ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) 491–492

Spin correlation and ﬁeld induced staggered magnetization in
the 2D orthogonal dimer spin system SrCu2 ðBO3Þ2
K. Kodama, K. Arai, M. Takigawa*, H. Kageyama1, Y. Ueda
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, JapanAbstract
We report the results of nuclear magnetic resonance (NMR) experiments on 11 B nuclei in SrCu2 ðBO3 Þ2 : The NMR
shift at a magnetic ﬁeld of 7 T shows a staggered component, which is ascribed to the buckling of the CuBO3 layers
giving rise to the staggered magnetization and the staggered hyperﬁne coupling. The temperature variation of theorientation dependence of the shift indicates that the staggered magnetization grows signiﬁcantly below 20 K; where theuniform magnetization decreases exponentially due to formation of the dimer singlet ground state.
r 2003 Elsevier B.V. All rights reserved.
PACS: 75.20.Ck; 75.30.Gw; 75.50.Ee; 76.60.Cq
Keywords: SrCu2 ðBO3 Þ2 ; Orthogonal dimer; NMR; Field-induced staggered magnetization

SrCu2 ðBO3 Þ2 has a two-dimensional network oforthogonal dimers of Cu2þ with spin-12; which isequivalent to the Shastry–Sutherland spin model [1]. It
has the dimer singlet ground state [2] and, in highmagnetic ﬁelds, shows plateaus in the magnetization
curve at 18; 14 and 13 of the fully saturated moment [3,4].
These plateaus have been considered as signature forlocalization of triplets forming superlattices. RecentNMR experiments [5] indeed conﬁrmed such a superlattice in the 18-plateau phase with a large rhomboid unit
cell containing 16 spins per layer.
Although these properties can be largely understoodby an isotropic Heisenberg spin model on the Shastry–
Sutherland lattice, small perturbations speciﬁc to thismaterial not included in the Shastry–Sutherland modelcause interesting phenomena. For example, the splittingof the triplet excitation energies observed by ESR [6] andneutron scattering [7] have been ascribed to theDzyaloshinskiy–Moriya (DM) interaction between twospins on neighboring dimers [7]. Another feature is thenon-coplanar structure of the CuBO3 layers. The upper*Corresponding author. Tel./fax: +81-4-7136-3225.
E-mail address: masashi@issp.u-tokyo.ac.jp (M. Takigawa).
1
Present address: Department of Chemistry, Faculty ofScience, Kyoto University, Sakyou-ku, Kyoto 606-8502, Japan.

and lower parts of Fig. 1 schematically show thestructure of the CuBO3 layer viewed along the c-axis
and the ð1 1% 0Þ-direction, respectively. The latter showsbuckling of the CuBO3 layers. The buckling invalidates
the inversion symmetry at the center of a dimer bond,giving rise to the DM interaction between two spins on
the same dimer. The buckling also results in tilting of theprincipal axes of the g-tensor. For the Cu(1) site inFig. 1, for example, one principal axis is along the ½1 1% 0
direction, while the other two make an angle f withthe [0 0 1] (c direction) and the [1 1 0] directions,
respectively. If the CuBO3 layers were ﬂat f should bezero. The g-tensors for other Cu sites are obtained bysuccessive rotation by 90 around the c-axis, yielding
alternation of the principal axes. This means that theuniform magnetic ﬁeld induces staggered magnetization
with four sublattices. The intradimer DM interaction
mentioned above also contributes to the staggeredmagnetization.
We have performed 11 B NMR measurements to detect
the ﬁeld-induced staggered magnetization. Fig. 2 shows
the hyperﬁne magnetic shift at the B(1) sites as afunction of the direction of the external ﬁeld rotated inthe ð1 1% 0Þ plane. By symmetry, the shift at the B(2) sites
is obtained by interchanging y with y: At 100 K; theextremal of the angular pattern is located 9 away from0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2003.11.205

ARTICLE IN PRESS
K. Kodama et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 491–492

492

0.1
(110)

Cu(3)

B

K (%)

B(4)

O

11Cu(4)
Cu(1)

0Cu

B(2)
B(1)

Cu(2)

H=7T

0.05

B(3)

-0.05
2.5K
-0.1

6K
10K

-0.15
c20K
100K

Cu(2)

Cu(1)
B(1)

Cu(3,4)

-0.2
-100

B(2)

(110)

Fig. 1. Schematic structure of CuBO3 layer in SrCu2 ðBO3 Þ2viewed along the c-axis (upper panel). Atomic arrangement ofCuBO3 layer along the [1 1 0]-direction (lower panel, not incorrect scale).

y ¼ 0 or 90. This deviation means different shift values
for the B(1) and B(2) sites, i.e., a staggered componentof the shift. Since the shift tensor is a product of thelocal magnetization and the hyperﬁne coupling tensor,staggered component in either or both of them will
result in staggeredness of the shift. What is remarkable isthat the position of the extremal moves rapidly below
20 K: Since the hyperﬁne coupling tensor should benearly independent of temperature, such strong temperature dependence can be explained only by rapid
growth of the staggered magnetization below 20 K: It isquite surprizing that large staggered magnetization
remains even at 2:5 K; where the uniform magnetizationis negligibly small due to formation of the singlet groundstate.

-500

θ (deg.)50100

Fig. 2. Angular dependence of the shift 11 K of B(1) site
obtained at various temperatures with the ﬁeld of 7 T applied
perpendicular to ½1 1% 0: y is the angle between the ﬁeld and thec-axis.

This work was supported by the Grant-in Aid for
Scientiﬁc Research on Priority Area (B) on ‘‘FieldInduced New Quantum Phenomena in Magnetic Systems’’ from the ministry of Education, Culture, Sports,
Science and Technology, Japan.

References[1] B.S. Shastry, B. Sutherland, Physica B 108 (1981) 1069.
[2] K. Kodama, et al., J. Phys.: Condens. Matter 14 (2002)
L319.
[3] H. Kageyama, et al., Phys. Rev. Lett. 82 (1999) 3168.
[4] K. Onizuka, et al., J. Phys. Soc. Jpn. 69 (2000) 1016.
[5] K. Kodama, et al., Science 298 (2002) 395.
[6] H. Nojiri, et al., J. Phys. Soc. Jpn. 68 (1999) 2906.
[7] O. C!epas, et al., Phys. Rev. Lett. 87 (2001) 167205.