The 3nd QLC young colloquium
Date & Time : Tuesday, June 1, 2021. 13:30～15:00
Speaker： Three winners of the 2nd QLC Young Researcher Award ,
Shun OKUMURA（University of Tokyo）
Tatsuki SATO（University of Tokyo）
Yusuke SHIMAMOTO（Osaka Prefecture University）
Place：online using “Zoom”
*If you wish to join this seminar, please register at this site.
*Zoom information will be sent on the day of this colloquium to those who registered by noon on the day of the event (Japan Time).
1) 13:30-14:00
Speaker：Shun OKUMURA（University of Tokyo）
Title：Theoretical study on magnetic hedgehog lattices in itinerant magnets
Abstract：
Recently, a three-dimensional topological magnetic texture, which is called magnetic hedgehog lattice (HL), has attracted many attentions in condensed matter physics. The HL hosts a periodic array of hedgehog and antihedgehog spin textures, which generate emergent magnetic fields regarded as magnetic monopoles and antimonopoles, respectively. In experiments, the HLs are observed in a noncentrosymmetric metal MnSi1-xGex and centrosymmetric metal SrFeO3, both of which exhibit the topological Hall effect [1]. Theoretically, however, their stabilization mechanism and topological properties remain elusive since there are few previous studies on the HLs.
In this seminar, I will introduce our theoretical works on the magnetic hedgehog lattices in itinerant magnets. We first find that the HLs are stabilized as a ground state by the long-range multiple-spin interactions, which are induced by the itinerant nature of electrons. In a noncentrosymmetric system, we clarify that the emergent magnetic field changes drastically thorough topological transitions due to pair annihilation of the monopoles and antimonopoles [2]. As a recent result, we also show the unique properties of a HL in a centrosymmetric system without the Dzyaloshinskii-Moriya interaction. In addition, I will mention our recent interests and perspectives on the HLs.
[1] Y. Fujishiro et al., Appl. Phys. Lett. 116, 090501 (2020).
[2] S. Okumura et al., Phys. Rev. B 101, 144416 (2020).
２）14:00～14:30
Speaker：Tatsuki SATO（University of Tokyo）
Title：Magnetochiral imaging of antiferromagnetic domain wall motion
Abstract：
Real-space imaging is one of the most powerful techniques to investigate magnetic domain patterns. Magnetic domain patterns can be visualized when each domain shows different optical responses to incident light. For example, magnetic circular dichroism enables visualization of ferromagnetic domain patterns and linear dichroism enables visualization of orientational domains in some antiferromagnets.
In the colloquium, I will present magnetochiral imaging of antiphase domain pattern in a collinear antiferromagnet MnTiO3. When the unpolarized light go through noncentrosymmetric materials without time-reversal symmetry, magnetochiral dichroism, which originate from the interference between oscillating electric and magnetic fields of light, arises as the perturbative contribution to absorption coefficient. Since the magnetochiral dichroism changes in sign for the time-reversal operation, magnetochiral dichroism can be exploited to visualize antiphase domain pattern. While previous studies focused on magnetochiral dichroism caused by net magnetization, we have recently reported magnetochiral dichroism in antiferromagnet with no magnetization. In this study, we demonstrate magnetochiral imaging of antiferromagnetic domains in MnTiO3. Exploiting improved exposure time of less than a second compared with previously used techniques, we observed motion of antiferromagnetic domain walls driven by electric and magnetic fields and clarified the effect of thermal fluctuation in the enhancement of mobility of antiferromagnetic domain walls.
３）14:30～15:00
Speaker：Yusuke SHIMAMOTO（Osaka Prefecture University）
Title：キラルスピンソリトン格子の磁気共鳴
Abstract：
キラル磁性体CrNb3S6では、キラルな結晶構造に起因するDyzaloshinskii‐Moriya（DM）相互作用が働く。Heisenberg交換相互作用およびZeeman効果との競合により、スピンが片巻きのらせん状にねじれた部位（ソリトン）が周期的に整列したキラルソリトン格子（CSL）が形成される。CSLの周期はらせん軸に垂直な静磁場に応じて変化する[1]。その形成過程は、カイラルサインゴルドン模型で記述され、楕円関数を用いて解析的に解くことができる。
本講演では、CSLの集団運動である「CSLフォノン」に焦点をあてる。磁気超格子の格子振動に該当するCSLフォノンでは、マグノンバンド内にCSL周期に応じたブルリアンゾーンが形成されることが期待される[2]。興味深いことに、磁場中でCSL周期が変わるとブルリアンゾーンが変化するため励起モードが変調されうる。しかしながら、これまで実験においてCSLフォノンは観測されていない。
マイクロ波分光法を用いて、CrNb3S6結晶の磁気共鳴特性を調べた。CSL相において臨界磁場に向かって収束する4本の共鳴モードを観測した。これらの高次モードはCSLフォノンモードに対応していると考えられる。さらに、理論モデル式を用いたフィッティングからDM相互作用の大きさを見積もった。当日は、磁気共鳴実験から得られた知見をもとにCSLの集団運動について議論する。
[1] Y. Togawa et al., Phys. Rev. Lett. 108, 107202 (2012).
[2] J. Kishine et al., Phys. Rev. B 79, 220405(R) (2009).
Committee Chair：Hiroki WADATI（University of Hyogo）