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。 ）承認日 Date of Approval 2017/10/16
承認者 Approver Takashi Ohara
提出日 Date of Report 2017/10/16

Experimental Report
課題番号 Project No.

装置責任者 Name of Instrument scientist

2016B0151

Takashi Ohara

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Determinationofthemagneticstructureofa

layered

BL-18

ExtremeEnvironment

Singleantiferromagnet EuMnBi2 with quasi 2D Dirac fermions

Crystal Neutron Diffractometer (SENJU)

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Hideaki Sakai

2 Feb 2017 – 10 Feb 2017

所属 Affiliation
Osaka 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.
EuMnBi2 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.

Dirac fermions in magnetic materials have attracted a growing interest, since the quantum anomalous Halleffect was observed at zero fields in magnetic topological insulator films. To further explore such distinct
quantum transport phenomena enriched by magnetic order, we have recently focused on a layered bulk
antiferromagnet EuMnBi2, where the Bi layer hosting quasi 2D Dirac fermions and the magnetic blocking layer(Eu and Mn-Bi layers) stack alternatively (see Fig. 1b). In this material, the antiferromagnetic order of Eu
sublattice has a marked impact on the Dirac fermion transport. However, the detailed magnetic structure hasremained unclear, which needs to be clarified to seek the origin of strong coupling between the Dirac fermionsand magnetic order.
In this experiment, to determine the type of the antiferromagnetic order of both Eu and Mn sublattices in
EuMnBi2, we were supposed to perform the neutron diffraction measurements on the single-crystalline
sample (inset to Fig. 1a) at magnetic fields. On the first day, we started the initial cooling of a superconducting
magnet. However, a needle valve equipped with the variable temperature insert (VTI) was blocked at some

2. 実験方法及び結果（つづき） Experimental method and results (continued)
the blocking at all.
On the second day, we stopped using the superconducting magnet and gave up the measurements atmagnetic fields. Instead, we decided to perform the measurements for magnetic structural analyses on theantiferromagnetic phase at zero field. We remounted the single-crystalline sample in the GM type fridge. This
fridge worked well during the rest of beam time and enabled us to measure the neutron diffraction at zero field
at 4 K (below TN) and at 30 K (above TN), where TN denotes the antiferromagnetic transition temperature ofEu sublattice.
Figure 1a shows the neutron diffraction intensity on the (H 0 L) plane at 4 K. As denoted by red arrows, weclearly observed the (H 0 L) reflections with H+L=odd, which completely disappear above TN. These
reflections hence arise from magnetic scattering associated with the antiferromagnetically ordered Eumoments. By checking other extinction rules, we have determined a plausible type of antiferromagnetic orderof Eu sublattice; the Eu moments order ferromagnetically in the ab plane and align along the c axis in thesequence of up-up-down- down, where the Bi square net intervenes between the Eu layers with magnetic
moments up and down (Fig. 1b). This result is consistent with what was deduced from the resonant x-ray
magnetic scattering. Full magnetic structural analyses considering both Eu and Mn sublattices are ongoing atpresent.

Fig. 1: a. Reciprocal-space map of neutron diffraction intensity for a single-crystalline EuMnBi2 measured atzero field. Red arrows denotes the magnetic scattering from the antiferromagnetically ordered Eu moments.
Inset shows a photograph of the measured sample. b. Plausible magnetic structure for EuMnBi2.