New magnetic intermediate state, "B - phase," in the cubic chiral magnet MnSi

It is well known that the archetype chiral magnet MnSi stabilizes a skyrmion lattice, termed "A-phase,"in a narrow temperature range in the vicinity of the paramagnetic boundary around Tc ∼29 K and Hc ∼2 kOe. Recently, it has been predicted that at much lower temperatures below Tc, the conical helicoid and the forced ferromagnetic (FFM) states could be separated by a new "unknown state."In order to detect this "unknown state,"we explored the phase diagram of MnSi oriented single crystals as a function of the d.c. magnetic field (H - dc) and the temperature (T) by using a.c. magnetization measurements. For H - dc∥ 〈111〉, we observed a new region, termed "B-phase,"in the magnetic phase diagram, characterized by a flat-valley-like anomaly on the in-phase component of the a.c. magnetization (m′), over 3.5 ≤ Hdc ≤ 6.2 kOe just below the low temperature (T < 6 K) FFM boundary. The observed frequency independence over 0.3-1000 Hz and the absence of any measurable absorption in the a.c. magnetization (m″) in the "B-phase"suggest a static nature. The "B-phase"was not observed for either H - dc∥ 〈100〉 or 〈110〉, revealing that the magnetic anisotropy could play a role in the stabilization of the phase. The "B-phase"could be compatible with the theoretical predictions if the new magnetic state is supposedly related with a relative reorientation of the four helices in MnSi.