Kharkiv Quantum Seminar

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).