Engineering magnetic states with light through nonlinear lattice excitation

Ankit Disa

Optical manipulation of magnetism offers enormous possibilities for enabling energy-efficient data storage and processing with ultrafast switching speeds. These advantages are especially pronounced in antiferromagnets, which are robust to environmental perturbations and whose fundamental magnetic modes typically lie at high frequencies. While absorption can result in heating-induced modifications of magnetic order, it also accompanied by detrimental incoherent effects. Thus, an important research effort has centered around understanding non-thermal coupling pathways of light with the magnetic order parameter in complex magnetic materials.
Recent studies have demonstrated the ability to drive magnetic changes via the coupling between the electromagnetic field of a light pulse and the spins or interaction parameters, which is often mediated through indirect electronic processes. Alternatively, the crystal structure can provide a direct handle on magnetism, dictating both local magnetic states and their interactions through the lattice symmetry and bonding environment.
In this talk, I discuss how one can dynamically control the crystal structure by resonantly exciting optical phonons in the mid-infrared and THz range, and I show how this approach can be applied to induce, enhance, and switch magnetic order. I focus on our recent experiment demonstrating light-induced ferrimagnetism in the prototypical antiferromagnet CoF2. By simultaneously driving degenerate lattice vibrations, we transiently realize a low-symmetry, non-equilibrium crystal structure, which generates a net magnetization via a “dynamical piezomagnetic effect.” The optically created magnetization is switchable by the light polarization, and has a magnitude that can be tuned up to 100 times larger than achievable in equilibrium, offering a potential platform for opto-magnetic applications. I also highlight a new experiment on the correlated magnet YTiO3 in which ferromagnetism is transiently enhanced and stabilized at a temperature well in excess of the equilibrium Tc. In both cases, optically driven symmetry breaking is enabled by selectively exploiting phonon nonlinearities, demonstrating new possibilities for rational designing non-equilibrium magnetism with light.