Czochralski Growth, Magnetic Properties and Faraday Characteristics of CeAlO₃ Crystals

Abstract

CeAlO₃ crystals were grown in different growth atmospheres by the Czochralski method. The lattice parameters and space group of CeAlO₃ crystal were determined by Rietveld structure refinement of X-ray diffraction (XRD) data. The influence of Ce⁴⁺ ions in the crystal on the transmittance and crystal color was confirmed by XPS analysis. Magnetization curve at room temperature and temperature dependencies of the magnetic susceptibility in two different directions were measured, indicating that CeAlO₃ crystal has remarkable magnetic anisotropy and there is an abnormal magnetic behavior in the vertical <001> direction in the temperature range of 50–150 K. Faraday characteristics of CeAlO₃ crystal were investigated at room temperature. Verdet constants of CeAlO₃ at 532, 635 and 1064 nm are about 2.1 times as large as those of CeF₃. The reason of large Verdet constants was analyzed based on the Van Vleck–Hebb theory and the magnetic circular dichroism (MCD) spectrum.

1. Introduction

Faraday isolators are important components currently used for high-power-laser machinery and advanced optical communications, guaranteeing a unidirectional light propagation in the laser systems [1,2]. Due to recent development in laser applications such as precise measurements and advanced display systems, the demand for optical Faraday devices using wavelengths of 400–1100 nm is increasing, where ferrimagnetic yttrium iron garnets (YIG) and doped YIG are inapplicable because of their poor transparency. Paramagnetic magneto-optical materials containing rare earth ions (such as Tb³⁺, Eu²⁺ and Ce³⁺) are suitable for use in this band, although their Verdet constants are smaller than those of doped YIG. As is known, crystals [3,4,5,6], glasses [7,8,9] and ceramics [10,11] containing Tb³⁺ ions have been extensively investigated as Faraday rotator materials due to its good magneto-optic effect in the visible and near-infrared bands and relatively easy preparation process. However, at the present, the crystals containing Ce³⁺ ions have drawn people's attention [12,13,14,15]. Compared to Tb³⁺ ions, Ce³⁺ ions have larger electron effective transition wavelengths and higher transition efficiency, and CeO₂ raw material is relatively cheap.

Rare earth aluminate (REAlO₃) crystallizes in a perovskite-type structure. It has a high content of rare earth ions per-unit volume, which is advantageous for enhancing the magneto-optical effect of the crystal. TbAlO₃ crystal belongs to an orthorhombic system with two optical axes, although exhibiting good magneto-optical properties [16], it is difficult to apply to Faraday devices in the 400–1100 nm band due to its low symmetry. Different from TbAlO₃, CeAlO₃ shows a tetragonal symmetry with a space group of P4/mmm or I4/mcm at room temperature [17,18,19,20]. In this series of rare earth aluminate, CeAlO₃ is an unusual one, undergoing three phase transitions above room temperature [21]. In the past three decades, the crystal structure and phase transition of CeAlO₃ compounds have been extensively studied and confirmed. Its magnetic and electrical properties have also been reported [22,23], but little research has been done on the growth and properties of bulk crystals. Until 2015, CeAlO₃ crystals with large sizes were obtained first time through the Edge-defined Film Fed Growth (EFG) techniques by Arhipov et al. [24]. The optical, luminescence and magnetic properties of CeAlO₃ were studied. They also tried to grow the CeAlO₃ crystals by varying different growth parameters using the Czochralski method, but did not obtain cylindrically shaped crystals.

To our knowledge, there are few reports on the magneto-optical properties of Ce-containing crystals. In this paper, Czochralski growth, magnetic properties and magneto-optical characteristics of CeAlO₃ crystals are investigated.

2. Experimental Procedure

The polycrystalline materials for crystal growth were prepared by solid-state reaction according to the following chemical reaction equation:

CeO₂ + 0.5Al₂O₃→CeAlO₃ + 0.25O₂↑

(1)

Stoichiometric amounts of CeO₂ (4N) and Al₂O₃ (4N) were weighed accurately, then the mixture was sintered at three different temperatures (1350, 1450 and 1500 °C) for 12 to 20 h in the flowing 5% H₂ + 95% Ar atmosphere each with intermediate grinding and pressing into tablets. Sintering needed to be performed multiple times until the strongest diffraction peak of CeO₂ near 28.5 degrees in the X-ray diffraction (XRD) patterns disappeared substantially. CeAlO₃ crystals were grown by the Czochralski method, in a Φ60 mm × 38 mm iridium crucible with radio frequency (RF) induction heating. It was found when pure Ar was used as protection atmosphere in the growth process, CeAlO₃ crystal obtained presented a flat shape and dark green color, as shown in Figure 1a. Parameters such as growth rate and rotation speed were adjusted, and the crystal shape did not change significantly. Finally, when 5% H₂ + 95% Ar was used as growth atmosphere, the CeAlO₃ crystal obtained was cylindrical and the green color was obviously lighter, as shown in Figure 1b. A portion of the greenish CeAlO₃ crystal was cut out and annealed under the following conditions: a flowing 5% H₂ + 95% Ar atmosphere, and a constant temperature of 1550 °C for 30 h. The annealed CeAlO₃ sample presents a light yellow color, as shown in Figure 1c.

X-ray powder diffraction was performed by a computer automatic diffractometer (Rigaku Ultima IV, Rigaku, Tokoy, Japan ) using Cu-Kα radiation (λ = 1.54056Å) in the range 10° ≤ 2θ ≤ 80° with a scanning step of 0.01° and a scanning rate of 0.15°/min. The annealed and unannealed greenish CeAlO₃ crystals were cut along (001) plane, which were oriented by X-ray diffraction, and then grounded and polished carefully to 1.0 mm thickness for spectra and XPS measurement. Transmission spectra were measured over the wavelength range 250–2500 nm (Lambda 900, Perkin-Elmer, Waltham, MA, USA). Ce valance band spectra of two samples were recorded by XPS ( ESCALAB 250XI, ThermoFisher, Waltham, MA, USA). The oriented CeAlO₃ sample with sizes 3.5 mm × 3.5 mm × 3.5 mm was used for magnetic susceptibility testing (JDAW-2000D vibrating sample magnetometer, YINGPU MAGNETIC, Changchun, China) and DC magnetization measurements (PPMS6000, Quantum Design, San Diego, CA, USA). Faraday rotation of annealed CeAlO₃ crystal at 532, 635 and 1064 nm were measured by the extinction method, and a commercial CeF₃ crystal (Beijing Scitlion Technology Corp., LTD, Beijing, China) was as a comparison during the test process. The magnetic field could be adjusted from 0 to 1.2 T continuously. Magnetic circular dichroism (MCD) spectra of annealed crystal samples with 0.2 mm thickness were measured by using a circular dichroism spectrometer (MOS-450, Bio-Logic, Auvergne-Rhône-Alpes, France) equipped with a magnetic field equipment of 2500 Oe intensity (the magnetic field paralleled to the propagation direction of probe light). All measurements were performed at room temperature.

3. Results and Discussion

3.1. Crystal Growth

CeAlO₃ is congruent compound with a melting point of about 2348 K [25], it could be grown by the Czochralski method. It is difficult to obtain CeAlO₃ crystals with high optical quality due to the high melting point and oxidation of Ce³⁺. When pure Ar was used as the growth atmosphere, A small amount of Ce³⁺ oxidation may be present in the melt, CeAlO₃ crystallized from the melt is dark in color and strongly absorbs thermal radiation, resulting in a decrease in temperature gradient between the melt and crystal, making crystal growth in the length direction becomes difficult. The longitudinal temperature gradient can be increased by the crucible position adjustment and the thickness decrease of the insulation layer of the post-heat chamber, but the surface temperature of Ir crucible may exceed 2600K, and Ir has a risk of melting (2727K). Arhipov et al. [24] used a tungsten crucible as growth container through CZ method, as-grown CeAlO₃ crystal also showed a flat shape. When the weakly reducing atmosphere, 5% H₂ + 95% Ar, was used, it can inhibit the oxidation of Ce³⁺ in the melt, resulting in the crystallized CeAlO₃ crystal becoming lighter in color, reducing the absorption of thermal radiation, and finally obtaining a cylindrical CeAlO₃ crystal with diameter 20 mm and length 22 mm.

3.2. Structure Determination

Tas and Akinc [26], and Shishido et al. [17,18] reported the structure of CeAlO₃ at room temperature with space group P4/mmm and the primitive tetragonal cell a = 3.7669(9) Å and c = 3.7967(7) Å. In 2004, Fu et al. reinvestigated the room temperature structure of CeAlO₃ through Rietveld refinement, and revealed a super cell a = 5.32489(6) Å and c = 7.58976(10) Å, with the space group I4/mcm [19]. In order to verify the rationality of these two space groups, powder X-ray diffraction data of CeAlO₃ crystal was used for Rietveld refinement according to the above two space group models, the refined results are listed in

Table 1. Refined structural parameters for the as-grown CeAlO₃ crystal according to the P4/mmm model.

Atoms	Site	x	y	z	B (Å2)
Ce	1d	0.5	0.5	0.5	0.0087 (4)
Al	1a	0	0	0	0.0232 (12)
O(1)	2f	0.5	0	0	0.056 (5)
O(2)	2f	0	0.5	0	0.01 (2)
O(3)	1b	0	0	0.5	0.000 (3)

Weighted profile R-factor Rwp = 9.78% and Profile residual Rp = 7.35% with the Goodness-of-fit of 1.76.

and

Table 2. Refined structural parameters for the as-grown CeAlO₃ crystal according to I4/mcm model.

Atoms	Site	x	y	z	B (Å2)
Ce	4b	0.5	0	0.25	0.0055 (6)
Al	4c	0	0	0	0.0076 (15)
O(1)	4a	0	0	0.25	0.005 (4)
O(2)	8h	0.2812 (11)	0.7812 (11)	0	0.005 (3)

Weighted profile R-factor Rwp = 8.64% and Profile residual Rp = 8.22% with the Goodness-of-fit of 1.79.

. Indeed, there were only small differences between the two agreement factors, but the refined thermal parameters based on the P4/mmm model were larger than those of based on I4/mcm, particularly, the thermal anisotropy between basal oxygen atoms which can be found in Table 1. So, I4/mcm is considered to be a more reasonable space group for CeAlO₃, the refined cell parameters are a = 5.32064(12) Å, c = 7.5810(3) Å. Figure 2 shows the observed, calculated and differences between X-ray diffraction profiles of the as-grown CeAlO₃ crystal.

3.3. Transmission Spectra and Ce Valence State Analysis

Figure 3 shows the transmission spectra of different growth atmosphere and annealed CeAlO₃ samples at 250–2500 nm waveband. The transmittance of CeAlO₃ grown in Ar was relatively low, which corresponds to the dark color presented by the crystal. Although the transmittance of CeAlO₃ grown in 5% H₂ + 95% Ar was improved, there was still a large absorption in the 500–1500 nm band. After annealing at 1550 ℃ for 30 h in a flowing 5% H₂ + 95% Ar, the transmittance of CeAlO₃ crystal increased obviously, up to 68% or higher at 550–2500 nm regions and the short wavelength edge of optical absorption was shifted to ~380 nm. In order to verify that the above transmittance and corresponding crystal color change were related to the presence of Ce⁴⁺ ions, the valence state of Ce ions in the crystal was analyzed. Figure 4; Figure 5 show the Ce3d XPS spectra collected from greenish and yellowish CeAlO₃ samples, respectively. Both XPS spectra have four peaks at almost the same position, located at 882.1 ± 0.1 eV, 886.0 ± 0.1 eV, 900.3 ± 0.1 eV and 904.4 ± 0.1 eV. The peaks at 882.1 ± 0.1 eV and 886.0 ± 0.1 eV correspond to the pairs of Ce 3d5/2 spin-orbit doublets, while peaks at 900.3 ± 0.1 eV and 904.4 ± 0.1 eV correspond to the pairs of Ce 3d3/2 spin-orbit doublets. The band energies of spin-orbit splitting between 3d5/2 and 3d3/2 are about 18.3 eV, in agreement with that of CePO₄ [27]. However, due to the overlap of the photo-electron peaks of Ce(III) and Ce(IV), these four peaks cannot be used to identify the valence state of the Ce ions. It is believed that the peak near 917eV is the fingerprint of the Ce(IV) compound [28]. The small peak observed at 916.7 eV in Figure 4 confirms the presence of Ce⁴⁺ ions in the greenish CeAlO₃ crystal, and estimates that its percentage ratio does not exceed 1%. However, no Ce^{4 +} ion was found in the yellowish CeAlO₃ crystal within the error of XPS measurement.

3.4. Magnetic Analysis

As shown in Figure 6, the magnetization curve displays that CeAlO₃ is a paramagnetic compound, which is magnetically anisotropic. The calculated mass magnetic susceptibility at room temperature in the vertical and parallel <001> directions are 1.04 × 10⁻⁷ m³/Kg and 1.78 × 10⁻⁷ m³/Kg, respectively. The degree of anisotropy increases by a factor of approximately six upon cooling from room temperature to 2 K. The magnetic behavior of paramagnetic lanthanide compounds was mainly influenced by dipole–dipole interactions and crystal-field (CF) effects, whereas super-exchange interactions were relatively unimportant. The influence of dipole–dipole interactions can be described by the Curie–Weiss law [29]. Temperature dependencies of the inverse magnetic susceptibility χ⁻¹ of CeAlO₃ crystal in the vertical and parallel <001> directions are shown in Figure 7. The curve of χ⁻¹ vs T in the parallel <001> direction shows a good linear trend over the 4–300 K range, fitted data based on the Curie–Weiss law gives an effective moment of 2.57µ_b, is consistent with the theoretical value 2.54µ_b for the free ion ²F_5/2 ground state of Ce³⁺. The corresponding Curie–Weiss temperature is approximately −2.34 K, which is different from the result of polycrystalline or unoriented single CeAlO₃ crystals reported by other authors [22,24]. But in the vertical <001> direction, the relationship between χ⁻¹ and T at 50−150 K range does not obey the Curie-Weiss law. In light of the interpretation of magnetic properties of oriented CeF₃ single crystals [30], this anomalous behavior of the magnetic susceptibility could be explained in the crystal-field model with Ce³⁺ ion energy levels based on the D_2d site symmetry. It is noted that abrupt magnetic changes in both directions of the CeAlO₃ crystal near 4 K were observed, which indicates the transition of the CeAlO₃ crystal from a paramagnetic phase to an antiferromagnetic one. This magnetic phase transition was also found in TbAlO₃ and DyA1O₃. Most analyses assume that the ordering of magnetic sublattices at low temperature is due to a combination of exchange and dipole interactions [31].

3.5. Faraday Rotation

It is well known that Faraday rotation in a paramagnetic material is proportional to the applied magnetic field, the length of light-passing medium and the Verdet constant, which is itself a function of the wavelength. Faraday rotations of CeAlO₃ and CeF₃ crystals vs the magnetic field in the range of 0–1.2 T are linear at three wavelengths, as shown in Figure 8. The Verdet constants of the two crystals can be calculated from the slope of lines, which are −389 rad/m·T at 532 nm, −270 rad/m·T at 635 nm and −79.7 rad/m·T at 1064 nm for CeAlO₃, −180 rad/m·T at 532 nm, −125 rad/m·T at 635 nm and −39.1 rad/m·T at 1064 nm for CeF₃, respectively. Compared with CeF₃ and TGG, Verdet constants of CeAlO₃ at the corresponding wavelengths are about 2.1 times those of CeF₃ and about two times those of TGG reported [32].

As for paramagnetic materials of glass or crystal, when the interaction between rare earth ions is small, based on the Van Vleck–Hebb theory with a single-oscillator model, the relationship between Verdet constants V and wavelength $\lambda$ is used as the following equation [33,34,35]:

$$V^{-1}=\frac{g{\mu }_{B}ch}{4{\pi }^{2}n\chi C_t}(1-\frac{{\lambda }^2}{{\lambda }_t^2})$$

(2)

where g is the Landé factor, ${\mu }_B$ the Bohr magneton, c the velocity of light, h the Planck constant, n the refraction index (it can be estimated by the transmittance of crystal [4]), $\chi$ the volume magnetic susceptibility and C_t the transition probability. The terms n, $\chi$ and C_t are functions of temperature, and when the temperature is constant, could be considered as a constant, so the inverse of Verdet constant is in linear relationship with proportional to the wavelength square. Fitting the wavelength-dependence data for two crystals to Equation (2) yields the effective transition wavelength λ_t and probability C_t at room temperature, listed in

Table 3. The fitting and calculated parameters of two crystals at room temperature according to Equation (2). (The refraction index and volume magnetic susceptibilities are also listed).

	Parameters	n (at 1064 nm)	$\mathit{\chi }$ (emu/cm3·T)	λt (nm)	Ct (10−45 J·cm3)
Crystals
CeAlO3		2.08	0.942	280	22.0
CeF3		1.64	0.686	239	26.2

together with n and $\chi$ for comparison. According to Equation (2) and Table 3, CeAlO₃ has larger Verdet constants than those of CeF₃ owing to its large refraction index, volume magnetic susceptibility and effective transition wavelength, although the 4f to 4f5d transition efficiency of Ce³⁺ in the CeAlO₃ perovskite is relatively small. Substituting the above calculated parameters into Equation (2), the relationships between Verdet constants and the wavelength of CeAlO₃ and CeF₃ were plotted, as shown in Figure 9.

3.6. Magnetic Circular Dichroism

Faraday magneto-optical effect of rare earth ions is mainly caused by the transitions of 4f–4f5d [36,37], which is essentially the circular birefringence induced by the applied magnetic field. MCD signal of the rare earth magnetic ions at the transition absorption can be indicative of their magneto-optical activity (MOA) [38]. Usually the shorter the transition absorption wavelength and the stronger the intensity, the stronger the corresponding MCD signal. Figure 10 shows the MCD spectra of CeAlO₃ and CeF₃ crystals at 2500 Oe magnetic fields. At the 350–800 nm waveband, MCD spectrum of CeAlO₃ shows two peaks centred at 366.3 nm and 372.7 nm, respectively, which can correspond to the 4f–5d transition of Ce³⁺ ions in the perovskite. Below 350 nm, no MCD signal can be detected because of the absorption of right-handed and left-handed light by CeAlO₃ sample exceeds the detection range of the instrument. Reducing the thickness of sample may detect the MCD signal below 350 nm, but it is difficult to process the CeAlO₃ sample. Due to the strong influence of the surrounding coordination ions and the weak crystal-field interaction, the 4f–5d transition absorption position of Ce³⁺ ions in the CeF₃ matrix is below 300 nm, so only a MCD signal of CeF₃ peaked at 271.3 nm can be found at the 200–800 nm waveband. The peak intensity is much smaller than that of CeAlO₃ at 366.3 nm and 372.7 nm, indicating the relatively weak MOA of Ce³⁺ ions in the CeF₃ matrix.

4. Conclusions

The CeAlO₃ crystal with a diameter of 20 mm and a length of 22 mm has been grown in 5% H₂ + 95% atmosphere by the Czochralski method. Rietveld structure refinement of XRD data confirms that the CeAlO₃ crystallizes in the tetragonal system at room temperature, space group I4/mcm. The green color and corresponding optical absorption exhibited by as-grown CeAlO₃ crystal are related to the presence of Ce⁴⁺ ions according to the XPS analysis. Secondary annealing of optimized conditions can greatly reduce the content of Ce⁴⁺ in CeAlO₃ crystal and improve the transmittance. Above 4 K, CeAlO₃ crystal exhibits paramagnetism and magnetic anisotropy. There is an anomalous behavior of the magnetic susceptibility in the vertical <001> direction over 50–150 K range, which can be explained in the crystal-field model. The present investigations demonstrate CeAlO₃ crystal have larger Verdet constants than those of CeF₃ and TGG at 532, 635 and 1064 nm wavelengths at room temperature. So CeAlO₃ crystal maybe a candidate magneto-optical material for Faraday devices in the visible and near-infrared regions. Based on the analysis of Van Vleck–Hebb theory, the co-contribution of the large refraction index, volume magnetic susceptibility and effective transition wavelength is the reason why CeAlO₃ have large Verdet constants, which will help to find other new magneto-optical crystals containing Ce³⁺ applied in the visible and near-infrared regions.