Study Of The Magnetism In Rare-earth Based Ferrimagnetic Films And Spin Transport In Their Heterostructures

Since the magnetic properties of ferrimagnet（FIM）are determined by two（or more）magnetic sublattices coupled in antiparallel,their basic magnetic properties can be substantially modulated by controlling the magnetic sublattice magnetism,and can exhibit richer physical properties than ferromagnets.In recent years,the research on the mechanism and technology of current-driven magnetic moment flip-flop in the field of spintronics has been intensified,and it has been found that the heterogeneous structure composed of ferrimagnetic materials can exhibit more excellent spintronics properties than ferromagnetic materials.For example,when the ferrimagnetic layer is close to the magnetization intensity compensation state,the magnetization flipping efficiency driven by the spin torque is nearly six or seven times larger than that of ferromagnetic materials;when the ferrimagnetic layer is close to the angular momentum compensation state,the rate of current-driven domain wall motion can break the theoretical limit in conventional ferromagnetic materials and can be increased in magnitude.Therefore,an in-depth understanding of the spin transport mechanism in ferrimagnetic heterostructures and the development and exploration of ferrimagnetic-based spintronics devices and functions are the current research frontiers in the field of spintronics.In this dissertation,we study the magnetic heterostructures,mainly transition metal and rare-earth（TM-RE）ferrimagnetic alloy thin films and Gd IG ferrimagnetic insulator thin films,and systematically measure their static and dynamic magnetic properties,focusing on the spin transport phenomena in these structures near the compensation point.The main innovative conclusions were obtained as follows.1)Based on the non-uniform distribution of components in the ferrimagnet alloy films,we propose a biphasic model of ferrimagnet alloys and better explain the anomalous anomalous Hall effect and spin Hall magnetoresistance in Co Gd alloy films near the magnetization intensity compensation point.Based on the model and experimental results,we fully realize that the nonuniform composition distribution is a common phenomenon in the ferrimagnet metal films,and the resulting nonuniformity of magnetic and transport properties will be amplified near the compensation point,thus exhibiting some non-intrinsic（pseudo）novelty effects.For example,the topological Hall effect,which has attracted much attention in ferrimagnet alloys.2)An innovative method is used to verify that Co Tb has a spin coherence length greater than 10 nm at the magnetization intensity compensation point.The general electrical measurement method tends to introduce voltage signals generated by other effects.We avoid the influence of other related factors and use variable temperature ferromagnetic resonance to measure the temperature dependence of the damping of Py/Cu/Co Tb（t nm）heterojunction and find that the spin decoherence effect is weakened near the temperature compensation point of the magnetization intensity,and verify that the spin coherence length of Co Tb alloy can reach 10 nm and above.This result enables us to recognize the longer spin decoherence length in TM-RE ferrimagnet alloy,and drives us to further explore the factors affecting the spin decoherence effect.3)We confirm that the ageing property（the change of film properties with time）of TM-RE ferrimagnet alloy,alloy films originates from the surface oxidation reaction and propose an experimental method that can effectively suppress the ageing property.We have systematically investigated the relationship between static and dynamic magnetic properties of TM-RE alloy films with time,obtained the influence of experimentally controllable conditions such as ferrimagnetic thickness,type and thickness of cover layer on the aging properties,and verified that the reaction of RE and TM elements in the films with oxygen in air is the source of such aging properties.This reaction increases the non-uniformity of the alloy composition distribution along the thickness direction on the one hand,and leads to an overall reduction of the relative content of RE elements in the films on the other hand.The results have important implications for the stability of the ferrimagnetic alloy film properties and for the fine control of the process,and lead to further insight into the details of the ferrimagnetic alloy film microstructure.4)The voltage signals generated by spin pumping（SP）effect and spin Seebeck effect（SSE）were measured simultaneously in the same device,avoiding the uncertainties introduced by sample preparation,device preparation,and measurement processes.By measuring the SP and SSE signals of Co Gd/Pt（Gd IG/Pt）heterojunctions before and after the magnetization intensity compensation point（temperature compensation point）,it is demonstrated that the SP effect in compensated ferrimagnet mainly originates from the contribution of its net magnetic moment,and the SSE effect originates from the contribution of the sublattice magnetic moment.The results provide a deeper understanding of the mechanism of the SP effect and the spin flow generated by the SSE.