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Czochralski growth, magnetic and magneto-optical properties of Na2Tb4(MoO4)7 crystal
Journal of Crystal Growth 421 (2015) 8–12
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Czochralski growth, magnetic and magneto-optical properties of Na2Tb4(MoO4)7 crystal Xin Chen, Min Ruan, Feiyun Guo n, Jianzhong Chen n College of Chemistry, Fuzhou University, Fuzhou 350116, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 March 2015 Accepted 1 April 2015 Communicated by S. Uda Available online 9 April 2015
Na2Tb4(MoO4)7 crystals have been grown by the Czochralski technique for the first time for magnetooptical applications. Structure refinement of XRD data confirms that the compound crystallizes in the tetragonal system I41/a, with scheelite structure. The as-grown crystal exhibits moderate thermal stability and weak thermal expansion anisotropy (αc =αa 1:08), and the hardness is about 4.7 Moh. Transmittance spectra indicated that the Na2Tb4(MoO4)7 crystal presents a high transparency from 600 nm to 1500 nm after annealing in O2 atmosphere at 973 K for 20 h. The Faraday effect and the temperature dependence of the magnetic susceptibility have been investigated, which demonstrate that the Na2Tb4(MoO4)7 crystal exhibits paramagnetic behavior over the experimental temperature-range 2– 300 K and yields a Faraday rotation which is about 1.6 times as large as that of Tb3Ga5O12 crystal. Na2Tb4(MoO4)7 crystal is therefore a promising Faraday material in particular for new magneto-optical applications in the visible–near IR wavelength region. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Faraday effect A1. Magnetism A2. Czochralski method B1. Molybdate B2. Magneto-optic material
1. Introduction The optical isolators (OIs) using the non-reciprocity of the Faraday effects play an important role in a modern optical fiber communication system to prevent optical feedback and noise from reaching a laser light source through scattering and reflections along the light path [1–5]. Yttrium iron garnet, Y3Fe5O12 (YIG), characterized by a high transparency in the IR region (1.2–5.0 μm) [6,7], a low saturation magnetization and a large Faraday rotation (FR) angle, is so far the most commonly used material in OIs. However, as the rapid development of fiber lasers, the demand of OIs operated at wavelengths below 1100 nm (i.e. the visible–near IR region of 400–1100 nm) is rapidly increasing [8–10], while the conventional YIG crystals are not practical due to their very poor transparency in this spectral region [11]. The most commonly used materials for the visible and nearinfrared regions are terbium-doped glasses [12–14] and terbiumgallium garnet, Tb3Ga5O12 (TGG) single crystals [1,15]. However, terbium-doped glasses as amorphous material cannot be used in high average power lasers because of its low thermal conductivity and low relatively stability [16]. TGG, a commonly used crystalline magnetooptic material, although it melts congruently at approximately 2100 K, its Czochralski growth is not exempt of difficulties. The decomposition
and evaporation of Ga2O3 from the TGG melt, which is easy to over flow the container during the crystal growth process due to its large infiltration, lead to a serious component deviation from the congruent melting [17,18]. Tb3 þ yields to the largest FR among all the rare-earth ions, so that electric dipole contribution of Tb3 þ dominates even over that of Fe3 þ in magnetic terbium-iron garnet [19–21]. Therefore, there is a clear need to find a new generation of terbium-based magneto-optic materials with high Verdet constant and low optical absorption loss in the visible–near IR region. During the past few years, interest arose concerning the rare-earth molybdate A2R4(Mo4)7 (A¼ alkali metal, R¼lanthanide) with a tetragonal scheelite-type structure because of its great potential for applications in laser host materials [22,23] and phosphors [24–26]. Taking into account the requirements for a magneto-optic material applied in the visible–near IR region, Na2Tb4(MoO4)7 (NTM) a member of the scheelite-type molybdate has come to draw more and more attention from our research group. NTM presents favorable growth characteristics, with strong crystallizability, low cost of materials, and no special absorption at the visible–near IR region. These features are advantages for a magneto-optical (MO) material candidate. In the present work, we attempt to employ the Czochralski technique to grow NTM single crystals. The spectral, magnetic and MO properties were discussed as well. In order to compare these
n
Corresponding authors. Tel.: þ 86 591 22866243; fax: þ 86 591 22866130. E-mail addresses: [email protected] (F. Guo), [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.jcrysgro.2015.04.001 0022-0248/& 2015 Elsevier B.V. All rights reserved.
with the reference material TGG, the latter has been also measured under equivalent experimental conditions.
X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
2. Experimental 2.1. Synthesis and crystal growth The polycrystalline materials used for single-crystal growth were prepared by the conventional solid-state reaction method according to the following chemical reaction equation: 1 Tb4 O7 þ7MoO3 þ Na2 CO3 -Na2 Tb4 ðMoO4 Þ7 þ CO2 ↑ þ O2 ↑ 2
ð1Þ
The initial reagents used were Tb4O7 with purity of 99.99%, MoO3 and Na2CO3 with analytical reagent. The stoichiometric amount of raw materials was weighed with the excess amount 2 wt% of MoO3 to compensate for its volatilization loss during the process of crystal growth, and then ground homogeneously in an alumina mortar with the ethanol. The uniformly mixed raw materials were pressed into tablets and then sintered at 833 K for 12 h in air to decompose the carbonate. They were then cooled to room temperature, reground, extruded into pieces and sintered again at 1173 K in air for 25 h. The purity of the sample was checked by X-ray powder diffraction. A single scheelite phase of NTM was obtained when repeated heat treatment caused no further changes in the X-ray powder diffraction. Since the DSC analysis indicates that NTM is a congruently melting material, the single crystals can be grown by the conventional RF-heating Czochralski technique which is known to be a very efficient and suitable method for the growth of large single crystals with good optical performance [27]. The polycrystalline pieces were melted in a platinum crucible with dimensions of Ø55 35 mm2, heated by a radiofrequency furnace. The polycrystalline material was heated up to a temperature of about 50 K higher than the melting point and kept this temperature for 2 h, and then the temperature was decreased to the crystallization point. After the growing temperature was accurately determined by repeated seeding trials, a seed crystal of o0014 orientation, cut from the initial NTM crystal which obtained by crystallization on the iridium rod, was dipped into the melt and then slowly withdrawn, so that the crystal began to grow along the c-axis with 1.5–2.0 mm h 1 pulling rate and 20– 25 rpm rotating rate. After crystallization, the crystal was pulled out of the melt and cooled down to room temperature at a rate of 10– 30 K h 1. The NTM-1 single crystal, grown in the atmosphere of pure N2, is black in color because of an oxygen-deficient atmosphere [28,29], as shown in Fig. 1(a). Meanwhile, the NTM-2 crystal (Fig. 1(b)), grown in air atmosphere, is transparent and crack free with dimensions of Ø20 25 mm2 with a yellowish-brown coloration. It is noteworthy that since NTM is an optically uniaxial crystal, in order to avoid birefringence in the MO measurements, crystals
9
should be oriented along the optic axis, which is coincident with the crystallographic c-axis. After oriented accurately by X-ray diffraction, the as-grown crystals were cut along the (001) crystal plane, and then wafers were ground and polished carefully for the following measurement.
2.2. Structure determination The X-ray powder diffraction (XRD) data for Rietveld analysis were recorded on a Rigaku Ultima III diffractometer using Cu Kα radiation (λ ¼1.54051 Å) from 101 to 901 (2θ) with a scanning step of 0.011 (2θ) and a count time of 5 s per step at room temperature. Crystal structure determination was performed using the Rietveld method implemented in the DBWS-9411 [30] PC program. The observed, calculated and their difference diffraction profiles of the as-grown NTM-2 crystal are displayed in Fig. 2. The structure was refined from its powder X-ray diffraction data in space group I41 =a, and the refinement showed comparable good fit value: Rwp ¼6.63%, which indicates unambiguously the existence of the scheelite structure with the tetragonal system. The refined lattice parameters of NTM-2 crystal are a ¼b¼ 5.2184(2) Å, c ¼11.4006(5) Å and V¼ 310.5(6) Å3, basically consistent with the previously reported values found in other scheelite-type rare-earth molybdate [31].
Fig. 2. Rietveld refinement of the X-ray powder diffraction pattern from the asgrown NTM-2 crystal at room temperature.
Fig. 1. Photographs of as-grown crystals (a) NTM-1 (N2 atmosphere) and (b) NTM-2 (air atmosphere).
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X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
2.3. Hardness and thermal expansion measurement The hardness of as-grown NTM-2 crystal, determined by using a 401MVA™ Vickers-microhardometer, is about 336.1 VDH (equal to about 4.7 Moh), and hence is favorable for crystal machining. NTM crystal belongs to the tetragonal system, and there are only two independent main thermal expansion features [29]. Therefore, the thermal expansion coefficients of NTM-2 single crystal along the a and c axes were evaluated respectively by using a Netzsch DIL402PC model thermo-mechanical analyzer equipment from room temperature to 973 K at a heating rate of 10 K min 1. 2.4. Transmission spectrum The transmission spectrum of as-grown crystals NTM was measured by using a Perkin-Elmer Lambda UV–vis–NIR spectrophotometer, over the wavelength range from 400 to 1500 nm with a 2 nm resolution at room temperature. 2.5. Variable-temperature magnetic susceptibility
Fig. 3. Transmittance spectra of as-grown NTM crystals in comparison with the transmittance of commercial TGG (5 mm thick, without antireflection coating).
The temperature dependence of the magnetic susceptibility was measured under both zero-field-cooled (ZFC) and field-cooled (FC) conditions in an applied field of 0.1 T over the temperature range 2–300 K using a Quantum Design MPMS XL SQUID magnetometer. The sample was placed into a diamagnetic gelatin capsule, and the data were corrected by extracting the contribution from diamagnetic ionic susceptibilities. 2.6. Faraday rotation measurement The specific FR of the as-grown NTM-2 crystal in the direction of the c-axis was measured by the extinction method [32] with two lasers emitting at three wavelengths 532, 633, and 1064 nm at room temperature. For each sample, the average values of six to nine data were used for interpretation. A commercial TGG crystal was used as the standard sample to correct the magnetic field. The magnetic field intensity could be adjusted from 0 to 1.2 T continuously. Fig. 4. Thermal expansion of as-grown NTM-2 crystal.
3. Results and discussion 3.1. Effect factors of crystal quality It is known that a very important factor during growth of the scheelite-type molybdate single crystals is the choice of growth ambient atmosphere [28,29]. In order to investigate the effect of the growth atmosphere on the crystal quality, NTM crystals were grown in the different kinds of atmosphere. NTM-1 single crystal, grown in the atmosphere of pure N2, appeared to have black color, and the transmittance spectra of NTM-1 crystal, as shown in Fig. 3, demonstrated a very broad absorption band in the visible–near IR wavelength region as well. This band was found to appear because of formation of color centers, based on oxygen vacancies. Besides the formation of oxygen vacancies and partial transition of Mo6 þ into Mo5 þ in NTM-1 crystal in case of low oxygen content at growth atmosphere, free electrons can localize at oxygen vacancies, which may lead to formation of color centers, responsible for the additional optical absorption and black coloration of crystals. As expected, in the case of NTM-2 single crystal, the oxidizing atmosphere of air decreased the oxygen vacancy defect and the concentration of color centers to a great extent, and then improved the optical transmittance obviously, as shown in Fig. 3. In order to further enhance the visible transparency, the asgrown NTM-2 crystal was annealed in O2 atmosphere at 973 K for
20 h, and the transmittance spectra of annealed NTM-2, in comparison with a commercial device-quality TGG, are shown in Fig. 3, which indicated that the NTM-2 crystal presents a high transparency from 600 nm to 1500 nm after annealing in O2 atmosphere. The sharp absorption peak around 488 nm is mainly related to the energy level transition 7F6-5D4 of Tb3 þ in the scheelite structure. 3.2. Thermal analysis The thermal expansion of as-grown NTM-2 crystal is shown in Fig. 4, which indicates that NTM exhibits moderate thermal stability with thermal expansion coefficients about 7.56 10 6 K 1 along a-axis and 8.16 10 6 K 1 along c-axis. Moreover, the similar thermal expansion coefficient along the a and c axes indicates a weak thermal expansion anisotropy (αc =αa 1:08), which is helpful to reduce the crystal cracking during the crystal growth. 3.3. Paramagnetic behavior The temperature dependence of the magnetic susceptibility (χ M ) for NTM measured in the temperature range of 2–300 K is shown in Fig. 5. Paramagnetic behavior was observed down to 2.0 K, i.e., no magnetic interaction was found in the experimental temperature range. In the inset of Fig. 5, the reciprocal magnetic susceptibility was
X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
Fig. 5. Temperature dependence of the magnetic susceptibility for NTM. The inset shows the reciprocal magnetic susceptibility vs. temperature curve. The solid line is the Curie–Weiss fitting.
11
Fig. 7. FR angles of NTM-2 crystal as a function of the magnetic induction intensity.
Table 1 The Verdet constants and Tb3 þ concentration of NTM-2 crystal in comparison with those of reference TGG crystal. Crystal
NTM-2 TGG
Fig. 6. FR angles of NTM-2 crystal as a function of the wavelength λ. A commercial TGG crystal was measured as reference, and the measured data were fitted by an equation in the form A/(λ2 λ20 ), with A and λ0 constants.
plotted against temperature, and no evidence of any magnetic transition has been recognized down to 2.0 K. The reciprocal susceptibilities of NTM exhibit Curie–Weiss behavior between 30 and 300 K (see the solid line in the inset graph of Fig. 5), but deviate from the Curie–Weiss law below 30 K. This result may indicate that the magnetic ions Tb3 þ in this compound are affected by the crystal field to some extent, and single ion anisotropy may take effect, as reported by Rosenkranz et al. [33,34]. A fit of the χ M 1 –T curve to the Curie– Weiss law above 200 K gives an effective paramagnetic moment (μef f ) of 9.634(3) mB/Tb3 þ , which agree well with the theoretical moment for a free Tb3 þ ion, i.e. μcal ¼9.721(1) mB [35], and the Weiss constant, θ¼ 1.9(2) K. 3.4. Magneto-optical property For applications, e.g., Faraday rotator, requiring a given rotation angle, a large Verdet constant is desirable to minimize the magnetic field and the optical path length. The MO response of as-grown NTM-2 crystal in the direction of the c-axis is shown in Fig. 6 as a function of the wavelength and the magnetic induction intensity. For each sample, under the same applied magnetic field conditions, the average values of six to nine data were used for interpretation. As shown in Fig. 6, NTM-2 crystal has a larger FR
NTb (1021 ions/cm3)
7.31 12.74
V (rad/Tm) at different wavelengths 532 nm
633 nm
1064 nm
306 190
216 133
65 40 [37]
than TGG sample, and the shorter the wavelength, the larger the FR angle. For paramagnetic material, the Verdet constants, V, is defined as V ¼ θ=HL, where θ is the FR angle, H is the component of the magnetic field in the direction of the light propagation, and L is the light path length through the medium. So V can be calculated from the slope of lines shown in Fig. 7, and the calculated values are listed in Table 1 together with the related Tb3 þ concentrations. The result indicates that NTM-2 crystal possesses a higher V value than that of TGG sample, with an increment of 60% independently of the considered wavelength. The larger values for V are not due to higher concentration of the MO active Tb3 þ ions, because the number of Tb3 þ ions cm 3, N, in NTM is much less than that in TGG, as shown in Table 1. According to the standard theory of FR, which considers a single electronic transition frequency, V is proportional to N and to the inverse of the wavelength square λ2 as follows: V¼
E λ2 λ20
ð2Þ
where λ0 corresponds to the transition wavelength (in this case the 4f–4f5d Tb3 þ transition), and E includes all the proportionality factor, e.g., the concentration of magnetic ions NRE, the Lande splitting factor, and the transition probability [36]. Fitting the wavelength-dependence data for the samples to Eq. (2) yields E and λ0, listed in Table 2. It is seen that the transition wavelengths of Tb3 þ in both compounds are close to each other at 259 and 262 nm, respectively. Hence, the V increment of 60% observed in NTM-2 is merely due to a higher rotation efficiency of the Tb3 þ ions, i.e., (E/N)NTM-2 4(E/N)TGG, in the NTM-2 matrix. 4. Conclusions Summarizing, the potential of scheelite-type NTM crystals for MO applications in the visible–near IR wavelength region is evaluated. NTM single crystals with large size and high optical quality could be
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X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
Table 2 The fitting parameters of NTM-2 and TGG according to Eq. (2). Crystal
E (107 rad nm2/Tm)
λ0 (nm)
NTM-2 TGG
6.9330 4.2595
259 262
grown by the Czochralski technique. The compound crystallizes in the tetragonal system with space group I41 =a. The thermal expansion coefficients of NTM crystal have evaluated and exhibited moderate thermal stability and weak thermal expansion anisotropy. As expected, NTM crystal exhibits paramagnetic behavior down to 2 K and presents a high transparency from 600 nm to 1500 nm after annealing in O2 atmosphere. In comparison with commercially available TGG, we demonstrate that the as-grown NTM crystals exhibit remarkably higher Verdet constants in the considered wavelength region. Presented results indicate therefore that NTM crystals are promising candidates in particular for potential MO applications in the visible–near IR wavelength region. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 91022025, 51072036 and 51272044), the Natural Science Foundation of Fujian Province (No. 2010J01284) and the Project of Capacity Improvement of National Basic Scientific Personnel Training Foundation of China (No. J1103303). References [1] E. Khazanov, N. Andreev, O. Palashov, A. Poteomkin, A. Sergeev, O. Mehl, D. Reitze, Appl. Opt. 41 (2002) 483. [2] V. Vasyliev, E. Villora, M. Nakamura, Y. Sugahara, K. Shimamura, Opt. Express 20 (2012) 14460. [3] V. Bedarev, M. Pashchenko, D. Merenkov, Y. Savina, V. Pashchenko, S. Gnatchenko, L. Bezmaternykh, V. Temerov, J. Magn. Magn. Mater. 362 (2014) 150. [4] K. Tsushima, N. Koshizuka, IEEE Trans. 23 (1987) 3473. [5] K. Tanaka, N. Tatehata, K. Fujita, K. Hirao, N. Soga, J. Phys. D: Appl. Phys. 31 (1998) 2622.
[6] Y. Shoji, T. Mizumoto, H. Yokoi, I. Hsieh, R.M. Osgood, Appl. Phys. Lett. 92 (2008) 071117. [7] K.B. Modi, R.P. Vara, H.G. Vora, M.C. Chhantbar, H.H. Joshi, J. Mater. Sci. 39 (2004) 2187. [8] L. Daechun, C. Kyungmook, K. Ki-Doo, P. Youngil, Opt. Commun. 285 (2012) 1767. [9] C. Kaiyun, Q. Jinguo, X. Zhengyuan, Opt. Commun. 298 (2013) 59. [10] D. Pankaz, P. Youngil, K. Ki-Doo, Opt. Commun. 324 (2014) 264. [11] M. Geho, T. Sekijima, T. Fujii, J. Cryst. Growth 267 (2004) 188. [12] A. Giesen, J. Speiser, IEEE J. Sel. Top. Quantum Electron. 13 (3) (2007) 598. [13] H. Tomokatsu, N. Masayuki, N. Nishi, N. Sawanobori, Chem. Mater. 14 (2002) 3223. [14] C. Qiuling, W. Hui, W. Qingwei, C. Qiuping, J. Non-Cryst. Solids 400 (2014) 51. [15] R. Yasuhara, S. Tokita, J. Kawanaka, T. Kawashima, H. Kan, H. Yagi, H. Nozawa, T. Yanagitani, Y. Fujimoto, H. Yoshida, M. Nakatsuka, Opt. Express 15 (2007) 11255. [16] N. Miyanaga, H. Azechi, K.A. Tanaka, T. Kanabe, T. Jitsuno, J. Kawanaka, Y. Fujimoto, R. Kodama, H. Shiraga, K. Knodo, K. Tsubakimoto, H. Habara, J. Lu, G. Xu, N. Morio, S. Matsuo, E. Miyaji, Y. Kawakami, Y. Izawa, K. Mima, J. Phys. IV 133 (2006) 81. [17] R.C. Linares, Solid State Commun. 2 (1964) 229. [18] J.R. Carruthers, M. Kokta, R.L. Barns, M. Grasso, J. Cryst. Growth 19 (1973) 204. [19] W.A. Crossley, R.W. Cooper, Phys. Rev. 181 (1969) 896. [20] S.B. Berger, C.B. Rubinstein, C.R. Kurkjian, A.W. Treptow, Phys. Rev. 133 (1964) A723. [21] D.R. MacFarlane, C.R. Bradbury, P.J. Newman, J. Non-Cryst. Solids 213 (1997) 199. [22] X. Lu, Z. You, J. Li, Z. Zhu, G. Jia, B. Wu, C. Tu, Solid State Commun. 146 (2008) 287. [23] W. Zhao, L.Z. Zhang, G.F. Wang, J. Cryst. Growth 311 (2009) 2336. [24] T. Kim, S. Kang, J. Lumin. 122–123 (2007) 964. [25] B. Yan, J.H. Wu, Mater. Chem. Phys. 116 (2009) 67. [26] M. Haque, H.I. Lee, D.K. Kim, J. Alloy. Compd. 481 (2009) 792. [27] J. Liebertz, Prog. Cryst. Growth Charact. 6 (1983) 361. [28] G.M. Kuz’micheva, D.A. Lis, K.A. Subbotin, V.B. Rybakov, E.V. Zharikov, J. Cryst. Growth 275 (2005) 1835. [29] X.Z. Li, Z.B. Lin, L.Z. Zhang, G.F. Wang, Opt. Mater. 29 (2007) 728. [30] R.A. Young, A. Shakthivel, T.S. Moss, C.O. Paiva Santos, J. Appl. Crystallogr. 28 (1995) 366. [31] W. Zhao, Z.B. Lin, L.Z. Zhang, G.F. Wang, J. Alloy. Compd. 509 (2011) 2815. [32] H. Yoshida, K. Tsubakimoto, Y. Fujimoto, K. Mikami, H. Fujita, N. Miyanaga, Opt. Express 19 (2011) 15181. [33] S. Rosenkranz, A.P. Ramirez, A. Hayashi, R.J. Cava, R. Siddharthan, B.S. Shastry, J. Appl. Phys. 87 (2000) 5914. [34] M. Gingras, B. Hertog, M. Faucher, J. Gardner, S. Dunsiger, L. Chang, B. Gaulin, N. Raju, J. Greedanm, Phys. Rev. B 62 (2000) 6496–6511. [35] R.L. Carlin, Magnetochemistry, Springer-Verlag, Berlin-Heidelberg, 1986. [36] J.R. Qiu, K. Tanaka, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 213 (1997) 193. [37] M.Y.A. Raja, D. Allien, W. Sisk, Appl. Phys. Lett. 67 (1995) 2123.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Czochralski growth, magnetic and magneto-optical properties of Na2Tb4(MoO4)7 crystal Xin Chen, Min Ruan, Feiyun Guo n, Jianzhong Chen n College of Chemistry, Fuzhou University, Fuzhou 350116, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 March 2015 Accepted 1 April 2015 Communicated by S. Uda Available online 9 April 2015
Na2Tb4(MoO4)7 crystals have been grown by the Czochralski technique for the first time for magnetooptical applications. Structure refinement of XRD data confirms that the compound crystallizes in the tetragonal system I41/a, with scheelite structure. The as-grown crystal exhibits moderate thermal stability and weak thermal expansion anisotropy (αc =αa 1:08), and the hardness is about 4.7 Moh. Transmittance spectra indicated that the Na2Tb4(MoO4)7 crystal presents a high transparency from 600 nm to 1500 nm after annealing in O2 atmosphere at 973 K for 20 h. The Faraday effect and the temperature dependence of the magnetic susceptibility have been investigated, which demonstrate that the Na2Tb4(MoO4)7 crystal exhibits paramagnetic behavior over the experimental temperature-range 2– 300 K and yields a Faraday rotation which is about 1.6 times as large as that of Tb3Ga5O12 crystal. Na2Tb4(MoO4)7 crystal is therefore a promising Faraday material in particular for new magneto-optical applications in the visible–near IR wavelength region. & 2015 Elsevier B.V. All rights reserved.
Keywords: A1. Faraday effect A1. Magnetism A2. Czochralski method B1. Molybdate B2. Magneto-optic material
1. Introduction The optical isolators (OIs) using the non-reciprocity of the Faraday effects play an important role in a modern optical fiber communication system to prevent optical feedback and noise from reaching a laser light source through scattering and reflections along the light path [1–5]. Yttrium iron garnet, Y3Fe5O12 (YIG), characterized by a high transparency in the IR region (1.2–5.0 μm) [6,7], a low saturation magnetization and a large Faraday rotation (FR) angle, is so far the most commonly used material in OIs. However, as the rapid development of fiber lasers, the demand of OIs operated at wavelengths below 1100 nm (i.e. the visible–near IR region of 400–1100 nm) is rapidly increasing [8–10], while the conventional YIG crystals are not practical due to their very poor transparency in this spectral region [11]. The most commonly used materials for the visible and nearinfrared regions are terbium-doped glasses [12–14] and terbiumgallium garnet, Tb3Ga5O12 (TGG) single crystals [1,15]. However, terbium-doped glasses as amorphous material cannot be used in high average power lasers because of its low thermal conductivity and low relatively stability [16]. TGG, a commonly used crystalline magnetooptic material, although it melts congruently at approximately 2100 K, its Czochralski growth is not exempt of difficulties. The decomposition
and evaporation of Ga2O3 from the TGG melt, which is easy to over flow the container during the crystal growth process due to its large infiltration, lead to a serious component deviation from the congruent melting [17,18]. Tb3 þ yields to the largest FR among all the rare-earth ions, so that electric dipole contribution of Tb3 þ dominates even over that of Fe3 þ in magnetic terbium-iron garnet [19–21]. Therefore, there is a clear need to find a new generation of terbium-based magneto-optic materials with high Verdet constant and low optical absorption loss in the visible–near IR region. During the past few years, interest arose concerning the rare-earth molybdate A2R4(Mo4)7 (A¼ alkali metal, R¼lanthanide) with a tetragonal scheelite-type structure because of its great potential for applications in laser host materials [22,23] and phosphors [24–26]. Taking into account the requirements for a magneto-optic material applied in the visible–near IR region, Na2Tb4(MoO4)7 (NTM) a member of the scheelite-type molybdate has come to draw more and more attention from our research group. NTM presents favorable growth characteristics, with strong crystallizability, low cost of materials, and no special absorption at the visible–near IR region. These features are advantages for a magneto-optical (MO) material candidate. In the present work, we attempt to employ the Czochralski technique to grow NTM single crystals. The spectral, magnetic and MO properties were discussed as well. In order to compare these
n
Corresponding authors. Tel.: þ 86 591 22866243; fax: þ 86 591 22866130. E-mail addresses: [email protected] (F. Guo), [email protected] (J. Chen).
http://dx.doi.org/10.1016/j.jcrysgro.2015.04.001 0022-0248/& 2015 Elsevier B.V. All rights reserved.
with the reference material TGG, the latter has been also measured under equivalent experimental conditions.
X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
2. Experimental 2.1. Synthesis and crystal growth The polycrystalline materials used for single-crystal growth were prepared by the conventional solid-state reaction method according to the following chemical reaction equation: 1 Tb4 O7 þ7MoO3 þ Na2 CO3 -Na2 Tb4 ðMoO4 Þ7 þ CO2 ↑ þ O2 ↑ 2
ð1Þ
The initial reagents used were Tb4O7 with purity of 99.99%, MoO3 and Na2CO3 with analytical reagent. The stoichiometric amount of raw materials was weighed with the excess amount 2 wt% of MoO3 to compensate for its volatilization loss during the process of crystal growth, and then ground homogeneously in an alumina mortar with the ethanol. The uniformly mixed raw materials were pressed into tablets and then sintered at 833 K for 12 h in air to decompose the carbonate. They were then cooled to room temperature, reground, extruded into pieces and sintered again at 1173 K in air for 25 h. The purity of the sample was checked by X-ray powder diffraction. A single scheelite phase of NTM was obtained when repeated heat treatment caused no further changes in the X-ray powder diffraction. Since the DSC analysis indicates that NTM is a congruently melting material, the single crystals can be grown by the conventional RF-heating Czochralski technique which is known to be a very efficient and suitable method for the growth of large single crystals with good optical performance [27]. The polycrystalline pieces were melted in a platinum crucible with dimensions of Ø55 35 mm2, heated by a radiofrequency furnace. The polycrystalline material was heated up to a temperature of about 50 K higher than the melting point and kept this temperature for 2 h, and then the temperature was decreased to the crystallization point. After the growing temperature was accurately determined by repeated seeding trials, a seed crystal of o0014 orientation, cut from the initial NTM crystal which obtained by crystallization on the iridium rod, was dipped into the melt and then slowly withdrawn, so that the crystal began to grow along the c-axis with 1.5–2.0 mm h 1 pulling rate and 20– 25 rpm rotating rate. After crystallization, the crystal was pulled out of the melt and cooled down to room temperature at a rate of 10– 30 K h 1. The NTM-1 single crystal, grown in the atmosphere of pure N2, is black in color because of an oxygen-deficient atmosphere [28,29], as shown in Fig. 1(a). Meanwhile, the NTM-2 crystal (Fig. 1(b)), grown in air atmosphere, is transparent and crack free with dimensions of Ø20 25 mm2 with a yellowish-brown coloration. It is noteworthy that since NTM is an optically uniaxial crystal, in order to avoid birefringence in the MO measurements, crystals
9
should be oriented along the optic axis, which is coincident with the crystallographic c-axis. After oriented accurately by X-ray diffraction, the as-grown crystals were cut along the (001) crystal plane, and then wafers were ground and polished carefully for the following measurement.
2.2. Structure determination The X-ray powder diffraction (XRD) data for Rietveld analysis were recorded on a Rigaku Ultima III diffractometer using Cu Kα radiation (λ ¼1.54051 Å) from 101 to 901 (2θ) with a scanning step of 0.011 (2θ) and a count time of 5 s per step at room temperature. Crystal structure determination was performed using the Rietveld method implemented in the DBWS-9411 [30] PC program. The observed, calculated and their difference diffraction profiles of the as-grown NTM-2 crystal are displayed in Fig. 2. The structure was refined from its powder X-ray diffraction data in space group I41 =a, and the refinement showed comparable good fit value: Rwp ¼6.63%, which indicates unambiguously the existence of the scheelite structure with the tetragonal system. The refined lattice parameters of NTM-2 crystal are a ¼b¼ 5.2184(2) Å, c ¼11.4006(5) Å and V¼ 310.5(6) Å3, basically consistent with the previously reported values found in other scheelite-type rare-earth molybdate [31].
Fig. 2. Rietveld refinement of the X-ray powder diffraction pattern from the asgrown NTM-2 crystal at room temperature.
Fig. 1. Photographs of as-grown crystals (a) NTM-1 (N2 atmosphere) and (b) NTM-2 (air atmosphere).
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X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
2.3. Hardness and thermal expansion measurement The hardness of as-grown NTM-2 crystal, determined by using a 401MVA™ Vickers-microhardometer, is about 336.1 VDH (equal to about 4.7 Moh), and hence is favorable for crystal machining. NTM crystal belongs to the tetragonal system, and there are only two independent main thermal expansion features [29]. Therefore, the thermal expansion coefficients of NTM-2 single crystal along the a and c axes were evaluated respectively by using a Netzsch DIL402PC model thermo-mechanical analyzer equipment from room temperature to 973 K at a heating rate of 10 K min 1. 2.4. Transmission spectrum The transmission spectrum of as-grown crystals NTM was measured by using a Perkin-Elmer Lambda UV–vis–NIR spectrophotometer, over the wavelength range from 400 to 1500 nm with a 2 nm resolution at room temperature. 2.5. Variable-temperature magnetic susceptibility
Fig. 3. Transmittance spectra of as-grown NTM crystals in comparison with the transmittance of commercial TGG (5 mm thick, without antireflection coating).
The temperature dependence of the magnetic susceptibility was measured under both zero-field-cooled (ZFC) and field-cooled (FC) conditions in an applied field of 0.1 T over the temperature range 2–300 K using a Quantum Design MPMS XL SQUID magnetometer. The sample was placed into a diamagnetic gelatin capsule, and the data were corrected by extracting the contribution from diamagnetic ionic susceptibilities. 2.6. Faraday rotation measurement The specific FR of the as-grown NTM-2 crystal in the direction of the c-axis was measured by the extinction method [32] with two lasers emitting at three wavelengths 532, 633, and 1064 nm at room temperature. For each sample, the average values of six to nine data were used for interpretation. A commercial TGG crystal was used as the standard sample to correct the magnetic field. The magnetic field intensity could be adjusted from 0 to 1.2 T continuously. Fig. 4. Thermal expansion of as-grown NTM-2 crystal.
3. Results and discussion 3.1. Effect factors of crystal quality It is known that a very important factor during growth of the scheelite-type molybdate single crystals is the choice of growth ambient atmosphere [28,29]. In order to investigate the effect of the growth atmosphere on the crystal quality, NTM crystals were grown in the different kinds of atmosphere. NTM-1 single crystal, grown in the atmosphere of pure N2, appeared to have black color, and the transmittance spectra of NTM-1 crystal, as shown in Fig. 3, demonstrated a very broad absorption band in the visible–near IR wavelength region as well. This band was found to appear because of formation of color centers, based on oxygen vacancies. Besides the formation of oxygen vacancies and partial transition of Mo6 þ into Mo5 þ in NTM-1 crystal in case of low oxygen content at growth atmosphere, free electrons can localize at oxygen vacancies, which may lead to formation of color centers, responsible for the additional optical absorption and black coloration of crystals. As expected, in the case of NTM-2 single crystal, the oxidizing atmosphere of air decreased the oxygen vacancy defect and the concentration of color centers to a great extent, and then improved the optical transmittance obviously, as shown in Fig. 3. In order to further enhance the visible transparency, the asgrown NTM-2 crystal was annealed in O2 atmosphere at 973 K for
20 h, and the transmittance spectra of annealed NTM-2, in comparison with a commercial device-quality TGG, are shown in Fig. 3, which indicated that the NTM-2 crystal presents a high transparency from 600 nm to 1500 nm after annealing in O2 atmosphere. The sharp absorption peak around 488 nm is mainly related to the energy level transition 7F6-5D4 of Tb3 þ in the scheelite structure. 3.2. Thermal analysis The thermal expansion of as-grown NTM-2 crystal is shown in Fig. 4, which indicates that NTM exhibits moderate thermal stability with thermal expansion coefficients about 7.56 10 6 K 1 along a-axis and 8.16 10 6 K 1 along c-axis. Moreover, the similar thermal expansion coefficient along the a and c axes indicates a weak thermal expansion anisotropy (αc =αa 1:08), which is helpful to reduce the crystal cracking during the crystal growth. 3.3. Paramagnetic behavior The temperature dependence of the magnetic susceptibility (χ M ) for NTM measured in the temperature range of 2–300 K is shown in Fig. 5. Paramagnetic behavior was observed down to 2.0 K, i.e., no magnetic interaction was found in the experimental temperature range. In the inset of Fig. 5, the reciprocal magnetic susceptibility was
X. Chen et al. / Journal of Crystal Growth 421 (2015) 8–12
Fig. 5. Temperature dependence of the magnetic susceptibility for NTM. The inset shows the reciprocal magnetic susceptibility vs. temperature curve. The solid line is the Curie–Weiss fitting.
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Fig. 7. FR angles of NTM-2 crystal as a function of the magnetic induction intensity.
Table 1 The Verdet constants and Tb3 þ concentration of NTM-2 crystal in comparison with those of reference TGG crystal. Crystal
NTM-2 TGG
Fig. 6. FR angles of NTM-2 crystal as a function of the wavelength λ. A commercial TGG crystal was measured as reference, and the measured data were fitted by an equation in the form A/(λ2 λ20 ), with A and λ0 constants.
plotted against temperature, and no evidence of any magnetic transition has been recognized down to 2.0 K. The reciprocal susceptibilities of NTM exhibit Curie–Weiss behavior between 30 and 300 K (see the solid line in the inset graph of Fig. 5), but deviate from the Curie–Weiss law below 30 K. This result may indicate that the magnetic ions Tb3 þ in this compound are affected by the crystal field to some extent, and single ion anisotropy may take effect, as reported by Rosenkranz et al. [33,34]. A fit of the χ M 1 –T curve to the Curie– Weiss law above 200 K gives an effective paramagnetic moment (μef f ) of 9.634(3) mB/Tb3 þ , which agree well with the theoretical moment for a free Tb3 þ ion, i.e. μcal ¼9.721(1) mB [35], and the Weiss constant, θ¼ 1.9(2) K. 3.4. Magneto-optical property For applications, e.g., Faraday rotator, requiring a given rotation angle, a large Verdet constant is desirable to minimize the magnetic field and the optical path length. The MO response of as-grown NTM-2 crystal in the direction of the c-axis is shown in Fig. 6 as a function of the wavelength and the magnetic induction intensity. For each sample, under the same applied magnetic field conditions, the average values of six to nine data were used for interpretation. As shown in Fig. 6, NTM-2 crystal has a larger FR
NTb (1021 ions/cm3)
7.31 12.74
V (rad/Tm) at different wavelengths 532 nm
633 nm
1064 nm
306 190
216 133
65 40 [37]
than TGG sample, and the shorter the wavelength, the larger the FR angle. For paramagnetic material, the Verdet constants, V, is defined as V ¼ θ=HL, where θ is the FR angle, H is the component of the magnetic field in the direction of the light propagation, and L is the light path length through the medium. So V can be calculated from the slope of lines shown in Fig. 7, and the calculated values are listed in Table 1 together with the related Tb3 þ concentrations. The result indicates that NTM-2 crystal possesses a higher V value than that of TGG sample, with an increment of 60% independently of the considered wavelength. The larger values for V are not due to higher concentration of the MO active Tb3 þ ions, because the number of Tb3 þ ions cm 3, N, in NTM is much less than that in TGG, as shown in Table 1. According to the standard theory of FR, which considers a single electronic transition frequency, V is proportional to N and to the inverse of the wavelength square λ2 as follows: V¼
E λ2 λ20
ð2Þ
where λ0 corresponds to the transition wavelength (in this case the 4f–4f5d Tb3 þ transition), and E includes all the proportionality factor, e.g., the concentration of magnetic ions NRE, the Lande splitting factor, and the transition probability [36]. Fitting the wavelength-dependence data for the samples to Eq. (2) yields E and λ0, listed in Table 2. It is seen that the transition wavelengths of Tb3 þ in both compounds are close to each other at 259 and 262 nm, respectively. Hence, the V increment of 60% observed in NTM-2 is merely due to a higher rotation efficiency of the Tb3 þ ions, i.e., (E/N)NTM-2 4(E/N)TGG, in the NTM-2 matrix. 4. Conclusions Summarizing, the potential of scheelite-type NTM crystals for MO applications in the visible–near IR wavelength region is evaluated. NTM single crystals with large size and high optical quality could be
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Table 2 The fitting parameters of NTM-2 and TGG according to Eq. (2). Crystal
E (107 rad nm2/Tm)
λ0 (nm)
NTM-2 TGG
6.9330 4.2595
259 262
grown by the Czochralski technique. The compound crystallizes in the tetragonal system with space group I41 =a. The thermal expansion coefficients of NTM crystal have evaluated and exhibited moderate thermal stability and weak thermal expansion anisotropy. As expected, NTM crystal exhibits paramagnetic behavior down to 2 K and presents a high transparency from 600 nm to 1500 nm after annealing in O2 atmosphere. In comparison with commercially available TGG, we demonstrate that the as-grown NTM crystals exhibit remarkably higher Verdet constants in the considered wavelength region. Presented results indicate therefore that NTM crystals are promising candidates in particular for potential MO applications in the visible–near IR wavelength region. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 91022025, 51072036 and 51272044), the Natural Science Foundation of Fujian Province (No. 2010J01284) and the Project of Capacity Improvement of National Basic Scientific Personnel Training Foundation of China (No. J1103303). References [1] E. Khazanov, N. Andreev, O. Palashov, A. Poteomkin, A. Sergeev, O. Mehl, D. Reitze, Appl. Opt. 41 (2002) 483. [2] V. Vasyliev, E. Villora, M. Nakamura, Y. Sugahara, K. Shimamura, Opt. Express 20 (2012) 14460. [3] V. Bedarev, M. Pashchenko, D. Merenkov, Y. Savina, V. Pashchenko, S. Gnatchenko, L. Bezmaternykh, V. Temerov, J. Magn. Magn. Mater. 362 (2014) 150. [4] K. Tsushima, N. Koshizuka, IEEE Trans. 23 (1987) 3473. [5] K. Tanaka, N. Tatehata, K. Fujita, K. Hirao, N. Soga, J. Phys. D: Appl. Phys. 31 (1998) 2622.
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