Rationally designing a crystal structure with light to control magnetism

In quantum materials, the crystal lattice shapes the interactions between electrons and governs, in large part, their emergent electronic and magnetic phases. Hence, a powerful approach for controlling functional properties and stabilizing hidden ground states is based on altering a material’s structure through, for instance, strain or interfacial engineering. Here, we show that a desired non-equilibrium crystal structure can be engineered with light to achieve macroscopic behavior beyond that achievable statically. In particular, we generate a highly polarized ferrimagnetic phase in the prototypical antiferromagnet CoF₂ by optically driving atomic motions with resonant THz pulses. By exploiting lattice anharmonicities, a symmetry-breaking structural distortion is created on the picosecond time scale, leading to a site-selective modulation of the Co crystal field and associated magnetic moment. This effect resembles the static piezomagnetic effect in CoF₂; however, the ultrafast magnetization is almost two orders of magnitude larger than that achievable statically. The realization of a targeted light-induced structure and functionality establishes the potential of nonlinear phononics and expands the ultrafast control of quantum materials towards the level of rational design.