0000000000707192
AUTHOR
Myung-hwa Jung
Exploring High-Energy Li-I(r)on Batteries and Capacitors with Conversion-Type Fe3O4-rGO as the Negative Electrode
We report a microwave-assisted solvothermal process for the preparation of magnetite (Fe3O4, ca. 5 nm)-anchored reduced graphene oxide (rGO). It has been examined as a prospective conversion-type negative electrode for multiple energy storage applications, such as Li-ion batteries (LIBs) and Li-ion capacitors (LICs). A LiFePO4/Fe3O4-rGO cell is constructed and capable of delivering an energy density of approximately 139 Wh kg−1 with a notable cyclability (ca. 76 %) after 500 cycles. Prior to the fabrication of a LIB, the Fe3O4-rGO is electrochemically pretreated to eliminate the irreversible capacity loss. In addition to the LIB, a high-energy LIC is also fabricated by using the pre-lithiat…
Field-driven domain wall motion under a bias current in the creep andflow regimes in Pt/[CoSiB/Pt](N) nanowires
AbstractThe dynamics of magnetic domain wall (DW) in perpendicular magnetic anisotropy Pt/[CoSiB/Pt]N nanowires was studied by measuring the DW velocity under a magnetic field (H) and an electric current (J) in two extreme regimes of DW creep and flow. Two important findings are addressed. One is that the field-driven DW velocity increases with increasing N in the flow regime, whereas the trend is inverted in the creep regime. The other is that the sign of spin current-induced effective field is gradually reversed with increasing N in both DW creep and flow regimes. To reveal the underlying mechanism of new findings, we performed further experiment and micromagnetic simulation, from which w…
Role of top and bottom interfaces of a Pt/Co/AlOx system in Dzyaloshinskii-Moriya interaction, interface perpendicular magnetic anisotropy, and magneto-optical Kerr effect
We investigate the role of top and bottom interfaces in inversion symmetry-breaking Pt/Co/AlOx systems by inserting ultra-thin Cu layers. Wedge-type ultrathin Cu layers (0-0.5 nm) are introduced between Pt/Co or Co/AlOx interfaces. Interface sensitive physical quantities such as the interfacial Dzyaloshinskii-Moriya interaction (iDMI) energy density, the interfacial perpendicular magnetic anisotropy (iPMA), and the magneto-optical Kerr effects (MOKE) are systematically measured as a function of Cu-insertion layer thickness. We find that the Cu-insertion layer in the bottom interface (Pt/Co) plays a more important role in iDMI, PMA, and MOKE. In contrast, the top interface (Co/AlOx) noticeab…
Studying the reduction of graphene oxide with magnetic measurements
Abstract The reduction of graphene oxide is one of the most facile methods to fabricate a large amount of graphene. The reduction rate is generally examined by various spectroscopic techniques, but each technique is applied for different purposes. Herein, we demonstrate the correlation between spectroscopic results and magnetic data, which plays an important role in determining the quality of reduced graphene oxides. The magnetic signals are related with the carbon-oxygen functional groups analyzed by spectroscopic tools. Especially, highly reduced sample exhibits the diamagnetic property similar to graphene-like materials. This report can provide an insight to determine the reduction rate …
Current induced chiral domain wall motion in CuIr/CoFeB/MgO thin films with strong higher order spin–orbit torques
We investigate the Dzyaloshinskii–Moriya interaction (DMI) and spin–orbit torque effects in CuIr/CoFeB/MgO heterostructures. To this end, harmonic Hall measurements and current induced domain wall motion experiments are performed. The motion of domain walls at zero applied field due to current demonstrates the presence of DMI in this system. We determine the strength of the DMI to be D = + 5 ± 3 μ J / m 2 and deduce right-handed chirality in domain walls showing a partial Neel type spin structure. To ascertain the torques, we perform a second harmonic measurement to quantify the damping- and field-like current induced effective fields as a function of the magnetization direction. From the a…
New Approach to Determine the Quality of Graphene
The reduction of graphene oxide is one of the most facile methods to fabricate a large amount of graphene and the reduction rate of graphene oxide is related with the quality of synthesized graphene for its possible application. The reduction rate is usually determined by using various spectroscopy measurements such as Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Here we propose that the magnetic data can be used as a means of determining the quality of graphene oxide (GO) and reduced graphene oxide (RGO) by the investigation of close relation between magnetic moment and chemical bonding state. Our experimental findings and previous theo…
The Azimuthal Dependence of Exchange Bias Effect and its Analysis by Spin Glass Model in Ni0.8Fe0.2/CoxNi1−xO Bilayers
Exchange bias (EB) effect has been vigorously researched for many years due to its possible applications in information storage and spintronics, especially in spin valves for magnetic recording devices. Even though many models have been expounded to this day, they do not prove convincingly the origins of EB effect. We attempt to establish the azimuthal dependence of EB effect with respect to varying the composition of the antiferromagnet CoxNi $_{\mathrm {1-x}}\text{O}$ and temperature. In this report, we deposited the bilayer thin films of Ni0.8Fe0.2/Co x Ni1− x O with $x$ varying from 0.4 to 0.8 by magnetron sputtering and studied the variation of exchange bias field and coercivity. The E…
Antiferromagnetic order competing with topological state in CexBi2−xTe3
The topological surface states in three-dimensional topological insulators are easily tuned by chemical doping, especially by magnetic impurities. We prepared single crystals of CexBi2−xTe3 with various x (=0.04, 0.06, 0.08, 0.10, and 0.12). The obtained crystals were characterized by X-ray diffraction and scanning electron microscopy. The magnetic susceptibility data revealed that the Ce atoms are well substituted for Bi into Bi2Te3. From the Curie-Weiss fits, we observed that the effective magnetic moments μeff are close to 2.54 μB for free Ce ion, and the paramagnetic Curie-Weiss temperatures θp are negatively increased from 2.87 K to −59.3 K with increasing x. The magnetization data cle…
Gradual phase transition from ferromagnetic tetragonal to antiferromagnetic cubic states in Mn Ga (1.80 ≤ x ≤ 3.03) thin films
Abstract The structural, magnetic, and electronic properties of MnxGa thin films are investigated as varying the Mn composition (1.80 ≤ x ≤ 3.03). The variation of x in MnxGa films dramatically changes the crystal structure as well as the magnetic properties. With increasing x, we observe the gradual phase transition from a ferromagnetic tetragonal state to an antiferromagnetic cubic state. The structural characterization reveals that the D022 tetragonal structure of Mn2Ga is slowly transformed to the L12 cubic structure of Mn3Ga. Two phases coexist around x = 2.4. The magnetization is systematically reduced as x increases, ending to an antiferromagnetic state of cubic Mn3Ga, and the electr…
Magnetic and structural phase transitions by annealing in tetragonal and cubic Mn3Ga thin films
Abstract Thermal Annealing is a simple and powerful tool to improve the crystallinity in general or promote the functionality for peculiar purposes, ultimately leading to metastable states with lower energy. We report the annealing effect focusing primarily on the structural and magnetic properties of two different Mn3Ga thin films. One is the D022 tetragonal ferrimagnetic phase Mn3Ga, and the other is the disordered-L12 cubic antiferromagnetic phase Mn3Ga. They were grown by RF/DC magnetron sputtering method on MgO substrate. After deposition, the thin films were annealed at various temperatures (200, 300, 400, 500, and 600 °C) and Ar pressures (10−3, 10−1, and 103 Torr). We find that the …
Fine tuning of the magnetic properties in Mn3-Co Ga Heusler films near the critical regime
Abstract Tunability of structural and magnetic properties of Mn3-xCoxGa films is presented by Co substitution, where critical behavior emerges 0.37 ≤ x ≤ 0.56 exhibiting a transition from tetragonal hard ferrimagnetic with perpendicular magnetic anisotropy (PMA) to a cubic soft ferrimagnetic phase with in-plane magnetic anisotropy (IMA). In the critical regime, coexisting state of tetragonal and cubic phases possesses significantly low coercive field (HC = 1.9 kOe) with relatively low saturation magnetization (MS = 100~150 emu/cc) while maintaining the PMA. From first-principles calculations, moments of two Mn sites do not change upon Co substitution. However, moments of substituted Co almo…
Highly Reduced Saturation Magnetization in Epitaxially Grown Ferrimagnetic Heusler Thin Films
The key of spintronic devices using the spin-transfer torque phenomenon is the effective reduction of switching current density by lowering the damping constant and the saturation magnetization while retaining strong perpendicular magnetic anisotropy. To reduce the saturation magnetization, particular conditions such as specific substitutions or buffer layers are required. Herein, we demonstrate highly reduced saturation magnetization in tetragonal D022 Mn3–xGa thin films prepared by rf magnetron sputtering, where the epitaxial growth is examined on various substrates without any buffer layer. As the lattice mismatch between the sample and the substrate decreases from LaAlO3 and (LaAlO3)0.3…