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.
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- Physics (PHY) [453]