Picosecond creation of switchable optomagnets with giant photoinduced Kerr rotations in polar antiferromagnetic (Fe${}_{1-x}$Zn${}_x$)${}_2$Mo${}_3$O${}_8$

On-demand spin orientation with long polarized lifetime and easily detectable signal is an ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe${}_{1-x}$Zn${}_x$)${}_2$Mo${}_3$O${}_8$, from which we can obtain tunable magnetization, spanning from -40$\mathrm{\%}$ to 40$\mathrm{\%}$ of a saturated magnetization that is created from zero magnetization in the antiferromagnetic state without magnetic fields. It is accomplishable via utilizing circularly-polarized laser pulses to excite spin-flip transitions in polar antiferromagnets that have no spin canting, traditionally hard to control without very strong magnetic fields. The spin controllability in (Fe${}_{1-x}$Zn${}_x$)${}_2$Mo${}_3$O${}_8$ originates from its polar structure that breaks the crystal inversion symmetry, allowing distinct on-site $d$-$d$ transitions for selective spin flip. By chemical doping, we exploit the phase competition between antiferromagnetic and ferrimagnetic states to enhance and stabilize the optomagnet effects, which result in long-lived photoinduced Kerr rotations. The present study, creating switchable giant optomagnet effects in polar antiferromagnets, sketches a new blueprint for the function of antiferromagnetic spintronics.