Spin Electronic Devices Research Articles

Binary magnetic moments of all-boron fullerene B40 by positioning the Co atom

Recently, the all-boron fullerene B40 is experimentally observed and theoretically confirmed. As the second single-element fullerene after C60, further study about this novel fascinating fullerene is thus quite desirable. In this work, the structures, stability and magnetic properties of the Co-doped B40-based metallofullerenes were systematically studied. The results reveal different stable configurations of the endohedral Co@B40 and exohedral Co-B40 metallofullerenes depending on the doped sites of the Co atom. The spin-polarized calculations indicate that all the Co-doped metallofullerenes have an interesting characteristic of the magnetic properties: the magnetic moments having binary alternative between 2.98 μB and 1.00μB, corresponding to the Co atom resident inside the center of the unwoven double rings and inside the center of multiple-membered holes, respectively. This interesting finding of the binary magnetic moments implies that the Co-encapsulated B40-based metallofullerenes may be a promising candidate for the single molecular spin electronic devices.

Bidirectional surface photovoltage on a topological insulator

Controlled extraction of spin-polarized currents from the surface of topological insulators (TIs) would be an important step to use TIs as spin-electronic device materials. One way is to utilize the surface photovoltage (SPV) effect, by which the surface current may flow upon irradiation of light. To date, unipolar SPV has been observed on TIs, while the realization of ambipolar SPV is crucial for taking control over the direction of the flow. By using time-resolved photoemission, we demonstrate the ambipolar SPV realized on the TI ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The topological surface states showed downward and upward photovoltaic shifts for the $n$- and $p$-type samples, respectively. We also discerned the photogenerated carriers accumulated in the surface states for $g4\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. We provide the keys besides the in-gap Fermi level to engineer the SPV on TIs.

Interface alloying of ultra-thin sputter-deposited Co2MnSi films as a source of perpendicular magnetic anisotropy

Abstract Novel spin-electronic devices require electrodes that inject electrons with both high spin-polarization and perpendicular magnetic anisotropy (PMA). Several full-Heusler compounds are expected to be half-metallic ferromagnets, e.g. chemically-ordered L21 or B2 Co2MnSi. However, most cubic full-Heusler alloys have small magneto-crystalline anisotropy meaning that PMA is difficult to achieve in thin film geometries of devices. Addressing this limitation, Butler et al. (2014) calculate PMA and full spin-polarization in ultra-thin (2–3 nm) chemically-ordered Co2MnSi with an epitaxial coherent interface to rock-salt MgO (0 0 1). Experimentally, PMA in sputter-deposited ultra-thin films with full-Heusler compositions was reported though with adjacent layers of Pd or Pt. We investigate structural origins of such PMA using a test case of ultra-thin Co2MnSi films prepared by magnetron sputter-deposition, adjacent to a MgO layer to represent a tunneling barrier, and to a Pd buffer layer. We measure PMA, with an energy density of 7.8 ± 1.8 Merg/cc at 5 K, when a Pd layer is adjacent to Co2MnSi, following annealing in a narrow temperature range, around 350 °C. This ferromagnetism originates from nanometer scale regions, below 5 nm in size, having a relatively low saturation magnetization, 550 ± 50 emu/cc at low temperatures. Following thermal annealing, significant compositional intermixing between Co2MnSi films and adjacent layers, Co with Pd and Mn with MgO, was measured by electron energy-loss and angle resolved X-ray photoelectron spectroscopies. Aberration-corrected transmission electron microscopy shows that Co2MnSi does not crystallize while at the interface with Pd, nanometer-scale crystallites of FCC solid solution CoPd with {1 1 1} texture are identified. We conclude that these Pd rich CoPd crystallites, characterized by large magnetostriction, are a source for PMA.

Mono-crystalline SnTe with micro-octahedroncharacteristic: One-pot facile synthesis and comprehensive crystallographic evidence

AbstractIn recent years, topological crystalline insulators have attracted increased attention due to practical demands of tunable electronic, spin electronic and thermoelectric devices. As an important topological crystalline insulator, mono-crystalline SnTe octahedra with {111} dominated surfaces, were successfully synthesized on a large scale via a surfactant-free hydrothermal synthesis route in this work. Important controlling factors for phase and morphology, i.e. reaction temperature, reactant concentration and stoichiometric ratio, are discussed in detail. The results indicated that high temperature is favorable for forming pure phase, and excessive Sn suppresses the appearance of SnTe with octahedral morphology. Lower reactant concentration is beneficial to preparing uniform SnTe octahedra via the selective growth mechanism. Crystallographic characteristics of the SnTe octahedra were investigated using focused ion beam, electron backscattered diffraction and transmission electron microscopy analysis. The hexagonal-like micro-plate (cut from an octahedron) was confirmed as mono-crystalline by the corresponding three Euler angle maps, the Kikuchi diffraction pattern and selected-area diffraction. It can be further deduced from the multiple experimental results that surfaces of octahedral SnTe are dominated by {111} crystallographic planes and the average size is 1–3 μm. Controllable mono-crystalline octahedra would effectively promote the development of topological crystalline insulators and their micro devices.

Electrostatic memory in KTaO3

Perovskite oxide materials with strong spin–orbit coupling (SOC) have tremendous potential for next generation spin-electronic devices. In this work, we present the creation of artificial electrical domains on the conducting surface of KTaO3 (KTO) having strong SOC. A conductive atomic force microscopy tip has been used to induce extremely small electrically active charge domains on the surface of KTO. The versatility of creating such nanodomains is that the features can be written and erased with nanometer scale precision. The written signal (phase difference between the written and unwritten regions) on KTO has turned out to be much higher than that on other reported perovskite oxides. Kelvin probe force microscopy measurements are performed to probe the surface potential and work function changes in the regions of nano-electrical domains. Magnetic force microscopy measurements suggest generation of a magnetic field when electrostatic charge is written in a dipolar configuration which can be utilized in future nano-electric devices.

Anisotropic magnetoelectronic structures and magneto-transport properties of topological Dirac semimetal nanowires

Abstract The magnetoelectronic structures, and the charge and spin conductance spectra of topological Dirac semimetal (TDS) nanowires are calculated by adopting the tight-binding Hamiltonian model in combination with the Green’s-function-based Landauer-Buttiker formula. Anisotropic electronic structures and magneto-transport properties are found in TDS nanowires with different confinement and magnetic field directions. With respect to in-plane-confinement TDS nanowires, trivial surface/corner states are observed and they are split into two spin-polarized states with oppositive signs when a weak out-of-plane magnetic field is applied. Moreover, the spin-polarized surface states will be transformed into bulk and corner states as the magnetic field strength is increased. Therefore, quantized spin conductance energy windows and fully-spin-polarized conductance can be achieved in the normal/out-of-plane-magnetic-field-modulated/normal TDS system in the case of the weak and strong fields, respectively. However, chiral surface states are generated when an in-plane magnetic field is applied, resulting in the perfect charge conductance steps of the normal/in-plane-magnetic-field-modulated/normal TDS system. With respect to the TDS nanowire with both the in-plane and out-of-plane confinements, nontrivial spin-momentum-locked helical surface states are observed. However, the helical character of the surface states will be destroyed and nontrivial-trivial topological phase transition happens as a high in-plane magnetic field is applied. In addition, the energy position of the surface subband inside the bulk energy gap is shifted so that the surface state transport can be switched on or off by tuning the magnetic field strength. Interestingly, the surface subband situated inside the bulk energy gap is split into two subbands with opposite spin signs as an out-of-plane magnetic field is added. Therefore, the topological phase transition between the quantum spin Hall and spin-polarized quantum Hall phases in the TDS nanowire, and the spin-polarized transport in the normal/out-of-plane-magnetic-field-modulated/normal TDS system, can be controlled by varying the magnetic field strength. These effects may benefit both the fundamental understanding of the magneto-electronic characters of the TDS nanostructures and the design of low-dissipation (spin) electronic devices.

Distinctive electronic and spin structures at the oppositely polarized ferroelectric BiAlO3(0001) surfaces

The ferroelectric Rashba semiconductors (FERSCs) have recently been regarded as a novel class of multifunctional materials due to the coexistence and coupling of ferroelectricity and bulk Rashba spin-splitting. However, the surface spin-orbit coupling (SOC) is often different from the bulk phase. In the present work, we provide first-principles calculations on electronic and spin structures of the oppositely polarized (0001) surfaces for R3c BiAlO3, which is the first known FERSC oxide [Phys. Rev. B 93 (2016) 245159]. Our results indicate that the surface electronic and spin structures display distinctive characteristics with respect to the bulk phase and exhibit significant dependence on the ferroelectric polarization direction as well as the topmost atomic layer. The predicted polarization- and termination-dependent surface electronic properties and spin-textures for the oppositely polarized BAO(0001) surfaces have practical implications for the FERSC-based spin-electronic devices.

Half-metallic behavior in ruthenium-cyclopentadienyl organometallic sandwich molecules.

We have theoretically investigated spin transport properties of one-dimensional ruthenium-cyclopentadienyl sandwich molecules, Run(Cp)n+1, between two gold electrodes. Calculations were performed based on the non-equilibrium Green's function formalism and density functional theory. The results clearly reveal that for n = 1, no spin-polarized behavior is observed due to the fully occupied valence shell of ruthenocene molecule, while for the higher n values, the total magnetic moments increase with increasing size of the cluster and a half metallic behavior has been achieved. This can be attributed to the altered ligand field of π-conjugate systems generated by metal centers. Interestingly, for n values higher than 1, not only do the current-voltage curves exhibit an efficient spin-current generation but also a pronounced negative differential resistance behavior can be detected in the bias region. Our results will be strongly informative for the design and control of high-performance organometallic sandwich molecules based on spin-electronic devices.

Dephasing effect of quantum spin topological states in HgTe/CdTe quantum well

HgTe/CdTe quantum well is a typical two dimensional topological material which supports the helical edge states and quantum spin Hall effect that is imposing in applying of spin electronic devices. The special plateau valued with <inline-formula><tex-math id="M2">\begin{document}$0.25\;h/e^{2}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20191072_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20191072_M2.png"/></alternatives></inline-formula> of nonlocal resistance in H-shaped four terminal devices can be used as the fingerprint of quantum spin Hall effect. Based on the HgTe/CdTe quantum well, with the aid of nonequilibrium Green's function theory and multi-terminal Landauer-Büttiker formula, we calculate the nonlocal resistance and study the dephasing effect of spin topological states in the presence of exchange field and external magnetic field. It is found the dephasing processes play a role completely different from exchange field and external magnetic field. The latter destroy time reversal symmetry and change the width and relative position of topological gap, but do not influent the topological stability of helical edge states. In the contrary, dephasing processes don't change the width and relative position, however, they broke the topological stability. We consider two kinds of dephasing: normal dephasing and spin dephasing. In the first kind, the carriers lose only the phase memory while maintaining the spin memory. In the second kind, the carriers lose both phase and spin memories. Because of the spin locking properties, normal dephasing almost have no influence on the helical edge states. While the spin dephasing will induce spin flip backscattering and finally destroy helical edge states seriously.

Structural, magnetic, and electron-transport properties of epitaxial Mn2PtSn films

The growth of new magnetic materials on suitable insulating substrates is an important part of the development of spin-electronics devices for memory or information processing. Epitaxial thin films of Mn2PtSn were grown on a MgO [001] substrate by magnetron co-sputtering of the constituents. Structural, magnetic, and electron-transport properties were investigated. The epitaxial Mn2PtSn film has an inverse tetragonal structure with the c-axis aligned in the plane of the MgO substrate. The lattice constants determined using XRD and TEM analysis are c = 6.124 Å and a = b = 4.505 Å. The orientation of Mn2PtSn c-axis which is 45° away from the a-axis of MgO has resulted in a small lattice mismatch of about 2.8%. The measured saturation magnetization is 5.3 μB/f.u., which is smaller than the first-principles calculated value of 6.4 μB/f.u. for ferromagnetic spin arrangement. Magnetization measurements determined the bulk magnetocrystalline anisotropy constant Kv of about 11.3 Merg/cm3 (1.13 MJ/m3). The electron-transport behavior is similar to that of normal magnetic metals. These results indicate that Mn2PtSn may have promising applications in spintronic devices.

Revisiting La2MMnO6 (M = Co, Ni, Cu, Zn) perovskites in view of 3d-electron configuration

We investigate the impact of 3d-electron configuration on the magnetic and DC conductive properties of La2MMnO6 (M = Co, Ni, Cu, Zn) and offer evidence to an enhanced ionic ordering degree with the increase in the 3d electron number of an M ion. A key role of the 3d electronic configuration that influences the B-site ionic arrangement is revealed by changing both the ionic radii and bond covalency. The position of the valence band edge is shifted away from the Fermi level with the increase in 3d electrons, which reveals a strengthened electron localization behavior. Magnetic characterization indicates that the Co, Ni, and Zn samples present magnetic behaviors with half- and full-filled eg configurations, whereas the Cu sample shows a special Jahn-Teller configuration (t2g6eg3), giving rise to the degenerated eg states and significant change in bond parameters. Consequently, the Goodenough-Kanamori rules are not followed and the ferromagnetic coupling is weakened greatly. Further, DC resistivity measurements of samples M = Co, Ni, and Zn present thermally activated conductive behavior, in good accordance with the position shift of valence band edge. However, due to the Jahn-Teller effect arising from Cu2+ ion, a strong electron-phonon coupling is evidenced. This effect provides a suitable hopping channel for small polarons resulting in a considerable rise of its conductivity. Such changes in magnetization and DC conductivity of La2MMnO6 perovskites by modulating the M elements would inspire us to develop novel spin-electronic devices.

Manufacturing of Sputtering Composite Targets Containing Phases of Co2FeSi or Co2MnSi Heusler Alloy

The paper presents a method for manufacturing mechanically strong sputtering composite targets containing the phase of the Co2FeSi or Co2MnSi Heusler alloy of the stoichiometric composition, which can be used for fabrication of spin electronic devices by high-frequency magnetron deposition and pulsed laser deposition of thin films.

Ultrathin nanosheets of Mn3O4: A new two-dimensional ferromagnetic material with strong magnetocrystalline anisotropy

Two-dimensional (2D) materials with robust ferromagnetism have played a key role in realizing nextgeneration spin-electronic devices, but many challenges remain, especially the lack of intrinsic ferromagnetic behavior in almost all 2D materials. Here, we highlight ultrathin Mn3O4 nanosheets as a new 2D ferromagnetic material with strong magnetocrystalline anisotropy. Magnetic measurements along the in-plane and out-of-plane directions confirm that the out-of-plane direction is the easy axis. The 2D-confined environment and Rashba-type spin-orbit coupling are thought to be responsible for the magnetocrystalline anisotropy. The robust ferromagnetism in 2D Mn3O4 nanosheets with magnetocrystalline anisotropy not only paves a new way for realizing the intrinsic ferromagnetic behavior in 2D materials but also provides a novel candidate for building next-generation spin-electronic devices.

Structural and magnetic properties of MoS2 monolayer zigzag nanoribbon doped by Ti, V, Cr, and Mn

In order to find new functions of monolayer MoS2 in nanoelectronics or spin electronic devices, using spin-polarized density functional theory (DFT) calculations with on-site coulomb interaction (U), we investigated substitutional doping of Mo atoms of monolayer zigzag MoS2 nanoribbon (ZZ-MoS2 NR) by transition metals (TM) (where TM = Ti, V, Cr, Mn) at the Mo-edge, S-edge, and the middle of the NRs. The results of this study indicate the NR widened irrespective of the doped TM position and type, and the Mo-edge was found as the easiest substitutional position. For ZZ-MoS2 NR doped by Mn, Cr or V atoms, the preferred magnetic coupling state is the edge atoms of S at the S-edge, exhibiting the same spin polarization with TM (named the FM1 state), attributing the NR with metallic magnetism. For Ti-doped monolayer ZZ-MoS2 NR, in addition to the FM1 state, other preferred magnetic coupling state was observed in which the edge atoms of S at the S-edge exhibit the opposite spin polarization with that of Ti (named the FM2 state). Thus, the NR doped by Ti atom possesses metallic (FM1 state) or half-metallic (FM2 state) magnetism. The total magnetic moments of the ZZ-MoS2 NR doped by TM follows a linear relationship as a function of the TM dopants (Mn, Cr, V, and Ti). Under >4% applied strain, the NR doped by Ti atom only presents the characteristics of half-metallic magnetism as the initial one in the FM2 state, and its total magnetic moment always remained 0 μB, i.e., it was not affected by the width of the NR. This study provides a rational route of tuning the magnetic properties of ZZ-MoS2 NRs for their promising applications in nanoelectronics and spin electronic devices.

Dependence of magnetic properties on different buffer layers of Mn3.5Ga thin films

D022-Mn3.5Ga thin films were prepared on MgO (100) single crystalline substrates with different buffer layer (Cr, Fe, Cr/Pt and Cr/Au) using an ultra-high-vacuum electron beam vapor deposition system. From XRD patterns, a fundamental (004) peak has clearly observed for all samples. The relatively low saturation magnetization (Ms) of 178 emu/cm3, high magnetic anisotropy (Ku) of 9.1 Merg/cm3 and low surface roughness (Ra) of 0.30 nm were obtained by D022-Mn3.5Ga film (20 nm) on Cr/Pt buffer layer at Ts = 300 °C, Ta = 400 °C (3h). These findings suggest that MnGa film on Cr/Pt buffer layer is a promising PMA layer for future spin electronics devices.

Spin-resolved electronic structure of ferroelectric α-GeTe and multiferroic Ge1−xMnxTe

Germanium telluride features special spin-electric effects originating from spin-orbit coupling and symmetry breaking by the ferroelectric lattice polarization, which opens up many prospectives for electrically tunable and switchable spin electronic devices. By Mn doping of the α-GeTe host lattice, the system becomes a multiferroic semiconductor possessing magnetoelectric properties in which the electric polarization, magnetization and spin texture are coupled to each other. Employing spin- and angle-resolved photoemission spectroscopy in bulk- and surface-sensitive energy ranges and by varying dipole transition matrix elements, we disentangle the bulk, surface and surface-resonance states of the electronic structure and determine the spin textures for selected parameters. From our results we derive a comprehensive model of the α-GeTe surface electronic structure which fits to experimental data and first principle theoretical predictions and we discuss the unconventional evolution of the Rashba-type spin splitting upon manipulation by external B- and E-fields.

Recent Advances in Two-Dimensional Materials with Charge Density Waves: Synthesis, Characterization and Applications

Recently, two-dimensional (2D) charge density wave (CDW) materials have attracted extensive interest due to potential applications as high performance functional nanomaterials. As other 2D materials, 2D CDW materials are layered materials with strong in-plane bonding and weak out-of-plane interactions enabling exfoliation into layers of single unit cell thickness. Although bulk CDW materials have been studied for decades, recent developments in nanoscale characterization and device fabrication have opened up new opportunities allowing applications such as oscillators, electrodes in supercapacitors, energy storage and conversion, sensors and spinelectronic devices. In this review, we first outline the synthesis techniques of 2D CDW materials including mechanical exfoliation, liquid exfoliation, chemical vapor transport (CVT), chemical vapor deposition (CVD), molecular beam epitaxy (MBE) and electrochemical exfoliation. Then, the characterization procedure of the 2D CDW materials such as temperature-dependent Raman spectroscopy, temperature-dependent resistivity, magnetic susceptibility and scanning tunneling microscopy (STM) are reviewed. Finally, applications of 2D CDW materials are reviewed.

Spin-up “pristine-like” Dirac cone in bridge-structure graphene on Ni(111)

Abstract We report a novel unshifted Dirac cone in energy and in k-space (pristine-like) for the spin-up electrons of experimentally verified bridge-structure of graphene on Ni(111) using density functional theory with van der Waals corrections. Also, we found that the spin-down electrons exhibit a metallic property. This co-existence of semi-metallic and metallic nature of electrons at the Fermi level can pave a way for effective ultra-thin spin-electronic devices such as memory devices, where different scattering of two spin-configurations of electrons is desired.

Optical and structural properties of GaSb-doped Mn based diluted magnetic semiconductor thin films grown via DC magnetron sputtering

In this work, diluted magnetic semiconductor GaSb:Mn thin films were grown via DC magnetron Co-sputtering on glass substrates. The effect of synthesis parameters on the optical and structural properties was determined through spectral transmittance measurements and X-ray diffraction (XRD), respectively. Substrate temperature was changed from 373 to 523 K and layer thicknesses were obtained between 200 and 330 nm. All samples were subjected to annealing process to 623 K in situ. It was found that the optical constants (refractive index (n), absorption coefficient (a), extinction coefficient (k), and energy gap (E1)) are significantly affected by the temperature and target power used during the synthesis process. In particular, a variation of the energy gap between 0.58 and 0.98 eV was obtained when the target power of GaSb varied between 80 and 140 W. GaSb, Mn2Sb and SiO2 phases were observed for lowest values of target power, where SiO2 phase is a contribution of the substrate. The dielectric function of the compound with a dependence on synthesis parameters such as, the substrate temperature and deposition time was obtained, both real (e1) and imaginary part (e2). Analysis of the XRD measurements allowed to find that the (GaSb)Mn films grow with a mixture of the GaSb and Mn2Sb phases, and an amorphous halo associated with the glass substrate. A correlation between synthesis parameters and optical properties is presented. Diluted magnetic semiconductors, like GaSb:Mn, are considered among promising materials for the development of new spin-electronic devices, high speed quantum-mechanical in computational information, other more; in this case, studies on magnetic properties in digital alloys of GaSb/Mn can be realized around of the optimization of Curie temperature.

Tunable electronic structure and magnetic moment in C2N nanoribbons with different edge functionalization atoms.

First principles calculations based on density functional theory were carried out to study the electronic and magnetic properties of C2N nanoribbons (C2NNRs). The electronic structure could be modified by different methods using saturated or co-saturated H, O, and F on the edges, which can provide a new pathway at the nanoscale for fabricating 2D spintronic materials. It was found that the pristine armchair C2NNR (A-C2NNR) is a nonmagnetic semiconductor with a direct band gap, while the pristine zigzag C2NNRs (Z-C2NNRs) can show either magnetic semiconductor with an indirect band gap or magnetic metallic behavior depending on its ribbon widths. A-C2NNRs with one type of atom (H, O or F) saturated on the edges are nonmagnetic, while H and O (F and O) co-saturated A-C2NNRs show magnetic ground states. H and O (F and O) co-saturated Z-C2NNRs share a larger magnetic moment compared to the case with H, O and F saturated on the edges. Furthermore, O-saturated Z-C2NNR is a spin "gapless" semiconductor. Additionally, there is no need to spin flip in the process of electronic transition near the Fermi level. Therefore, C2NNRs might have potential applications in photoelectronic and spinelectronic devices.