Direct observation of multiple magnetic transitions in the La3NiGe2-type compounds

By using Lorentz transmission electron microscopy, we show the real-space observation of magnetic domain structures in orthorhombic polycrystalline (Tb1−xGdx)3NiSi2 (x = 0, 0.4) compounds. The significant changes in magnetic domains related to multiple magnetic transitions were identified. A spin reorientation transition was further studied and a type of magnetic domain structure with two mutually perpendicular domain walls was observed, where the magnetic vortices and antivortices appeared as an array. This magnetic domain pattern evolves with temperature, which corresponds well to the spin reorientation transition and is related to the change in ferromagnetic and noncollinear magnetic components.