2022 Fiscal Year Research-status Report
Opto-spintronics in atomic-layer materials
| Project/Area Number |
21K13889
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| Research Institution | Institute of Physical and Chemical Research |
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Principal Investigator |
王 子謙 国立研究開発法人理化学研究所, 創発物性科学研究センター, 特別研究員 (00898934)
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| Project Period (FY) |
2021-04-01 – 2024-03-31
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| Keywords | Magnon-sideband / vdW antiferromagnet / SHG |
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| Outline of Annual Research Achievements |
Efficient photogeneration of magnons is important for realizing ultrafast optospintronics. Direct photoexcitation of exciton-magnon pairs, observed as the one-magnon sideband of Frenkel exciton in MnPS3, suggests a promising approach to generate magnons and manipulate spins in two-dimensional (2D) van der Waals (vdW) antiferromagnets. However, the details of the paired excitation as well as the properties of magnons so generated are far from being well understood. Using resonant second-harmonic generation spectroscopy combined with group theoretical analysis, we identified two subbands within the magnon sideband of MnPS3, originating from the splitting of magnon density of states due to significant exciton-magnon interactions. A theory developed by us, incorporating modeling and formulation, reproduces the observed sideband lineshape and offers quantitative evidence for the unconventional magnon splitting under perturbation by exciton. Our finding of the large exciton-induced magnon splitting in MnPS3, not identified in conventional three-dimensional antiferromagnets, suggests the enhanced correlation among charge, spin and orbital degrees of freedom in the 2D vdW magnetic systems. Our study also suggests the potential ultrafast multimode magnon control through the exciton-magnon excitations.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
1. Our primary objective last year was to understand the behaviors of magnetic excitations, in particular, exciton and exciton-magnon, in vdW antiferromagnet MnPS3. We accomplished a comprehensive study of the SHG spectra by investigating the light polarization, temperature, and magnetic field dependences, with a specific focus on the exciton and exciton-magnon resonances. Our findings unveiled novel phenomena, including the exciton-magnon splitting and excitonic linear magnetoelectric effect. This systematic research offers new insights into magnetic quasiparticles and quasiparticle correlations in 2D vdW antiferromagnets.
2. Last year we also aimed to understand the reported inversion-symmetry breaking in the nominal magnetically centrosymmetric vdW antiferromagnet FePS3 and NiPS3. Our systematic Raman anisotropy and mapping study of the phonons and the magnon in our in-house grown bulk FePS3 sample revealed that the centrosymmetry in the lattice and spin system was preserved. We recognized that the dimensionality effect could be significant and, therefore, are in the process of establishing a combined SHG microscopy/spectroscopy system, which is more powerful and suited for symmetry analysis using microscale ultrathin flake samples.
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| Strategy for Future Research Activity |
Based on the progress made so far, future works are planned as follows:
1. Ferrotoroidic domain physics in 2D MnPS3 and MnPSe3: MnPS3 and MnPSe3 exhibit a linear magnetoelectric effect coupled with antiferromagnetic order, and their antiferromagnetic domains coincide with ferrotoroidic domains. We are setting up a microscopic SHG imaging/spectroscopy system that can apply electric and magnetic fields of sufficient strength to a thin-flake sample while observing its microscopic domain structures. With this system, we plan to explore the dynamic control of the ferrotoroidic order and study the ferrotoroidic domain physics in 2D MnPS3 and MnPSe3 at the micrometer scale with light and external fields.
2. Electrical control of magneto-optic effects in 2D MnPS3 and MnPSe3: A prior neutron diffraction study showed that magnetoelectric annealing can align antiferromagnetic domains in bulk MnPS3, implying a large linear magnetoelectric effect. A theoretical study also predicted the gate-controllable magneto-optical Kerr effect in bilayer MnPSe3. However, the magnitude of these effects and the efficiency for electric control of magneto-optical effects in realistic 2D MnPS3 and MnPSe3 are still unclear. To investigate this, we plan to use our ultra-sensitive scanning Sagnac interferometer to detect the magnetization induced by external electric fields and explore the possibility of a large electric-field modulation of the magneto-optical effects.
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| Causes of Carryover |
In our new microscopic SHG imaging/spectroscopy system, the objective lens and the sample/cryostat are in close proximity, so we will use permanent magnets, specifically Halbach arrays, instead of a bulky electromagnet to apply magnetic fields. I will use the remaining budget to purchase multiple Halbach arrays with varying magnetic field strengths to achieve a range of discrete magnetic field values. The budget will also go towards purchasing essential optical, mechanical, and electrical components required to assemble the entire system.
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