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Single crystal growth of LuFe2O4, LuFeCoO4 and YbFeMgO4 by the floating zone method
Journal ol (‘rystal Grow Oi 102 (1990) ~98 400 North Holland
SINGLE CRYSTAL GROWTH OF LuFe2O4, LuFeCoO4 AND YbFeMgO4 BY THE FLOATING ZONE METHOD Junji IIDA
*
insiiiuie for Material Resear(h, Tohoku (‘no cr5115, 2 / 1 Aaiahira,$e,islai, ti0 agi 980, Japan
and Shunji TAKEKAWA and Noboru KIMIZIJKA \atjonal iirsiitute br Re5earch in Inorganic Materials, I I Naniik,. Tsukuha. Iharahi ?0~.Japan Receised 6 September 1989: manuscript received in final form 10 Januar\ 1990
Single cr~stalsof LuFe,04, LuFeCoO4 and YbFeMgO4 (YhFe~04 structure with space group R3m) were grown h~the floating zone method. The atmosphere of C0~ CO gas mixture was used for growth of LuFe~()4cr’sstal, and the flowing air for growing both LuFeCoO4 and YbFeMgO4 crystals. The growth rate was 1 mm h. and the growth direction was perpendicular to the c axis in the hexagonal system for all cases. Single crystals attaining 3 >< 2 X 15 mm, 1 X S x 10 mm and 5 XIX 20 mm were obtained for LuFeX)4, LuFeC 004 and YbFeMgO4. respectively
I. Introduction
2. Growth experiments
The crystal of RFe2O4 (R Y, Ho, Er, Tm, Yb. Lu and In) belongs to the trigonal system, with space group R3m [1]. The crystal has a layer structure consisting of alternating Fe202 and R01 layers perpendicular to the c-axis in the hexagonal system. As the crystal contains 50% of divalent and trivalent iron ions, other divalent cations such as Mg, Co. Mn, Cu and Ztt can occupy the divalent iron site [2]. The magnetic properties of these compounds are very interesting because of their magnetic two-dimensionality in low temperatures. Although single crystals are necessary to study the details of their magnetism, only YFe2O4 and YbFe2O4 have been grown so far by the floating zone method [3.4]. Thus we tried to grow single crystals of LuFe,04, LuFeCoO4 and YbFeMgO4. In this paper. the growth conditions are reported.
2.1
*
Present address’ Department of Physics, Ochanomizu Uni versit~.2 1 1 Otsuka. Bunkyo, Tokyo 112. Japan
0022 0248
90 $03 50
1990
General
The crystal growth experiment was performed h~ using an infrared radiation convergence type apparatus with a radiation source of 1.5 kW halogen lamp [5]. In this apparatus. the growth atmosphere can be controlled precisely. Powders of CoO (99.9%). Lu,05 (99.9%), Yb~O5 (99.9%), MgO (99%) and Fe2O~ (99.99%) were used as raw materials and the reagents except CoO were heated in air at 1000°C for I day in order to eliminate the absorbed gas such as H-,O. 2.2. LuFe,04 The mixture of Lu2O4/Fe2O~ 1/2 in mole ratio was reacted at 1200°C under C0/H1 3.0 mixed gas atmosphere for approximately 6 h, and quenched to ice temperature. The obtained polycrystalline LuFe2O4 was then crushed and ground into fine powders. This powder was charged into a
Elsesier Science Publishers By, (North Holland)
J.
Jida et al. / Single crystal growth of LuFe ~O4,LuFeCoO
rubber tube and pressed hydrostatically 2 tounder obtaina pressure of approximately 1000 kg/cm a rod of 7 mm in diameter and 70 mm in length. The rod was sintered at 1200°C for 2 h under the same atmosphere as described above. It is well-known that the stability field of compounds containing transition metal ions is strongly dependent upon the temperature and the oxygen partial pressure. The CO 2 CO gas mixture was used for an atmosphere in crystal growth of LuFe2O4, because a CO2 H2 mixture whose mixing ratio is near unity generates much water, giving an undesirable effect on single crystal growth. The optimum mixing ratio of CO2/CO was determined by the trial-and-error method. The crystal was grown at a rate of 1 mm/h in all experiments. The solidified rod in C02/CO 2.5 was cut, polished and observed by using a differential interference microscope and X-ray diffraction method. The specimen does not contain any inclusions and is a mono-phase of LuFe2O4. In addition, it turned out from chemical analysis that the quenched part of the molten zone consisted of Lu0 5Fe3 430X. This suggests that LuFe2O4 melts incongruently, and thus the traveling solvent floating zone method should be adopted to grow single crystals of LuFe2O4 under a gas mixture with C02/CO 2.5. As shown in fig. 1, the obtained boule was typically 5 mm in diameter and 50 mm in length, and composed of several grains. It was difficult to grow larger single crystal grains in only one crystal growth experiment. Therefore crystal growth experiments were carried out several times using a new feed rod. Since this material melts incongruently, the grown crystal and the quenched molten zone at the end were used as a seed crystal and a first molten zone, respectively, in the next growth experiment. A similar procedure was applied in the growth of the other two compounds. The crystal of LuFe2O4 can be cleaved easily along the growth direction. Fig. 2 shows a cleaved surface which was etched in 20% HC1 aqueous solution at 50°Cfor 13 mm and photographed using a phase contrast microscope. The etch pits take on a triangular shape. Also, the three-fold symmetric pattern was obtained in a Laue photograph by X-ray. The cleavage plane is thus determined to be (001)
4 and YbFeMgO4 by FZM
399
a
b
C
~
~
Fig. 1. Portion of as-grown single crystal boules of (a) LuFe2O4, (b) LuFeCoO4 and (e) YbFeMgO4 Except for YhFeMgO4, several single crystal grains are contained in each of them. Marker represents 1 cm
plane. The crystal of LuFe2O4 was grown perpendicular to its c-axis. The same relation in growth direction and shape of etch pits holds for the other two compounds.
=
—~
_____
Fig. 2. The triangular etch pits on the (001) cleavage surface of LuFe.8J4 single crystal.
400
J. Jida et al.
Single crystal growth of LuFe ,04, LuFeCoO
4 and YbFeMgO4 by FZM
gruently. (Fe,Mg)O and Yb201 crystallize alternately, whereas YbFe1 ±5Mg1 P4 solidifies evenly
Table I Lattice parameter and growth conditions of each crystal Lattice parameter a (A) LuFe2O4 LuFeCoO
3.429 4 3.419 YbFeMgO4 3.423
c
(A)
25.25 25.29 25.18
Melting Growth point atmosphere (° C) 1300 1490 1630
C02/CO Air Air
2.5
2.3. LuFeCoO4 and YbFeMgO4 In order to grow single crystals of LuFeCoO4 and YbFeMgO4, mixtures of the ratio Lu 203. Fe2O3 : CoO 1: 1: 2 and Yb203: Fe2O3: MgO 1: 1: 2 in mole, respectively, were pressed into a rod and sintered at 1380°C and 1590°Cfor 2 h under oxygen atmosphere. Although oxygen was tried at first as the growth atmosphere, it was found that bubbles were formed in the molten zone, leading to a remarkably unstable molten zone. It has been known that the occurrence of bubbles in the molten zone can be suppressed by reducing the partial oxygen pressure [6]. Thus we examined flowing air with a linear velocity of 1.8 mm/s, and found that this improved the conditions very much, as expected. In the case of LuFeCoO4 crystal growth, the quenched molten zone consists of Lu0 86Fe105 Co1 28°r’ and this composition is close to that of the solidified part LuFeCoO4. This is very different from the case of LuFe2O4, whose molten zone consists of Lu05Fe3430~.The difference in the compositions between the solidified and molten part is smaller in LuFeCoO4 than in LuFe2O4. Since large single crystal grains were grown more easily in the case of LuFeCoO4 than in LuFe2O4, this difference may have some relation to the difficulty in growing single crystals. In the case of YbFeMgO4, the initially solidified part in the growth experiment was composed of YbFe1 ~~Mg1 X04, (Fe,Mg)0, Yb203 and YbFeO3, indicating that YbFeMgO4 melts incon-
throughout the whole region of the boule. It is assumed that YbFeMgO4 was reduced to some extent at the melting temperature in air and de3~Fe2~Mg 2~. composed into YbFe 1c04, (Fe Mg)O and Yb2O1. In order to grow a single crystal of YbFe1÷~Mg1~O4 in air, the value of x was determined on the specimen by means of EPMA to be 0,12. Based on these, we prepared a sintered rod with composition of YbFe1 12Mg0 8804 and carried out single crystal growth experiments of this compound in air. A crystal of YbFe1 12Mg088 04 was grown more easily than in the case of LuFe2O4. One of the reasons of the difficulty met in single crystal growth of RFe2O4 is the difficulty involved in controlling the atmosphere. Homogeneous control of oxygen partial pressure in the molten zone is not easy because the crystal grows very fast. The lattice parameters and the growth conditions of the crystals are shown in table 1.
Acknowledgements The authors express their thanks to Professor Y. Nakagawa (Tohoku University) and Drs. K. Siratori (Osaka University), S. Kimura, E. Takayama-Muromachi and N. Iyi (NIRIM) for helpful discussions.
References [11K.
Kato, I. Kawada, N. Kimizuka and T Katsura, Z. Knst. 141 (1975) 314 [21 N. Kimizuka and E. Takayama. J. Solid State Chem. 40 (3981) 109. [311. Shindo. N. Kimizuka and S. Kimura. Mater. Res. Buli. 11(1976) 637. [41 S. Takekawa, private communication. [51T. Akashi, K. Matsui, I Okada and T. Mizutani, IEEE Trans. Magnetics MAG-5 (1969) 285. [61 N. Iyi, private communication.
SINGLE CRYSTAL GROWTH OF LuFe2O4, LuFeCoO4 AND YbFeMgO4 BY THE FLOATING ZONE METHOD Junji IIDA
*
insiiiuie for Material Resear(h, Tohoku (‘no cr5115, 2 / 1 Aaiahira,$e,islai, ti0 agi 980, Japan
and Shunji TAKEKAWA and Noboru KIMIZIJKA \atjonal iirsiitute br Re5earch in Inorganic Materials, I I Naniik,. Tsukuha. Iharahi ?0~.Japan Receised 6 September 1989: manuscript received in final form 10 Januar\ 1990
Single cr~stalsof LuFe,04, LuFeCoO4 and YbFeMgO4 (YhFe~04 structure with space group R3m) were grown h~the floating zone method. The atmosphere of C0~ CO gas mixture was used for growth of LuFe~()4cr’sstal, and the flowing air for growing both LuFeCoO4 and YbFeMgO4 crystals. The growth rate was 1 mm h. and the growth direction was perpendicular to the c axis in the hexagonal system for all cases. Single crystals attaining 3 >< 2 X 15 mm, 1 X S x 10 mm and 5 XIX 20 mm were obtained for LuFeX)4, LuFeC 004 and YbFeMgO4. respectively
I. Introduction
2. Growth experiments
The crystal of RFe2O4 (R Y, Ho, Er, Tm, Yb. Lu and In) belongs to the trigonal system, with space group R3m [1]. The crystal has a layer structure consisting of alternating Fe202 and R01 layers perpendicular to the c-axis in the hexagonal system. As the crystal contains 50% of divalent and trivalent iron ions, other divalent cations such as Mg, Co. Mn, Cu and Ztt can occupy the divalent iron site [2]. The magnetic properties of these compounds are very interesting because of their magnetic two-dimensionality in low temperatures. Although single crystals are necessary to study the details of their magnetism, only YFe2O4 and YbFe2O4 have been grown so far by the floating zone method [3.4]. Thus we tried to grow single crystals of LuFe,04, LuFeCoO4 and YbFeMgO4. In this paper. the growth conditions are reported.
2.1
*
Present address’ Department of Physics, Ochanomizu Uni versit~.2 1 1 Otsuka. Bunkyo, Tokyo 112. Japan
0022 0248
90 $03 50
1990
General
The crystal growth experiment was performed h~ using an infrared radiation convergence type apparatus with a radiation source of 1.5 kW halogen lamp [5]. In this apparatus. the growth atmosphere can be controlled precisely. Powders of CoO (99.9%). Lu,05 (99.9%), Yb~O5 (99.9%), MgO (99%) and Fe2O~ (99.99%) were used as raw materials and the reagents except CoO were heated in air at 1000°C for I day in order to eliminate the absorbed gas such as H-,O. 2.2. LuFe,04 The mixture of Lu2O4/Fe2O~ 1/2 in mole ratio was reacted at 1200°C under C0/H1 3.0 mixed gas atmosphere for approximately 6 h, and quenched to ice temperature. The obtained polycrystalline LuFe2O4 was then crushed and ground into fine powders. This powder was charged into a
Elsesier Science Publishers By, (North Holland)
J.
Jida et al. / Single crystal growth of LuFe ~O4,LuFeCoO
rubber tube and pressed hydrostatically 2 tounder obtaina pressure of approximately 1000 kg/cm a rod of 7 mm in diameter and 70 mm in length. The rod was sintered at 1200°C for 2 h under the same atmosphere as described above. It is well-known that the stability field of compounds containing transition metal ions is strongly dependent upon the temperature and the oxygen partial pressure. The CO 2 CO gas mixture was used for an atmosphere in crystal growth of LuFe2O4, because a CO2 H2 mixture whose mixing ratio is near unity generates much water, giving an undesirable effect on single crystal growth. The optimum mixing ratio of CO2/CO was determined by the trial-and-error method. The crystal was grown at a rate of 1 mm/h in all experiments. The solidified rod in C02/CO 2.5 was cut, polished and observed by using a differential interference microscope and X-ray diffraction method. The specimen does not contain any inclusions and is a mono-phase of LuFe2O4. In addition, it turned out from chemical analysis that the quenched part of the molten zone consisted of Lu0 5Fe3 430X. This suggests that LuFe2O4 melts incongruently, and thus the traveling solvent floating zone method should be adopted to grow single crystals of LuFe2O4 under a gas mixture with C02/CO 2.5. As shown in fig. 1, the obtained boule was typically 5 mm in diameter and 50 mm in length, and composed of several grains. It was difficult to grow larger single crystal grains in only one crystal growth experiment. Therefore crystal growth experiments were carried out several times using a new feed rod. Since this material melts incongruently, the grown crystal and the quenched molten zone at the end were used as a seed crystal and a first molten zone, respectively, in the next growth experiment. A similar procedure was applied in the growth of the other two compounds. The crystal of LuFe2O4 can be cleaved easily along the growth direction. Fig. 2 shows a cleaved surface which was etched in 20% HC1 aqueous solution at 50°Cfor 13 mm and photographed using a phase contrast microscope. The etch pits take on a triangular shape. Also, the three-fold symmetric pattern was obtained in a Laue photograph by X-ray. The cleavage plane is thus determined to be (001)
4 and YbFeMgO4 by FZM
399
a
b
C
~
~
Fig. 1. Portion of as-grown single crystal boules of (a) LuFe2O4, (b) LuFeCoO4 and (e) YbFeMgO4 Except for YhFeMgO4, several single crystal grains are contained in each of them. Marker represents 1 cm
plane. The crystal of LuFe2O4 was grown perpendicular to its c-axis. The same relation in growth direction and shape of etch pits holds for the other two compounds.
=
—~
_____
Fig. 2. The triangular etch pits on the (001) cleavage surface of LuFe.8J4 single crystal.
400
J. Jida et al.
Single crystal growth of LuFe ,04, LuFeCoO
4 and YbFeMgO4 by FZM
gruently. (Fe,Mg)O and Yb201 crystallize alternately, whereas YbFe1 ±5Mg1 P4 solidifies evenly
Table I Lattice parameter and growth conditions of each crystal Lattice parameter a (A) LuFe2O4 LuFeCoO
3.429 4 3.419 YbFeMgO4 3.423
c
(A)
25.25 25.29 25.18
Melting Growth point atmosphere (° C) 1300 1490 1630
C02/CO Air Air
2.5
2.3. LuFeCoO4 and YbFeMgO4 In order to grow single crystals of LuFeCoO4 and YbFeMgO4, mixtures of the ratio Lu 203. Fe2O3 : CoO 1: 1: 2 and Yb203: Fe2O3: MgO 1: 1: 2 in mole, respectively, were pressed into a rod and sintered at 1380°C and 1590°Cfor 2 h under oxygen atmosphere. Although oxygen was tried at first as the growth atmosphere, it was found that bubbles were formed in the molten zone, leading to a remarkably unstable molten zone. It has been known that the occurrence of bubbles in the molten zone can be suppressed by reducing the partial oxygen pressure [6]. Thus we examined flowing air with a linear velocity of 1.8 mm/s, and found that this improved the conditions very much, as expected. In the case of LuFeCoO4 crystal growth, the quenched molten zone consists of Lu0 86Fe105 Co1 28°r’ and this composition is close to that of the solidified part LuFeCoO4. This is very different from the case of LuFe2O4, whose molten zone consists of Lu05Fe3430~.The difference in the compositions between the solidified and molten part is smaller in LuFeCoO4 than in LuFe2O4. Since large single crystal grains were grown more easily in the case of LuFeCoO4 than in LuFe2O4, this difference may have some relation to the difficulty in growing single crystals. In the case of YbFeMgO4, the initially solidified part in the growth experiment was composed of YbFe1 ~~Mg1 X04, (Fe,Mg)0, Yb203 and YbFeO3, indicating that YbFeMgO4 melts incon-
throughout the whole region of the boule. It is assumed that YbFeMgO4 was reduced to some extent at the melting temperature in air and de3~Fe2~Mg 2~. composed into YbFe 1c04, (Fe Mg)O and Yb2O1. In order to grow a single crystal of YbFe1÷~Mg1~O4 in air, the value of x was determined on the specimen by means of EPMA to be 0,12. Based on these, we prepared a sintered rod with composition of YbFe1 12Mg0 8804 and carried out single crystal growth experiments of this compound in air. A crystal of YbFe1 12Mg088 04 was grown more easily than in the case of LuFe2O4. One of the reasons of the difficulty met in single crystal growth of RFe2O4 is the difficulty involved in controlling the atmosphere. Homogeneous control of oxygen partial pressure in the molten zone is not easy because the crystal grows very fast. The lattice parameters and the growth conditions of the crystals are shown in table 1.
Acknowledgements The authors express their thanks to Professor Y. Nakagawa (Tohoku University) and Drs. K. Siratori (Osaka University), S. Kimura, E. Takayama-Muromachi and N. Iyi (NIRIM) for helpful discussions.
References [11K.
Kato, I. Kawada, N. Kimizuka and T Katsura, Z. Knst. 141 (1975) 314 [21 N. Kimizuka and E. Takayama. J. Solid State Chem. 40 (3981) 109. [311. Shindo. N. Kimizuka and S. Kimura. Mater. Res. Buli. 11(1976) 637. [41 S. Takekawa, private communication. [51T. Akashi, K. Matsui, I Okada and T. Mizutani, IEEE Trans. Magnetics MAG-5 (1969) 285. [61 N. Iyi, private communication.