_Tb³⁺ is consistent with that of the Verdet constant. Hence, C_Tb³⁺ can effectively predict the MO property.

 

Fig. 4. The effective concentration of Tb ion (N_Tb) with different concentrations of (a) TiO and (b) Tb₂O₃. The concentration of Tb³⁺ (C_Tb³⁺) with different concentrations of (c) TiO and (d) Tb₂O₃

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Fig. 5. Verdet constant of MO glasses with different concentrations of (a) TiO and (b) Tb₂O₃ at 650 nm, (c) Verdet constant of magneto-optical glass from recent literatures.

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3.4.2 Verdet constant of MO glass

As shown in Fig. 5(a), the addition of an appropriate amount of TiO is beneficial for the Verdet constant. The maximum Verdet constant at 650 nm is 59.4 rad/(T·m) for the 20T10Ti because the Tb³⁺/Tb⁴⁺ ratio is the highest at 10 mol% TiO, and C_Tb³⁺ is also at its maximum. As shown in Fig. 5(b), the Verdet constant increases continuously with the increase of Tb₂O₃ when the TiO concentration is fixed. The MO property of paramagnetic glass originates from Tb³⁺ ions. With the increase of Tb₂O₃, more Tb³⁺ ions enter the glass, which increases the MO property.

Currently, terbium gallium garnet (TGG) crystal is the main commercial MO material with a high Verdet constant of 121 rad/(T·m) at 650 nm [31]. In this work, the maximum Verdet constant of 117 rad/(T·m) at 650 nm is achieved when the Tb₂O₃ is 40 mol% (V_40T10Ti). The MO crystals are limited in terms of long-distance light transmission and rotation. In contrast, magneto-optical glass can be quickly processed into any shape and size, and glass manufacturing is a mature technology, highlighting the potential for magneto-optical glass to replace TGG crystals. As shown in Fig. 5(c), this work significantly improves the Verdet constant compared to previously reported works [5,10,15].

4. Conclusion

Tb-doped TiO-B₂O₃-Al₂O₃-Na₂O glasses were prepared using the melt-quenching method, and their structures were analyzed using Raman and infrared spectroscopy. The addition of an appropriate amount of TiO facilitates the simultaneous processes of Ti²⁺→Ti³⁺→Ti⁴⁺ and Tb⁴⁺→Tb³⁺, which is favorable for enhancing the Verdet constant. The variation trend of the effective concentration of Tb ions in this system cannot effectively predict the change in the Verdet constant, whereas the introduction of C_Tb³⁺ shows high consistency with the Verdet constant. When the glass doped with 20 mol% Tb₂O₃ and 10 mol% TiO (V_20T20Ti), the Verdet constant reaches its maximum value of 59.4 rad/(T·m), which is 19% higher than that of V_20T0Ti. With the increase of Tb₂O₃ concentration, more Tb³⁺ ions enter the glass network, leading to an increase in the Verdet constant. Consequently, a recorded Verdet constant of 117 rad/(T·m) at 650 nm was achieved with 40 mol% Tb₂O₃ doped MO glass. These results demonstrate that introducing TiO as a glass matrix can enhance the C_Tb³⁺ and obtain a higher Verdet constant.