Spin Transport Properties In Ferromagnetic Insulator/Metal Heterostructures

Spin information transport plays the important role in spintronic devices such as magnetic memory and spin logic,and traditional spin information transport depends on the conduction electrons in metals.The flow of electron spins always accompanys by a charge current,which results in the Joule heating.Magnons are quasiparticles of low excited states of ferromagnets,and each magnon carries a spin angular momentum of-h.Especially the pure magnon spin information propagation in ferromagnetic insulators without any charge current is one of the most desirable properties for devices with dramatically reduced power consumption.This thesis focuses on the spin transport properties in ferromagnetic insulator/metal heterostructures,and the research work includes the following five parts:(1)Spin Hall magnetoresistance is studied in CoFe2O4/Pt heterostructures.By measuring the magnetic circular dichroism spectra(XMCD);we can obtain the induced magnetic moment of Pt by magnetic proximity effect is less than 0.004 ?B/Pt,which could be negligible.The dependence of Pt resistance with the direction of CoFe2O4 magnetic moment proves that this type of magnetoresistance comes from the spin Hall effect induced spin current communication between Pt and CoFe2O4,rather than the traditional anisotropic magnetoresistance.(2)Hanle magnetoresistance(HMR)is studied in pure Pt and YIG/Pt systems.By fitting the Pt thickness dependence of HMR,we can obtain the spin diffusion length and spin Hall angle of Pt.We can also get the spin lifetime of Pt by fitting the magnetic field dependence of HMR.By comparing the HMR in pure Pt and YIG/Pt systems,the role of edge spin accumulation and interfacial spin current in HMR could be distinguished.(3)The NiFe/IrMn exchange bias structure is proposed to study the inverse spin Hall effect(ISHE)in ferromagnetic metals.The exchange bias effect could separate the magnetization reversal process of NiFe and YIG respectively,by using transversal and longitudinal spin Seebeck measurements,the ISHE signals and planar(anomalous)Nernst siganls which are related to the magnetization of YIG and NiFe respectively could also be separated as well.A large spin Hall angle of NiFe is obtained,which is comparable with Pt with strong spin-orbit coupling.(4)Observation of magnon-mediated electric current drag in the Pt/YIG/Pt sandwich structure.The magnon drag voltage shows a linear relationship with the injection current,and the electrical drag coefficient at room temperature reaches 10-4 orders of magnitude and satisfies the T2/5 temperature dependence.The magnon drag voltage satisfies the Vt ? my dependence,where my is the y-component of YIG magnetization.By fitting the YIG thickness dependence of the electrical drag coefficient,we can get the magnon decay length of the polycrystalline YIG is 38 nm.(5)We investigated the spin Seebeck driven YIG/Au/YIG spin valve structure.The spin current across the spin valve maximized for the parallel magnetization state and minimized for the anti-parallel magnetization state,which could be measured by the ISHE voltage in top Pt layer(parallel state V??,anti-parallel state V??,By fitting the Au thickness dependence of(V??-V??)/V??.we can obtain the spin diffusion length of Au(15.1 nm).(V??-V??)/V?? shows the T5/2 temperature dependence,which is in accordance with the theoretical temperature dependence of the spin convertance between electron spin and magnon spin in ferromagnet/non-magnetic metal heterostructures.