Deciphering the new magnetic state, “B-Phase”, found in MnSi at Low Temperatures | Prof. J. Campo

In cubic chiral magnets, Dzyaloshinskii-Moriya (DM) interactions within the chiral crystal structure result in diverse magnetic textures, including skyrmion lattices (SkL) and chiral soliton lattices, which hold promise for spintronic and magnonic devices. Among these, MnSi has been extensively studied due to the SkL formation in the so-called “A-phase” just below Tc [1]. Recently, it was suggested theoretically that at low temperatures (T), the conical helimagnetic (CH) and forced-ferromagnetic (FFM) phases in MnSi might not be directly connected but separated by another SkL phase, possibly metastable, or a new phase of unknown nature near the critical magnetic field (Bc) [2]. The theoretical prediction of the new SkL phase at low T is in good agreement with the experiments reported in MnSi and Cu2OSeO3 [3,4]. On the other hand, by using careful ac susceptibility measurements at low temperature, we determined the magnetic phase diagrams of oriented crystals of MnSi [5]. A new anomalous region, termed “B-phase”, was observed when the magnetic field was applied along the main diagonal (111). To clarify the nature of the “B-phase”, we performed small-angle neutron scattering (SANS) measurements at TAIKAN in J-PARC and transverse field (TF)-μSR experiments at TRIUMF. At low temperatures and fields near Bc, SANS patterns revealed two peaks along the horizontal axis, corresponding to the magnetic Bragg peaks of the CH state. Notably, no diffraction peaks indicative of a six-fold-symmetric SkL were observed. Meanwhile, μSR results showed a distinct internal magnetic field distribution in the “B-phase”, different from those in the CH or FFM phases, suggesting that the “B-phase” could involve a reorientation of Mn helices within the unit cell. In the presentation, we will discuss these SANS and μSR findings in detail and their implications for understanding the spin texture in the “B-phase”. References 1. S. Mühlbauer et al., Science 323, 915 (2009). 2. V. Laliena and J. Campo, Phys. Rev. B 96, 134420 (2017). 3. T. Nakajima et al., Sci. Adv. 3, e1602562 (2017). 4. A. Chacon et al., Nature Phys 14, 936–941 (2018). 5. M. Ohkuma et al., APL Mater. 10, 041104 (2022). ------------------------------ The International Institute for Sustainability with Knotted Chiral Meta Matter (WPI-SKCM2) is on a mission to introduce a new interdisciplinary research paradigm called “knotted chiral meta matter” (KCM2). Using this paradigm, our team of researchers cross-pollinates mathematical knot theory and chirality knowledge across disciplines and scales. This work includes the creation of designable artificial knot-like particles that exhibit highly unusual and technologically useful properties, which will both deepen our understanding of nature as well as be useful for technological innovation to solve global problems, such as climate change. While pursuing this research, we will create a testbed for research-based graduate education reforms in Japan & beyond, connecting young talent globally. Follow us here: Website | https://wpi-skcm2.hiroshima-u.ac.jp/ Instagram |   / skcm2   LinkedIn |   / wpi-skcm2-hiroshima-university   X | https://x.com/SKCM2_HU WPI-SKCM2 is funded by the Japanese Ministry of Education, Culture, Sports, Science and Technology’s (MEXT) World Premier International Research Center Initiative (WPI). WPI-SKCM2 is hosted by Hiroshima University, located in Higashi-Hiroshima, which is approximately 30 km east of Hiroshima City.