Probing the Ultrafast Dynamics of the Magnetic Phase Transition in FeRh Nanostructures

Abstract

Materials that undergo a coupled phase transition offer a window into the relationship between electrons, nuclei, and magnetic spins in condensed matter. The development of ultrafast techniques where materials can be probed in the sub-ps time regime have provided the means to provide new insights into the exchanges of energy that occur between these systems. This can be applied to magnetocaloric, memory storage, and spintronics devices. This work investigated the dynamics of the FeRh coupled phase transition, where the magnetic ordering change from Anti-Ferromagnetic (AF) to FerroMagnetic (FM) at temperatures moderately above room temperature. The specific focus of this work is on the structural transformations and the effects of lateral confinement on the transition. An x-ray based probe of anti-parallel Fe spin lattice in the AF phase of FeRh is demonstrated experimentally. Non-resonant x-ray magnetic scattering relies upon long-range spin order being established. We demonstrate the temperature dependence of the long-range ordering and confirm that this order only disappears following complete establishment of the FM moment. As a consequence, it allows for a probe of the mixed AF/FM phase of FeRh. This technique allowed for an estimation of the AF domain size suggesting dimensions are limited by the microstructure of the thin film (~40 nm). Time-resolved X-Ray Diffraction (XRD) studies were carried out at the x-ray Free Electron Laser (x-FEL) at SACLA, Japan. We observed structural changes through the phase transition on a timescale not previously reported and show a fluence dependence that indicates the importance of considering non-equilibrated states in the growth and relaxation dynamics of FeRh. A model is presented which demonstrates that such non-equilibria states can be explained using non-trivial electron-phonon coupling. Complementary heated XRD measurements are consistent with the hypothesis that the paramagnetic phase of FeRh is accessed on ps timescales. The effects of lateral confinement were examined in FeRh nanowire arrays to determine if mesoscale magnetic interactions affect magnetisation dynamics. In order to understand the results obtained, heat dissipation was modelled using finite-element software so as to separate magnetic and thermal contributions. Pump-probe Magneto- Optical Kerr Effect (MOKE) investigations alongside static electrical measurements demonstrate that the orientation of external magnetic fields influences the transition behaviour in FeRh wires. FM stabilisation is observed when the external field is applied along the nanowire length. This orientation dependence was not observed in thin films and is ascribed to the shape anisotropy which may influence the FM domain growth mechanism - shifting the phase transition temperature by up to 10 K at applied magnetic fields of 1 T.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Thomson (Supervisor)