Journal of Crystal Growth 206 (1999) 65}67

Crystal growth of the two-dimensional spin gap system
SrCu (BO )H. Kageyama, K. Onizuka, T. Yamauchi, Y. Ueda*
Material Design and Characterization Laboratory, The Institute for Solid State Physics, The University of Tokyo, Roppongi 7-22-1, Minato-ku,
Tokyo 106-866, Japan
Received 1 March 1999; accepted 28 May 1999
Communicated by T. Nishinaga

Abstract
Our high-quality large single crystals of SrCu (BO ) have been successfully grown by a travelling solvent #oatingzone (TSFZ) method using LiBO as a solvent. We present the growth condition and the result of the temperature
dependent magnetic susceptibilities measured along the directions parallel and perpendicular to the tetragonal caxis.  1999 Elsevier Science B.V. All rights reserved.
PACS: 81.10
Keywords: SrCu (BO ) ; Single crystal; TSFZ method

1. Introduction
For theorists in the "eld of condensed matterphysics, models achieving an exact dimer groundstate have been one of the interesting subjects [1,2].
However, in spite of vigorous e!orts of experimentalists, no materials realizing the models havebeen found. Quite recently, it was found that
SrCu (BO ) is a two-dimensional spin gap systemwith the exact dimer ground state [3,4], which
indeed realizes the Shastry}Sutherland model[5,6]. This cuprate crystallizes in space group
I4 2m with a unit cell of dimensions: a"8.995 As ,c"6.649 As , and <"537.9 As . The structure is* Corresponding author. Tel.: #81-3-3478-8459; fax: #813-3478-8459.
E-mail address: yueda@kodama.issp.u-tokyo.ac.jp (Y. Ueda)

characterized by a novel two-dimensional magnetic
network of the Cu> ions with S" (topologicallyequivalent to the Shastry}Sutherland lattice), in
which neighboring Cu>}Cu> dimers connect orthogonally. Furthermore, quantized plateaux at and  of the full Cu> moment were observed in themagnetization. The previous experiments, however,were performed using a polycrystal so that large
single crystals of high quality have been indispensable for further elucidation of the intrinsic nature of
the ground state as well as the excited states. Although Smith and Keszler obtained single crystals
by a #ux method using LiBO [7], the crystal size
(0.20 mm;0.08 mm;0.03 mm) is not enough formore sophisticated physical measurements. Toachieve this goal, we employed a vertical travelling
solvent #oating zone (TSFZ) method using LiBO
as a solvent. Consequently, we obtained su$ciently

0022-0248/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 3 1 3 - 9

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H. Kageyama et al. / Journal of Crystal Growth 206 (1999) 65}67

large single crystals, and investigated the magnetic
properties using the obtained crystals.

2. Experimental procedure
Since SrCu (BO ) melts incongruently in air ata peritectic melting point of 9703C, a solutiongrowth technique with the aid of an appropriate
solvent is necessary for the purpose of its crystal
growth. Polycrystalline SrCu (BO ) was preparedby a solid state reaction. Starting materials of
SrCO , CuO and B O with 99.99% purity were mixed, ground, followed by heat treatment at9003C in #owing oxygen for 1 week with several
intermediate grindings. The resulting blue powder
was isostatically water-pressed under 400 kg/cm
in the form of rods (6}10 mm in diameter and&100 mm in length), yielding densities of about
70%. The rods were subsequently sintered at 8503C
for 1 day under oxygen #ow. We also prepareda solvent disk (2}3 mm in thickness) composed ofa mixture of LiBO and SrCu (BO ) in the weight
ratio of 1 : 3.
The crystal growth was carried out using
FZ-T10000N 10 KW high-pressure type (CrystalSystem. Inc.) with four halogen lamps as heat sources. The solvent disk was placed between the upper
and the lower rods. By gradually increasing thelamp power, the solvent disk was molten. Immediately, the upper rod was moved downwards inorder to establish a #oating zone between the lower
and the upper rods. Growth rates were0.3}1.0 mm/h and both rods were rotated at about
10 rpm in opposite directions to secure the liquidhomogeneity as well as the homogeneous temperature distribution within the liquid zone. Only oxygen gas (1 atm, 99.99%) was allowed to #ow during
the growth process.
The X-ray powder di!raction pattern indicateda single-phase product. Single crystallinity of thegrown material was checked by means of Laue
X-ray back-re#ection. The temperature ¹ dependence of the magnetic susceptibility was measured in
the magnetic "eld of 1.0 T, using a superconducting
quantum interference device (SQUID) magnetometer (Quantum Design, MPMS) in a ¹ range,
2 K(¹(350 K.

3. Result and discussion
We have succeeded in growing crystals ofSrCu (BO ) . The crystals obtained are dark blueand easily cleaved along the tetragonal ab plane as
con"rmed by Laue X-ray back-re#ection. Fig. 1(a)shows a photo of a typical crystal rod (6 mm indiameter) containing a single domain 43 mm long,where the feed rod with 6 mm diameter and thesolvent LiBO with 0.05 g weight were applied togrow this crystal. In Fig. 1(b), we show a single
domain (6 mm;6 mm;3 mm) with the glossy
cleavage plane, i.e., the ab plane. It was found thatthe crystal size is entirely dependent on the crystallization rates. When a crystallization rate wasFig. 1. The representative photos of a crystal rod (a) and a single
domain (b) of SrCu (BO ) . The upward arrow in (a) indicates
the starting point of the crystal growth.

H. Kageyama et al. / Journal of Crystal Growth 206 (1999) 65}67

Fig. 2. The temperature-dependent magnetic susceptibilities ofSrCu (BO ) , s (䊐) and s (䉭). The solid line denotes the  ,
susceptibility for the powdered sample [3]. Inset: the susceptibility of the powdered sample (solid) and the calculated isotropic
susceptibility of the crystal (broken).

1.0 mm/h, the crystal grew easily along the ab plane(typically 5 mm;5 mm), while the growth along
the c-axis was exceedingly small (less than 0.5 mm).
In addition, some Laue photos taken along thec-axis have several weak spots around a strong
spot, indicating some kind of imperfections in thesingle crystal.
Accordingly, a slower growth rate of 0.5 mm/h
was then employed. As a result, the crystals becamemonodomain after several centimeters of growth as
seen in Fig. 1(a), and the Laue-photo analysis has
proven a single crystal product without any imperfections. Since the liquid zone contains a solvent
and therefore the composition is di!erent from thatof the crystal and the charge rods, we consider slow
growth rates would be necessary for su$cient di!usion to take place at the solid}liquid interface.
Fig. 2 demonstrates the temperature dependenceof magnetic susceptibilities of SrCu (BO ) , s and  s , in which the magnetic "eld was applied parallel,and perpendicular to the c-axis, respectively. Thesolid line represents the previous data for the powdered sample [3]. The di!erence between s ands is attributable to the anisotropic g-factor as,

67

observed by electron spin resonance (ESR) [8]. At
high temperatures, both s and s obey the Curie},
Weiss law s"Ngk S(S#1)/[3k (¹!h)], where
N denotes Avogadro number, k , the Bohr magneton, k , Boltzmann constant, g, the g-factor of theCu> electron spin, and h, Weiss constant. UsingS" for the Cu> ion and g "2.28 and g "2.07,
determined by the ESR measurement, we "ttedthe experimental data in the temperature range
between 250 and 350 K to the above formula, andobtained h"!102.5 K together with a constantsusceptibility s "!2.603;10\ emu/mol Cu.
Shown in the inset of Fig. 2 is a comparison between the susceptibility of the powdered sample
and the isotropic susceptibility ("s #s ) of   ,the crystal. We would like to emphasize that aCurie-like upturn below 4 K presumably comingfrom defects in the crystal is much smaller than thatfor the powdered sample, clearly showing the highquality of the present crystal.
4. Conclusion
In summary, we have grown single crystals ofSrCu (BO ) by means of a TSFZ method with animage furnace using LiBO as a solvent. The X-ray
di!raction analysis and the relatively small Curielike upturn in the low-temperature susceptibility
ensure high quality of the grown crystal. Other
physical measurements such as nuclear magnetic
resonance, high-"eld magnetization, and neutron
scattering using the crystals are now in progress.
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[2] T. Oguchi, H. Kitatani, J. Phys. Soc. Jpn. 64 (1994) 612, andreferences therein.
[3] H. Kageyama, K. Yoshimura, R. Stern, N.V. Mushnikov,K. Onizuka, M. Kato, K. Kosuge, C.P. Slichter, T. Goto,Y. Ueda, Phys. Rev. Lett. 82 (1999) 3168.
[4] H. Kageyama, K. Onizuka, Y. Ueda, N.V. Mushnikov,T. Goto, K. Yoshimura, K. Kosuge, J. Phys. Soc. Jpn. 67(1998) 4305.
[5] B.S. Shastry, B. Sutherland, Physica B 108 (1981) 1069.
[6] S. Miyahara, K. Ueda, Phys. Rev. Lett. 82 (1999) 3701.
[7] R.W. Smith, D.A. Keszler, J. Solid State Chem. 93 (1991) 430.
[8] H. Nojiri, private communication.