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Growth of Tb0.27DY0.73Fe2 magnetostrictive single crystals
j........ C R Y S T A L
Journal of Crystal Growth 128 (1993) 1092-1094 North-Holland
GROWTH
Growth of Yb0.27DY0.73Fe 2 magnetostrictive single crystals * Q i a n g Li, Y.L. Z h a n g , R . Z . Y u a n , X.H. H u a n g a n d D.J. Jin Advanced Material Research Institute, Wuhan University of Technology, Wuhan, Hubei 430070, People's Republic of China
Tb0.27DY0.73Fe 2 single crystal is a new kind of excellent magnetostrictive material with high As at room temperature. We have grown large-size Tb0.27DY0.73Fe2 (Terfenol) single crystals completely without contamination using a magnetic levitation cold crucible CZ (Czochralski) technique. In the growth by this method, the starting materials Tb (99.95 wt%), Dy (99.98 wt%), Fe and Mn (99.99 wt%) are used to synthesize the precursor alloy. The weight of each loading is generally 80-100 g. A pulling rate of 1-10 mm/h and a rotation of 5-20 rpm are chosen. The single crystals of Tb0.27DY0.73Fe2 grown under such conditions usually have a growth direction close to [112]. In order to improve the magnetostrictive properties of the Terfenol single crystal, Mn-doped alloy (Tbo.27DYo.73Fe2_xMnx) single crystal is also prepared under similar conditions. The difference between the growth conditions is discussed also in this paper.
1. Introduction Since the discovery of large magnetostrictive strains in dysprosium, terbium and their alloys with iron [1,2], attention has been given to Tbo.27Dyo.73Fe 2 which is highly magnetostrictive at room temperature. This alloy is a superior material for applications by virtue of its high Curie temperature and in some cases, lower magneto-crystalline anisotropy, suitable for devices at room temperature with relatively small applied field [3]. It has been the object of a series of experimental studies since it is a desirable candidate for use in commercial transductive devices, ultrosonic tranducers and microstoning devices, etc. [4,5]. As the magnetic anisotropies are encountered in this special alloy, grain-orientation crystal or single crystal is essential for achieving high magnetostriction. It is known that single crystal Tbo.27Dyo.73Fe 2 can be obtained by an induction vertical float zone or Bridgman-
* Supported by the National Key Laboratoryof Crystal Materials, Shandong University, and by the National Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences.
Stockbarger method [6]. However, the contamination of the alloy by the container materials makes it difficult to get high-quality single crystals. The magnetic levitation cold crucible, reported by Hukin [7], has been used to eliminate the contamination from the ordinary crucible materials. By using a cold crucible, the CZ technique makes it possible to grow bulk Tbo.27Dyo.73Fe 2 single crystals in certain directions. In order to study the effect of Mn in Tbo.27Dyo.73Fe 2 materials, Mn-doped single crystal (Tbo.27Dyo.73Fe2_xMnx), which can be used to examine the magnetic properties of Mn substitution for Fe, has also been prepared in our laboratory.
2. Experiments The magnetic levitation cold crucible, was recommended by D.A. Hukin and manufactured by Crystalox Ltd. (UK). It is in a column shape, and has been cut into 16 pieces with a hole in the middle of the bottom. The water-in and -out tubes have been set in each insulating piece. The whole crucible is made of copper, gold-coated to
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Qiang Li et al. / Growth of Tbo.27DYo.73Fe2magnetostrictive single crystals"
improve its conductivity and protect it from contamination. The size of the cold crucible is 25 mm in diameter and 25 ml in volume. The purities of the starting materials are 99.95% for Tb, 99.98% for Dy and 99.99% for both Mn and Fe. Hukin's cold crucible has been used not only for crystal growth, but also for alloy synthesis. Tb0.27DY0.73 Fe 2 alloy can be directly synthesized from the starting materials in the cold crucible. The weight of each loading is generally 80-100 g. The power supply of the RF generator is about 18 kW. Pure argon (99.999 wt%) should be filled in the chamber to keep the melting alloy from oxidation. The formal condition for CZ grown to Tb0.27Dy0.73Fe2 adopted in our experiments is 50-10 m m / h for pulling rate and 15-20 rpm for rotation. Tb0.zyDyo.73Fez_xMnx single crystals can be prepared under similar conditions. The introduction of Mn, causes the growth to become more difficult.
3. Results and discussion
(1) T h e sizes of single crystals of Yb0.27Dy0.v3Fe2 are usually about 5-15 mm in diameter (see fig. 1). At the very beginning of crystal growth, a highly pure iron bar (0.5 mm diameter) was used as a seed crystal. We could obtain single crystal Tb0.27DY0.v3Fe2 using the neck-in and elimination method. Single crystal seeds of Ybo.27Dyo.73Fe2 single crystal with direction [112] were then produced. Yb0.z7DY0.73Fe 2 alloy can be synthesized directly from the starting materials in Hukin's cold crucible. The advantage of this method is that it can reduce the contamination during the process of alloy preparation. When melted, the liquid alloy just levitates over the bottom of the cold crucible (see fig. 2). Un-
Fig. 1. Single crystal of Tbo.27DYo.v3Fe2.
1093
Fig. 2. Growth with cold crucible.
melted solids and impurities are removed automatically from the surface of the liquid alloy. That will benefit both the crystal seeding and growth processes. As the cold crucible is out of contact with the melting alloy, there will be no contamination from the crucible material during the alloy preparation process and crystal growth. The melting alloy should be kept for no less than 15-30 min to make their composition homogenized and uniform before seeding. The rotation rate should be controlled in the range of 15-20 rpm. At such a rate, a seed crystal can be put into the liquid and the growing interface of the crystal is flat and uniform. The pulling rate is generally controlled at 5-10 m m / h . When the pulling rate is over 15 r a m / h , the quality of crystal (such as dislocations, density, etc.) obviously becomes worse. Over 20 m m / h , the seed crystal will easily part contact with the alloy melt. The constituent of the crystal is usually Tb0.zvDY0.v3Fel.98_2.01 under such growth conditions. The purity is better than 99.9%. (2) The substitution of Mn in the Terfenol alloy generally makes crystal growth by the CZ technique more difficult. A suitable amount of Mn (x = 0.1-0.2 at%) will improve its magnetostrictive properties and Curie temperature, as well as the growth speed and crystal quality. Otherwise excessive Mn will certainly impair both. If the substitution of Mn is over 0.3 at%, there will be more than one phase in the annealed single crystals. That will be very harmful to their
1094
Qiang Li et al. / Growth of Tbo.27DYo.73Fe2magnetostrictive single crystals
magnetic properties. With the addition of Mn, the melting alloys should be kept for a longer time before the growth of the crystal. However, the longer the time, the more the volatile loss of the rare earth metals. The heating time is about 30-45 min, according to our experience. Comparing with the growth of Tb0.27DY0.73Fe2 single crystal, the pulling and rotation rate for the Mn-doped crystal cannot be higher than the former. The higher the pulling rate, the more likely the seed crystal will separate from the melting alloy. In our experiments, the rotation rate has remained between 5 and 10 rpm, while the pulling rate should be as slow as possible in order to make the crystal grow under a stable environment. If the pulling rate is higher than 5 m m / h , the homogeneity of the crystal composition and their mechanical properties will become worse. An additional harmful phase and too many micro defects may exist in the prepared single crystal. Therefore, the typical CZ condition should be 1-3 m m / h for pulling and 5-10 rpm for rotation rate. The overall dimension of Mn-doped Terfenol single crystal is usually about 3 - 6 mm in diameter grown under the mentioned conditions (see fig. 3). (3) Both the Tb0.27Dy0.73Fe 2 and the Tb0.27Dy0.73Fe2_~Mn x single crystals should be annealed for about 200 h at 1000°C under vacuum conditions. Otherwise, the crystal would
Fig. 3. Single crystal of Tb0.27DY0.73Fel.9Mn0. I.
break perpendicularly to be growth direction. Both of the single crystals, grown under such conditions, usually have a grown direction close to [112], sometimes with a deviation of 3°-10 °. Chemical and X-ray analyses showed nearly single phase RFe 2 single crystal with a high purity no less than 99.9 wt%. Measuring with single crystal of Tb0.a7DY0.73Fe2, the saturation applied magnetic field can be reduced to less than 2 kOe, and As over 1500 ppm (under a stress of 4 MPa). Fundamental measurements for the effects of Mn on the properties of the single crystal of Yb0.27DY0.73Fe2 have been made using special equipment. The relating data and results about it will be reported later.
Acknowledgments The authors would like to thank Professor H.C. Chen and D.L. Shun, Shandong University, and Professor J.Y. Li and Dr. X.G. Zhao, Institute of Physics, Chinese Academy of Sciences, for their help and cooperation.
References [1] A.E. Clark, H.T. Savage and R.M. Bozarth, Phys. Rev. 138 (1965) 216. [2] A.E. Clark, Ferromagnetic Materials, Vol. 1 (North-Holland, Amsterdam, 1980) ch. 7, pp. 533-534. [3] A.E. Clark, J.P. Teter and O.D. McMasters, IEEE Trans. Magnetics MAG-23 (1987) 3526. [4] V. Joyce, J.S. Abell, R.D. Greenough and K.C. Pitman, J. Magnetism Magnetic Mater. 54-57 (1986) 877. [5] S.W. Meks and R.W. Timme, J. Acoust. Soc. Am. (1977) 1158. [6] O.D. McMasters, G.E. Holland and K.A. Gschneider, J. Crystal Growth 43 (1978) 577. [7] D.A. Hukin, British Patent No. 3,702,368.
Journal of Crystal Growth 128 (1993) 1092-1094 North-Holland
GROWTH
Growth of Yb0.27DY0.73Fe 2 magnetostrictive single crystals * Q i a n g Li, Y.L. Z h a n g , R . Z . Y u a n , X.H. H u a n g a n d D.J. Jin Advanced Material Research Institute, Wuhan University of Technology, Wuhan, Hubei 430070, People's Republic of China
Tb0.27DY0.73Fe 2 single crystal is a new kind of excellent magnetostrictive material with high As at room temperature. We have grown large-size Tb0.27DY0.73Fe2 (Terfenol) single crystals completely without contamination using a magnetic levitation cold crucible CZ (Czochralski) technique. In the growth by this method, the starting materials Tb (99.95 wt%), Dy (99.98 wt%), Fe and Mn (99.99 wt%) are used to synthesize the precursor alloy. The weight of each loading is generally 80-100 g. A pulling rate of 1-10 mm/h and a rotation of 5-20 rpm are chosen. The single crystals of Tb0.27DY0.73Fe2 grown under such conditions usually have a growth direction close to [112]. In order to improve the magnetostrictive properties of the Terfenol single crystal, Mn-doped alloy (Tbo.27DYo.73Fe2_xMnx) single crystal is also prepared under similar conditions. The difference between the growth conditions is discussed also in this paper.
1. Introduction Since the discovery of large magnetostrictive strains in dysprosium, terbium and their alloys with iron [1,2], attention has been given to Tbo.27Dyo.73Fe 2 which is highly magnetostrictive at room temperature. This alloy is a superior material for applications by virtue of its high Curie temperature and in some cases, lower magneto-crystalline anisotropy, suitable for devices at room temperature with relatively small applied field [3]. It has been the object of a series of experimental studies since it is a desirable candidate for use in commercial transductive devices, ultrosonic tranducers and microstoning devices, etc. [4,5]. As the magnetic anisotropies are encountered in this special alloy, grain-orientation crystal or single crystal is essential for achieving high magnetostriction. It is known that single crystal Tbo.27Dyo.73Fe 2 can be obtained by an induction vertical float zone or Bridgman-
* Supported by the National Key Laboratoryof Crystal Materials, Shandong University, and by the National Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences.
Stockbarger method [6]. However, the contamination of the alloy by the container materials makes it difficult to get high-quality single crystals. The magnetic levitation cold crucible, reported by Hukin [7], has been used to eliminate the contamination from the ordinary crucible materials. By using a cold crucible, the CZ technique makes it possible to grow bulk Tbo.27Dyo.73Fe 2 single crystals in certain directions. In order to study the effect of Mn in Tbo.27Dyo.73Fe 2 materials, Mn-doped single crystal (Tbo.27Dyo.73Fe2_xMnx), which can be used to examine the magnetic properties of Mn substitution for Fe, has also been prepared in our laboratory.
2. Experiments The magnetic levitation cold crucible, was recommended by D.A. Hukin and manufactured by Crystalox Ltd. (UK). It is in a column shape, and has been cut into 16 pieces with a hole in the middle of the bottom. The water-in and -out tubes have been set in each insulating piece. The whole crucible is made of copper, gold-coated to
0022-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
Qiang Li et al. / Growth of Tbo.27DYo.73Fe2magnetostrictive single crystals"
improve its conductivity and protect it from contamination. The size of the cold crucible is 25 mm in diameter and 25 ml in volume. The purities of the starting materials are 99.95% for Tb, 99.98% for Dy and 99.99% for both Mn and Fe. Hukin's cold crucible has been used not only for crystal growth, but also for alloy synthesis. Tb0.27DY0.73 Fe 2 alloy can be directly synthesized from the starting materials in the cold crucible. The weight of each loading is generally 80-100 g. The power supply of the RF generator is about 18 kW. Pure argon (99.999 wt%) should be filled in the chamber to keep the melting alloy from oxidation. The formal condition for CZ grown to Tb0.27Dy0.73Fe2 adopted in our experiments is 50-10 m m / h for pulling rate and 15-20 rpm for rotation. Tb0.zyDyo.73Fez_xMnx single crystals can be prepared under similar conditions. The introduction of Mn, causes the growth to become more difficult.
3. Results and discussion
(1) T h e sizes of single crystals of Yb0.27Dy0.v3Fe2 are usually about 5-15 mm in diameter (see fig. 1). At the very beginning of crystal growth, a highly pure iron bar (0.5 mm diameter) was used as a seed crystal. We could obtain single crystal Tb0.27DY0.v3Fe2 using the neck-in and elimination method. Single crystal seeds of Ybo.27Dyo.73Fe2 single crystal with direction [112] were then produced. Yb0.z7DY0.73Fe 2 alloy can be synthesized directly from the starting materials in Hukin's cold crucible. The advantage of this method is that it can reduce the contamination during the process of alloy preparation. When melted, the liquid alloy just levitates over the bottom of the cold crucible (see fig. 2). Un-
Fig. 1. Single crystal of Tbo.27DYo.v3Fe2.
1093
Fig. 2. Growth with cold crucible.
melted solids and impurities are removed automatically from the surface of the liquid alloy. That will benefit both the crystal seeding and growth processes. As the cold crucible is out of contact with the melting alloy, there will be no contamination from the crucible material during the alloy preparation process and crystal growth. The melting alloy should be kept for no less than 15-30 min to make their composition homogenized and uniform before seeding. The rotation rate should be controlled in the range of 15-20 rpm. At such a rate, a seed crystal can be put into the liquid and the growing interface of the crystal is flat and uniform. The pulling rate is generally controlled at 5-10 m m / h . When the pulling rate is over 15 r a m / h , the quality of crystal (such as dislocations, density, etc.) obviously becomes worse. Over 20 m m / h , the seed crystal will easily part contact with the alloy melt. The constituent of the crystal is usually Tb0.zvDY0.v3Fel.98_2.01 under such growth conditions. The purity is better than 99.9%. (2) The substitution of Mn in the Terfenol alloy generally makes crystal growth by the CZ technique more difficult. A suitable amount of Mn (x = 0.1-0.2 at%) will improve its magnetostrictive properties and Curie temperature, as well as the growth speed and crystal quality. Otherwise excessive Mn will certainly impair both. If the substitution of Mn is over 0.3 at%, there will be more than one phase in the annealed single crystals. That will be very harmful to their
1094
Qiang Li et al. / Growth of Tbo.27DYo.73Fe2magnetostrictive single crystals
magnetic properties. With the addition of Mn, the melting alloys should be kept for a longer time before the growth of the crystal. However, the longer the time, the more the volatile loss of the rare earth metals. The heating time is about 30-45 min, according to our experience. Comparing with the growth of Tb0.27DY0.73Fe2 single crystal, the pulling and rotation rate for the Mn-doped crystal cannot be higher than the former. The higher the pulling rate, the more likely the seed crystal will separate from the melting alloy. In our experiments, the rotation rate has remained between 5 and 10 rpm, while the pulling rate should be as slow as possible in order to make the crystal grow under a stable environment. If the pulling rate is higher than 5 m m / h , the homogeneity of the crystal composition and their mechanical properties will become worse. An additional harmful phase and too many micro defects may exist in the prepared single crystal. Therefore, the typical CZ condition should be 1-3 m m / h for pulling and 5-10 rpm for rotation rate. The overall dimension of Mn-doped Terfenol single crystal is usually about 3 - 6 mm in diameter grown under the mentioned conditions (see fig. 3). (3) Both the Tb0.27Dy0.73Fe 2 and the Tb0.27Dy0.73Fe2_~Mn x single crystals should be annealed for about 200 h at 1000°C under vacuum conditions. Otherwise, the crystal would
Fig. 3. Single crystal of Tb0.27DY0.73Fel.9Mn0. I.
break perpendicularly to be growth direction. Both of the single crystals, grown under such conditions, usually have a grown direction close to [112], sometimes with a deviation of 3°-10 °. Chemical and X-ray analyses showed nearly single phase RFe 2 single crystal with a high purity no less than 99.9 wt%. Measuring with single crystal of Tb0.a7DY0.73Fe2, the saturation applied magnetic field can be reduced to less than 2 kOe, and As over 1500 ppm (under a stress of 4 MPa). Fundamental measurements for the effects of Mn on the properties of the single crystal of Yb0.27DY0.73Fe2 have been made using special equipment. The relating data and results about it will be reported later.
Acknowledgments The authors would like to thank Professor H.C. Chen and D.L. Shun, Shandong University, and Professor J.Y. Li and Dr. X.G. Zhao, Institute of Physics, Chinese Academy of Sciences, for their help and cooperation.
References [1] A.E. Clark, H.T. Savage and R.M. Bozarth, Phys. Rev. 138 (1965) 216. [2] A.E. Clark, Ferromagnetic Materials, Vol. 1 (North-Holland, Amsterdam, 1980) ch. 7, pp. 533-534. [3] A.E. Clark, J.P. Teter and O.D. McMasters, IEEE Trans. Magnetics MAG-23 (1987) 3526. [4] V. Joyce, J.S. Abell, R.D. Greenough and K.C. Pitman, J. Magnetism Magnetic Mater. 54-57 (1986) 877. [5] S.W. Meks and R.W. Timme, J. Acoust. Soc. Am. (1977) 1158. [6] O.D. McMasters, G.E. Holland and K.A. Gschneider, J. Crystal Growth 43 (1978) 577. [7] D.A. Hukin, British Patent No. 3,702,368.