Probing magnetism from static order to ultrafast dynamics with resonant X-ray scattering

Understanding spin excitations and their dynamics is key to unravelling the behaviour of strongly correlated materials, from complex magnetic order to ultrafast non-equilibrium phenomena. In this talk, I will show how resonant X-ray scattering techniques‒particularly RIXS‒can be used to study magnetism from static order to excitations and ultrafast dynamics.

I will first discuss my PhD work on the frustrated multiferroic YBaCuFeO₅, where the interplay between chemical disorder and magnetic interactions stabilizes unconventional spin states relevant for high-temperature multiferroicity. This includes single-crystal growth by floating-zone techniques and the characterization of long-range chiral magnetic order using neutron diffraction and polarimetry, together with resonant magnetic X-ray diffraction under applied external fields. I will then present my work at PSI on momentum-resolved magnetic excitations in YBaCuFeO₅, where RIXS reveals coherent spin waves in an entropy-driven mixed phase, demonstrating that disorder can enhance, rather than suppress, collective excitations. Finally, I will focus on recent advances in time-resolved resonant diffuse scattering, where we track the ultrafast evolution of spin excitations following photoinduced demagnetization in an antiferromagnet. These results reveal hierarchical energy transfer pathways involving spins, charges, and lattice degrees of freedom, and demonstrate the generation and relaxation of non-thermal magnons across the Brillouin zone. This establishes the unique capabilities of resonant diffuse scattering for probing coupled degrees of freedom in non-equilibrium states.

These results highlight the power of resonant X-ray techniques to bridge equilibrium and non-equilibrium magnetism, and motivate future research directions at ESRF ID32.