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Open Access Publications from the University of California

Scholarly Works (16 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2024)

This dissertation mainly focuses on the unconventional spin textures in artificial magnetic nanostructures and van der Waals magnetic heterostructures, and the magnetization self-switching in van der Waals materials via the spin-orbit torque mechanism. Various state-of-the-art experimental techniques were used to characterize the unconventional phenomena; theoretical analyses were provided for every discovery; and micromagnetic simulations were performed in several different ways to build the consistency between experimental results and theoretical analyses.

In Chapter 2, I present our investigation of magnetic anisotropy in permalloy antidot square lattice. Samples were characterized by magnetic transmission soft x-ray microscopy and rotational magneto-optic Kerr effect (ROTMOKE). We also found a general artifact of the ROTMOKE technique that can show up when probing various kinds of artificial magnetic nanostructures, and we developed a method of data analysis to overcome this artifact. All the results were supported by micromagnetic simulations.

In Chapter 3, I present our discovery of spin-frustration-induced unconventional spin twisting state in van der Waals Fe5GeTe2/NiPS3 heterostructures. we find that C-type in-plane antiferromagnet NiPS3 develops three equivalent antiferromagnetic domains which are robust against external magnetic field and magnetic coupling with Fe5GeTe2. Consequently, spin frustration at the Fe5GeTe2/NiPS3 interface was shown to develop a perpendicular Fe5GeTe2 magnetization in the interfacial region that switches separately from the bulk of the Fe5GeTe2 magnetizations. In particular, we discover an unconventional spin twisting state that the Fe5GeTe2 spins twist from perpendicular direction near the interface to in-plane direction away from the interface in Fe5GeTe2/NiPS3. Our finding of the twisting spin texture is a unique property of spin frustration in van der Waals magnetic heterostructures.

In Chapter 4, I present our discovery of a new magnetic state in van der Waals ferromagnet Fe3GeTe2 induced by interfacial coupling to Co magnetic vortex, using X-ray photo-emission electron microscopy. Fe3GeTe2 domains become narrower, and some break into elongated skyrmions exhibiting a preferred orientational order along the Co vortex circulations, representing a topological vortex-nematic liquid crystal state of the skyrmions. Moreover, the Fe3GeTe2 stripe domains outside the Co vortex regions are also affected with a strong thickness-dependent behavior. Further micromagnetic simulations not only replicate the experimental results, but also reveal the thermal-history origin of the domain behaviors outside the vortex regions. Our result provides a new opportunity for future investigations into magnetic spin textures and spintronics in van der Waals magnetic heterostructures.

In Chapter 5, I present our experimental observation of spin-orbit torque magnetization self-switching at room temperature in a layered polar ferromagnetic metal, Fe2.5Co2.5GeTe2. The spin-orbit torque is generated from the broken inversion symmetry along the c-axis of the crystal. Our results provide a direct pathway toward applicable two-dimensional spintronic devices.

    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2016)

    This dissertation explores interface interactions, topology of spin structures, and the use of interface interactions in manipulating the topology of spin structures. First, I will present our work on interface interactions in magnetic thin film heterostructures. A majority of interesting phenomena in magnetic heterostructures are caused primarily by the interface interactions. In exchange bias system CoO/MgO/Fe/Ag(001), the role of frozen and rotatable antiferromagnetic spins in exchange bias system was investigated using X-ray Magnetic Linear Dichroism (XMLD). In the Py/FeMn/Ni/Cu(001) system, the indirect coupling through an antiferromagnetic spacer layer was studied through anisotropy measurements using Rotating-Field Magneto-Optical Kerr Effect (ROTMOKE).

    Second, I will present our work on artificial topological spin structures. Traditionally, research on nanostructured magnetic materials has focused on the study of uniformly magnetized thin films. With the increasing capability to prepare and manipulate non-uniform spin configurations in recent years, various topological spin structure have been realized. In magnetic thin films, magnetic vortex states can be stabilized by reducing the lateral dimension of the magnetic thin film to the nanoscale due to the increased shape anisotropy. By combining this shape anisotropy and interlayer exchange coupling at the interface, we showed that more complex topological spin structures such as spin skyrmions and hedgehog spin merons can be stabilized and manipulated. By utilizing uniaxial strain in a ferroelectric-ferromagnetic system Co/Cu/PMN-PT(011), we demonstrated a method to manipulate the circulation of a magnetic vortex state using strain.

      Cover page: Interface and Topology in Magnetism
      • Article
      • Peer Reviewed
      LBL Publications (2015)
      Tailoring the spin orientation at the atomic scale has been a key task in spintronics technology. While controlling the out-of-plane to in-plane spin orientation has been achieved by a precise control of the perpendicular magnetic anisotropy at atomic layer thickness level, a design and control of the in-plane magnetic anisotropy has not yet been well developed. On well aligned atomic steps of a 6° vicinal Cu(001) surface with steps parallel to the [110] axis, we grow Py/Ni overlayer films epitaxially to permit a systematic exploration of the step-induced in-plane magnetic anisotropy as a function of both the Py and the Ni film thicknesses. We found that the atomic steps from the vicinal Cu(001) induce an in-plane uniaxial magnetic anisotropy that favors both Py and Ni magnetizations perpendicular to the steps, opposite to the behavior of Co on vicinal Cu(001). In addition, thickness-dependent study shows that the Ni films exhibit different magnetic anisotropy below and above ~6 ML Ni thickness.
        Cover page: Tailoring the magnetic anisotropy of Py/Ni bilayer films using well aligned atomic steps on Cu(001).Creative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        LBL Publications (2014)
        Nonlinear dynamics of the magnetic vortex state in a circular nanodisk was studied under a perpendicular alternating magnetic field that excites the radial modes of the magnetic resonance. Here, we show that as the oscillating frequency is swept down from a frequency higher than the eigenfrequency, the amplitude of the radial mode is almost doubled to the amplitude at the fixed resonance frequency. This amplitude has a hysteresis vs. frequency sweeping direction. Our result showed that this phenomenon was due to a Duffing-type nonlinear resonance. Consequently, the amplitude enhancement reduced the vortex core-switching magnetic field to well below 10 mT. A theoretical model corresponding to the Duffing oscillator was developed from the Landau-Lifshitz-Gilbert equation to explore the physical origin of the simulation result. This work provides a new pathway for the switching of the magnetic vortex core polarity in future magnetic storage devices.
          Cover page: Duffing oscillation-induced reversal of magnetic vortex core by a resonant perpendicular magnetic field.Creative Commons 'BY-NC-ND' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic circular dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process modulated by the sub-nm thick Co inserting layer in a single crystalline Fe/Co/CoO/MgO(001). The CoO AFM domain switching occurs at lower temperature for the thicker Co interlayer, and the activation energy barrier of CoO AFM domain switching decreases as the Co interlayer thickness increases. The exchange coupling strength between the AFM spins in CoO layer and the ferromagnetic spins in the Fe/Co bilayer is found to be independent of the Co layer thickness. Our results suggest an approach to modulate the dynamic properties of AFM domains with an interfacial modification.
            Cover page: Antiferromagnetic domain switching modulated by an ultrathin Co interlayer in the Fe/Co/CoO/MgO(001) system
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2021)
            van der Waals (vdW) magnetic materials provide an ideal platform to study low-dimensional magnetism. However, observations of magnetic characteristics of these layered materials truly distinguishing them from conventional magnetic thin film systems have been mostly lacking. In an effort to investigate magnetic properties unique to vdW magnetic materials, we examine the exchange bias effect, a magnetic phenomenon emerging at the ferromagnetic-antiferromagnetic interface. Exchange bias is observed in the naturally oxidized vdW ferromagnet Fe3GeTe2, owing to an antiferromagnetic ordering in the surface oxide layer. Interestingly, the magnitude and thickness dependence of the effect is unlike those expected in typical thin-film systems. We propose a possible mechanism for this behavior, based on the weak interlayer magnetic coupling inherent to vdW magnets, demonstrating the distinct properties of these materials. Furthermore, the robust and sizable exchange bias for vdW magnets persisting up to relatively high temperatures presents a significant advance for realizing practical two-dimensional spintronics.
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2023)
              We investigate the interaction between two magnetic layers separated with a normal metal insertion layer (Ti, Pt, and Ru) using x-ray ferromagnetic resonance (XFMR). We measure the amplitude and phase of the ferromagnetic resonance of both layers. Our results indicate that a ferromagnetic exchange coupling between two layers is a dominant coupling mechanism for a thick insertion metal layer. Based on the exchange coupling model, we extract the smallest value of the indirect exchange coefficient of 1.2 μJ/m2, which corresponds to an exchange field of about 0.36 mT. While this value is difficult to measure with other experimental tools, we were able to measure the small value because XFMR detects a resonance phenomenon of a thin layer generated by an oscillating indirect exchange and the Oersted fields with a phase and layer resolved observation.
                • Article
                • Peer Reviewed
                UC Berkeley Previously Published Works (2021)
                The Berry phase, which reveals the intimate geometrical structure underlying quantum mechanics, plays a central role in the anomalous Hall effect. In this work, we observed a sign change of Berry curvatures at the interface between the ferromagnet SrRuO3 (SRO) layer and the SrIrO3 (SIO) layer with strong spin-orbit coupling. The negative Berry curvature at the interface, induced by the strongly spin-orbit-coupled Ir 5d bands near the Fermi level, makes the SRO/SIO interface different from the SRO layer that has a positive Berry curvature. These opposite Berry curvatures led to two anomalous Hall effect (AHE) channels with opposite signs at the SRO/SIO interface and in the SRO layer, respectively, resulting in a hump-like feature in the Hall resistivity loop. This observation offers a straightforward explanation of the hump-like feature that is usually associated with the chiral magnetic structure or magnetic skyrmions. Hence, this study provides evidence to oppose the widely accepted claim that magnetic skyrmions induce the hump-like feature.
                  • Article
                  • Peer Reviewed
                  UC Berkeley Previously Published Works (2020)
                  Insulating antiferromagnets have recently emerged as efficient and robust conductors of spin current. Element-specific and phase-resolved x-ray ferromagnetic resonance has been used to probe the injection and transmission of ac spin current through thin epitaxial NiO(001) layers. The spin current is found to be mediated by coherent evanescent spin waves of GHz frequency, rather than propagating magnons of THz frequency, paving the way towards coherent control of the phase and amplitude of spin currents within an antiferromagnetic insulator at room temperature.
                    Cover page: Coherent Transfer of Spin Angular Momentum by Evanescent Spin Waves within Antiferromagnetic NiO
                    • Article
                    • Peer Reviewed
                    UC Berkeley Previously Published Works (2024)
                    Polymorph engineering involves the manipulation of material properties through controlled structural modification and is a candidate technique for creating unique two-dimensional transition metal dichalcogenide (TMDC) nanodevices. Despite its promise, polymorph engineering of magnetic TMDC monolayers has not yet been demonstrated. Here we grow FeSe2 monolayers via molecular beam epitaxy and find that they have great promise for magnetic polymorph engineering. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we find that FeSe2 monolayers predominantly display a 1T' structural polymorph at 5 K. Application of voltage pulses from an STM tip causes a local, reversible transition from the 1T' phase to the 1T phase. Density functional theory calculations suggest that this single-layer structural phase transition is accompanied by a magnetic transition from an antiferromagnetic to a ferromagnetic configuration. These results open new possibilities for creating functional magnetic devices with TMDC monolayers via polymorph engineering.
                      Cover page: Synthesis and Polymorph Manipulation of FeSe2 MonolayersCreative Commons 'BY-NC-ND' version 4.0 license
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