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
2014/5/19

課題番号 Project No.

装置責任者 Name of responsible person

2014A0258

Toru Ishigaki

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Study of the magnetic structure in the localized-4d system

BL20

La5Mo4O16

実施日 Date of Experiment

実験責任者名 Name of principal investigator

2014/5/9 13:00 - 2014/5/10 13:00

Ryoichi Kajimoto(1 day)所属 Affiliation
J-PARC Center
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.
3.7 g powder La5Mo4O16.

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.

Generally, with an atomic number increasing, a spin-orbit (LS) coupling becomes significant for physicalproperties. Due to the strong LS coupling, 5d electron systems, for instance, show unconventional phenomena
such as a topological insulator in SrIrO3. On the other hand, in 3d electron systems, it is well known that the LS
coupling can be treated perturbatively as a single-ion anisotropy. However, how the LS coupling works in the 4delectron systems still remains as a question. In addition, localized magnetism of the 4d electrons was only
discovered in few compounds, and thus is not investigated much. Therefore, we are investigating La5Mo4O16, a4d electron material with localized magnetic moment. The most intriguing phenomenon we found in La5Mo4O16
so far is the long-time decay of the magnetization as mentioned below, and to clarify its origin is the purpose ofthis work. Here, we performed the powder neutron scattering experiment on La5Mo4O16 to study the magnetic
structures without an external magnetic field.
From bulk measurements, a phase diagram of La5Mo4O16 is reported [1]. At zero magnetic field there is aphase transition from a paramagnetic to an antiferromagnetic phase at TAF = 190 K, and then another one fromthe antiferromagnetic to a ferrimagnetic phase at TF = 70 K. Since it is difficult to make La5Mo4O16 system, the1

2. 実験方法及び結果（つづき） Experimental method and results (continued)
magnetic structure has not been investigated by neutron scattering. Instead, the magnetic structure of V-doped
La5Mo2.75V1.25O16 was reported; a collinear ferrimagnetic structure develops in ab-plane and it stacks
antiferromagnetically along c-axis [2]. Speculating from La5Mo2.75V1.25O16, we expect that similar magnetic
structures in the antiferromagnetic and ferrimagnetic phases of La5Mo4O16, and to confirm them by means ofneutron diffraction technique is purpose of this experiment.
Neutron diffraction measurements were performed at BL20 iMATERIA using a dual frame. Approximately
3.7 g of La5Mo4O16 was packed into a vanadium can. The can was sealed by an indium wire. Diameter andheight of the can were 6 and 65 mm, respectively. The sample can was filled with 4He gas, which was then set toa closed-cycle refrigerator. Measurements were performed at T = 4, 40, 100, and 220 K.
Figure 1 shows the neutron powder diffraction patterns of La5Mo4O16 measured at (a) T = 4, (b) 100, and (c)220 K. Almost all Bragg peaks in the paramagnetic phase at T = 220 K are indexed by the reported structure of
La5Mo4O16 [3]. There are several magnetic Bragg peaks observed at T = 4 and 100 K in the long range magnetic
ordered state. Contrary to the bulk measurement results, however, the diffraction patterns at T = 4 and 100 K arealmost same; as mentioned above the bulk measurements reported the phase transition from theantiferromagnetic to ferrimagnetic phase at 70 K. Both at 4 and 100 K, magnetic Bragg peaks were observed atQ = 1.135, 1.175, 1.4, and 1.49 Å-1, which are indexed as (-1 1 0.5), (1 1 0.5), (-1 1 1.5), and (1 1 1.5),respectively; we assume the C/2m space group in this report. Since symmetry of La5Mo4O16 is monoclinic,
magnetic propagation vector is k = (0, 0, 0.5). Thus, the model spin structure expected from the bulk
measurements below 70 K, the ferrimagnetic structure with k = (0, 0, 0), is not correct. Our powder diffraction
measurements revealed that the magnetic structure of La5Mo4O16 below TN ~ 190 K is an antiferromagneticstructure, which is very similar to that of La5Mo2.75V1.25O16 [2]: a collinear ferrimagnetic structure in ab-planewith antiferromagnetic stacking along c-axis. We are now doing Rietveld refinements of both the crystal andmagnetic structures to determine the magnetic structure in La5Mo4O16.
[1] K. Kobayashi and T. Katsufuji, Phys. Rev. B 83, (2011) 100411(R).
[2] F. Ramezanipour et al., J. Sol. Stat. Chem. 184, (2011) 3366.
[3] M. Ledesert et al., J. Sol. Stat. Chem. 105, (1993) 143.

Fig. 1. Neutron diffraction patterns of La5Mo4O16 at (a) T = 4, (b) 100, and (c) 220 K using iMATERIA.
Magnetic reflections are indicated by solid arrows.
2