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Open Access Publications from the University of California

Scholarly Works (11 results)

  • Thesis
  • Peer Reviewed
UC Riverside Electronic Theses and Dissertations (2018)

The Nobel prize was awarded in 2007 due to the big discovery of the giant magnetoresistance effect (GMR) in 1988 giving birth to the new, spin-based electronics or spintronics. In addition to the charge degree of freedom, spin degree of freedom of electrons has attracted a great deal of research interest in the last two decades. Based on electron spin, technology of spintronics has emerged with new device functionalities and decreased electric power consumption. For example, applications such as computer read-heads and magnetoresistive random access memories are based on spin-dependent effects that originate from interaction between spin of the carriers and the magnetic properties of the materials. Therefore, making new magnetic materials and understanding their magnetic properties is very important. This work focuses mainly on rare-earth iron garnet magnetic insulator (REIG-MI) materials and heterostructures of REIG-MI and heavy metals (strong spin-orbit coupling). These REIG-MI materials have unique properties of large electronic band gap, room temperature ferrimagnetism and long spin wave propagation length and thus are ideal materials for low-power spintronics. Heterostructures of REIG-MI and heavy metals give rise to a variety of interesting effects such as the spin Seebeck effect and the spin Hall magnetoresistance effect.

This dissertation summaries all my work in past five years and is divided into seven chapters. The first chapter starts off with a short introduction to magnetism by explaining fundamental concepts as an entrance to spintronics field and then some important transport phenomena in spintronics. The last part is devoted to the importance of thin films and critical role of interface properties and interface engineering.

The second chapter presents the state-of-the-art growth of yttrium iron garnet (YIG). A special emphasis is placed on optimizing growth conditions using pulsed laser deposition (PLD). Epitaxial growth with the layer-by layer mode, growth of strained single crystal YIG and polycrystalline YIG development are discussed in detail with full characterization results.

The third chapter shows a breakthrough of the growth of YIG on the top of Pt, an inverted bilayer structure, where we overcome the Pt stability problems related to high temperature growth or annealing. Mastering growth control leads to single crystal phase for YIG and Pt in the inverted structure with flat and sharp interface.

The fourth chapter mainly discusses the experimental observation of theoretically predicted magnon-mediated current drag effect in HM/FMI/HM heterostructures, here HM denoting heavy metal either Pt or Ta, and FMI denoting ferrimagnetic insulator YIG. This system is ideal to realize SHE in one HM layer and detect ISHE in the second layer of HM. More importantly, distinctive behaviors of this effect are studied via nonlocal voltage response as a function of magnetic field strength, angles and temperature with comparison to local signal of SMR and SSE.

The fifth chapter shows the observation of a giant magnetoresistance (GMR) effect in spin valve-like structure consisting of two identical magnetic insulator layers of YIG where they are separated by a non-magnetic metal layer. Magnetization of anti-parallel state between two layers gives rise to high resistance. SSE measurements confirm individual contribution of thermal spin current from each YIG layer.

Chapter six describes the interface modulated spin dynamics of magnetic insulator (YIG)/Pt bilayers with engineering different resistivities of Pt layer. Gilbert damping constant is tuned based on Pt resistivities and interfacial spin mixing is investigated. Both cavity FMR and broadband FMR with coplanar waveguide set-up are used.

Chapter seven discusses strain effect to tune magnetic anisotropy from in-plane to out-of-plane via magnetostriction effect utilizing strain modulation over a wide range of thicknesses ranging from 5 to 100 nm in TbIG. Study of anomalous Hall and compensation effects on these thicknesses are investigated.

    Cover page: Garnet Materials Development and Spin Transport Phenomena in Magnetic Insulator/ Normal Metal Heterostructures
    • 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
      UC Riverside Previously Published Works (2016)
      Pure spin current, a flow of spin angular momentum without flow of any accompanying net charge, is generated in two common ways. One makes use of the spin Hall effect in normal metals (NM) with strong spin-orbit coupling, such as Pt or Ta. The other utilizes the collective motion of magnetic moments or spin waves with the quasi-particle excitations called magnons. A popular material for the latter is yttrium iron garnet, a magnetic insulator (MI). Here we demonstrate in NM/MI/NM trilayers that these two types of spin currents are interconvertible across the interfaces, predicated as the magnon-mediated current drag phenomenon. The transmitted signal scales linearly with the driving current without a threshold and follows the power-law T(n) with n ranging from 1.5 to 2.5. Our results indicate that the NM/MI/NM trilayer structure can serve as a scalable pure spin current valve device which is an essential ingredient in spintronics.
        Cover page: Observation of magnon-mediated current drag in Pt/yttrium iron garnet/Pt(Ta) trilayers
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2018)
        Quantum anomalous Hall state is expected to emerge in Dirac electron systems such as graphene under both sufficiently strong exchange and spin-orbit interactions. In pristine graphene, neither interaction exists; however, both interactions can be acquired by coupling graphene to a magnetic insulator (MI) as revealed by the anomalous Hall effect. Here, we show enhanced magnetic proximity coupling by sandwiching graphene between a ferrimagnetic insulator yttrium iron garnet (YIG) and hexagonal-boron nitride (h-BN) which also serves as a top gate dielectric. By sweeping the top-gate voltage, we observe Fermi level-dependent anomalous Hall conductance. As the Dirac point is approached from both electron and hole sides, the anomalous Hall conductance reaches 1/4 of the quantum anomalous Hall conductance 2e2/h. The exchange coupling strength is determined to be as high as 27 meV from the transition temperature of the induced magnetic phase. YIG/graphene/h-BN is an excellent heterostructure for demonstrating proximity-induced interactions in two-dimensional electron systems.
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2019)
          We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown FeF_{2}(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy over the temperature range of 3.8-250 K. Both the magnetic-field and temperature-dependent SSE signals below the Néel temperature (T_{N}=70  K) of the FeF_{2} films are consistent with a theoretical model based on the excitations of AFM magnons without any net induced static magnetic moment. In addition to the characteristic low-temperature SSE peak associated with the AFM magnons, there is another SSE peak at T_{N} which extends well into the paramagnetic phase. All the SSE data taken at different magnetic fields up to 12 T near and above the critical point T_{N} follow the critical scaling law very well with the critical exponents for magnetic susceptibility of 3D Ising systems, which suggests that the AFM spin correlation is responsible for the observed SSE near T_{N}.
            Cover page: Spin Seebeck Effect from Antiferromagnetic Magnons and Critical Spin Fluctuations in Epitaxial FeF2 Films
            • 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 (2018)
              We show tunable strain-induced perpendicular magnetic anisotropy (PMA) over a wide range of thicknesses in epitaxial ferrimagnetic insulator Eu3Fe5O12 (EuIG) and Tb3Fe5O12 (TbIG) thin films grown by pulsed-laser deposition on Gd3Ga5O12 with (001) and (111) orientations, respectively. The PMA field is determined by measuring the induced anomalous Hall loops in Pt deposited on the garnet films. Due to positive magnetostriction constants, compressive in-plane strain induces a PMA field as large as 32.9 kOe for 4 nm thick EuIG and 66.7 kOe for 5 nm thick TbIG at 300 K and relaxes extremely slowly as the garnet film thickness increases. In bilayers consisting of Pt and EuIG or Pt and TbIG, robust PMA is revealed by squared anomalous Hall hysteresis loops in Pt, the magnitude of which appears to be only related to the net magnetic moment of iron sublattices. Furthermore, the magnetostriction constant is found to be 2.7 × 10−5 for EuIG and 1.35 × 10−5 for TbIG, comparable with the values for bulk crystals. Our results demonstrate a general approach of tailoring magnetic anisotropy of rare earth iron garnets by utilizing modulated strain via epitaxial growth.
                • Article
                • Peer Reviewed
                UC Riverside Previously Published Works (2018)
                We experimentally study nanowire-shaped spin-Hall nano-oscillators based on nanometer-thick epitaxial films of Yttrium Iron Garnet grown on top of a layer of Pt. We show that, although these films are characterized by significantly larger magnetic damping in comparison with the films grown directly on Gadolinium Gallium Garnet, they allow one to achieve spin current-driven auto-oscillations at comparable current densities, which can be an indication of the better transparency of the interface to the spin current. These observations suggest a route for improvement of the flexibility of insulator-based spintronic devices and their compatibility with semiconductor technology.
                  Cover page: Spin Hall-induced auto-oscillations in ultrathin YIG grown on 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 (2018)
                    We experimentally study nanowire-shaped spin-Hall nano-oscillators based on nanometer-thick epitaxial films of Yttrium Iron Garnet grown on top of a layer of Pt. We show that, although these films are characterized by significantly larger magnetic damping in comparison with the films grown directly on Gadolinium Gallium Garnet, they allow one to achieve spin current-driven auto-oscillations at comparable current densities, which can be an indication of the better transparency of the interface to the spin current. These observations suggest a route for improvement of the flexibility of insulator-based spintronic devices and their compatibility with semiconductor technology.
                      Cover page: Spin Hall-induced auto-oscillations in ultrathin YIG grown on Pt
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