Investigating spin transport across magnetic insulators and their interfaces

Abstract

Spin transport across magnetic insulator/heavy metal (MI/HM) interfaces has been a topic of interest in spintronics. The spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR) are two phenomena that have garnered much attention. The SSE studies magnon spin current induced by thermal effects, while SMR investigates the change in HM resistivity due to spin transfer torque at the MI/HM interface. This thesis investigates the use of electrical insulating magnetic materials for spin information transmission at room temperature, with a focus on understanding spin transport phenomena across magnetic insulators and their interfaces. The first part of the thesis presents the work on detecting spin-Hall magnetoresistance (SMR) on a crystalline b-plate of Ho0.5Dy0.5FeO3 (HDFO)/Pt hybrid. The SMR measurements were conducted at various temperatures, ranging from 11 to 300 K. The first set of experiments focused on measuring the angular dependence of SMR at room temperature under fields above and below the critical field, revealing anomalies in the signal. These anomalies were then explained through the simulation of the SMR signal using a simple Hamiltonian model. Further analysis of SMR measurements was conducted under a constant field above the critical field at different temperatures, and the results were discussed. The second part of the thesis describes research on the measurement of SMR and SSE on a polycrystalline Sr3Co2Fe24O41 (SCFO)/Pt heterostructure, a room-temperature magneto-electric multiferroic material. The amplitude of SMR data obtained from two measurement sets shows a non-monotonic behaviour with a sign reversal from negative to positive as the external magnetic field is varied. The observed SMR data in SCFO is analysed using a simple Hamiltonian model. Additionally, longitudinal SSE measurements are performed, which resemble the dc magnetization results at 300 K. In the last part, spin transport (SMR and SSE) was investigated on trilayer devices consisting of MgO/Ni0.8Zn0.2Fe2O4 (NZFO)/NiO/Pt heterostructures with varying NiO thicknesses. SMR (1ω) measurements were conducted at various temperatures, followed by current-induced heating to detect 2ω signal. The lock-in detection technique was used to measure 2ω signals by varying magnetic field, current, and temperature that shows non-sinusoidal SSE signal. This non-sinusoidal SSE signal was attributed to unidirectional anisotropy (UDA), caused by ferrimagnetic/antiferromagnetic exchange coupling, using a simple Hamiltonian model. Overall, this thesis contributes to the advancement of spintronic research by exploring the potential of electrical insulating magnetic materials as carriers of spin information and developing a simple Hamiltonian model for analysing spin-related phenomena in these materials.