Switching of an antiferromagnet controlled by spin canting in a laser-induced hidden phase

During laser-induced phase transitions, fast transformations of electronic, atomic, and spin configurations often involve emergence of hidden and metastable phases. Being inaccessible under any other stimuli, such phases are indispensable for unveiling mechanisms and controlling the transitions. We experimentally explore spin kinetics during ultrafast first-order 90$^{\circ}$ spin-reorientation (SR) transition in a canted antiferromagnet Fe$_3$BO$_6$, and reveal that the transition is controlled by the canting between the magnetic sublattices. Laser-induced perturbation of the Dzyaloshinskii-Moriya interaction results in a change of the intersublattice canting within first picoseconds, bringing Fe$_3$BO$_6$ to a hidden phase. Once this phase emerges, laser-induced heating activates precessional 90$^\circ$ spin switching. Combination of the spin canting and heating controls the final spin configuration comprising coexisting initial and switched phases. Extended phase coexistence range is in a striking contrast to the narrow SR transition in Fe$_3$BO$_6$ induced by conventional heating.