Structure,Magnetic And Transport Properties Of Epitaxial Pt(111)/Co/Ta And Epitaxial FeSi/nonmagnetic Metal Heterojunctions

The new generation of electronic technology with spin as the transmission and storage of information will help break through the bottleneck of the information storage with semiconductor technology as the core.The spin current-related transport phenomena are the focus of research in spintronics.In recent years,the pure spin current without net charge currents and Oersted stray magnetic field effects have attracted much attention.The research of using spin current to switch the magnetization of magnetic recording layers in spintronics requires a deep understanding of the magnetic and spin relaxation mechanisms associated with spin current.Compared with polycrystalline and amorphous nano-heterojunctions,epitaxial magnetic nano-heterojunctions are the best choice to study the spin dynamics and spin transport properties owning to their well-defined crystal structure with fewer crystal impurities and defects.It is also more beneficial to study the physical mechanisms related to spin transport.Pt with large spin orbit coupling effects could produce pure spin current and is one of the most extensively studied materials due to that its relatively low resistivity could minimize the Joule heating and current shunting effects in heterostructures.The current research mainly focuses on the generation of the spin current in Pt with polycrystalline structure.In this thesis,the dependence of the transmission of spin current on the orientations of epitaxial single crystal Pt in heterojunction with perpendicular and in plane magnetic anisotropy is mainly studied.The details are as follows:?1?The MgO/Pt/Co/Ta with perpendicular magnetic anisotropy were prepared on?111?and?001?MgO single crystal substrates by direct current magnetron sputtering techniques.High-resolution X-ray diffraction?HXRD?shows that Pt in MgO?111?/Pt/Co/Ta heterojunction has an epitaxial properties with the epitaxial relationship of Pt?111?[110]//MgO?111?[110].In the case of MgO?001?/Pt/Co/Ta heterojunctions,Pt shows polycrystalline fcc structure with?111?and?001?preferred orientations.In terms of the MgO/Pt/Co/Ta heterojunction,the strength of the spin orbital torque and the corresponding spin Hall angle(?_SH)are characterized by the second harmonic Hall voltage measurements when the applied current is along different in-plane crystal directions of Pt.The estimated values of?_SH are 0.057?0.008 and 0.083?0.008 when the applied current are parallel to the MgO MgO[112?]and MgO[11?0]for the MgO?111?/Pt/Co/Ta heterojunctions,respectively.As for the MgO?001?/Pt/Co/Ta heterojunctio ns,the estimated value of?_SH are 0.048?0.007 and 0.051?0.01 when the applied current are parallel to the MgO[100]and MgO[010],respectively.The obtained results from the critical switching current density experiments verify that the spin Hall angles of Pt?111?depend on the in-plane crystal directions of single crystal Pt.The experime nt a l results show that the spin Hall angle of single crystal Pt is obviously dependent on its crystal directions,which is of great significance for understanding and enginee r ing SOT-based spintronic devices.?2?The FeSi?001?/Pt?111?,FeSi?001?/Pt?001?,FeSi/AlO_x and FeSi?001?/Ta bilayers were epitaxially grown on MgO?001?single crystal substrates by RF magnetron sputtering.The high-resolution X-ray diffraction techniques was used to obtain the epitaxial relationship between FeSi and Pt:Pt?001?[110]//FeSi?001?[100]//MgO?001?[110]and Pt?111?[110]//FeSi?001?[100]//MgO?001?[110].Ta and AlOx show amorphous properties with non-obvious diffraction peaks.The angular dependent static magnetic properties measured by the vibrating sample magnetometer and the angular dependent ferromagnetic resonance spectra obtained from the electron spin paramagnetic resonance spectrometer both show the four-fold symmetric magnet ic anisotropy of FeSi?001?films,which is consistent with the epitaxial properties of FeSi characterized by HXRD.In addition,the magnetic anisotropy originates from the cubic magnetic anisotropy of FeSi?001?.The damping constants estimated from the ferromagnetic resonance are anisotropic for the samples of FeSi?001?/Ta and FeSi?001?/AlO_x,which indicates that there exist anisotropic spin current injection when spin current is injected from FeSi into the heavy metal Ta and AlO_x layers,since the heavy metal Ta and AlO_x are amorphous with isotropic properties.As for MgO?001?/FeSi?001?/Pt?111?with the variation of the thicknesses of Pt,the magnetic relaxatio n behaviors were measured by the ferromagnetic resonance.Comparing with the anisotropic damping constants of MgO?001?FeSi?001?/AlO_x sample,there is an additional anisotropic magnetic relaxation mechanism caused by spin pumping related to the Pt crystal orientation in this system.Moreover,these results are consistent with the results obtained from the Pt?111?/Co/Ta perpendicular magnetic samples.The damping-like spin orbit torque obtained by the second harmonic Hall voltage measurement for in-plane magnetic samples with the same structure is indeed anisotropic,indicating that the anisotropic magnetic relaxation phenomenon is related to the crystal orientation of the single crystal Pt.The transmission of anisotropic spin current was explained by the Kambersky micromodel,El iott-Yafet scattering theory and spin-orbit coupling theory.