実験報告書様式（一般利用課題・成果公開利用）

MLF Experimental Report
課題番号 Project No.

提出日 Date of Report
1 June, 2016
装置責任者 Name of responsible person

2015A0209

Kenji Nakajima

実験課題名 Title of experiment

装置名 Name of Instrument/（BL No.）

Observations of spin-orbit-lattice excitations in frustrated spinel

AMATERAS (BL14)

MgV2O4 with orbital degree of freedom by single-crystal inelastic

実施日 Date of Experimentneutron scatteringfrom 22 February to 1 March, 2016

実験責任者名 Name of principal investigator
Keisuke Tomiyasu
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.
Spinel MgV2O4, 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.

I. Introduction
Frustrated magnets are promising sources for exotic spin liquid/liquid-like states. Spinel AB2O4 provides
the representative spin-frustrated systems, in which magnetic B ions form a corner-sharing tetrahedral latticecalled a pyrochlore lattice. Insulating spinel MgV2O4 bears orbital degree of freedom [V3+: d2, spin S = 1, (t2g)2]
and its spin-orbital-coupled frustration has attracted much interest both theoretically and experimentally.
Although the stoichiometric sample successively exhibits rather complex spin ordering and orbital-latticeordering, a tiny amount of excess Mg holds the system disordered without any specific-heat anomaly at leastdown to 2 K [A. T. M. Nazmul Islam et al. (2012)].

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2. 実験方法及び結果（つづき） Experimental method and results (continued)
Interestingly, recent ultrasound measurements found a novel macroscopic property in this disordered
MgV2O4, that is, several elastic moduli significantly soften with decreasing the temperature from at least 150 K
to 30 K [T. Watanabe et al. (2014)]. Because there is no phase transition, this lattice softening is not due to thewell-studied precursor phenomena to structural transition. Further, the sample exhibits spin freezing at Tf = 13 K,
hence, the softening to 30 K does not go towards the spin freezing. Instead, the ultrasound results arequantitatively analyzed as thermal activation effects of excitations around 5 meV to 15 meV, which are probably
novel spin-orbit-lattice-coupled excitations.
Thus, by single-crystal inelastic neutron scattering, we observed the overall spin excitations up to ~ 30meV, including this suggested energy range, in the disordered MgV2O4.
II. Experimental method
Standard inelastic neutron scattering experiments were performed using CCR on BL14 (AMATERAS).
III. Results & discussion
As shown in Figs. (a) and (b), the orientation-averaging
spectrum structure extending along energy axis is observed.
The scattering is gapless quasielastic at 15 K, which opens a
finite gap ~2 meV at 4 K below Tf = 13 K. This excitation
continues at least 30 meV (not shown).
Figure(c)showstheorientation-resolvedQdependence, sliced at constant energy. No energy dependence
is detected within the entire measured energy range (notshown). The character of intensity pattern is well reproducedby that calculated for antiferromagnetic nearest-neighbordimer or tetramer (tetrahedron), as shown in Fig. (d). These
two models show almost the same patterns so cannot be
distinguished. Further, these models are spatially rather
isotropic, which is consistent with the fact that the ultrasoundmeasurements detect no special symmetry for the excitations

Figure. Experimental orientation-averaging
data measured at 4 K (a) and 15 K (b). (c)Experimental and (d) calculated Q distribution
map in the (h k 1/2) plane.

[T. Watanabe et al. (2014)].
IV. Conclusions
We successfully observed and analyzed the molecular spin excitations in the disordered MgV2O4, whichmost probably correspond to the origin of the anomalous elastic property. Further details are under consideration.

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