In antiferromagnets, where quantum mechanical exchange interactions dictate spin behaviour, understanding the dynamics of magnons—collective spin wave excitations that naturally reach terahertz frequencies and supersonic velocities—is essential for both fundamental science and emerging technologies. Femtosecond optical pulses offer a powerful means to coherently excite these magnons across the full Brillouin zone and to manipulate their spectral characteristics. Yet, achieving such control has remained difficult, as it requires ultrafast and sustained tuning of the underlying exchange interaction. Here we demonstrate an optically driven renormalization of the terahertz magnon spectrum in the insulating antiferromagnet DyFeO₃. Our results show that this transformation arises from a substantial transient reduction of the exchange interaction within a nanoscale region near the surface. These findings reveal a route to light-induced, nanoscale control of antiferromagnetic spin dynamics, opening opportunities for reconfigurable, ultrafast magnonic and spintronic functionalities.

Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making them ideal candidates for future magnonic devices operating at THz frequencies. A major focus of current research is the investigation of optical methods for the efficient generation of coherent magnons in antiferromagnetic insulators. In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling enables spin dynamics through the resonant excitation of low-energy electric dipoles such as phonons and orbital resonances which interact with spins. However, in magnetic systems with zero orbital angular momentum, microscopic pathways for the resonant and low-energy optical excitation of coherent spin dynamics are lacking. Here, we consider experimentally the relative merits of electronic and vibrational excitations for the optical control of zero orbital angular momentum magnets, focusing on a limit case: the antiferromagnet manganese phosphorous trisulfide (MnPS3), constituted by orbital singlet Mn2+ ions. We study the correlation of spins with two types of excitations within its band gap: a bound electron orbital excitation from the singlet orbital ground state of Mn2+ into an orbital triplet state, which causes coherent spin precession, and a vibrational excitation of the crystal field that causes thermal spin disorder. Our findings cast orbital transitions as key targets for magnetic control in insulators constituted by magnetic centers of zero orbital angular momentum.

CoPS₃ stands out in the family of the van der Waals antiferromagnets XPS₃ (X = Mn, Ni, Fe, and Co) due to the unquenched orbital momentum of the magnetic Co²⁺ ions, which is known to facilitate the coupling of spins to both electromagnetic waves and lattice vibrations. Here, using a time-resolved magneto-optical pump-probe technique, we experimentally study the ultrafast laser-induced dynamics of mutually correlated spins and lattice. It is shown that a femtosecond laser pulse acts as an ultrafast heater and, thus, results in the melting of the antiferromagnetic order. At the same time, the resonant pumping of the ⁴T_1g → ⁴T_2g electronic transition in Co²⁺ ions effectively changes their orbital momentum, giving rise to a mechanical force that moves the ions in the direction parallel to the orientation of their spins, thus generating a coherent B_g phonon mode at the frequency of about 4.7 THz.

Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS₃. Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.