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Open Access Publications from the University of California

Scholarly Works (13 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2019)

Spintronics has been an active research area for decades, as pure spin current which is free of Joule heating oers a promising possibility to build next-generation electronic devices with revolutionarily high energy efficiency. Spin-related transport phenomena such as anomalous Hall effect (AHE), spin Seebeck effect (SSE) and spin Hall magnetoresistance (SMR), serving as reliable probes, help researchers understand the generation, propagation and conversion of spin current as well as the mechanisms behind in ferromagnetic-materialbased and antiferromagnetic-material-based spintronic systems. In Chapter 1, I will go through a brief introduction to those spin-related transport phenomena especially in ferromagnetic insulators (FMI) and antiferromagnetic insulators (AFMI) related systems.

FMI-based heterostructures with normal metals (NM) that have strong spin-orbit coupling, such as yttrium iron garnet YIG/Pt bilayers, have attracted a great deal of attention. In such systems, electric signal can only be generated in the metal layers adjacent to the FMI layer, which allows for more valid investigations into the pure spin current related phenomena. To better study various spin-related transport phenomena in FMI/NM bilayers, good control over the magnetic anisotropy of FMI is essential. In Chapter 2, I will go through the techniques we use to engineer and manipulate magnetic anisotropy in FMI by choosing proper substrates that provide desired strain, tuning thickness of FMI and varying temperature. These tuning capabilities enable us to discover topological Hall effect (THE) in TmIG/Pt bilayers at room temperature and conduct systematically study to explore its physical origin.

In FMI/NM bilayers where NM has strong spin-orbit coupling, there have been some debates on whether those spin-related transport phenomena originate from the magnetic proximity effect (MPE) that NMs get magnetized by adjacent FMI layer or come from pure spin current contribution via spin Hall and inverse spin Hall effects in the NM layer. In Chapter 3, I will discuss my work on temperature dependence study of AHE in TmIG/Pt and TmIG/Cu/Pt heterostructures. The distinct behaviors in AHE temperature dependence w/o Cu spacer indicate that MPE dominates the AHE signal.

In recent years, great efforts have been devoted to the study of spintronics in antiferromagnetic insulator(AFMI) related heterostructures thanks to its unique properties of ultrafast spin dynamics and robustness against external field. AFMI working either as sources generating terahertz spin current or as a spacer in between FMI and NM that manipulates magnon propagation offer numerous interesting questions to be fully explored. Cr2O3 which is an uniaxial AFMI, has been a popular candidate for these study purposes. In Chapter 4, I will review the progress of recent AFMI related spin transport study. Then discuss in details of my research on the study of YIG/Cr2O3/Pt heterostructures, which begins from the growth and characterization of Cr2O3 thin films on FMI YIG and then the observed unique behaviors in SSE measurements. The results are to be associated with the characteristic properties of Cr2O3 which reveal the existence of AFM magnon contribution in SSE.

    Creative Commons 'BY' version 4.0 license
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2021)
      Cover page: Ultrafast measurements of the interfacial spin Seebeck effect in Au and rare-earth iron-garnet bilayers
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2013)
      Arabidopsis thaliana cryptochrome 2 (CRY2) mediates light control of flowering time. CIB1 (CRY2-interacting bHLH 1) specifically interacts with CRY2 in response to blue light to activate the transcription of FT (Flowering Locus T). In vitro, CIB1 binds to the canonical E-box (CACGTG, also referred to as G-box) with much higher affinity than its interaction with non-canonical E-box (CANNTG) DNA sequences. However, in vivo, CIB1 binds to the chromatin region of the FT promoter, which only contains the non-canonical E-box sequences. Here, we show that CRY2 also interacts with at least CIB5, in response to blue light, but not in darkness or in response to other wavelengths of light. Our genetic analysis demonstrates that CIB1, CIB2, CIB4, and CIB5 act redundantly to activate the transcription of FT and that they are positive regulators of CRY2 mediated flowering. More importantly, CIB1 and other CIBs proteins form heterodimers, and some of the heterodimers have a higher binding affinity than the CIB homodimers to the non-canonical E-box in the in vitro DNA-binding assays. This result explains why in vitro CIB1 and other CIBs bind to the canonical E-box (G-box) with a higher affinity, whereas they are all associated with the non-canonical E-boxes at the FT promoter in vivo. Consistent with the hypothesis that different CIB proteins play similar roles in the CRY2-midiated blue light signaling, the expression of CIB proteins is regulated specifically by blue light. Our study demonstrates that CIBs function redundantly in regulating CRY2-dependent flowering, and that different CIBs form heterodimers to interact with the non-canonical E-box DNA in vivo.
        Cover page: Multiple bHLH Proteins form Heterodimers to Mediate CRY2-Dependent Regulation of Flowering-Time in Arabidopsis
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2013)
        Plants possess multiple photoreceptors to mediate light regulation of growth and development, but it is not well understood how different photoreceptors coordinate their actions to jointly regulate developmental responses, such as flowering time. In Arabidopsis, the photoexcited cryptochrome 2 interacts with the transcription factor CRYPTOCHROME-INTERACTING basic helix-loop-helix 1 (CIB1) to activate transcription and floral initiation. We show that the CIB1 protein expression is regulated by blue light; CIB1 is highly expressed in plants exposed to blue light, but levels of the CIB1 protein decreases in the absence of blue light. We demonstrate that CIB1 is degraded by the 26S proteasome and that blue light suppresses CIB1 degradation. Surprisingly, although cryptochrome 2 physically interacts with CIB1 in response to blue light, it is not the photoreceptor mediating blue-light suppression of CIB1 degradation. Instead, two of the three light-oxygen-voltage (LOV)-domain photoreceptors, ZEITLUPE and LOV KELCH PROTEIN 2, but not FLAVIN-BINDING KELCH REPEAT 1, are required for the function and blue-light suppression of degradation of CIB1. These results support the hypothesis that the evolutionarily unrelated blue-light receptors, cryptochrome and LOV-domain F-box proteins, mediate blue-light regulation of the same transcription factor by distinct mechanisms.
          Cover page: Arabidopsis CRY2 and ZTL mediate blue-light regulation of the transcription factor CIB1 by distinct mechanisms
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2021)
          The interplay between chirality and magnetism generates a distinct physical process, the magneto-chiral effect, which enables one to develop functionalities that cannot be achieved solely by any of the two. Such a process is universal with the breaking of parity-inversion and time-reversal symmetry simultaneously. However, the magneto-chiral effect observed so far is weak when the matter responds to photons, electrons, or phonons. Here we report the first observation of strong magneto-chiral response to excitons in a twisted bilayer tungsten disulfide with the amplitude of excitonic magneto-chiral (ExMCh) anisotropy reaches a value of ~4%. We further found the ExMCh anisotropy features with a spectral splitting of ~7 nm, precisely the full-width at half maximum of the excitonic chirality spectrum. Without an externally applied strong magnetic field, the observed ExMCh effect with a spontaneous magnetic moment from the ferromagnetic substrate of thulium iron garnet at room temperature is favorable for device applications. The unique ExMCh processes provide a new pathway to actively control magneto-chiral applications in photochemical reactions, asymmetric synthesis, and drug delivery.
            Cover page: Observation of strong excitonic magneto-chiral anisotropy in twisted bilayer van der Waals crystals
            • Article
            • Peer Reviewed
            UC Riverside Previously Published Works (2018)
            Spin-orbit coupling (SOC) in graphene can be greatly enhanced by proximity coupling it to transition metal dichalcogenides (TMDs) such as WSe2. We find that the strength of the acquired SOC in graphene depends on the stacking order of the heterostructures when using hexagonal boron nitride ( h-BN) as the capping layer, i.e., SiO2/graphene/WSe2/ h-BN exhibiting stronger SOC than SiO2/WSe2/graphene/ h-BN. We utilize photoluminescence (PL) as an indicator to characterize the interaction between graphene and monolayer WSe2 grown by chemical vapor deposition. We observe much stronger PL quenching in the SiO2/graphene/WSe2/ h-BN stack than in the SiO2/WSe2/graphene/ h-BN stack and, correspondingly, a much larger weak antilocalization (WAL) effect or stronger induced SOC in the former than in the latter. We attribute these two effects to the interlayer distance between graphene and WSe2, which depends on whether graphene is in immediate contact with h-BN. Our observations and hypothesis are further supported by first-principles calculations, which reveal a clear difference in the interlayer distance between graphene and WSe2 in these two stacks.
              Cover page: Effect of Distance on Photoluminescence Quenching and Proximity-Induced Spin–Orbit Coupling in Graphene/WSe2 Heterostructures
              • Article
              • Peer Reviewed
              UC Riverside Previously Published Works (2018)
              This work demonstrates markedly modified spin dynamics of magnetic insulator (MI) by the spin momentum-locked Dirac surface states of the adjacent topological insulator (TI), which can be harnessed for spintronic applications. As the Bi concentration x is systematically tuned in 5-nm-thick (Bi x Sb1-x )2Te3 TI films, the weight of the surface relative to bulk states peaks at x = 0.32 when the chemical potential approaches the Dirac point. At this concentration, the Gilbert damping constant of the precessing magnetization in 10-nm-thick Y3Fe5O12 MI films in the MI/TI heterostructures is enhanced by an order of magnitude, the largest among all concentrations. In addition, the MI acquires additional strong magnetic anisotropy that favors the in-plane orientation with similar Bi concentration dependence. These extraordinary effects of the Dirac surface states distinguish TI from other materials such as heavy metals in modulating spin dynamics of the neighboring magnetic layer.
                Cover page: Dirac surface state–modulated spin dynamics in a ferrimagnetic insulator at room temperature
                • Article
                • Peer Reviewed
                UC Riverside Previously Published Works (2019)
                Non-volatile memory and computing technology rely on efficient read and write of ultra-tiny information carriers that do not wear out. Magnetic skyrmions are emerging as a potential carrier since they are topologically robust nanoscale spin textures that can be manipulated with ultralow current density. To date, most of skyrmions are reported in metallic films, which suffer from additional Ohmic loss and thus high energy dissipation. Therefore, skyrmions in magnetic insulators are of technological importance for low-power information processing applications due to their low damping and the absence of Ohmic loss. Moreover, they attract fundamental interest in studying various magnon-skyrmion interactions11. Skyrmions have been observed in one insulating material Cu2OSeO3 at cryogenic temperatures, where they are stabilized by bulk Dzyaloshinskii-Moriya interaction. Here, we report the observation of magnetic skyrmions that survive above room temperature in magnetic insulator/heavy metal heterostructures, i.e., thulium iron garnet/platinum. The presence of these skyrmions results from the Dzyaloshinskii-Moriya interaction at the interface and is identified by the emergent topological Hall effect. Through tuning the magnetic anisotropy via varying temperature, we observe skyrmions in a large window of external magnetic field and enhanced stability of skyrmions in the easy-plane anisotropy regime. Our results will help create a new platform for insulating skyrmion-based room temperature low dissipation spintronic applications.
                  Cover page: Topological Hall effect at above room temperature in heterostructures composed of a magnetic insulator and a heavy metal
                  • Article
                  • Peer Reviewed
                  UC Riverside Previously Published Works (2017)
                  The quantum anomalous Hall effect (QAHE) that emerges under broken time-reversal symmetry in topological insulators (TIs) exhibits many fascinating physical properties for potential applications in nanoelectronics and spintronics. However, in transition metal-doped TIs, the only experimentally demonstrated QAHE system to date, the QAHE is lost at practically relevant temperatures. This constraint is imposed by the relatively low Curie temperature (Tc) and inherent spin disorder associated with the random magnetic dopants. We demonstrate drastically enhanced Tc by exchange coupling TIs to Tm3Fe5O12, a high-Tc magnetic insulator with perpendicular magnetic anisotropy. Signatures showing that the TI surface states acquire robust ferromagnetism are revealed by distinct squared anomalous Hall hysteresis loops at 400 K. Point-contact Andreev reflection spectroscopy confirms that the TI surface is spin-polarized. The greatly enhanced Tc, absence of spin disorder, and perpendicular anisotropy are all essential to the occurrence of the QAHE at high temperatures.
                    Cover page: Above 400-K robust perpendicular ferromagnetic phase in a topological insulator
                    • Article
                    • Peer Reviewed
                    UC Riverside Previously Published Works (2019)
                    We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.
                      Cover page: Exploring interfacial exchange coupling and sublattice effect in heavy metal/ferrimagnetic insulator heterostructures using Hall measurements, x-ray magnetic circular dichroism, and neutron reflectometry
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