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Open Access Publications from the University of California

Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2015)

Multiferroic magnetoelectric (ME) materials, which simultaneously show ferroelectricity and magnetic ordering, have attracted a huge scientific attention due to both the intriguing fundamental importance and the great technological potential, as a result of the coupling between the dual ferroic orderings. However there are only a few single-phase multiferroic materials in nature and the performance is limited by the low ordering temperature, the small polarization/magnetization or the weak coupling efficiency. Another promising pathway to engineer ME coupling is through designing heterostructures. The current studies of ME coupling (electric field control of magnetism) in heterostructures can be divided into 3 routes: (1) Using piezoelectric effects to change the strain (lattice); (2) Using ferroelectric polarization to tune the carriers (charge); (3) Using multiferroic materials to manipulate the moments by magnetic coupling (spin). Previous studies of an all-oxide model heterostructure system that consists of the ferromagnet La0.7Sr0.3MnO3 (LSMO) and the BiFeO3 (BFO) reveals an interesting charge and spin interactions at the interface. Following the same route, the first part of the dissertation focuses on two different pathways to improve the ME coupling in manganite/BFO model system. Chapter 3 demonstrates that the ME coupling in LSMO/BFO is dramatically different by changing the atomic stack-ing sequence at the interface. In chapter 4, another model system La0.5Ca0.5MnO3 (LCMO), which is at the ferromagnetic/antiferromagnetic phase boundary, is studied instead of LSMO to explore the limit within this route. The results complementarily suggest the importance of a more comprehensive design rule at the atomic scale in order to achieve a better ME coupling efficiency.

Furthermore, although the coupling between the lattice, charge and spin has been intensively studied in terms of ME coupling, the orbital degree of freedom has been neglected so far. In principle, orbital is strongly coupled to lattice. Therefore a large ME coupling could be expected if a strong spin-orbit interaction could be established. The second part of the dissertation concentrates on pursuing this new ideal through two different pathways. Chapter 5 presents a systematic study of the in-situ strain effect on the magnetic and orbital orderings of a model system Nd0.5Sr0.5MnO3. The results demonstrate the close correlation between the orbital ordering parameter and the strain. However the magnetic ordering parameter is less sensitive to strain in this system. With the implication from chapter 5, 5d transition metal oxide SrIrO3(SIO), which hosts a strong intrinsic spin-orbit coupling due to the large atomic number, is studied in chapter 6. Superlattice LSMO/SIO shows an interesting novel magnetic state with a large orbital momentum in the nominally paramagnetic SIO, which is likely to be a very promising candidate to engineer the ME coupling at the interface.

In summary, our studies on engineering the ME effects in heterostructures have revealed two key implications: 1. in the traditional routes that have been extensively studied, further improvements are possible by carefully engineering the interface; 2. the coupling between spin and orbital degrees of freedom is likely to be another promising route to investigate the ME coupling in heterostructures.

    Cover page: Engineering the Magnetoelectric Coupling in Complex Oxides Heterostructures
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2021)
    Perpendicular magnetic anisotropy (PMA) has recently been shown to emerge at interfaces of 3d and 5d transition-metal oxides (TMOs). However, strategies to systematically stabilize such interface-driven PMA still remains elusive, hindering further applications of this design approach. Here, tuning crystal symmetry is shown to be an effective means to engineer this interfacial phenomenon. The evolution of PMA strength as a function of ferromagnetic oxide thickness quantitatively reveals the competition between volume- and interface-specific contributions that determine the magnetic anisotropy. By applying different degrees of epitaxial strain, the relative contributions to PMA are modulated, clearly revealing their correlations with crystal symmetries. To be more specific, the volume anisotropy energy is found to be correlated with the tetragonal distortion of the ferromagnetic layer, while the interface anisotropy energy is mainly modulated by the octahedral tilting at the interface. With these insights, superlattices with enhanced interface-driven PMA and higher Curie temperature are realized. These findings reveal a route to engineering interface-driven PMA and associated magnetic phenomena in TMO heterostructures for future spintronic applications.
      Cover page: Enhanced Interface-Driven Perpendicular Magnetic Anisotropy by Symmetry Control in Oxide Superlattices
      • Article
      • Peer Reviewed
      UC San Francisco Previously Published Works (2023)
      The naringin extraction process was optimised using response surface methodology (RSM). A central component design was adopted, which included four parameters: extraction temperature (X1), material-liquid ratio (X2), extraction time (X3), and ultrasonic frequency (X4) of 74.79 °C, 1.58 h, 1:56.51 g/mL, and 28.05 KHz, respectively. Based on these optimal extraction conditions, naringin was tested to verify the model's accuracy. Naringin yield was 36.2502 mg/g, which was equivalent to the predicted yield of 36.0124 mg/g. DM101 macroporous adsorption resin was used to purify naringin. The effects of loading concentration, loading flow rate, and sample pH on the adsorption rate of naringin and the effect of ethanol concentration on the desorption rate of naringin were investigated. The optimum conditions for naringin purification using macroporous resins were determined. The optimal loading concentration, sample solution pH, and loading flow rate were 0.075 mg/mL, 3.5, and 1.5 mL/min, respectively. Three parallel tests were conducted under these conditions, and the average naringin yield was 77.5643%. Naringin's structure was identified using infrared spectroscopy and nuclear magnetic resonance. In vitro determination of the lipid-lowering activity of naringin was also conducted. These results showed that naringin has potential applications as a functional food for lowering blood lipid levels.
        Cover page: Optimisation of the Extraction Process of Naringin and Its Effect on Reducing Blood Lipid Levels In Vitro
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2023)
        Spin-state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low-spin to high-spin transition in LaCoO3 thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen-vacancy formation, spin-state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO3 thin films is systematically modulated by varying the oxygen pressure during thin-film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen-vacancy and spin-ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low-spin to high-spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation-off-stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO3 thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation-stoichiometry engineering.
          Cover page: Manipulation of the Ferromagnetism in LaCoO3 Thin Films Through Cation‐Stoichiometric EngineeringCreative Commons 'BY-NC-SA' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2019)
          We report the observation of a linear magnetoresistance in single crystals and epitaxial thin films of the pyrochlore iridate Bi2Ir2O7. The linear magnetoresistance is positive and isotropic at low temperatures, without any sign of saturation up to 35 T. As temperature increases, the linear field dependence gradually evolves to a quadratic field dependence. The temperature and field dependence of magnetoresistance of Bi2Ir2O7 bears strikingly resemblance to the scale invariant magnetoresistance observed in the strange metal phase in high Tc cuprates. However, the residual resistivity of Bi2Ir2O7 is more than two orders of magnitude higher than the curpates. Our results suggest that the correlation between linear magnetoresistance and quantum fluctuations may exist beyond high temperature superconductors.
            Cover page: Possible scale invariant linear magnetoresistance in pyrochlore iridates Bi2Ir2O7
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2016)
            Magnetic anisotropy (MA) is one of the most important material properties for modern spintronic devices. Conventional manipulation of the intrinsic MA, i.e., magnetocrystalline anisotropy (MCA), typically depends upon crystal symmetry. Extrinsic control over the MA is usually achieved by introducing shape anisotropy or exchange bias from another magnetically ordered material. Here we demonstrate a pathway to manipulate MA of 3d transition-metal oxides (TMOs) by digitally inserting nonmagnetic 5d TMOs with pronounced spin-orbit coupling (SOC). High-quality superlattices comprising ferromagnetic La2/3Sr1/3MnO3 (LSMO) and paramagnetic SrIrO3 (SIO) are synthesized with the precise control of thickness at the atomic scale. Magnetic easy-axis reorientation is observed by controlling the dimensionality of SIO, mediated through the emergence of a novel spin-orbit state within the nominally paramagnetic SIO.
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2017)
              Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La_{1-x}Sr_{x}MnO_{3} and SrIrO_{3}, we find that all superlattices (0≤x≤1) exhibit ferromagnetism despite the fact that La_{1-x}Sr_{x}MnO_{3} is antiferromagnetic for x>0.5. PMA as high as 4×10^{6}  erg/cm^{3} is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.
                Cover page: Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La1-xSrxMnO3)/(SrIrO3) Superlattices
                • Article
                • Peer Reviewed
                Advanced Light Source (2020)
                Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized transition metal oxide that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up this class of materials for voltage-controlled functionality.
                  Cover page: Emergent electric field control of phase transformation in oxide superlattices
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