実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report

提出日 Date of Report

課題番号 Project No.

装置責任者 Name of responsible person

2012A0071

Prof. Yasuhiro Miyake

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

muSR study on frustrated spin system (CuCl)La(Nb(1-x)Tax)2O7

D1

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Seiko Kawamura

31 May 2012 - 03 June 2012

所属 Affiliation
J-PARC Center, JAEA
試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
(CuCl)LaTa2O7powder 3g(CuCl)LaNb2O7 powder 3g

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

(CuCl)LaB2O7 (B = Nb or Ta) is quantum spin systems on a
two-dimensional square lattice. In these systems, spin
frustration is expected to operate due to competition between
the nearest and next-nearest exchange interactions, J1 and J2,of the magnetic Cu ions on the square lattice. The magnetic
ground state of (CuCl)LaNb2O7 has been suggested to bespin-singlet with an excitation gap of ΔE ~ 2.3 meV [1],while (CuCl)LaTa2O7 exhibits an antiferromagnetic (AFM)
order with collinear spin structure below TN = 7 K [2]. The
Magnetic phase diagram ofphase diagram for their mixed compound is reported as Fig. 1 :(CuCl)La(NbTa
P stands for the1-xx)2O7.
shown in Fig. 1 [2]. We proposed μSR experiments onparamagnetic state, S the spin-singlet state and(CuCl)La(Nb1-xTax)2O7 for several Ta concentration x in order
CAF collinear AFM ordered state.

12. 実験方法及び結果（つづき） Experimental method and results (continued)
to understand their spin dynamics which reflects different
ground states and commonly operating spin frustration.
However, in the present experiment, we concentrated on thetwo pure systems, (CuCl)LaNb2O7 and (CuCl)LaTa2O7
because of limited beamtime.
The ZF- and LF-μSR measurements on powder samplesof the above-mentioned systems were carried out at D1.
Powder sample (~3 g for each) shaped into a disk was fixed
onto a sample holder of a vertical flow-type 4He cryostatand cooled down to ~2 K. Longitudinal field up to 1500 G Fig. 2 :ZF-μSR(CuCl)LaTa2O7.
was applied.

timespectrafor

ZF-μSR time spectra for (CuCl)LaTa2O7 are shown inFig. 2. The static internal field due to nuclear dipoles isobserved above 15 K, and the muon spin relaxation is
gradually enhanced as temperature approaches TN = 7 K by acritical slow-down effect. Below TN, rapid damping of theasymmetry is observed due to the long-range AFM order. Ina previous μSR study on this material using a DC muon
beam by Uemura et al., clear muon spin precession isobserved [3] with high precession frequencies which areunobservable in the pulsed μSR setup. Our ZF-μSR result isFig. 3 : LF-μSR time spectra under LF of 100
consistent with the previous one though the fast oscillation is G for (CuCl)LaTa2O7averaged. In the LF-μSR time spectra just above TN, themuon spin depolarization due to strong spin fluctuation was expected. However, the obtained time spectra
indicate rather static internal field as shown in Fig. 3. It is necessary to do quantitative discussion based on theresults of the LF decoupling measurements. On the other hand, the μSR time spectra for (CuCl)LaNb2O7 showno remarkable enhancement of the muon spin relaxation down to ~2 K in contrast to (CuCl)LaNb2O7, reflecting
different magnetic ground state. Detailed analysis will be proceeded.
References[1] H. Kageyama et al., J. Phys. Soc. Jpn. 74 (2005) 1702.
[2] A. Kitada et al., Phys. Rev. B 80 (2009) 174409.
[3] Y. J. Uemura et al., Phys. Rev. B 80 (2009) 174408.

2