Spin Dependent Transport Phenomena In Magnetic Epitaxial Nano-heterostructures

Spin dependent transport phenomena are the core research contents in the modern spintronics.The transportation of spin angular momentum does not rely on the flow of charge currents,which leads a reduced Joule heating effect and minimized Oersted fields of spin currents in condensed matter system compared to charge currents.Therefore,the utilizing of spin as a carrier for information delivery and storage will be favorable for the future electronic techniques to break the bottleneck in the current development of electronics based on semiconductors.On the other hand,magnetic heterostructures with epitaxial crystal structures are the ideal platforms for the study of spin dependent transport phenomena owing to their well-defined material properties and clear crystalline structures with little disorder.Especially when it comes to the research related to magnetic nano-sized thin films,the epitaxial stacks allow for a more reliable illumination of fundamental physical mechanism compared to polycrystalline and amorphous thin films,which are highly disordered and uncontrollable.This work will focus on the spin dependent transport phenomena in magnetic epitaxial nano-heterostructures,which consist of both metal and insulators.Primary experiments and results are as follows:(1)The spin Hall magnetoresistance(SMR)in epitaxial magnetic insulator gadolinium iron garnet(Gd3Fe5O12,GdIG)are studied.The magnetic structure of GdIG shows prominent temperature dependence below its Curie temperature and it is non-collinear near the compensation temperature where the spontaneous magnetization disappears,and is collinear far from the compensation temperature.Within the collinear temperature range,the SMR in GdIG/Pt shows usual angular dependence that was well established.The Fe3+ in both GdIG and YIG potentially dominate the spin transfer to the Pt while Gd3+ play a much smaller role for the SMR.In the non-collinear regime,both a SMR signal with inverted sign and a more complex multi-peak angular dependence that are sensitive to both temperature and magnetic field strength are observed.This complex SMR behavior can be understood in terms of the non-collinear magnetic structures and the multi-domain states of GdIG.This work also shows that SMR measurements can be used as an alternative approach for identifying the magnetization state in non-collinear ferrimagnets.(2)The spin Hall magnetoresistance(SMR)in heterostructures based on epitaxial ?-Fe2O3/NiO stacks are studied.The angular dependence of SMR in Fe2O3/NiO/Pt shows a sign change with the increasing of NiO thickness,while at a fixed thickness of NiO,the sign of SMR ratio remains unchanged with the variation of temperature.These results help to clarify the origin of the sign change upon the decreasing of temperature found in yttrium iron garnet/NiO/Pt with polycrystalline NiO,and indicate that the Neel orders in antiferromagnetic NiO which are exchange coupled to magnetization are responsible for the negative SMR due to its equivalent role as magnetization in terms of exchanging spin angular momentum with spin currents.Our results also imply a thickness dependent exchange coupling mode between the NiO and ?-Fe2O3 layers,comprising parallel alignment for thin NiO and perpendicular alignment when the NiO is thick.(3)The spin dependent transport phenomena in Co-based fully epitaxial single crystalline heterostructures are studied.In PtMn/Co/Ir stacks with perpendicular magnetic anisotropy,an antisymmetric magnetoresistance which could be ascribed to the anomalous hall contribution to the longitudinal resistance induced by the 180°domain wall is found.The spin Hall effect(SHE)in highly ordered Pt(111)thin films is studied through spin torque ferromagnetic resonance measurement in epitaxial Pt(111)/Co stacks,and the spin Hall angle(SHA)of 0.080 in Pt.The relative high value of SHA Pt(111)compared with the reported values from polycrystalline Pt indicate a facet dependent SHA and the potential advantage of single crystalline thin film in the study of SHE.