Yihong Wu Group



Characterization of spin-charge interconversion using spin-torque bridge





Quantification of spin-orbit torque (SOT) is extremely important for developing an understanding of the spin-charge interconversion process as well as to exploit the SOT for practical applications. The SOT can be characterized using different techniques including spin-torque ferromagnetic resonance (ST-FMR) [Phys. Rev. Lett. 106, 036601 (2011)], harmonic Hall voltage (Appl. Phys. Lett. 97, 162507 (2010) and Phys. Rev. B 89, 144425 (2014)), and 2nd order planar Hall effect (Nat. Comm. 5, 3042 (2014)). We have developed a spin-torque bridge method to characterize the fieldlike SOT in films with in-plane magnetic anisotropy. The spin torque method has a high immunity to thermal drift and thus produces much cleaner signals as compared to other methods





Appl. Phys. Lett. 111, 032402 (2017) J. Appl. Phys., 122, 193904 (2017) Adv. Mater. Technol. 3, 1800073 (2018) Phys. Rev. B 100, 094413 (2019) Phys. Rev. Applied 11, 064021 (2019).


THz emission based on anomalous Hall effect from a single layer FM





Recently several groups have demonstrated efficient THz emission from femtosecond (fs) laser excited magnetic heterostructures via either the inverse spin Hall effect (ISHE) or inverse Rashba-Edelstein effect (IREE). We investigate THz emission from a single layer ferromagnet via the anomalous Hall effect (AHE). It involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by fs laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the AHE, thereby generating the THz radiation. The THz emission can be either enhanced or suppressed, or even the polarity can be reversed, by introducing a magnetization gradient in the thickness direction of the ferromagnet. Unlike THz emitters based on the ISHE or IREE, the AHE-based emitter does not require a non-magnetic layer with large spin-orbit coupling or a Rashba interface.




Phys. Rev. Applied 12, 054027 (2019).



Spin-orbit torque from single layer ferromagnet





Spin-orbit torque (SOT) has been demonstrated as a promising technique to manipulate the magnetization of ferromagnet or Neel vector of anti-ferromagnet. But so far, the SOT was only observed in systems with either broken bulk or structure inversion asymmetry. Particularly, for the practically useful transition metal ferromagnet and related alloys, one has to resort to a non-magnet (NM)/ferromagnet (FM) heterostructures with broken structure inversion symmetry in order to obtain a sizable SOT.


Recently, we have demonstrated the presence of SOT in a single FM layer which lacks both bulk and structure inversion asymmetry. Specifically, we characterized the SOT in single Fe0.8Mn0.2 layers using both the nonlinear magnetoresistance and second-order planar Hall effect methods, and found that the current-induced SOT in single-layer Fe0.8Mn0.2 is as efficient as in Pt/NiFe bilayers with the same FM thickness. Since the SOT is more prominent in samples with large spin-orbit coupling and small thickness, we attribute the SOT to spin rotation at the vicinity of the surface via spin-orbit dependent scattering. Due to the scattering asymmetry for spin-up and spin-downs electrons, the torques from the top and bottom surfaces simply add up instead of cancelling out with each other, which effectively removes the requirement of spatial inversion asymmetry for generating SOT.





Z.Y. Luo et al, Phys. Rev. Applied 11, 064021 (2019)



Anomalous hall magnetoresistance from a single layer ferromagnet





Magnetoresistance in ferromagnetic materials and related heterostructures plays essential roles both in fundamental understanding of magnetism and electron transport in these structures and in various technological applications. Recently, several new types of magnetoresistance effects of different origins have attracted much attention, which include spin Hall magnetoresistance (SMR) in ferromagnet/heavy metal (FM/HM) bilayers, Rashba-Edelstein magnetoresistance (REMR) in ferromagnet/nonmagnetic metal (FM/NM) heterostructures, and Hanle magnetoresistance (HM) in heavy metals (see illustration below a-c). One key aspect of all these newly observed MR phenomena is that they all originate from a two-step charge-spin conversion process, i.e., in the first step charge current is converted to spin current through either the spin Hall effect or the Rashba-Edelstein effect, and in the second step part of the reflected spin current is converted back to charge current by the respective inverse effects.


We have recently reported on the observation of a new type of magnetoresistance, dubbed anomalous Hall magnetoresistance (AHMR). The AHMR originates from the anomalous Hall effect (AHE) and its inverse (i.e., IAHE). Although it also involves a two-step charge-spin conversion process, unlike the SMR which is typically present in FM/HM bilayers, the AHMR was observed in a single layer of FM. In addition to serving as an important tool for studying spin-charge conversion processes in FM, the AHMR is also promising for spintronics applications as the polarization of the spin current generated by the AHE can be controlled by the magnetization direction.





Y.M. Yang et al., Nature Communications 9, 2255 (2018).



Spin-orbit torque in antiferromagnet/heavy metal bilayer





Antiferromagnet (AFM) plays an important role in magnetic storage including magnetic media, magnetic read head and magnetic random access memory. As the main role of AFM is to assist its ferromagnet (FM) counterpart to attain good thermal stability rather than to serve as an active element, its role is often overshadowed by the FM. However, recently, there is an increasing interest in exploiting the AFM as an active element in spintronic devices arising from its advantages of negligible shape anisotropy and good thermal stability as compared to the FM. To this end, however, one must find a viable way to obtain sizable output signal from the AFM directly related to its spin configuration and an efficient mechanism to manipulate its spin state, both of which are known to be more difficult than the case of FM. Focusing on the second aspect, we have demonstrated experimentally the presence of spin-orbit torque (SOT) in Pt/FeMn bilayers and Pt/FeMn/NiFe trilayers. A large SOT effective field was obtained, which is able to induce clear spin-flop like spin canting in FeMn and rotation of magnetization of NiFe in Pt/FeMn/NiFe trilayers via exchange coupling at the FeMn/NiFe interface.





Y. M. Yang et al., Phys. Rev. B 93, 094402 (2016).



Spin-orbit torque in FeMn/Pt multilayers





Current-induced switching of magnetization based on spin-orbit torque (SOT) is actively studied for potential spintronic applications. However, most of the SOT-induced magnetization switching researches have been focused on ferromagnet (FM) / heavy metal (HM) bilayers with an ultrathin FM, which may have thermal stability issues in practical applications.


As an attempt to mitigate the thermal issue, we stacked ultrathin FeMn and Pt alternately to form multilayer structures. Despite the fact that bulk FeMn is an antiferromagnet and Pt is a paramagnet, the FeMn/Pt multilayers were found to exhibit ferromagnetic properties above room temperature. We performed systematic current-driven magnetization switching experiments and confirmed that the magnetization of FeMn/Pt multilayers can be switched reversibly by its own current without the need of an external field and/or a thick HM layer for providing the necessary torque. Both the strength of the current-induced effective field and the current-driven magnetization switching process can be explained reasonably well using theoretical models.





Y. J. Xu et al., Scientific Reports 6, 26180 (2016).





Department of Electrical and Computer Engineering

National University of Singapore

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