Theoretical and experimental investigation of magnetic layer coupling in spin-valves and magnetic tunnel junctions

Thesis

Heterostructures composed of ferromagnetic layers that are mutually interacting through a nonmagnetic spacer are at the core of magnetic sensor and memory devices. In the present study, layer-resolved ferromagnetic resonance was used to investigate the coupling between the magnetic layers of a Co/MgO/Permalloy magnetic tunnel junction (MTJ) and Co/Sn/Py spin valves.

Elemental tin in the a-phase is an intriguing member of the family of topological quantum materials. In thin films, with decreasing thickness, a-Sn transforms from a 3D topological Dirac semimetal to a 2D topological insulator (TI). Getting access to, and making use of its topological surface states is challenging and requires interfacing to a magnetically ordered material.

For both types of samples two magnetic resonance peaks were observed for both magnetic layers, as probed at the Co and Ni L₃ x-ray absorption edges, showing a strong interlayer interaction through the insulating MgO barrier. A theoretical model based on the Landau-Lifshitz-Gilbert-Slonczewski equation was developed, including exchange coupling and spin pumping between the magnetic layers. Fits to the experimental data were carried out, both with and without a spin pumping term, and the goodness of the fit was compared using a likelihood ratio test.

Evidence of two types of magnetic layer coupling were found for the studied MgO MTJ. A Likelihood ratio test performed between competing models showed that a model with only exchange coupling is insufficient, and the correct description of the experimental data requires inclusion of spin pumping coupling between magnetic layers. The values characterising both the EC and the spin pumping were estimated. A recipe has been developed for spin-vale fabrication incorporating a a-Sn TI spacer layer. Up to 2nm thick a-Sn layers were deposited onto a Co surface. The X-ray detected ferromagnetic resonance (XFMR) study of the a-Sn system has shown a strong exchange coupling interaction between the magnetic layers with no clear evidence for spin pumping.

The methods developed in this work can be used to interpret XFMR data, not only in the discussed cases, but potentially for all types of measurements in general. The explored idea of incorporating the TI a-Sn into a spin-valve has shown promising results, and will serves as a solid basis for further research.

Authors: Gladczuk, L

• DOI: 10.5287/ora-kzdr6jrd6
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: MTJ; spin pumping; MgO; Landau-Lifshitz-Gilbert-Slonczewski; a-Sn; XFMR; spin-valve; likelihood ratio test; magnetic tunnel junctions; topological insulator
• Subjects: Condensed matter