Reflectivity ferromagnetic resonance for layer-resolved dynamic study of multi-layered systems

Thesis

This thesis develops and applies Reflectivity Ferromagnetic Resonance (RFMR) as a novel technique for probing depth-resolved magnetisation dynamics in magnetic thin films and multilayers. By combining the depth resolution of X-ray Resonant Magnetic Reflectivity (XRMR) with the temporal control of X-ray detected Ferromagnetic Resonance (XFMR), RFMR enables the reconstruction of layer-specific precessional motion on the nanometre scale with picosecond resolution.

After establishing the necessary understanding of optical properties in RFMR and the advanced modelling required to interpret the data, we demonstrate the capabilities of RFMR through three progressive case studies. First, we examine a NiFe single-layer magnetic film, which exhibits a uniform mode alongside indications of damped precession at the interfaces. Here, we show that the RFMR signal is sensitive to the waveform of the RF driving field. This sensitivity is leveraged to distinguish the FMR modal structure under different waveform excitations(sinusoidal versus distorted).

In the second case, we investigate magnetic trilayers with different ferromagnetic layers separated by a Chromium spacer. Vibrating Sample Magnetometry (VSM) and Vector Network Analyzer Ferromagnetic Resonance (VNA-FMR) reveals distinct, interlayer-coupled modes (acoustic and optical). RFMR measurements reveal this allows for the engineering of the trilayer’s depth-profile by tuning the applied static field across the resonance conditions.

Finally, we explore a complex [Ta/CoFeB/MgO]4 heterostructure stack, where the FMR modes exhibit depth-dependent profiles that can be uniquely identified only in RFMR. Using VSM, VNA FMR, Magnetic Force Microscopy and micromagnetic simulations, we identify a Spin-Reorientation Transition (SRT) as 𝑡CoFeB is varied across 1.50nm. XRMR and RFMR measurements reveal that the depth profile of the magnetisation undergoes significant changes across the SRT, with the dynamics becoming increasingly complex in thinner layers. Through RFMR measurements at multiple polarisations, we reconstruct the full three-dimensional magnetisation trajectories, uncovering signatures of nonlinearity critical for future spintronic computing architectures.

Authors: Bollard, J

• DOI: 10.5287/ora-nzn7dpobo
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: x-ray scattering; time-resolved; genetic algorithms; XMCD; synchrotron; condensed matter physics; neuromorphic; magnetism
• Subjects: Spin valves; Condensed matter; Thin film devices; Genetic algorithms; X-ray spectroscopy; Thin films, Multilayered; Magnetic memory (Computers); Magnetism; X-ray absorption near edge structure