Magnetic proximity effect and interfacial spin dependent transport in ferromagnet/heavy metal thin films

Abstract

Platinum (Pt) has widely been used for interface driven spintronics applications due to its strong spin-orbit interaction. Pt has been reported to experience spin polarisation when placed in close proximity to a ferromagnetic (FM) material thereby influencing many spintronic phenomena. Consequently, the effect of proximity induced magnetization (PIM) in Pt is studied in this thesis with a detailed investigation of the mechanism and the implications of PIM on magnetoresistance measurements. In this work, CoFeTaB (CFTB) is the FM material investigated. Structural and magnetic characterisation of CFTB/Pt, Pt/CFTB and Pt/CFTB/Pt samples were made in order to investigate PIM at the interface. X-ray reflectivity (XRR) and x-ray diffraction (XRD) were performed on these samples, where asymmetry in the Pt density and the crystalline texture were observed at the top and bottom interfaces. XRD measurements show the Pt crystalline texture depends on the CFTB thickness for the CFTB/Pt interface, but no significant thickness dependence was observed for the Pt/CFTB interface. The magnetic depth profile of the CFTB layer was obtained with polarised neutron reflectivity (PNR), which shows magnetisation grading. An asymmetry in Pt magnetisation was found between the two interfaces using x-ray resonant magnetic reflectivity (XRMR) with a higher moment at the top interface and lower at the buffer interface, giving a similar CFTB thickness dependence as the XRD results. This indicated that the Pt magnetisation depends slightly on Pt texture at the interface. No PIM was found in a YIG/Pt bilayer and the Pt XRD texture was poor, supporting a possible link of Pt polarisation to crystalline the morphology at the interface.
Magnetoresistance investigations in three geometries performed on Pt/CFTB and CFTB/Pt bilayers were used to decouple the magnetoresistance contributions as a result of the anisotropy of the sample, spin Hall effect and other processes. The spin Hall MR ratio obtained was ~0.2 %, with an additional contribution with a cos θ dependence of ~0.1 %, which is a result of the impact of PIM generated spin current. No evidence of the Rashba effect was found in the symmetric CFTB/Pt/CFTB sample. Also, residual plots indicated the presence of higher harmonics that are dependent on the magnetisation direction.
Temperature dependent proximity induced magnetism in Pt in contact with CFTB was presented, with PNR providing the magnetic sensitivity to the FM layer while the XRMR provide sensitivity to Pt magnetisation. PIM scales linearly with CFTB magnetisation which is inconsistent with the Pauli susceptibility. Significantly, a threshold CFTB magnetization is required for PIM to occur. Therefore the asymmetry in PIM at Pt/CFTB and CFTB/Pt interface is attributed to different magnetic susceptibilities at these interfaces.