Yihong Wu Group



Nanoscale characterization using nanoprobes





We have developed a combined nano-fabrication and characterization tool consisting of (i) a scanning electron microscope with spin-polarization analysis (SEMPA), (ii) a scanning tunnel microscope (STM) or spin-dependent STM (SPSTM), (iii) four nano-probes (including the STM probe), (iv) a focused ion beam (FIB) and (v) a sample preparation and fabrication chamber with variable temperature and magnetic field features. This is a powerful system for spintronics and 2D materials research because of its unique capabilities which are not often in other places.





In-situ study of oxygen exposure effect on spin-orbit torque in Pt/Co bilayers in ultrahigh vacuum





Oxygen incorporation has been reported to increase the current-induced spin-orbit torque in ferromagnetic heterostructures, but the underlying mechanism is still under active debate. We carried out in-situ study of the oxygen exposure effect on spin-orbit torque in Pt/Co bilayers via controlled oxygen exposure, Co and Mg deposition, and electrical measurements in ultrahigh vacuum. We found that the oxygen exposure on Pt/Co indeed leads to an increase of spin-orbit torque, but the enhancement is not as large as those reported previously. Similar enhancement of spin-orbit torque is also observed after the deposition of an MgO capping layer. The results of ab initio calculations on the Rashba splitting of Pt/Co and Pt/Co/O suggest that the enhancement is due to enhanced Rashba-Edelstein effect by surface-adsorbed oxygen.





Scientific Reports volume 9, Article number: 17254 (2019).



In-situ oxygen exposure study of PMA in Pt/Co heterostructures





Co(Fe)/MOx (M = Mg, Al) heterostructures with perpendicular anisotropy (PMA) is an important building block for magnetic tunnel junction based spintronics. Studies reported in literature all point to the important role of Co(Fe)-O bond in the realization of PMA. However, given the fact that in this case, the oxygen forms part of the oxide instead of in the form of free oxygen atoms or molecules, a question naturally arises is: is the Co(Fe)-O bond or Co(Fe)-O-M bond playing the more determinant role in PMA? To shed light on this question, we systematically studied the oxygen exposure and Mg dusting effect on ultra-thin Pt/Co bilayers using an ultrahigh vacuum system with a base pressure < 5×10-9 mbar, which allows to perform in-situ deposition of Co and Mg, controlled adsorption of oxygen and anomalous Hall effect measurements without breaking the vacuum. It was found that the adsorbed oxygen reduces the effective magnetic thickness of Co and thereby affects its magnetic anisotropy and the subsequent Mg dusting can recover the magnetic moment as well as the magnetic anisotropy to a large extent. These observations are apparently different from those reported in Pt/Co(Fe)/MOx, where a maximum PMA is usually obtained upon oxidation under optimal conditions, and thus suggesting the important role of M in M-O bond of the structure. Ab initio calculations were further performed to elucidate the roles of Co-O-Mg bond. It was found that upon O adsorption the reduction of exchange splitting by charge transfer quenches the moment of the topmost Co layer, whereas the subsequently adsorbed Mg adatoms recover the moment by transferring some electrons back to Co. Our combined experimental and theoretical suggest that the role of Co-M-O bond in the realization of PMA at (Co)Fe/MOx interfaces may have been overlooked in previous studies, which stresses mostly the importance of Co(Fe)-O bond.






Y.M. Yang et al., Phys. Rev. B 95, 094417 (2017)



In-situ oxygen adsorption study of MoS2





Few-layer MoS2 is always n-type regardless of how it is prepared. Some reports attribute the origin of n-doping to the abundance of sulphur vacancies, and one of the evidences frequently used in literature to substantiate the argument is that oxygen adsorption induces p-doping in MoS2. However, the experiments conducted so far were not conducted in a controlled environment. We carried out a systematic study of the oxygen adsorption effect on the transport properties of MoS2 in an ultrahigh vacuum (UHV) system. The results revealed otherwise, i.e., chemisorption of oxygen at defect sites results in n-doping of MoS2, rather than p-doping reported in literature. First-principles calculations revealed that the n-doping effect originates from oxygen-induced lowering of the MoS2 conduction-band edge, which allows more electrons to be excited to the conduction band at room temperature. This is the first time that the role of chemisorption and physisorption of oxygen is probed and studied separately in MoS2. The results have greatly enhanced our understanding of oxygen adsorption on TMDCs, which have significant implications for future device applications of these materials. We have also found that surface passivation by chalcogen elements is able to suppress the defect-induced n-doping effect to a different degree of extent mainly through sulfur vacancy reduction. First-principles calculations support the experimental observations. The work has led to a simple passivation technique which can be applied to all TMDCs.







L. Qi et al., Appl. Phys. Lett. 108, 063103 (2016).



Imaging of magnetic vortices simultaneously with topography





Much work has been done to investigate spin configurations of permalloy circular disks by varying systematically the diameter and thickness of the patterned structures. When the thickness, diameter or both parameters are reduced, the magnetization configuration of these dots changes from the vortex state to single domain state. We have investigated the effect of successive in-situ FIB trimming on the spin configurations of hexagonal shaped ferromagnetic elements at remanent state, which was imaged by SEMPA. In contrast to the work reported elsewhere, where the variation of thickness was generally realized by fabricating structures with different pre-determined thicknesses, the advantage of using successive FIB-trimming is that the results obtained would be based on same structures, eliminating the effect of irregularity in edges caused by the fabrication processes. In addition, SEMPA offers a non-intrusive imaging which is able to provide direct information about the in-plane components of the surface spins of ferromagnetic samples. Having both the FIB and SEMPA in a same UHV system allows them to complete a series of experiments by avoiding exposing the samples to air.





Y[1] S. Y. H. Lua, et al., “Spin configuration of hexagonal shaped ferromagnetic elements in different arrangement”, J. Appl. Phys. 105, 07A319 (2009) S. Y. H. Lua et al., “Effect of in-situ FIB trimming on the spin configurations of Hexagonal Shaped Ferromagnetic Elements imaged by SEMPA”, IEEE Trans.Magn. 44, 3229 - 3232 (2008) [3] S. Y. H. Lua et al., “Chirality control and switching of vortices formed in hexagonal shaped ferromagnetic elements”, Appl. Phys. Lett. 93, 122504 (2008).




Department of Electrical and Computer Engineering

National University of Singapore

4 Engineering Drive 3, Singapore 117583

Tel: +65 65162139 Fax: +6567791103

Email: elewuyh@nus.edu.sg