Growth and characterization of magneto-optical YFeO3 crystals

Cryst. Res. Technol.42, No. 10, 943 – 947 (2007) / DOI

10.1002/crat.200710972

? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Growth and characterization of magneto-optical YFeO3 crystals

Hui Shen1,2, Jiayue Xu*1, Anhua Wu1, Jinxing Yu3, Yan Liu1, and Liqing Luo1

1Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China

2Graduate School of the Chinese Academy of Sciences, Beijing 100039, P.R. China

3School of materials science and engineering, Shanghai Jiao Tong University, Shanghai 200030, P.R. China

Received 19 March 2007, revised 2 August 2007, accepted 7 August 2007

Published online 24 August 2007

Key words floating zone technique, magneto-optical materials, single crystal growth.

PACS81.10.Fq

The floating zone growth of magneto-optical crystal YFeO3 has been investigated. The polycrystalline feed

rod was prepared by a pressure of 250MPa and sintering at about 1500°C. A crack- free YFeO3single crystal

has been successfully grown. The crystal preferred to crystallize along <100> direction with about 10°

deviation. The X-ray rocking curve of the crystal has a FWHM of 24 arcsec, confirming the high crystal

quality of the sample. The (100) plane was etched by hot phosphoric acid and the dislocation density was

about 104/cm2. A thin outer layer with Y2O3-rich composition was found at the periphery of as-grown

crystals, which was attributed to the Fe2O3evaporation during growth.

? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

YFeO3 and other rare earth substituted crystals with distorted orthorhombic pervoskite-like structure (space group Pbnm) have attracted much attention due to their high magneto-optical figure of merit in near infrared range and the highest domain wall motion velocity [1, 2]. They are considered as promising materials with more outstanding performances than the famous garnet YIG in several practical magneto-optical device applications, such as fast latching type optical switches, light spot position measurements, magneto-optical current sensors and magnetic field sensors [3-6]. More recently, laser-induced ultrafast spin reorientation was found in TmFeO3 crystal, which may have far-reaching influence on emerging spintronic devices [7]. These potential applications have created a high demand for availability of sizable and high quality RFeO3crystals.

So far, single crystal YFeO3has been grown by several methods. Flux method was most often used [8-12]. However, little and very small crystals were obtained. Moreover, second phase inclusions including oxyfluorides, garnet and PbFe12O19were also inevitably incorporated. Czochralski method yielded only small defective crystals [13]. Voids, bubbles, twins and optical density variations are commonly observed in crystals grown by Hydrothermal and Seeded Bridgman methods [14,15]. All above mentioned methods have difficulties in growing high quality YFeO3crystals. The floating zone growth of YFeO3has advantages of a fast growth rate and no contamination [16-18]. Although YFeO3crystal up to 1cm diameter has been obtained, laminar twins and optical non-uniformity were still observed in the crystals. So far, the supply of high quality YFeO3 crystal is limited.

In the present work, the floating zone growth of YFeO3 crystals has been systematically investigated and high quality YFeO3crystal was obtained by optimized process. The crystal quality was characterized by X-ray rocking curve and the growth defects were revealed by electron probe micro analysis and chemical etching.

____________________

* Corresponding author: e-mail: crystalxu@https://www.360docs.net/doc/d016348571.html,

944 Hui Shen et al.: Magneto-optical YFeO 3 crystals

? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim https://www.360docs.net/doc/d016348571.html,

2 Experimental procedure

Y 2O 3 (99.99%) and Fe 2O 3 (99.99%) powders were used as starting materials. They were weighted according to stoichiometric composition and milled in the present of absolute ethanol for 24h. After dried and screened, the mixture was calcined at 1100°C for 20h in air with an intermediate grinding to improve its homogeneity. Then the well-mixed powders were ground and formed into round rods using a cold isostatic pressure of 250MPa. The rods were then sintered at 1500-1550°C for 15h in air. The typical dimension of the sintered rod was 9-11mm diameter and 70-90mm length. The polycrystalline rods were used as feed and seed rods.

A floating zone furnace equipped with two ellipsoidal mirrors was employed for single crystal growth (ASGAL Co. LTD), using two 3.5KW halogen lamps as heat sources. The feed rod was suspended at the upper shaft and the seed was attached to the lower shaft. Both the feed rod and seed rod moved downwards and the melting zone was formed. The feed and seed shafts rotated at 5-20 rpm in opposite directions and the crystal was grown from the bottom to top in a vertical direction. The traveling rate of the melting zone was in the range of 2.5-7.5 mm/h in 1 bar of oxygen.

As-grown crystal was cut perpendicular to the growth direction. Some pieces were ground into fine powder and the phase identification was performed by X-ray diffraction. A piece of the crystal sample was oriented and polished for the measurement of rocking curve to check the crystal perfection. Another slice was polished for the microstructural and compositional analysis which was carried out by EPMA and EDS. Etch patterns of a polished (100) wafer were revealed by hot phosphoric acid.

3 Results and discussion

Preparation of the feed rod The preparation of the feed rods is a key process for the successful growth. First, the compound of YFeO 3 must be synthesized from the mixture of Y 2O 3 and Fe 2O 3 so that homogeneous melt can be achieved during the growth. Second, a dense rod is necessary because lower density of the rod may result in large shrinkage. Additionally, too many pores in the rods will introduce a large amount of defects in as-grown crystals. Figure 1 shows the XRD patterns of the feed rods sintered at different temperatures. It demonstrated that pure perovskite YFeO 3 phase was obtained when sintered at a temperature of above 1500oC. In our experiment, the relative density of the feed rods was nearly 90% of the theoretical value. It was expected

that finer powder was helpful to form a 97% dense rod.

Fig. 1XRD patterns of feed rods at different

sintering temperatures.

Crystal growth Due to lack of seeds, YFeO 3 crystals were grown by spontaneous nucleation. Many nuclei were formed at the initial growth stage and they all grew along the axial direction by competition, which resulted in large crystals in the center part. Figure 2(a) shows the transverse section of a polycrystalline boule. Many grains of about 1-3mm size were observed near the outside of the boule, while comparatively larger grains were found in the center. The grain boundary may cause the boule cracking when it was taken from the furnace. Figure 2(b) shows a crack on the surface of the boule grown at a rate of 5mm/h. The cracking goes along the growth direction. It is known that there is a narrow molten zone during the floating zone growth. Thus, a large axial temperature gradient was formed near the solid-liquid interface, resulting in large thermal stress in as-grown crystals. The thermal stress was easy to induce cracking at the grain boundary.

Cryst. Res. Technol. 42, No. 10 (2007) 945

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? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 2

(a) Tansverse section of a polycrystalline boule; (b) cracking along growth direction.

Fig. 3(a) A typical crack-free YFeO 3single crystal; (b) picture of the cross-sectional view of

YFeO 3 crystal.

By carefully controlling the nucleation and cracking, a YFeO 3single crystal without cracks has been successfully grown. Figure 3(a) shows as-grown crack-free crystal boule. The growth rates of 5, 2.5 and

7.5mm/h were employed from the start to the end to perform the effect of the growth rates on the growth behavior. The white lines in the picture divide the boule into four parts. There was a necking process at the beginning and then it was grown at a rate of 5mm/h. After growing about 17mm, the growth rate reduced to

2.5mm/h for 3h. Finally, the crystal was grown at a rate of 7.5mm/h to complete the growth. The boule has different diameter grown by varied growth rates. The diameter at 5 and 7.5mm/h is about 9mm. However, the diameter of the boule at 2.5mm/h is only about 6mm. It was suggested that the feed rod was not so dense that some melt infiltrated into the feed rod when a slow rate was employed. This gave rise to the decease in the volume of the molten zone and the crystal diameter consequently decreased.

Crystal quality The as-grown crystal was cut perpendicular to the growth direction, as shown in figure 3(b). From figure 3(b), we observed a thin shell at the periphery of the grown boule. The shell was poorly crystallized and the thickness was about 1mm. And the inner part was a single crystal up to 7mm in diameter. They were clearly separated by the boundary. Figure 4 shows the powder XRD patterns of the two parts. According to JCPDS card No. 39-1489, the inner part was pure perovskite YFeO 3 phase and can be well indexed. Whereas there are strong peaks of Y 2O 3 phases in the outer region as well as the main phase of YFeO 3. This demonstrated that some of Fe 2O 3 component was lost during the growth process.

X-ray orientation confirmed that the crystal preferred to grow along the <100> direction with a deviation of about 10°. In order to check the crystal perfection, a (100) wafer was carefully polished for measuring the rocking curve by high resolution X-ray diffraction (HR-XRD). Figure 5 shows the X-ray rocking curve. The profile of the (200) Bragg-reflections has a symmetrical peak with a full-width at half-maximum (FWHM) of 24 arcsec. The nearly perfect shape without any shoulder clarified the absence of any sub-grain, indicating the excellent quality of the crystal.

Surface morphology As-grown crystal surface shows a series of small striations with a slight fluctuation of its diameter. The depth and width of the striations varied with the growth rates. Figure 6 shows the microstructure images of the striations taken by EPMA. Many fine striations were observed in the sample grown at a rate of 7.5mm/h, the depth and width of the striations were about 50μm and 50-80μm, respectively.

946 Hui Shen et al.: Magneto-optical YFeO 3 crystals ? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim https://www.360docs.net/doc/d016348571.html,

The sample grown with a rate of 5mm/h showed the depth of 30μm and width of 120-180μm, as shown in figure 6(b). The striations were less pronounced for the slowly grown sample. Figure 6(b) and (c) show the influence of the feed rod density on the striations. With the same growth rate, the surface of sample grown from a 97% dense rod looked much flatter than that of sample from 85% dense rod. During the floating zone growth, the temperature gradient and surface tension gradient may give rise to molten zone oscillations, resulting in slight variation of the growth rate which contributes to tiny fluctuation in diameter [19]. These regular striations were attributed to the periodic oscillations of the molten zone. Thus, a denser feed rod and a

slow growth rate were applied to enhance the uniformity and stability of the melt.

Fig. 4Powder XRD patterns of the as-grown YFeO 3 crystal.

(a) inner single crystal; (b) outer layer. Fig. 5The X-ray rocking curve of (100)

oriented wafer, recorded for (200) reflection. Fig. 6Microstructure images of small striations on the original surface of YFeO 3crystal (a) growth rate of 7.5mm/h with 85% feed rod; (b) growth rate of 5mm/h with 85% dense feed rod; (c) growth rate of 5mm/h with 97% dense feed rod.

Defects Crystal defects were characterized by means of EPMA and chemical etching. Figure 7 (a) and (b) present backscattered electron images (BEIs) for the inner area and outer layer, respectively. The inner large area was smooth and homogenous. On the other hand, some voids and micro cracks were observed clearly in the outside layer. There were also several inclusions (the white area) randomly dispersed in this region. Table 1 summarizes the chemical compositions of different regions, which were taken by EDS analysis. EDS analysis results showed the white area was almost Y 2O 3, while compositions of the other regions were quite close to the nominal compositions of YFeO 3 crystal. No Fe 2O 3-rich phase was found in the inner crystal or the outer layer, which indicates the evaporation of Fe 2O 3 during the growth. In fact, the quartz tube became opaque and some black particles were found in the inner wall of the furnace. The evaporation of Fe 2O 3 resulted in compositional deviation in the melt. However, a convex solid-liquid interface was achieved during the growth, as shown in figure 8. Thus, many impurities and rich Y 2O 3 melt were excluded to the circumference to form the outer layer of as-grown boule. At last, YFeO 3single crystal without inclusions has grown by mechanically removing the thin shell at the circumference. The polished sample with (100) plane was also etched in fresh phosphoric acid at 250°C for 10min. The etched surface was examined by optical microscopy. The etch pits of the dislocations

Cryst. Res. Technol. 42, No. 10 (2007) https://www.360docs.net/doc/d016348571.html, ? 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim on the (100) plane are regular rectangles, with dimension of 5-10μm. The etch pit density was estimated to be 104/cm 2

.

Fig. 7Backscattered electron images (BEIs) of a polished surface of (a)

the inner part and (b) the outer layer. Fig. 8The solid-liquid interface along growth direction.

Table 1EDS analysis results of the inner area and inclusions at the periphery of the crystal (wt %).

component The inner area (spectrum 1 site) The inclusion (spectrum 2 site) The theoretical value

Y 2O 3 60.53 98.86 58.58 Fe 2O 3 39.47 1.14 41.42

4 Conclusions

Magneto-optical crystal YFeO 3 has been grown by the floating zone method. The growth conditions were discussed based on the growth results. A crack-free single crystal has been successfully obtained by optimized growth parameters. The crystal has a preferred growth direction of <100>. X-ray rocking curve verified that as-grown crystal has good quality with a FWHM of 24 arcsec, An Y 2O 3-rich thin layer was observed at the circumference of the as-grown boule and was ascribed to the evaporation of Fe 2O 3 during the growth. The (100) plane was etched by hot phosphoric acid and the dislocation density was about 104/cm 2.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 50672110) and National High Technology Research and Development Program of China (Grant No. 2006AA03Z419).

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