Elastically induced magnetization at ultrafast time scales in a chiral helimagnet

Chiral helimagnetic materials have recently attracted much attention for spintronic applications due to their long-range helical magnetic order, topological spin textures, and potential for hosting skyrmions. Their robust spin texture would provide a new concept of ultrafast magnetic memory if it can be controlled by an ultrafast optical pulse. Using time-resolved magneto-optical Kerr spectroscopy, we show that magnetism in the single-crystalline chiral helimagnet ${\text{Cr}}_{1/3}{\text{NbS}}_{2}$ can be induced by an ultrafast optical pulse. At low temperatures and in the absence of magnetic fields, ${\text{Cr}}_{1/3}{\text{NbS}}_{2}$ exhibits a chiral helical magnetic phase with a long-range helical spin order, but it contains zero net magnetization. However, after the laser pulse excitation we observe magnetization forming over tens of picoseconds, far exceeding the duration of the laser pulse. We attribute this peculiar behavior to a laser-induced phase transition from the chiral helimagnetic phase, with zero net magnetization, to a chiral conical helimagnetic phase, with finite magnetization. Ab initio density functional calculations provide a detailed microscopic picture of the mechanism behind the observation. The resonant magnon-phonon coupling with specific phonons leads to an elastic deformation of the chiral helimagnetic phase. Finally, at finite magnetic fields, the net magnetization is excited by the laser pulse and precesses around the equilibrium position, leading to anomalous magnetization precession as a function of the external magnetic field at specific magnetic phases.