All-optical re-switching has been investigated in the half-metallic Heusler ferrimagnet Mn 2 Ru 0.9 Ga, where Mn atoms occupy two inequivalent sites in the XA-type structure. The effect of a second 200 fs 800 nm pump pulse that follows a first pulse, when both are above the threshold for switching, is studied as a function of t 12 , the time between them. The aims are to identify the physical mechanisms involved and to determine the minimum time needed for re-switching. The time trajectory of the switching process on a plot of sublattice angular momentum, S 4a vs S 4c , is in three stages; When t < 0.1 ps, the sublattice moments are rapidly disordered, but not destroyed, while conserving net angular momentum via optical spin-wave excitations. This leads to transient parallel alignment of the residual Mn spins in the first quadrant. The net angular momentum associated with the majority sublattice then flips in about 2 ps, and a fully-reversed ferrimagnetic state is then established via the spin-lattice interaction, which allows re-switching provided t 12 > 10 ps.Single-pulse all-optical switching of magnetization (SP-AOS) is of both fundamental and technological interest [1][2][3]. Despite intense scrutiny over the last two decades, the microscopic origin of the effect is still poorly understood, but the possibility of switching the magnetisation of a thin film between two stable states on a picosecond timescale without recourse to an external magnetic field is intriguing and technologically relevant in the quest for ever-faster and more energy-efficient information technologies [4][5][6]. Here we establish the minimum time that must elapse between two pulses, if the second one is to re-establish the original state. Our results advance the fundamental understanding of SP-AOS and highlight its potential for future application in technology.

Thin films of Mn 4 N and related ternary metallic perovskites with perpendicular anisotropy are interesting for spintronics, but their magnetic structures differ from the triangular ferrimagnetism of the bulk. A temperature-independent anomalous Hall conductivity of −90 -1 cm -1 was found in Mn 4 N films, in addition to the normal temperature-dependent contribution. Based on known spin structures of bulk Mn 3 ZN compounds and the distance dependence of the Mn-Mn exchange for first-and second-neighbor Mn-Mn pairs, we propose a topological noncollinear spin structure for the Mn 4 N films with perpendicular anisotropy. The Letter shows how a small change in symmetry of the spin structure can influence the magnetotransport properties of frustrated ferrimagnetic films.

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