Study Of Anisotropic Magnetoresistance And Surface Effect In Ferromagnetic-Metal/Nonmagnetic-material Heterostructures

The development of spintronics is based upon electronic spin-dependent transport properties,which paved the way for high-impact technological devices.The spin-related phenomena include anisotropic magnetoresistance,planar Hall effect,anomalous Hall effect and spin Hall effect.Using spin-polarized electrons to store and process information in nanostructures has triggered enormous activity and effort to understand and control spin-dependent transport.Nowadays,magnetoresistance effects have been widely used for spin-related device such as magnetic random access memories?MRAM?.The element of most of such devices is consists of nanoscale multilayer structures with ferromagnetic layers.In order to understand and optimize the magnetoresistance of such heterostructures and nanostructures,it is important to develop an improved understanding of the magnetotransport properties of ferromagnetic metal/nonmagnetic material heterostructure films.In this way we systematically study the anisotropic magnetoresistance?AMR?in thin magnetic films and find different types of MR observed in various systems so far,including tranditional AMR,AMR with geometric size effect,interfacial effect.In this thesis we investigate spin-related transport properties of ferromagnetic metal/nonmagnetic material heterostructures from three parts,including traditional AMR modulation,perpendicular AMR discovery and perpendicular AMR regulation.And the specific contents are as follows:?1?Oxygen migration plays important roles in magnetic and transport properties of the FM/oxide heterostructure.Reversible control of interfacial oxidation state has been successfully achieved via applying electric field.Whether oxygen migration could be achieved in a single annealing process is still not clear.Based on this,we investigate the effect of thermodynamic properties on transport properties of oxide/NiFe heterostructures?oxide = SiO₂,MgO or HfO₂?.We report that the magnetic and electronic transport properties in oxide/NiFe?2 nm?/oxide film are strongly influenced by the electronic structure of NiFe/oxide interface.Magnetic measurements show that the SiO₂ sandwiched film exhitbits the lowest saturated magnetization?Ms?,MR and thermodynamic stability,and that the MgO sandwiched film exhibits improved Ms,MR and thermodynamic stability compared to the SiO₂ sandwiched film,while the HfO₂ sandwiched film exhibits the most outstanding Ms,MR and thermodynamic stability?For the NiFe?10nm?,the MR ratio remain 3.17%after annealing at 550??.For the ultrathin NiFe?2nm?,in the ultrathin SiO₂ sandwiched film no magnetoresistance?MR?is detected,while in the ultrathin MgO sandwiched film and HfO₂ sandwiched film the MR ratios reach 0.35%and 0.88%,respectively.The investigation by X-ray photoelectron spectroscopy reveals that the distinct interfacial redox reactions,which are dependent on the oxide layers,lead to the variation of magnetic and transport properties in different oxide/NiFe/oxide heterostructures.The elemental electronegativity and formation enthalpy of the oxides play important roles in the interfacial electronic variation.This work suggests a thermodynamic approach to select alternative oxide interlayer and provides further insight into control of interfacial structure in NiFe-based magnetic sensors,spin valves and magnetic random access memory?MRAM?.?2?In recent years,an anomalous MR in ferromagnetic-insulator/heavy-metal heterostructures has been observed when the magnetization vector is swept on the plane perpendicular to the current,and the argument of whether spin hall magnetoresistance or magnetic proximity effect could contribute to the anomalous MR follows.Considering that AMR effect has played a key role in measurements of spin Hall angle as well as spin torque generation,the study of angular AMR of ferromagnetic-metal related heterostructures is urgent.However,most of angular AMR focused on heavy metal.Based on this,we investigate MgO/Fe heterostructures without heavy metal.And a large perpendicular anisotropic magnetoresistance?AMR?is obtained in a MgO/Fe/Cu heterostructure,with a maximum AMR ratio of 0.59%.The perpendicular AMR is approximately inversely dependent on the thickness of the Fe layer above 2.5 nm.It can be increased by enhanced structural asymmetry,which implies an interface effect mainly derived from interfacial Rashba spin-orbit scattering in the MgO/Fe/Cu hetero structure.The perpendicular AMR is temperature dependent.This work on perpendicular AMR exhibits potential applications in three-dimensional magnetic sensors in combination with conventional in-plane AMR.Our study also may provide a reminder of implications for measurement of SHE or MPE?3?Owing to texture-related geometrical-size effect ptrans is larger than pperp in pure Ni or Co thin films,where ptrans and pperp are the resistivity when the saturation magnetic field in,transverse and perpendicular direction?magnetization M oriented relative to current?,respectively.In contrast to geometrical-size effect,Ptrans is smaller than ?perp in Pt/Co/Pt sandwich.What type of interface effect induced the different anomalous AMR in Co/Pt layers is still needed to be investigated.Besides,it is also need to figure out the relationship between the interface effect and the geometrical-size effect.Based on this,we investigate the perpendicular AMR of CoO/Co/Pt heterostructures.And a reversal in sign of perpendicular anisotropic magnetoresistance?AMR?is found in the film CoO?3 nm?/Co?3 nm?/Pt?3 nm?compared to the film CoO?3 nm?/Co?3 nm?/CoO?3 nm?,which is associated with Pt.Moreover,perpendicular AMR is tunable in the films CoO?3 nm?/Co?3 nm?/CoO?t?/Pt?3 nm?and CoO?3 nm?/Co?3 nm?/Pt?t?/CoO?3 nm?at will,either with positive or negative value,by varying the inserting layer thickness.The spin-related transport and XPS studies manifest that magnetic proximity effect,surface scattering,together with oxygen migration contribute to the tunable function of Pt.The present work would also deepen the understanding on the magnetoresistance of heterostructures in spintronic devices such as magnetic random access memories?MRAM?.