Magnetic phase diagram of MnSi inferred from magnetization and ac susceptibility

We report simultaneous measurements of the magnetization and the ac susceptibility across the magnetic phase diagram of single-crystal MnSi. In our study we explore the importance of the excitation frequency, excitation amplitude, sample shape, and crystallographic orientation. The susceptibility, μ₀0.16em0exdM/dB, calculated from the magnetization, is dominated by pronounced maxima at the transition from the helical to the conical and the conical to the skyrmion lattice phase. The maxima in μ₀0.16em0exdM/dB are not tracked by the ac susceptibility, which in addition varies sensitively with the excitation amplitude and frequency at the transition from the conical to the skyrmion lattice phase. The same differences between μ₀0.16em0exdM/dB and the ac susceptibility exist for Mn_1-xFeₓSi ($x=0.04$) and Fe_1-xCoₓSi ($x=0.20$). Taken together our study establishes consistently for all major crystallographic directions the existence of a single pocket of the skyrmion lattice phase in MnSi, suggestive of a universal characteristic of all B20 transition metal compounds with helimagnetic order.