Study Of The Effects Of Interfacial Electronic Structures On Magnetic Properties In Nanoscale Multilayers

The development of nano-scale materials in recent years boosts the researches on the surface/interface effect which is closely related to the size and dimensions. As ones of the most importance materials for applications on spintronic devices, the nanoscale magnetic multilayers containing ferromagnet, antiferromagnet, nonmagnetic metal and oxide have attracted many researchful and industrial interests because they possess high thermal stability, high magnetoresistance ratio, high mageto-optical Kerr effect, low curren density for current drived magnetization switching and good compatibility with semiconductor technology. It has been demonstrated that the film thickness, the lattice matching at interface, the strain, the defects, the interface roughness and the atom diffusion play important roles in many magnetic properties, including the magnetization, the coredvity, the magnetic anisotropy, the magnetic damping, the exchange bias effect, and so forth. However, to our best knowledge, little attention has been paid to the interfacial electronic structures (such as chemical bonding and orbital hybridization) and its effects on the magnetic properites. Therefore, in this Ph.D dissertation, we study the effects of interfacial electronic structures on the magnetic properties in nanoscale magnetic multilayers which are promising endicates for applications on high-performance spintronic devices. We aim to clarify the relationship between interfacial electronic structure and magnetic properties, and improve the magnetic properties by tuning interfacial electronic structures. The following researches have been carried out:(1) We have observed several unexpected phenomena when a trace amount of Fe atoms was deposited onto the CoFeB/MgO interface in Ta/CoFeB/MgO/Ta multilayers. X-ray photoelectron spectroscopy results showed that the electronic structures of CoFeB/MgO interface can be tuned by forming FeOx(x<1), Fe2O3 and Fe3O4 as the nominal thickness of the introduced Fe atoms was increased, leading to the modulation of Fe 3d-O 2p orbital hybridization at CoFeB/MgO interface. As a concequence, the perpendicular magnetic anisotropy can be enhanced or changed to in-plane magnetic anisotropy dependeing on the interfacial electronic structures. On the other hand, it was found that the introduction of Fe atoms also helped to reduce the effective magnetic damping of Ta/CoFeB/MgO/Ta multilayers. This study demonstrated that the interfacial electronic structures play a critical role in the magnetic anisotropy of nanoscale magnetic heterostructures.(2) The Blocking temperature (TB) of tunneling-anisotropic-magnetoresistance-like Pt/NiFe/IrMn/MgO/Pt multilayers was greatly enhanced from far below room temperature (RT) to above RT by inserting 1 nm thick Mg layer at IrMn/MgO interface. Furthermore, the exchange bias field (Heb) was increased as well. The evidence for a significant fraction of Mn-O bonding at IrMn/MgO interface without Mg insertion layer was provided by X-ray photoelectron spectroscopy. The bonding between Mn and O can decrease the antiferromagnetism of IrMn film, leading to lower value of TB in Pt/NiFe/IrMn/MgO/Pt multilayers. Ultrathin Mg film inserted at IrMn/MgO interface acting as an oxygen sinking layer can suppress the oxidation reactions between Mn and O and reduce the formation of Mn-0 bonding greatly. The oxidation suppression results in the recovery of the antiferromagnetism of IrMn film, which can enhance TB and Heb.This study will enhance the understanding of physics in antiferromagnet-based spintronic devices.(3) Effects of ultrathin Fe layer on the perpendicular magnetic anisotropy (PMA) in Co/Pt multilayers have been investigated. It was shown that the PMA and its annealing stability can be enhanced by doping Fe atoms at Pt/Co interface. The enhancement of PMA was attributed to the improvement of interfacial electronic structures due to the introduction of Fe atoms.(4) Effects of ultrathin Pt insertion layer on the perpendicular magnetic anisotropy (PMA) and its annealing stability of (Co/Ni)-based multilayers have been studied. It was shown that the PMA and its annealing stability can be significantly enhanced by inserting 6 A thick Pt layer at Ni/Co interface. The improvement of PMA and its annealing stability was attributed to the enhancement of interfacial orbit hybridization due to the insertion of Pt. This study is of interest for perpendicularly magnetized spintronic devices which require high thermal stability and high annealing stability.(5) Interfacial oxygen migration effect and its induced magnetic anisotropy evolution in Pt/Co/MgO/Pt and Pt/Co/SiO2/Pt multilayers have been study. Experimental results showed that oxygen atoms from the MgO and SiO2 targets combined with the neighboring Co atoms to form CoO at Co/MgO and Co/SiO2 interfaces and migrated back to MgO and SiO2 layers during the deposition of MgO and SiO2 layers and the subsequent annealing treatment, respectively. The oxygen migration effect was achieved by the redox reaction between Co and O at the Co/MgO and CO/SiO2 interfaces. The interfacial oxygen migration effect can improve both the Co 3d-O 2p orbital hybridization and the quality of Co/MgO and Co/SiO2 interfaces, leading to the evolution of magnetic anisotropy from in-palne magnetic anisotropy to PMA. This study demonstrated that the interfacial oxygen migration effect is a universal phenomenon at the ferromagnet/oxide interface and plays an important role on the magnetic properties of nanoscale magnetic multilayers.