委員会を2023年7月10日に開催した。2024年度中性子共同利用S1型実験課題及び装置調整課題の公募要項について審議し、申請書の提出期限を2023年11月10日とした。2023B期の中性子共同利用S1型課題へのビームタイム配分について審議した。マルチプローブ共同利用実験課題について、1課題の中間評価を了承した。
◆ 研究成果
ハニカム格子反強磁性体FeTiO3の超音波印加効果
社本真一、赤津光洋、Kwangwoo Shin、Jae-Ho Chung、植田大地、Lieh-Jeng Chang、Amer Hassan、今井正樹、家田淳一、伊藤晋一、横尾哲也
POLANOでは2019年度より非偏極中性子を用いた一般課題利用をおこなっている。本課題はコロナ禍による各国間における出入国制限が緩和され、日本・韓国・台湾・エジプトの国際共同研究として行われた課題である（図1）。白金薄膜を蒸着したY3Fe5O12では、縦型スピンゼーベック効果が観測され、超音波印加ではその電圧の符号反転が観測されている[1]。その電圧起源はFe2+不純物のスピン・格子結合とされる。FeTiO3では、Feの価数はすべて2+であることから、スピン流での巨大応答が期待される。
今回、FeTiO3単結晶（2.5 g）を用いてPOLANOの非偏極実験を実施した。結晶は[110]-[001]面でアラインし、[110]に沿って32 MHz縦波の超音波を印加した。4 Kで2倍以上の増大が磁気ブラッグピークである(1,1,4.5)で観測された（図2）。その増大率の温度変化は低磁場でのスピンゼーベック効果の温度変化と一致していた。今後、温度変化と共に、非弾性散乱スペクトルでの変化を加えて発表予定である。
[1] S. Shamoto, M. Akatsu et al., Phys. Rev. Research 4, 013245 (2022).
図１. 超音波印加下での国際共同研究中性子散乱実験（左から植田助教、Amer Hassan、今井研究員、張教授、社本コーディネーター（PI）、赤津助教、辛博士研究員、Jae-Ho Chung教授、横尾教授、POLANOキャビンにて）
図2. 超音波印加下で増大したFeTiO3のブラッグピーク。左：T= 6 K、右：T= 65 K
◆ 研究成果
Direct observations of spin fluctuations in hedgehog--anti-hedgehog spin lattice states in MnS1-xGex (x = 0.6 and 0.8) at zero magnetic field
The interplay between conduction electrons and noncoplanar magnetism is a topic of growing interest in condensed matter physics. Noncoplanar magnetic orders give rise to a finite scalar spin chirality which acts as an effective magnetic field on conduction electrons resulting in anomalous Hall effect of unconventional origin or topological Hall effect (THE). One example showing THE phenomenon is MnGe [1]. This material has magnetic structure of a hedgehog--anti-hedgehog spin lattice (HSL) state which can be described by a superposition of three proper-screw-type magnetic modulations with the q-vectors of (q,0,0), (0,q,0) and (0,0,q). Such non-trivial spin textures induce THE due to having a finite scalar spin chirality.
By substituting Si for Ge, the magnetic structure of MnSi1-xGex changes from cubic-3q (x = 0.7-1) to tetrahedral-4q (x = 0.3-0.6) HSL states [2]. Interestingly, it was reported that the sign of Hall resistivity turns from negative into positive near the critical temperature as the temperature increases for both cubic-3q and tetrahedral-4q HSL states. Moreover, the behavior of Hall resistivity on tetrahedral-4q SHAH lattice state is more complex since the positive sign of Hall resistivity was also observed at low temperature. A recent theoretical study has pointed out that the origin of the positive Hall resistivity is due to spin fluctuations with finite scalar spin chirality while the negative one is induced by a static long-range order of HSL [3].
To corroborate the scenario described on the theoretical study, we need to quantitatively investigate how spin fluctuations develop with varying temperature. For this purpose, we performed the time-of-flight (TOF) neutron inelastic scattering and MIEZE-type neutron spin-echo (NSE) experiments on MnSi1-xGex polycrystalline samples with x = 0.6 (tetrahedral-4q) and x = 0.8 (cubic-3q). Our main results shows that these samples exhibit relatively large spin fluctuations centered at the transition temperatures Tc and also reveal the correspondences between the temperature ranges where the positive Hall resistivity and spin fluctuations are observed as shown in Fig. 3. They agree very well with each other for the whole temperature range in the x = 0.8 sample, but show a discrepancy at low temperatures in the x = 0.6 sample. This discrepancy cannot be explained by the fluctuating spin-cluster mechanism. Another possible mechanism needs to be addressed for future study.
This work was accepted for publication in Physical Review B [4]. The TOF neutron inelastic scattering at HRC and neutron resonance spin echo measurements at VIN ROSE were performed under the user program (Proposal Nos. 2021S01, 2022S01, 2019S07, and 2020B0424).
図3. The correspondences between the spin fluctuations and positive Hall resistivity for x = 0.8 (left) and x = 0.6 (right), respectively. Temperature dependence of the static fraction S0 (a)-(b), and the topological Hall resistivity at μ0H = 1.5 T, [ρyxT (μ0H = 1.5 T) − ρyxT (0)] (c)-(d). The static fractions are derived from the present MIEZE-NSE measurements, while the topological Hall resistivity data were extracted from Ref. [2].
References:
[1] N. Kanazawa et al., Phys. Rev. Lett. 106, 156603 (2011).
[2] Y. Fujishiro et al., Nat. Commun. 10, 1059 (2019).
[3] H. Ishizuka and N. Nagaosa, Sci. Adv. 4, eaap9962 (2018).
[4] Seno Aji, Tatsuro Oda, Yukako Fujishiro, Naoya Kanazawa, Hiraku Saito, Hitoshi Endo, Masahiro Hino, Shinichi Itoh, Taka-hisa Arima, Yoshinori Tokura, and Taro Nakajima, Phys. Rev. B 108, 054445 (2023).
◆ 研究成果
Direct observations of spin fluctuations in hedgehog--anti-hedgehog spin lattice states in MnS1-xGex (x = 0.6 and 0.8) at zero magnetic field
（BL12と同一のため省略）
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