Š111‹ oriented and twin-free single crystals of Terfenol-D grown
by Czochralski method with cold crucible
Guang-heng Wu, Xue-gen Zhao, Jing-hua Wang, Jing-yuan Li, Ke-chang Jia,
and Wen-shan Zhan
State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080,
People’s Republic of China
~Received 3 March 1995; accepted for publication 25 July 1995!
The growth of twin-free single crystals of Tb0.3Dy0.7Fe2 with ^111& orientation by the Czochralski
method with cold crucible is reported. Various characterizations demonstrated the twin-free and
single-crystalline quality of the crystals. The optimized growth parameters have been
experimentally found to avoid constitutional supercooling. Decanting method was used to provide
direct evidence that the unstable growth is the origin of the dendrite growth in this material and
made the previous growth of ^111& single crystal a failure. © 1995 American Institute of Physics.
Early studies by Clark and co-workers1 have established
that the rare-earth iron compound of Tb0.3Dy0.7Fe2, known as
Terfenol-D, exhibits very large magnetostrictive strains and
has many potential applications. Since optimum magnetostrictive properties are found for single crystals, various
growth techniques, such as Bridgman method,2 floating-zone
method, and Czochralski method ~CZ!3– 6 have been used to
attempt a single-crystal growth, especially in ^111& orientation in which the magnetostriction is the greatest. Unfortunately, ^111& direction oriented single crystal of this material
has not been grown by those techniques for at least 10 years
since the Terfenol-D material was developed. Under previous
growth conditions, it is only possible to grow crystals along
^112& direction and the crystals were usually occupied by
lamellar twins with the plated face parallel to $111%.7 It was
believed that such ^112& twinned growth was caused by the
dendrite.
Recently, our investigations on crystal growth of this
material in ^112& and ^100& directions indicated that the origin which caused such dendrite growth was an unstable
growth state, constitutional supercooling.8 With the high
axial temperature gradient and the strong electromagnetic
stirring, the CZ method with cold crucible system is an ideal
technique to overcome the constitutional supercooling in a
nonstoichiometric growth environment. In this letter, we report that single crystals of Tb0.3Dy0.7Fe2 along ^111& growth
orientation and with twin-free quality have been grown successfully by Czochralski technique with an induction-heated
magnetic levitation cold crucible.
Starting materials of composition Tb0.3Dy0.7Fex , with
x51.80–1.95, were prepared from metals of the purity of
99.95%. ^111& oriented single-crystal bars ;232310 mm3
were used as seeds. Crystals were grown by the MCGS-3 CZ
instrument. The growth rates of 15–25 mm/h and the rotation rate of 30 rpm were adopted. Owing to the special convection style and the large radial temperature gradient in a
cold crucible system, a normal flat growth interface might
become concave during growth. For the present compositions of the melts, it may initiate a constitutional supercooling at the growth interface. Therefore, the elimination of interface conversion was a prime consideration for determining
growth parameters.
Appl. Phys. Lett. 67 (14), 2 October 1995
Single crystals of Tb0.3Dy0.7Fe2 with ^111& axial orientation were obtained with diameters of 6 –12 mm and a maximum length of 80 mm. The single-crystal rods, as shown in
Fig. 1, usually exhibit flat growth interfaces and symmetric
facets indicating the threefold symmetry of ^111& direction
along the growth axis and good growth conditions. The twinfree single crystallinity of the crystals has been determined
by the characterizations with x-ray detection, metallographic
examinations, and scanning electron microscopy. The characteristic diffraction peaks of ~111!, ~222!, and ~333! were
obtained solely by the x-ray diffraction experiments on the
transverse sections of the crystals. Dozens of the same and
perfect Laue spot photographs ~one is shown in Fig. 2! were
obtained by the x-ray Laue back-reflection technique from
different positions across one transverse section and from
different transverse sections cut from the crystals. Usually
the characteristic pattern of twins can be verified easily in
Laue photographs,8 but no sign of twins was observed in our
single-crystal samples. Some crystals contain netlike defect
patterns known as Widmanstantten precipitates ~WSP! in the
parts close to the seeds. Various reagents for this material,
such as Vilellar’s ~1 g picric acid in 5 ml HCl plus 95 ml
FIG. 1. Single-crystal rods of Tb0.3Dy0.7Fe2 with ^111& growth axes grown
by CZ technique with cold crucible. Grid size is in 2 mm.
0003-6951/95/67(14)/2005/3/$6.00
© 1995 American Institute of Physics
2005
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FIG. 4. Equilateral triangular islands on the growth surface decanted in a
stable growth state.
FIG. 2. Laue x-ray diffraction pattern of the transverse section of the ^111&
Tb0.3Dy0.7Fe2 single crystal.
methanol! and Nital’s ~2% nitric acid in ethanol! were employed to carefully examine those WSP or WSP-free
samples. Because some samples are without WSP, above reagents could not etch any conventional pattern on them, but
their quality of twin-free single crystallinity had been demonstrated. On the samples with WSP, equilateral trianglelike
networks on the ~111! faces, as shown in Fig. 3, are quite
different to the parallelogramlike WSP pattern on the ~112!
face.6 The pyramidal etch hillocks which appeared on the
~111! faces are similar to those of some other fcc materials,
such as Si and CdTe on their ~111! faces.
Our experimental results indicated that the major obstacle to growing the ^111& single crystal of Tb0.3Dy0.7Fe2 by
the CZ method is the occurrence of the constitutional supercooling. That occurs even with a flat growing interface. Employing the compositions mentioned above, the residual rare
earth is about 3–10 mol % in the starting materials. It would
increase up to 20 mol % in the melt left at the end of growth
because stoichiometric crystals of Tb0.3Dy0.7Fe2 were grown
from the nonstoichiometric melts. In order to avoid the unstable growth state in such a nonstoichiometric environment,
FIG. 3. WSP patterns and hillocks on ^111& oriented transverse section
~etched by the Vilellar reagent!.
2006
Appl. Phys. Lett., Vol. 67, No. 14, 2 October 1995
an empirical combination of growth conditions has been
adopted to dynamically retain a large ratio of the temperature
gradient to the growth rate for achieving a stable growth. It
mainly includes: ~1! the growth rates were decreased gradually from 25 mm/h to about 15 mm/h in the later growth
stages; ~2! the crystal rotation rates and the rf power were
increased slowly in the whole growth process and meanwhile, ~3! the temperature gradient was retained consistently
by adjusting the crucible position in relation to the inducting
coil. This combination allowed the growth to avoid constitutional supercooling and improve the quality of crystals, so
that various dimensions of ^111& single crystals of
Tb0.3Dy0.7Fe2 could be grown in a high reproducibility.
The decanting method9 has been used in the present
work to reveal the affection of the interface stability to the
crystal growth. In the different growth periods, the growing
crystals were suddenly extracted out of the melts to prepare
decanted samples. By scanning electron microscopy, it has
been found that equilateral triangular islands, as shown in
Fig. 4, always appear on a flat interface, indicating a normal
^111& growth of cubic structure.10 On the contrary, a typical
dendrite morphology ~Fig. 5! which corresponds to the ^112&
twinned structure in grown crystal appears when the decanted surface becomes concave. Correspondingly, high density of rare-earth-rich inclusions were observed in the related
area by chemical etching. This strongly suggests that the
constitutional supercooling would happen once a concave
interface formed. Thus, ^112& oriented dendrite occurred and
FIG. 5. Typical dendritic morphology of the growth surface decanted in a
constitutional supercooling situation.
Wu et al.
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replaced the original ^111& growth. Based upon the early
studies on metals,9 the ^112& dendrite seems the characteristic interface morphology of Tb0.3Dy0.7Fe grown in the unstable state. These decanted results provide direct evidence
that the origin of ^112& dendrite growth in Tb0.3Dy0.7Fe is
constitutional supercooling. It inhibits the growth of normal
^111& crystals.
In summary, the ^111& oriented single crystals of
Tb0.3Dy0.7Fe have been obtained using CZ method with cold
crucible. Various characterizations demonstrated the twinfree single-crystal quality of the crystals. The empirical combination of growth parameters was experimentally found to
eliminate the constitutional supercooling and support a reproducible growth of the single crystals. The decanting
method revealed the characteristic interface morphologies
formed in the stable and the unstable growth states, respectively. The results provided direct evidence that the origin
which causes the dendrite growth and then results in the
Appl. Phys. Lett., Vol. 67, No. 14, 2 October 1995
failure of growth of the ^111& single
Tb0.3Dy0.7Fe2 is constitutional supercooling.
crystal
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Wu et al.
2007
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