| 研究課題/領域番号 |
17F17706
|
|---|
| 研究機関 | 東北大学 |
|---|
研究代表者 |
佐藤 卓 東北大学, 多元物質科学研究所, 教授 (70354214)
|
|---|
| 研究分担者 |
REIM JOHANNES 東北大学, 多元物質科学研究所, 外国人特別研究員
|
|---|
| 研究期間 (年度) |
2017-04-26 – 2019-03-31
|
| キーワード | Skyrmion / Antiferromagnetism / Frustrated magnetism / Chiral magnet / Neutron scattering |
|---|
| 研究実績の概要 |
In the investigation of the stabilization of skyrmion-lattice I am focusing on two chiral materials: Cu2OSeO3 (ferromagnetic FM skyrmion), and CaBaCo2FeO7 (candidate for antiferromagnetic AFM skyrmion). Previously, we observed a particular dependence on the history of temperature and field when stabilizing the skyrmion-lattice in Cu2OSeO3 (published April 2017), which has to be considered also in its AFM counterpart. For this work I have been awarded a prize by 籏野奨学基金.
Recently, we have finished our study of the general magnetic properties of CaBaCo2FeO7. Specifically, we have separated two types of order using neutron scattering, a short range and a long range one. The first one was successfully described by a Heisenberg nearest neighbor model allowing us to derive the exchange interactions. However, the latter one shows additional precursors of a long-periodic spin structure. Using symmetry analysis we found that such structures are actually favorable and even a weak ferromagnetic moment (observed experimentally) is allowed (published April 2018). Further neutron scattering experiments focusing on the long periodic spin structure have been conducted and evaluated. The experiment at the neutron diffractometer MIRA (MLZ, Germany) revealed that the long range order including the precursors is a superposition of two periodic structures with different periodicities coexisting in a wide range of the AFM phase. Usually the longer periodic structure vanishes at lower temperatures while the remaining modulation can be described by a skyrmion-lattice like spin structure.
|
| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
Currently, I am working on evidencing the AFM skyrmion-lattice in CaBaCo2FeO7 at low temperatures. The previous spin structure model was found - considering the Heisenberg nearest neighbor model - to be energetically less favorable. The new approach will use Monte Carlo simulation considering the symmetry of the magnetic space group and the scattering results. Additionally, I will extract integrated intensities from a large peak set measured at SENJU (J-Parc, Japan) to determine the spin structure this way. Ideally both structures should correspond and exhibit a non-vanashing topological quantum number denoting the AFM skyrmion-lattice. In order to understand the stabilization mechanism, at MIRA we also studied the influence of external magnetic field. Cooling the sample in a magnetic field applied in any direction enhances the stability of the longer periodic structure slightly. However, in contrast to a previous experiment the skyrmion-like structure still becomes dominant at low temperatures. Applying magnetic fields (up to 5T) appears to have no influence on the magnetic structure, which supports the existence of a 3q structure required for a skyrmion-lattice.
Continuing the investigation of the established skyrmion material Cu2OSeO3 we used the neutron scattering instrument TAIKAN (J-Parc, Japan). I am currently working on the evaluation of the data. First results show indications for a new phase at low temperatures and an interference between the crystallographic and skyrmion lattice.
|
| 今後の研究の推進方策 |
For this year three experiments are planned to complete both studies.
For CaBaCo2FeO7, I have submitted a proposal to the neutron scattering instrument SANS-1 (MLZ, Germany) in order to distinguish a 3q structure from a 3-domain structure. This instrument provides sufficient resolution to measure the higher order peaks of the long-periodic order (absent for a 3-domain structure), low scattering background and also the Q-space coverage to reach first order peaks. Due to the conditions of the instrument MIRA the maximum electric field of 0.5kV/mm was insufficient to evidence multiferroic properties. Instead I now intend to measure the magnetically induced polarization in collaboration with a neighboring institute. For this we have to grow new single crystals in our laboratory’s floating zone furnace. Based on these and previous observations and the interplay between the Dzyaloshinskii-Moriya and exchange interactions in its model, I plan to derive rules for magnetic materials under which an AFM skyrmion-lattice can be stabilized.
The published data on Cu2OSeO3 showed an intriguing dependence on the magnetic field ramp used to stabilize the Skyrmion phase. I have proposed a new experiment using an electromagnet at the instrument QUOKKA (ANSTO, Australia). Being able to instantly apply the magnetic field we expect to influence the stabilization more strongly. Specifically, we intend to trap the spin structure in a glassy phase, reported earlier. This will provide further insight on the stabilization of the skyrmion-lattice in general.
|
