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Scholarly Works (8 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2022)

The recent advent of 2D atomically layered ferromagnetic materials has brought a great deal of excitement in condensed matter physics and materials science. Despite the intense ongoing research, the physics of many fascinating phenomena is still not completely understood. We explore several physical properties of a special type of 2D ferromagnets: Fe3GeTe2 (FGT) and Fe5Ge2Te2 (FG2T) in my dissertation research. FGT and FG2T are layered metallic ferromagnets with the phase transition from paramagnetic to ferromagnetic phase near room temperature, the highest among other known 2D magnets. The high Curie temperature, strong magnetic anisotropy, and the metallicity make them the best candidate materials for the next generation of high-density memory and logic devices.In this dissertation, I will present my study of the 2D FGT and FG2T nanodevices from the perspective of magneto-transport measurements. I present and discuss the fabrication process challenges in fabricating FGT and FG2T flakes with nanometer dimensions, the layer dependence of the low-temperature Kondo behaviors and the linear magnetoresistance, and the layer dependence of the perpendicular magnetic anisotropy field, Curie temperature, longitudinal and anomalous Hall conductivities. In addition, I will discuss the effect of the spin current generated in Pt on the FGT magnetization reversal due to the damping-like spin-orbit torque. Also, we quantify the magnitude of the SOT effect by measuring the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane.

  • 5 supplemental PDFs
Cover page: Layered Dependence of the Magneto Transport Properties of Metallic Ferromagnets: Fe5Ge2Te2, Fe3GeTe2
  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2016)

Introduction:

Since there is a lack in knowing the etiology of OLP “Oral Lichen Planus”, there is still a gap in the understanding and subsequently correct management of OLP.

OLP is a chronic immunologic mucocutaneous disease affecting on average 0.5-3% of the population with a significant female predilection, and a potential risk of developing into a carcinoma at advanced stages.

Still used widely among practitioners, steroids appear to remain the most reliable treatment modality among other options including the newer photodynamic therapy “PDT”.

PDT is a minimally invasive procedure, kills damaged cells by the use of oxygen, chemistry and light. Among its many advantages, this modality doesn’t usually result in systemic involvement as in the case of steroids; and it’s done in a series of appointments to monitor results and stage the need for additional therapy accordingly.

The importance of this research stems out of the awareness of an existing gap in the knowledge, and the importance of the lesion by itself with its malignant potential.

The aims of this research are to investigate whether or not PDT poses a better treatment option for OLP in adult patients than topical steroids, and out of the two concentrations of the most frequently prescribed topical corticosteroid, which one is more effective than the other.

Methods:

The search strategy included search engines and hand searching to obtain systematic reviews and randomized clinical trials relevant to the search question. Independent readers validated the quality of the evidence prior to conducting the acceptable sampling analysis.

Results:

For the main hypothesis, the bibliome consisted of four clinical trials, of which only one was yielded by the acceptance sampling process.

For the correlatory hypothesis, the bibliome consisted of one clinical trial, which was retained following the acceptable sampling step.

Qualitative assessment was performed on the findings of both search results and a qualitative consensus was made based on the best available evidence detected.

Conclusion:

Laser phototherapy is more effective for treatment of atrophic-erosive oral lichen planus than topical steroids. There is no significant difference between the two concentrations of topical Clobetasol of 0.025% and 0.05% in the treatment of OLP.

Key words: Oral lichen planus, photodynamic therapy, steroids, evidence-based dentistry, CER, Clobetasol.

    Cover page: Comparative effectiveness research in photodynamic therapy for oral lichen planus
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2024)
    The purpose of the present study was to understand students school readiness as a function of student and teacher behaviors but also school size and class size using both linear and non-linear analytical approaches. Data came from 21,903 schools distributed across 80 countries as per the 2018 cohort of the PISA database. Results pointed to a preference for the Cusp model in that the relationship between school and class sizes with achievement proved to be best described by the non-linearity of the Cusp catastrophe model. The critical benchmarks were a school size of 801 students and a class size of 27 students for which increases beyond those thresholds were linked to nonlinearity and unpredictability in school readiness. For this reason, we suggest using the cusp catastrophe model from Nonlinear Dynamical Systems Theory (NDST) to understand more fully such complex phenomena.
      Cover page: The effect of school size and class size on school preparedness.
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2024)
      We have investigated the electrical transport properties of nanodevices fabricated from exfoliated flakes of two-dimensional metallic ferromagnets Fe3GeTe2 (FGT) and Fe5Ge2Te2 (FG2T) down to below three layers in thickness. The per-layer anomalous Hall conductivity even in thick FGT and FG2T devices is found to be much smaller than ∼e2h, the approximate value calculated for thick undoped crystals. Moreover, we obtain a power-law scaling relation between the per-layer anomalous Hall and per-layer longitudinal conductivities with an exponent close to 1.6, which agrees with the universal value for poor ferromagnetic conductors. Both FGT and FG2T devices show clear layer-dependent Curie temperatures and layer-dependent perpendicular magnetic anisotropy, with FG2T dominating the former and FGT dominating the latter for all thicknesses. Despite their declining trend as the device thickness decreases, both Curie temperature and magnetic anisotropy retain a significant fraction of their bulk values (>60% and >80% of the bulk values, respectively, even in the thinnest FG2T device), indicating attractive potential for practical applications.
        Cover page: Layer-dependence study of two-dimensional ferromagnets: Fe3GeTe2 and Fe5Ge2Te2
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2019)
        Among van der Waals (vdW) layered ferromagnets, Fe3GeTe2 (FGT) is an excellent candidate material to form FGT/heavy metal heterostructures for studying the effect of spin-orbit torques (SOT). Its metallicity, strong perpendicular magnetic anisotropy built in the single atomic layers, relatively high Curie temperature (Tc ∼ 225 K), and electrostatic gate tunability offer a tantalizing possibility of achieving the ultimate high SOT limit in monolayer all-vdW nanodevices. In this study, we fabricate heterostructures of FGT/Pt with 5 nm of Pt sputtered onto the atomically flat surface of ∼15-23 nm exfoliated FGT flakes. The spin current generated in Pt exerts a damping-like SOT on FGT magnetization. At ∼2.5 × 1011 A/m2 current density, SOT causes the FGT magnetization to switch, which is detected by the anomalous Hall effect of FGT. To quantify the SOT effect, we measure the second harmonic Hall responses as the applied magnetic field rotates the FGT magnetization in the plane. Our analysis shows that the SOT efficiency is comparable with that of the best heterostructures containing three-dimensional (3D) ferromagnetic metals and much larger than that of heterostructures containing 3D ferrimagnetic insulators. Such large efficiency is attributed to the atomically flat FGT/Pt interface, which demonstrates the great potential of exploiting vdW heterostructures for highly efficient spintronic nanodevices.
          Cover page: Highly Efficient Spin–Orbit Torque and Switching of Layered Ferromagnet Fe3GeTe2
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2020)
          By exploiting proximity coupling, we probe the spin state of the surface layers of CrI3, a van der Waals magnetic semiconductor, by measuring the induced magnetoresistance (MR) of Pt in Pt/CrI3 nano-devices. We fabricate the devices with clean and stable interfaces by placing freshly exfoliated CrI3 flake atop pre-patterned thin Pt strip and encapsulating the Pt/CrI3 heterostructure with hexagonal boron nitride (hBN) in a protected environment. In devices consisting of a wide range of CrI3 thicknesses (30-150 nm), we observe that an abrupt upward jump in Pt MR emerge at a 2 T magnetic field applied perpendicularly to the layers when the current density exceeds 2.5 × 1010 A m-2, followed by a gradual decrease over a range of 5 T. These distinct MR features suggest a spin-flop transition which reveals strong antiferromagnetic interlayer coupling in the surface layers of CrI3. We study the current dependence by holding the Pt/CrI3 sample at approximately the same temperature to exclude the joule heating effect, and find that the MR jump increases with the current density, indicating a spin current origin. This spin current effect provides a new route to control spin configurations in insulating antiferromagnets, which is potentially useful for spintronic applications.
            Cover page: Current-induced CrI3 surface spin-flop transition probed by proximity magnetoresistance in Pt
            • Article
            • Peer Reviewed
            UC Riverside Previously Published Works (2019)
            Two-dimensional ferromagnet Cr2Ge2Te6 (CGT) is so resistive below its Curie temperature that probing its magnetism by electrical transport becomes extremely difficult. By forming heterostructures with Pt, however, we observe clear anomalous Hall effect (AHE) in 5 nm thick Pt deposited on thin (<50 nm) exfoliated flakes of CGT. The AHE hysteresis loops persist to ∼60 K, which matches well to the Curie temperature of CGT obtained from the bulk magnetization measurements. The slanted AHE loops with a narrow opening indicate magnetic domain formation, which is confirmed by low-temperature magnetic force microscopy (MFM) imaging. These results clearly demonstrate that CGT imprints its magnetization in the AHE signal of the Pt layer. Density functional theory calculations of CGT/Pt heterostructures suggest that the induced ferromagnetism in Pt may be primarily responsible for the observed AHE. Our results establish a powerful way of investigating magnetism in 2D insulating ferromagnets, which can potentially work for monolayer devices.
              Cover page: Probing Magnetism in Insulating Cr2Ge2Te6 by Induced Anomalous Hall Effect in Pt
              • Article
              • Peer Reviewed
              UC Riverside Previously Published Works (2022)
              Emergent color centers with accessible spins hosted by van der Waals materials have attracted substantial interest in recent years due to their significant potential for implementing transformative quantum sensing technologies. Hexagonal boron nitride (hBN) is naturally relevant in this context due to its remarkable ease of integration into devices consisting of low-dimensional materials. Taking advantage of boron vacancy spin defects in hBN, we report nanoscale quantum imaging of low-dimensional ferromagnetism sustained in Fe3GeTe2/hBN van der Waals heterostructures. Exploiting spin relaxometry methods, we have further observed spatially varying magnetic fluctuations in the exfoliated Fe3GeTe2 flake, whose magnitude reaches a peak value around the Curie temperature. Our results demonstrate the capability of spin defects in hBN of investigating local magnetic properties of layered materials in an accessible and precise way, which can be extended readily to a broad range of miniaturized van der Waals heterostructure systems.
                Cover page: Wide field imaging of van der Waals ferromagnet Fe3GeTe2 by spin defects in hexagonal boron nitride
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