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
課題番号 Project No.

提出日 Date of Report
装置責任者 Name of responsible person

2015A0183
実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Investigation of High Magnetic Field Phases in multi-ferroic

BL10

Ba2CoGe2O7 with nematic interaction

実施日 Date of Experiment

実験責任者名 Name of principal investigator

2016/06/11-13

Hiroyuki Nojiri

2017/01/26-28

所属 Affiliation
Institute for Materials Research, 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.
Ba2CoGe2O7

Single Crystal

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.

The purpose of this proposal is to investigate the high magnetic field phase in two-dimensionalsquare lattice antiferromagnet Ba2CoGe2O7, in which a magnetic field induced electric polarization
is observed. [Phys. Rev. Lett. 105, 137202 (2010)] The Neel order appears below TN=6.7 K and astaggered antiferromagnetic structure in the (001) plane with slight canting is found. Although themagnetic structure is trivial, it is found that the magnetization curve is not linear to external
magnetic field.
We have measured the field dependence of (1, 0, 0) magnetic peak at by applying the magnetic
field along the c-axis. The figures show the temperature dependence of (1, 0, 0) magnetic peak andthe comparison of TOF spectrum measured at 25 T with two zero field spectra. We have also tried
other magnetic fields, but the statistics is too low to judge their field variations. It was caused by
the cut off of the beam power to 150 kW for the target trouble. In this beam power, we need 3-4times longer accumulation time and an future extension in another proposal is needed to complete
this task.

12. 実験方法及び結果（つづき） Experimental method and results (continued)

As shown in the figures, we found the large increase of the magnetic peak intensity at 25 T. The
uniform moment is about 69-65 % at this field and the antiferromagnetic moments contributing to
the Bragg peak should be reduced. The experimental result is not very simple in the sense that theintensity variation does not meet with the simplest expectation. This is partly caused by the loss ofthe canting moment, but the domain redistribution must be checked by the zero field experiment
for more quantitative analysis. We also confirmed that there is no clear shift of the wave vector
between 0 and 25 T, but we found a small change of the TOF peak between 4.2 K and 2.5 K. This
point will be examined in successive proposal.
In summary, the non-monotonic field dependence of the magnetic Bragg peak is found in therange of the field where a non-linear magnetization was observed. However, this change isconfirmed to have no change of the magnetic wave vector in the resolution of the present
experiment. The successive experiments is needed to examine the field dependence of the magnetic
structure between 25 T and 35 T.

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