Magnetism and spin transport in heavy metal/ferrimagnetic insulator multilayers

Abstract

Semiconductor electronic devices are approaching their performance limits as their dimensions continue to shrink. Therefore, it is necessary to explore non-traditional materials and construct new devices based on mechanisms other than electronic charges. Currently, the transport of spin current has been realized in certain magnetic insulator (MI) materials, and the interconversion between charge current and spin current has also been achieved in heavy metal (HM)/MI heterojunctions. Devices based on this phenomenon show promise as the next generation of memory devices, and further research in this field is urgently needed. In my study, terbium iron garnet (Tb3Fe5O12,TbIG) was chosen as the MI layer and platinum (Pt) was the HM layer. TbIG thin films were deposited using pulsed laser deposition (PLD) and Pt was prepared by magnetron sputtering. Microstructural characterization showed that TbIG layer grew on Gd3Ga5O12 (GGG) single crystal substrates demonstrated good epitaxy and low surface roughness. A strong perpendicular magnetic anisotropy was observed in the Pt/TbIG sample. By measuring the anomalous Hall effect (AHE) resistance, the compensation temperature (Tcomp) was identified as around 210 K. The measured spin Hall magnetoresistance (SMR) of the sample showed strong temperature dependence, with an obvious inflection point near the Tcomp of the TbIG layer. The spin mixing conductance at 300 K of the sample was also calculated. In the second part of my study, the effect of TbIG thickness on Tcomp was systematically investigated by observing the AHE and SMR in Pt/TbIG bilayers. The results showed a unique behaviour of AHE at temperatures near Tcomp and demonstrated thickness dependence of Tcomp in TbIG thin films. A Tcomp above room temperature was observed from ultrathin TbIG thin films, and the possible origins behind this phenomenon were discussed. In addition, the AHE and SMR were also measured on Pt/YIG/TbIG trilayer samples. When a YIG layer was inserted, the interaction between the Pt and the TbIG magnetization was disrupted, and the coupling behaviour between Pt and TbIG layers was strongly affected. Finally, the interlayer coupling in TbIG/YIG samples deposited on Y3Al5O12 (YAG) substrates was explored. At low temperatures, this interlayer coupling behaviour was manifested in the form of an inverted magnetic hysteresis (M-H) loop. The coupling exhibited clear temperature dependence as well as directional dependence. The impact of various parameters on the interlayer coupling in TbIG/YIG samples (such as temperature, orientation, and film thickness) was studied. The coupling strength of the system was calculated using the Stoner-Wohlfarth model.