Magnetostriction of the 2D Orthogonal Dimer Spin System
SrCu 2 (B0 3 ) 2 under Pulsed High Magnetic Fields
Yoshiki Sawai*, Yasuo Narumi1*, Tetsuya Takeuchi**, Masahiro Matsumiya1,Hiroshi Kageyama1, Kazuyoshi Yoshimura1, Yoshitami Ajiro{, Masayuki
Hagiwara* and Koichi Kindo1**KYOKUGEN, Osaka University, Toyonaka, Osaka 560-8531, Japan"fInstitute for SolidState Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan**Low Temperature Center, Osaka University, Toyonaka, Osaka 560-0043, Japan^Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, JapanAbstract. We have measured the magnetostriction on the 2D orthogonal dimer system SrCii2(B03)2 under pulsed magnetic
fields up to about 55 T, in order to investigate the lattice instability in the field regions where 1/4 and 1/3 magnetization
plateaus appear. Theoretically, superstructure is expected to be formed at each plateau region and it is considered that spinlattice effects are necessary to fully describe the spin density profile of the plateaus. We observe the lattice deformation whichaccompanies the increase in magnetization.
Keywords: magnetostriction, high magneticfields,SrCii2(B03)2, spin-lattice coupling
PACS: 75.10.Jm,75.80.+q

INTRODUCTION

tion experiments on SrCu2(BOs)2 under pulsed magneticfields which reach the 1/4 and 1/3 plateau regions.

After the discovery of the 2D orthogonal dimer spin
system SrCu2(BOs)2 [1], a large number of experimental and theoretical research have been done on this material. SrCii2(B03)2 has a dimerized singlet ground state
separated from the excited triplet states by a finite gap
A ^ 35 K, as reported in the magnetic susceptibility [2]and in inelastic neutron scattering [3]. Application of an
external magnetic field changes the density of the triplet
excitations. Many interesting phenomena have been observed on this material under high magnetic fields. Experimentally, magentization plateaus were observed at
1/8, 1/4 and 1/3 of the fully saturated moment [4] andthe magnetic superlattice for the 1/8 plateau with broken translational symmetry was dicrectly observed by
NMR experiments [5]. The superstructure at each magnetization plateau was proposed theoretically. The 1/3and 1 /4 plateaus are expected to have magnetic superstructures of stripe form, while the 1/8 plateau has arhomboid cell [6]. Miyahara et al. considered the S = 1 /2
orthogonal dimer model coupled to adiabatic phonons,
and suggested that the superstructure in the rhomboid
unit cells for the 1/8 plateau was qualitatively consistent with NMR results [7]. These results indicate a presence of a spin-lattice coupling to SrCu2(B03)2, and it
is expected that a lattice deformation occurs under magnetic field with the appearance of a magnetization andthe change of the magnetic superstructures for the 1/8,
1/4 and 1/3 plateaus. We report results of magnetostric-EXPERIMENTThe high-field magnetostriction of SrCu2(BOs)2 wasmeasured at T = 1.3 K with strain gauges and a multilayer pulse magnet at KYOKUGEN, Osaka University.
The pulsed magnetic field was produced by the dischargeof a capacitor bank which has a maximum energy is
1 MJ. The maximum field of the long pulse magnet is55 T. The pulse duration is about 20 msec. The magnetic fields were applied along the [100] direction andthe magnetostrictions along the [100] and [110] directions were measured. The uniaxial magnetostriction wasmeasured using strain gauges glued onto the surface ofthe sample and a quartz dummy sample. The sample anddummy gauges were connected in series in order to compensate background signals, and remaining field-induced
signals were canceled using a compensation coil. Twosets of data were recorded changing polarity of currents
and were subtracted to extract the voltage change AV/V
due to the magnetostriction e (= AL/L) of the sample.

RESULTS AND DISCUSSION
The magnetostriction along the [100] and [110] direction show the same tendencies. Figure 1 shows the mag-CP850, Low Temperature Physics: 24th International Conference on Low Temperature Physics;
edited by Y. Takano, S. P. Hershfield, S. O. Hill, P. J. Hirschfeld, and A. M. Goldman© 2006 American Institute of Physics 0-7354-0347-3/06/$23.00
1053
Downloaded 05 Aug 2009 to 130.34.226.199. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp

netostriction which is parallel to the [100] direction under pulsed magnetic fields applied up to about 55 T along
the [100] direction, at 1.3K, with the magnetization curveof SrCu2(B03)2. There is almost no length change below about 11 T. The magnetostriction begins to increase
monotonically from 11 T to about 35 T. Comparable hysteresis is observed in the magnetostriction and the magnetization vs magnetic field. While there isn't a plateau
at the field region of the 1/8 magnetization plateau. It isthought that the increase of the magnetostriction which
is linearly for magnetic field from 11 T to 35 T is not
dominated by the change of the magnetic superstructure
but by the appearence of magnetization.
1.3K:

H||[i,c ,0]\\ ^ 1ykk

<.2•

j^

REFERENCES
1. H. Kageyama, K. Yoshimura, R. Stern, N. V. Mushnikov,K. Onizuka, M. Kato, K. Kosuge, C. P. Slichter, T. Goto
and Y. Ueda, Phys. Rev. Lett, 82, 3168-3171 (1999).
2. H. Kageyama, K. Onizuka, T. Yamauchi, Y Ueda,
S. Hane, H. Mitamura, T. Goto, K. Yoshimura andK. Kosuge, J. Phys. Soc. Jpn., 68, 1821-1823 (1999).
3. H. Kageyama, M. Nishi, N. Aso, K. Onizuka,T. Yosihama, K. Nukui, K. Kodama, K. Kakurai andY Ueda, Phys. Rev. Lett., 84, 5876-5879 (2000).
4. K. Onizuka, H. Kageyama, Y Narumi, K. Kindo, Y Ueda
and T. Goto, J. Phys. Soc. Jpn., 69, 1016-1018 (2000).
5. K. Kodama, M. Takigawa, M. Horvatic, C. Berthier,H. Kageyama, Y. Ueda, S. Miyahara, F. Becca andF. Mila, Science, 298, 395-399 (2002).
6. S. Miyahara and K. Ueda, Phys. Rev B, 61, 3417-3424(2000).
7. S. Miyahara, F. Becca and F. Mila, Phys. Rev B, 68,024401 (2003).

i-1-0:

ii:
0\\102030

405060Magnetic field [T]

FIGURE 1. The high-field magnetostricition along the [100]direction of SrCii2(B03)2 under magnetic field applied along
the [100] direction at T = 1.3 K, with the magnetization under
the same conditions.
The magnetostriction curve has a plateau from 35 T
to 43 T where the 1/4 magnetization plateau appears
and the length change above 43 T is very small. Whileit exhibits no jumps at 35 T and 43 T where the magnetization jumps. It is suggested that the lattice deformation is small in the field range between the 1 /4 and1/3 plateaus. These results support the idea that latticechanges stabilize the magnetization superstructure at thehigh magnetic field 1/3 and 1/4 plateaus. We can observe the change of the structure at each plateau indirectly by measuring the magnetostriction.

ACKNOWLEDGMENTS
This work was supported by The 21 st Century COE
Program named AgTowards a new basic science: depthand synthesisi\h, and by Giant-in-Aid for Science Research from the Japanese Ministry of Education, Science,Sports, Culture and Technology.

1054
Downloaded 05 Aug 2009 to 130.34.226.199. Redistribution subject to AIP license or copyright; see http://proceedings.aip.org/proceedings/cpcr.jsp