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
2, April, 2013

課題番号 Project No.

装置責任者 Name of responsible person

2012B0048

Touru Ishigaki

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Magnetic structure on electron-doped VO2iMATERIA

実験責任者名 Name of principal investigator

実施日 Date of Experiment

Daisuke Okuyama

21, January, 2013

所属 Affiliation
CMRG, ASI, RIKEN
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.
W-doped VO2: (V0.85W0.15O2) 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.

In the rutile-type VO2, the metal-insulator phase transition takes place accompanying the structure phasetransition with the V-dimerized distortion. Then, the magnetic ordering of V ions is not observed down tolowest temperature. In the W-doped VO2/TiO2 (V1-xWxO2) thin-film, in which V4+ is partially substituted by W6+
with doping the electron in V ion, the W-doped dependence of the transition temperature of metal-insulatortransition was extensively studied. With increasing x-value, the ground state changes from smaller-x insulator
to metal and then to another larger-x insulator. In contrast, the little study of the bulk sample of W-doped VO2
has been investigated only at the composition near Insulator due to the difficulty of synthesizing the sample.
Recently, we successfully synthesized rutile-type V1-xWxO2 powder sample for larger-x insulator phase at

x=0.15. In this sample, the huge decrease of the magnetization induced by the formation of the V-dimer(spin-Peierls transition) is no longer observed, but the antiferromagnetic like anomaly of magnetization isobserved at low temperature as shown. To clarify the origin of the observed anomaly and determine themagnetic ground state, we performed powder neutron diffraction experiment at low temperature.
12. 実験方法及び結果（つづき） Experimental method and results (continued)
The four-gram V0.85W0.15O2 powder sample was filled in V cylinder cell, and mounted on He-closed cycle
refrigerator of iMATERIA. The obtained diffraction data are shown in Fig. 1. The indexes of Fig. 1 arecalculated by using the tetragonal P42/mnm symmetry. At 90 K (blue line), the observed diffraction data canbe well explained by the nuclear scattering from tetragonal P42/mnm symmetry, which is same with that of themetallic phase of VO2. Below TMI~70 K (red line), we cannot unfortunately observe the additional magnetic
diffraction. However, we found structural phase transition, as shown Fig. 1. The (1 0 1) and (1 1 1) reflections
are split into two peaks, which indicate that the structural phase transition from tetragonal to monoclinic. In
the synchrotron x-ray experiment of V0.89W0.11O2 thin-film, the forbidden (h 0 l) with h+l=odd and (0 0 l) with

l=odd reflections are observed. Then, the possible monoclinic space group from the subgroup of P42/mnm isPm, Pc, P2/m, C2/m, or P2/c. From the simulation by using these candidates, it is clarified that the most
reasonable space group is C2/m. In this space group, V ions have two different sites, which indicates apossibility of charge disproportionation. In the larger-x insulator phase, the driving mechanism of insulator
phase is still debated. If two V sites have different bond valence sums, a charge ordering of V ions may takeplace. Therefore, we think there is a possibility of charge-ordering-induced (Mott-Hubbard-type) mechanismof insulator phase for this larger-x (x=0.15) W-doped VO2, in contrast to Spin-Peierls-type mechanism forsmaller-x (same with bulk VO2) insulator phase.

Fig. 1: The neutron diffraction profiles at 6.3 K (red) and 90 K (blue). The anomaly ofthe magnetization curve was observed at ~70 K.

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