Magnetic phase transitions of the 4f skyrmion compound EuPtSi studied by magnetization measurements

In this study, we measure the magnetic field and temperature variations of the magnetization M(H,T) of the cubic chiral compound EuPtSi and extensively investigate the magnetic phase transitions in a wide temperature range down to 60 mK. The incommensurate-commensurate transition at T_N^∗≃2.5 K in the helical state, which has not been observed in transport or thermodynamic measurements, is exclusively detected in M(T) for H || <111>. T_N^∗ shows a rapid decrease in H. For H || [100] and [111], M(H) at T < 100 mK showed that the helical q vectors are depinned from the preferred orientations at H≳10 kOe and gradually rotate toward the field directions as H increases. The skyrmion lattice phase (A-phase) field-induced in H || <111> only above 0.4 K in equilibrium conditions can be created at 60 mK under field cooling, with a sharp tilted plateau structure developing in M(H) of the A-phase. A similar metastable state is also created below 100 mK for the A′ phase induced by H || <100> at T≳0.3 K in equilibrium conditions. The magnetic phase diagrams established for the three directions [111], [110], and [100] are highly anisotropic and discussed herein on the basis of the q vectors rather strongly pinned to the lattice and transform under the lattice symmetry operations. (author)