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High Spatiotemporal Resolution of Magnetic Dynamics in 
Mn
-
Ni
-
Ga
 via Four-Dimensional Lorentz Microscopy

High spatiotemporal resolution study of magnetic dynamic transitions in (Mn0.5Ni0.5)65Ga35 (hereafter referred to as Mn-Ni-Ga) is carried out using Lorentz ultrafast transmission electron microscopy (UTEM). The Lorentz UTEM images with a subpicosecond temporal and sub-50-nm spatial resolution reveal remarkable features of magnetic transient states after a femtosecond pulsed laser excitation and three successive dynamic processes involving four distinct magnetic states are evidently observed in Mn-Ni-Ga crystals. Moreover, a full reversible dynamic cycle for the photoinduced ferroparamagnetic transition in a time span of up to tens of microseconds is also examined by using nanosecond-laser-generated photoelectrons. The temporal changes of electron, spin, and lattice in the present system are qualitatively analyzed based on a three-temperature approximation. Our Lorentz UTEM observations highlight the feasibility of direct imaging ultrafast demagnetization and relevant dynamic natures in complex spin textures with a nanometer-picosecond spatiotemporal resolution.