Physica B 294}295 (2001) 68}70

High-"eld ESR measurements of SrCu (BO )Shojiro Kimura *, Susumu Hirai , Yasuo Narumi , Koichi Kindo ,Hiroyuki Nojiri, Hiroshi Kageyama, Kenzo Onizuka, Yutaka Ueda
KYOKUGEN, Osaka University, Machikaneyama 1-3, Toyonaka 560-8531, Japan
CREST, Japan Science and Technology Corporation (JST), Kawaguchi 332-0012, Japan
Institute of Material Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan
Institute of Solid State Physics, University of Tokyo, Roppongi 7-22-1, Tokyo 106-8666, JapanAbstract
High "eld ESR measurements of the S" two-dimensional spin gap system SrCu (BO ) have been performed in thefrequency region from 526 to 1620 GHz using pulsed high magnetic "elds up to 55 T. Besides the ESR signal which isobserved near the EPR line, a number of new ESR signals are observed in the magnetic phase, where the plateau at  of
the saturated magnetization appears on the magnetization curve.  2001 Elsevier Science B.V. All rights reserved.
Keywords: Magnetization plateau; S" two-dimensional spin gap system; High "eld ESR measurement

1. Introduction
Recently, the two-dimensional spin gap system
SrCu (BO ) has attracted much attention. Thismaterial has a stacked-layer structure, composed ofalternating stacks of CuBO and Sr planes alongthe c-axis [1]. In the CuBO plane, Cu> ionswith S", form dimer units, and an orthogonalarrangement of these dimers is realized in theplane perpendicular to the c-axis. Various experimental and theoretical studies have revealed thatSrCu (BO ) shows interesting characteristics,
re#ecting this peculiar spin structure. This material
has a spin singlet ground state with an energy gap,
and the intra- and the inter-dimer exchange interaction, symbolized by J and J, and the energy gap
* Corresponding author. Tel.: #81-6-8501-6687; fax: #816-8501-6662.
E-mail address: kimura@hmf.mag.rcem.osaka-u.ac.jp (S. Kimura).

E were estimated to be J "100 K, J"68 K andE "34.7 K, respectively [3,5]. It was theoreticallypointed out that the ground state of this material
can be exactly described by the direct product ofthe singlet dimers, and the triplet excitations areextremely localized [2,3]. One of the most interesting features of this material is the magnetization
plateaus under an external magnetic "eld, which isconsidered to originate from the extremely localized nature of the triplet excitations [2}4,6}8]. In thehigh-"eld magnetization measurements, plateaus at,  and  of the saturated magnetization were observed [2,4]. Theoretical studies suggested thatthe occurrence of the plateaus is a consequence ofthe crystallization of the triplet dimers [6}8].
Moreover, it was predicted that the superstructuresof the triplet dimers of stripetype occur at themagnetization plateaus of  and  [6}8].

Recently, submillimeter wave ESR measurements of SrCu (BO ) using pulsed magnetic "elds


0921-4526/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 6 1 0 - 4

S. Kimura et al. / Physica B 294}295 (2001) 68}70

69

up to 30 T, were performed by Nojiri et al. [5].
These authors observed the direct transitions fromthe ground state to the "rst excited triplet state.
Moreover, a second-gap excitation and a numberof ESR absorption lines with zero "eld gap wereobserved in these measurements. In order to investigate the high-"eld phase of this material, we have
performed high-"eld ESR measurements ofSrCu (BO ) in the frequency region from 526 to1620 GHz using the pulsed high magnetic "eld upto 55 T.
2. Experimental
The high-"eld ESR measurements of SrCu (BO )single crystal were performed at KYOKUGENOsaka university using a far-infrared laser and thepulsed magnet up to 55 T. A magnetically enhanced
InSb detector made by QMC instruments wasused. An external magnetic "eld was applied along
the c-axis.
3. Results and discussion
Fig. 1 shows the frequency dependence of theESR absorption lines observed for Hc at 1.4 K.
The dashed lines show the critical "elds observedby the previous high-"eld magnetization measurements at 1.4 K [4]. We de"ne the critical "elds asfollows: a "nite magnetization appears above H ,and ,  and  plateaus of the magnetization occur in the region H (H(H , H (H(H , andH (H, respectively. In this measurement, almostno ESR signal is observed below H due to thelimitation of the sensitivity of our ESR system.
However, the ESR signal A , which appears above
H , is observed near the resonance "eld of theelectron paramagnetic resonance (EPR). As thefrequency is increased, the resonance "eld ofthe ESR signal A increases, and the intensity ofthis ESR signal increases with increasing resonance"eld. Re#ecting the magnetic phase transition,
the ESR signal A suddenly disappears at H ,while another ESR signal A appears justabove H . As in the case of the ESR signal A , the
resonance "eld of ESR signal A increases with
increasing the frequency. The ESR signal splits intoFig. 1. Frequency dependence of the ESR absorption lines observed for Hc at 1.4 K. The dashed lines show the critical "eldsobserved by the previous high-"eld magnetization measurements.

two absorption lines above H , where the  plateauof the magnetization is observed. Moreover, in thismagnetic phase, ESR signals B accompanied bya number of sharp structures are observed in thefrequency region from 716 to 977 GHz. These ESR
signals appear only well above the critical "eld H .
Fig. 2 shows the temperature dependence of theESR absorption lines observed at 847 GHz for Hc.
The observed temperature dependences are di!erent between the ESR signals A (i"1, 2) and B. AsGthe temperature is increased, the intensities of theESR signals A and A decrease, suggesting thatthese ESR signals correspond to the excitations
from the ground state, while the ESR signals
B broaden rapidly. Thus, the temperature dependence of the intensity of the ESR signals B is notclear. The ESR signal P, which appears above
2.2 K, is considered to come from the transition
between the excited states. Fig. 3 shows the frequency}"eld relation of the ESR signals observed
at 1.4 K for Hc. The ESR signals A 's and B areGshown by the open circles and the open diamonds,respectively. The magnetization curve observed at
1.4 K for Hc [4], is shown by the solid line in70

S. Kimura et al. / Physica B 294}295 (2001) 68}70

Fig. 2. Temperature dependence of the ESR absorption lines
observed at 847 GHz for Hc.

Fig. 3. The frequency}"eld relation of the ESR signals observed
at 1.4 K for Hc. The ESR signals A 's and B are shown by theopen circles and the open diamonds, respectively. The magnetization curve observed at 1.4 K for Hc are shown by the solid line.

Fig. 3. The ESR branch A strongly deviates from
the EPR line of g"2.28, which was determined by
the EPR measurement at 25 K [5], just above H .
This ESR branch was also observed by the submillimeter wave ESR measurement of Nojiri et al. [5].
The magnetization process shows an anomalous
behavior in that the "nite magnetization appears atH "14.5 T which is much lower than the critical"eld H "22.6 T, calculated from E "g H .
The observed frequency dependence of the ESRbranch A may re#ect the "nite mixing between theinitial spin-singlet ground state and the excitedstates, which is inferred from the magnetization
process [2,4], or it may suggest the possibility of"eld induced antiferromagnetic order around H .
An anomaly of the ESR branch can be seen at H ,suggesting the discontinuous change of the spin
structure at H . In the magnetic phase H (H(H of the  plateau, the ESR branch A slight
ly deviates from the EPR line to higher magnetic"eld. In the range H (H(H , however, the ESRbranches A and A almost coincide with the EPRline. The origin of the ESR branch shown by theopen circle is not clear at the moment, but
we consider that this ESR branch may be due to thespin #ip of the triplet dimer. Next we discuss theESR signals B observed in the magnetic phase ofthe  plateau. We speculate that these ESR signals
come from the excitations of the dimers such as thetransitions of the triplet to the singlet dimer state or
those of the singlet to the triplet dimer state. However, it is di$cult to calculate the energies of suchexcitations at the moment; in order to explain theESR signals, the theory of the excitations in themagnetic phase of -plateau is required.
In conclusion, high-"eld ESR measurements ofSrCu(BO ) using pulsed magnetic "elds up to
55 T have been performed. The ESR signal isobserved near the resonance "eld of EPR in themagnetic phase above H . Moreover, a number ofnew ESR signals are observed in the magnetic
phase of the -plateau for the "rst time. These ESR
signals suggest the presence of magnetic excitationspeculiar to the magnetic phase of the -plateau.
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