Multi-magnetic phases of FeRh films induced by optical excitation and ion irradiation

Abstract

In this dissertation, ultrafast electrodynamics during a unique first-order phase transition between antiferromagnetic (AFM) and ferromagnetic (FM) states transition of metallic compound FeRh are explored using a terahertz time-domain spectroscopy and various time-resolved techniques. Additionally, we investigate the effect of hydrogen ion irradiation on the spatial magnetic distribution and spin precession dynamics in FeRh. First, we utilized THz time-domain spectroscopy (THz-TDS) and optical pump THz probe (OPTP) techniques to explore how free electron dynamics in FeRh respond to both thermally-driven and photo-induced phase transitions. During the photo-induced phase transition, the transient conductivity exhibits distinct behavior from that of the thermally-driven phase transition as pump fluence surpasses the threshold pump fluence (2.9 mJ/cm²), where the optical pulse induces rapid changes in free electron dynamics driven by photo-excited electrons and notable spin fluctuations. To gain a comprehensive view of the temporal evolution of electronic, lattice, and magnetic dynamics during the photo-induced phase transition, we incorporate time-resolved reflectivity (TR-R) and time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. Furthermore, to understand the fundamental physics of the transition in a non-equilibrium state, we investigated how free electron dynamics respond under various pump fluences around the critical pump fluence for the photo-induced phase transition, with temperature dynamics analyzed based on the three-temperature model (3TM). Second, we explored the potential of FeRh for creating the FM/AFM multilayer within a single material by exploiting its tunability of the temperature-dependent AFM-FM transition with hydrogen ion (H⁺) irradiation. We investigated bulk and surface magnetic states separately based on the magneto-optical Kerr effect and magnetization-induced second harmonic generation, respectively, and revealed that FeRh can host the FM (surface)/AFM (bulk) magnetic multilayer within a single layer of FeRh even at room temperature, prepared with a H⁺-ion dose of 2.0 × 10¹⁵ H⁺/cm². As the FM and AFM states are stabilized with a well-defined spatial separation as manifested by the exchange bias effect, we expect the FeRh-based FM/AFM bilayer to alleviate limitations arising from the interfaces formed by otherwise different materials. Finally, we investigate the impact of H⁺-ion irradiation induced defect concentration on spin precession dynamics in FeRh thin films. To elucidate the effects of ion irradiation, we first examine variations in coherent spin precession during the temperature-driven antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in as-grown and H⁺-ion irradiated FeRh samples. Additionally, by comparing spin precession dynamics between laser-induced and temperature-driven phase transitions, we reveal distinct transition pathways resulting from differences in different heat-distributions. Furthermore, field-dependent measurements of spin precession dynamics in the FM state of H⁺-ion irradiated FeRh at room temperature demonstrate the tunability of spin precession dynamics on ion irradiated FeRh films.