The Interplay of Magnetic Order with the Electronic Scattering and Crystal-Field Effects in a Metallic Ferromagnet

The interplay between magnetic order, charge dynamics, and crystal field excitations underpins the emergent ground states of rare-earth intermetallics. Using time-domain terahertz spectroscopy, we probe this coupling in PrSi, a metallic ferromagnet. The optical response exhibits pronounced Drude-Smith behavior over a broad temperature range, indicating persistent carrier scattering. A classical Kondo-lattice model (CKLM) attributes this non-Drude conductivity to scattering of itinerant electrons by localized magnetic moments, persisting down to temperatures well below the magnetic ordering scale. At lower temperatures, beyond the scope of CKLM, our experiment reveals that the response is dominated by crystal field excitations, with sharp transitions at 0.6 and 1.54 THz. The mode at 1.54 THz shows a dynamic correlation with the onset of ferromagnetic order, marking the onset of a crystal-field-governed low temperature regime.