実験報告書様式（一般利用課題・成果公開利用）（ ※本報告書は英語で記述してください。ただし、産業利用課題として採択されている方は日本語で記述していただいても結構です。 ）MLF Experimental Report

提出日 Date of Report
June 30, 2011.

課題番号 Project No.

装置責任者 Name of responsible person

2010B0094

Takashi Kamiyama

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

High-resolution neutron diffraction study of La2O2Fe2OSe2

BL-08

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Sungdae Ji

2011/01/31 11:00 – 2011/02/02 11:00

所属 Affiliation
Tohoku University
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.
Fe-oxychalcogenide (La2O2Fe2OSe2) powder

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.

Recently, a Mott-insulating Fe-oxychalcogenide (La2O2Fe2OSe2) has drawn a lot of attention with respect toan abnormal magnetic ground state behavior. It crystalizes in tetragonal, I4/mmm structure and has magnetictransition at TN = 90 K. The magnetic structure below transition temperature forms double diagonal stripe
magnetic order which is geometrically frustrated up to the 2nd nearest neighbor interaction and has been
exceptionally realized in metallic but iso-structural Fe1+ySexTe1-x among Fe-based superconductors. While themagnetic frustration effect in Fe- is still controversial due to de-localized electrons and lattice distortion
accompanied by the magnetic order, that in La2O2Fe2OSe2 has been believed to be very strong since electron
are localized and lattice distortion has not been observed. There have been experimental reports showing
some precursors of magnetic frustration effect. Curie-Weiss temperature obtained from the inverse magnetic
susceptibility is 650 K seven times higher than TN and we observed a strong spin fluctuation still exists even at
300 K above TN, which implies that magnetic frustration suppresses to form a long-range order below
Curie-Weiss temperature being consistent with the magnetic susceptibility measurement. A heat capacity
measurement showed that the magnetic entropy is not fully released by magnetic transition which indicates12. 実験方法及び結果（つづき） Experimental method and results (continued)
still remains below magnetic transition, but it has not been examined by a microscopic measurement.
Moreover, if we remind that most geometrically frustrated magnets undergo a structural transition when they
form a full or partial magnetic order, it is demanded to re-confirm existence of structural distortion using
higher-resolution diffractometer.
Our main goals are 1) to find a magnetic frustration effects below magnetic transition such as magnetic peakbroadening and 2) to confirm global or local distortion of crystal structure in Fe-oxycha using BL-8 beamline
in J-PARC, a time-of-flight powder diffractometer with high flux and resolution. Since magnetic Bragg peaks
appear mostly in the lower-Q region but lattice distortion effect is more distinctive in the higher-Q region,
three detector banks were utilized simultaneously for the purpose. Temperature dependence of whole powder
patterns cover to from 5 K to 300 K using a closed-cycle refrigerator.
Fig. 1 shows a selected region of the powder pattern at T = 5 K with the lower bank. While the peak profile ofnuclear Bragg peaks is a shape of Gaussian and resolution limited, that of magnetic Bragg peaks is a shape ofLorentzian and broader than nuclear Bragg peaks. This indicates that magnetic order La2O2Fe2OSe2 issuppressed by geometrical magnetic frustration in magnetically ordered phase. More quantitative analysis is
now on process. However, as shown in Fig. 2, it is confirmed that any clear change of lattice distortion below
and above TN was not observed even using the back scattering bank which is the option of highest
instrumental resolution in BL-8. The result gives us clear evidences that La2O2Fe2OSe2 is the first system
realizing a frustrated double diagonal stripe magnetic order without a lattice distortion and moreover, it willshed light on understanding of geometrical frustration effect on newly emerged Fe-based superconductors.

Fig. 1: Selected region of powder pattern at 5 K

Fig. 2: powder patterns of 5 K and 300 K in high‐Q region

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