Large Spin Polarization Research Articles

Enhanced Intersystem Crossing and Transient Electron Spin Polarization in a Photoexcited Pentacene-Trityl Radical.

Identifying and characterizing systems that generate well-defined states with large electron spin polarization is of high interest for applications in molecular spintronics, high-energy physics, and magnetic resonance spectroscopy. The generation of electron spin polarization on free-radical substituents tethered to pentacene derivatives has recently gained a great deal of interest for its applications in molecular electronics. After photoexcitation of the chromophore, pentacene-radical derivatives can rapidly form spin-polarized triplet excited states through enhanced intersystem crossing. Under the right conditions, the triplet spin polarization, arising from mS-selective intersystem crossing rates, can be transferred to the tethered stable radical. The efficiency of this spin polarization transfer depends on many factors: local magnetic and electric fields, excited-state energetics, molecular geometry, and spin-spin coupling. Here, we present transient electron paramagnetic resonance (EPR) measurements on three pentacene derivatives tethered to Finland trityl, BDPA, or TEMPO radicals to explore the influence of the nature of the radical on the spin polarization transfer. We observe efficient polarization transfer between the pentacene excited triplet and the trityl radical but do not observe the same for the BDPA and TEMPO derivatives. The polarization transfer behavior in the pentacene-trityl system is also investigated in different glassy matrices and is found to depend markedly on the solvent used. The EPR results are rationalized with the help of femtosecond and nanosecond transient absorption measurements, yielding complementary information on the excited-state dynamics of the three pentacene derivatives. Notably, we observe a 2 orders of magnitude difference in the time scale of triplet formation between the pentacene-trityl system and the pentacene systems tethered with the BDPA and TEMPO radicals.

Very large thermal rectification in ferromagnetic insulator-based superconducting tunnel junctions

We investigate electronic thermal rectification in ferromagnetic insulator-based superconducting tunnel junctions. Ferromagnetic insulators coupled to superconductors are known to induce sizable spin splitting in the superconducting density of states and also lead to efficient spin filtering if used as tunnel barriers. The combination of spin splitting and spin filtering is shown to yield a substantial amount of self-amplification of the electronic heat diode effect due to breaking of the electron-hole symmetry in the system, which is added to the thermal asymmetry of the junction. Large spin splitting and large spin polarization (≳90%) can potentially lead to thermal rectification efficiencies exceeding ∼5×104% for realistic parameters in a suitable temperature range, thereby outperforming up to a factor of ∼250, the heat diode effect achievable with conventional superconducting tunnel junctions. These results are relevant for improved control of heat currents in innovative phase-coherent caloritronic nanodevices and for enhanced thermal management of quantum circuits at the nanoscale.

Structural, elastic, electronic, and magnetic properties of Si-doped Co2MnGe full-Heusler type compounds

The structural, electronic, magnetic and elastic properties of Co2MnGe1-xSix (x = 0, 0.25, 0.50, 0.75, and 1) compounds are investigated by first-principles calculations within the generalized gradient approximation (GGA). The calculated structural parameters of these compounds show a slightly decreasing lattice constants and similarly decreasing lattice volumes and enthalpies of formation on increasing Si substitution x. The band structure calculations estimate that these compounds at their optimized lattice constants are half-metallic ferromagnets. The calculated total magnetic moment values are all integers, which is typical for half-metallic materials with a half-metallic bandgap (EHM) in the minority states. Besides, the total magnetic moments of these compounds are fully compatible with the Slater-Pauling rule showing the half-metallicity and large spin polarization desired for spintronics applications. The partial substitution of the Ge atom by the Si does not affect the atomic magnetic moment and total magnetic moment. The obtained values of structural parameters, total and atomic magnetic moments for x = 0 and 1 stoichiometric compounds are in good agreement with experimental and theoretical results. The elastic constants are studied for all compositions of x in order to verify the mechanical stability of these compounds. Born’s stability criterion implemented on elastic constants of Co2MnGe1-xSix (x = 0, 0.25, 0.50, 0.75, and 1) compounds confirm that these materials are mechanically stable. Other elastic parameters like bulk modulus (B), shear modulus (G), ratio of B/G, Young’s modulus (E), Poisson’s ratio (ν), and Shear anisotropic factor (A), which are the significant elastic moduli for technological applications have been thoroughly investigated. Consequently, these compounds, especially Co2MnGe1-xSix (x = 0.25, 0.50, 0.75) mixed systems, are promising candidates for practical applications in the field of spin electronics.

Electronic and magnetic properties of 3d transition-metal adatoms on Mn/W(110)

Using density functional theory, we investigate the electronic and magnetic properties of $3d$ transition-metal adatoms adsorbed on a monolayer of Mn on W(110). Mn/W(110) has a noncollinear cycloidal spin-spiral ground state with an angle of 173$^\circ$ between magnetic moments of adjacent Mn rows. It allows to rotate the spin orientation of an adsorbed magnetic adatom quasi-continuously. Therefore, this surface is ideally suited for manipulating the spin direction of individual atoms and exploring their magnetic properties using scanning tunneling microscopy (STM). The adsorbed V and Cr transition-metal adatoms couple antiferromagnetically to the nearest neighbor Mn atom of Mn monolayer while Mn, Fe, Co, and Ni couple ferromagnetically. The magnetic moments of the $3d$ adatoms are large and show a Hund's rule type of trend with a peak in the middle of the series. We find large spin splitting of the $3d$ transition-metal adatoms, large spin polarization of the local vacuum density of states up to 73\% at the Fermi energy, and significant tunneling anisotropic magnetoresistance enhancement up to 27\%. We conclude that such large values stem from the strong hybridization between the adatoms and the Mn atoms of the monolayer. Furthermore, identification of spin orientations of the adatom using spin-polarized STM is only possible for Co and V adatoms.

Prediction of two-dimensional antiferromagnetic ferroelasticity in an AgF2 monolayer.

Two-dimensional multiferroics, simultaneously harboring antiferromagneticity and ferroelasticity, are essential and highly sought for miniaturized device applications, such as high-density data storage, but thus far they have rarely been explored. Herein, using first principles calculations, we identified two-dimensional antiferromagnetic ferroelasticity in an AgF2 monolayer that is dynamically and thermally stable, and can be easily fabricated from its bulk. The AgF2 monolayer is an antiferromagnetic semiconductor with large spin polarization, and with great structural distortion due to its intrinsic Jahn-Teller effect when thinning the AgF2 down to a monolayer. Additionally, it features excellent ferroelasticity with high transition signal and a low switching barrier, rendering the room-temperature nonvolatile memory accessible. Such coexistence of antiferromagneticity and ferroelasticity is of great significance to the study of two-dimensional multiferroics and also renders the AgF2 monolayer a promising platform for future multifunctional device applications.

Large Current Driven Domain Wall Mobility and Gate Tuning of Coercivity in Ferrimagnetic Mn4N Thin Films.

Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that Mn4N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn4N thin films grown epitaxially on SrTiO3 substrates possess remarkable properties, such as a perpendicular magnetization, a very high extraordinary Hall angle (2%) and smooth domain walls at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin-polarized currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetization and a large spin polarization. Finally, we show that the application of gate voltages through the SrTiO3 substrates allows modulating the Mn4N coercive field with a large efficiency.

Perpendicular magnetic anisotropy in Pt/Co-based full Heusler alloy/MgO thin-film structures

Perpendicular magnetic anisotropy (PMA) in ultrathin magnetic structures is a key ingredient for the development of electrically controlled spintronic devices. Due to their relatively large spin-polarization, high Curie temperature and low Gilbert damping the Co-based full Heusler alloys are of special importance from a scientific and applications point of view. Here, we study the mechanisms responsible for the PMA in Pt/Co-based full Heusler alloy/MgO thin films structures. We show that the ultrathin Heusler films exhibit strong PMA even in the absence of magnetic annealing. By means of ferromagnetic resonance experiments, we demonstrate that the effective magnetization shows a two-regime behavior depending on the thickness of the Heusler layers. Using Auger spectroscopy measurements, we evidence interdiffusion at the underlayer/Heusler interface and the formation of an interfacial CoFe-rich layer which causes the two-regime behavior. In the case of the ultrathin films, the interfacial CoFe-rich layer promotes the strong PMA through the electronic hybridization of the metal alloy and oxygen orbitals across the ferromagnet/MgO interface. In addition, the interfacial CoFe-rich layer it is also generating an increase of the Gilbert damping for the ultrathin films beyond the spin-pumping effect. Our results illustrate that the strong PMA is not an intrinsic property of the Heusler/MgO interface but it is actively influenced by the interdiffusion, which can be tuned by a proper choice of the underlayer material, as we show for the case of the Pt, Ta and Cr underlayers.

Two-dimensional spin-valley locking spin valve

Valleytronics is an emerging field of research which employs energy valleys in the band structure of two-dimensional (2D) electronic materials to encode information. A special interest has been triggered by the associated spin-valley coupling which reveals rich fundamental physics and enables new functionalities. Here, we propose exploiting the spin-valley locking in 2D materials with a large spin-orbit coupling and electric-field reversible valley spin polarization, such as germanene, stanene, a $1{T}^{\ensuremath{'}}$ transition metal dichalcogenide (TMDC) monolayer, and a $2H$-TMDC bilayer, to realize a valley spin valve (VSV). The valley spin polarization in these materials can be switched by an external electric field, which enables functionalities of a valley spin polarizer or a valley spin analyzer. When placed in series, they constitute the proposed VSV---a device whose conductance state is ON or OFF depending on the relative valley spin polarization of the polarizer and the analyzer. Using quantum-transport calculations based on an adequate tight-binding model, we predict a giant VSV ratio of nearly 100% for both germanene- and stanene-based VSV devices. Our results demonstrate the implication of the spin-valley coupling in 2D materials for the novel device concept promising for valleytronics.

Extremely Large Non-equilibrium Tunnel Magnetoresistance Ratio in CoRhMnGe Based Magnetic Tunnel Junction by Interface Modification.

Equiatomic quaternary Heusler compounds (EQHCs) generally have the advantages of high Curie temperature, large spin polarization and long spin diffusion length, and they are regarded as one of the most promising candidates for spintronics devices. Herein, we report a theoretical investigation on an EQHC CoRhMnGe based magnetic tunnel junction (MTJ) with (i) MnGe-terminated interface and (ii) modified pure Mn terminated interface, i.e., MnMn-terminated interface. By employing first principle calculations combined with non-equilibrium Green's function, the local density of states (LDOS), transmission coefficient, spin-polarized current, tunnel magnetoresistance (TMR) ratio and spin injection efficiency (SIE) as a function of bias voltage are studied. It reveals that when the MTJ under equilibrium state, TMR ratio of MnGe-terminated structure is as high as 3,438%. When the MTJ is modified to MnMn-terminated interface, TMR ratio at equilibrium is enhanced to 2 × 105%, and spin filtering effects are also strengthened. When bias voltage is applied to the MTJ, the TMR ratio of the MnGe-terminated structure suffers a dramatic loss. While the modified MnMn-terminated structure could preserve a large TMR value of 1 × 105%, even bias voltage rises up to 0.1 V, showing a robust bias endurance. These excellent spin transport properties make the CoRhMnGe a promising candidate material for spintronics devices.

All-electric magnetization switching and Dzyaloshinskii-Moriya interaction in WTe2/ferromagnet heterostructures.

All-electric magnetization manipulation at low power is a prerequisite for a wide adoption of spintronic devices. Materials such as heavy metals1-3 or topological insulators4,5 provide good charge-to-spin conversion efficiencies. They enable magnetization switching in heterostructures with either metallic ferromagnets or with magnetic insulators. Recent work suggests a pronounced Edelstein effect in Weyl semimetals due to their non-trivial band structure6,7; the Edelstein effect can be one order of magnitude stronger than it is in topological insulators or Rashba systems. Furthermore, the strong intrinsic spin Hall effect from the bulk states in Weyl semimetals can contribute to the spin current generation8. The Td phase of the Weyl semimetal WTe2 (WTe2 hereafter) possesses strong spin-orbit coupling6,9 and non-trivial band structures10 with a large spin polarization protected by time-reversal symmetry in both the surface and bulk states9-11. Atomically flat surfaces, which can be produced with high quality12, facilitate spintronic device applications. Here, we use WTe2 as a spin current source in WTe2/Ni81Fe19 (Py) heterostructures. We report field-free current-induced magnetization switching at room temperature. A charge current density of ~2.96 × 105 A cm-2 suffices to switch the magnetization of the Py layer. With the charge current along the b axis of the WTe2 layer, the thickness-dependent charge-to-spin conversion efficiency reaches 0.51 at 6-7 GHz. At the WTe2/Py interface, a Dzyaloshinskii-Moriya interaction (DMI) with a DMI constant of -1.78 ± 0.06 mJ m-2 induces chiral domain wall tilting. Our study demonstrates the capability of WTe2 to efficiently manipulate magnetization and sheds light on the role of the interface in Weyl semimetal/magnet heterostructures.

Endohedral Fullerene Fe@C28 Adsorbed on Au(111) Surface as a High-Efficiency Spin Filter: A Theoretical Study.

We present a theoretical study on the adsorption and spin transport properties of magnetic Fe@C28 using Ab initio calculations based on spin density functional theory and non-equilibrium Green’s function techniques. Fe@C28 tends to adsorb on the bridge sites in the manner of C–C bonds, and the spin-resolved transmission spectra of Fe@C28 molecular junctions exhibit robust transport spin polarization (TSP). Under small bias voltage, the transport properties of Fe@C28 are mainly determined by the spin-down channel and exhibit a large spin polarization. When compressing the right electrode, the TSP is decreased, but high spin filter efficiency (SFE) is still maintained. These theoretical results indicate that Fe@C28 with a large magnetic moment has potential applications in molecular spintronics.

Toward Achieving the Theoretical Limit of Electron Spin Polarization in Covalently Linked Radical-Chromophore Dyads.

Electron spin systems with non-Boltzmann spin distributions are commonly observed in photophysical and photochemical processes involving free radicals. Understanding the origin of electron spin polarization (ESP) reveals detailed insight into the spin-dependent interactions occurring in these processes. The ability to transfer ESP to the nuclear spins of the solvent to produce large nuclear polarization is itself an active area of research in the field called dynamic nuclear polarization (DNP). Harnessing ESP in such fields demands a large magnitude of ESP and importantly on free radicals that are stable. In this work, we explored various factors that could play a prominent role in generation of ESP on the stable nitroxyl radical. By exploiting the dependence of ESP on the zero-field splitting parameter DT of the chromophore triplets and a careful choice of the chromophores for efficient quenching of their excited states by a free radical, we present here our strategies for designing covalently linked chromophore-free radical dyads, which could generate high magnitudes of ESP. Syntheses and EPR studies on a series of such dyads to demonstrate successful realization of our strategy are reported here. Our work reveals that, along with the DT value and the triplet quenching efficiency, the relative orientational dynamics of the radical and chromophore moiety and an efficient intersystem crossing of the chromophore primarily govern the ESP of the chromophore-radical dyads. One such molecule, an anthraquinone moiety linked to a TEMPO free radical that optimally satisfies the above criteria, produces a very large value of ESP-about 300 times its Boltzmann value-reaching almost half of the theoretical maximum of the radical-triplet pair mechanism of ESP. Coupled with its excellent photostability in benzene and the use of ambient conditions, this molecule should prove to be highly desirable in photoinduced DNP studies and other applications such as highly sensitive magnetometers, which require generation of large nuclear spin polarization in the solvent by transferring the electron spin polarization.

La0.67(Ca0.24Sr0.09)MnO3:xAg2O (0 ≤ x ≤ 0.25) composites with improved room–temperature TCR and MR for advanced uncooling infrared bolometers and magnetic sensors

In this research, high–density La0.67(Ca0.24Sr0.09)MnO3:xAg2O (LCSMO: xAg2O, x = 0, 0.05, 0.1. 0.15, 0.2, and 0.25) composites were successfully fabricated through the conventional sol-gel method. Scanning electron microscopy (SEM), X–ray photoelectron spectroscopy (XPS) and XRD data confirmed presence of silver at A sites (La3+/Ca2+/Sr2+) as Ag+ and segregates at grain boundaries (GBs) to form metallic silver. The electrical properties greatly improved by adding Ag2O. Significant enhancements in peak temperature coefficient of resistivity (TCR) and magnetoresistance (MR) at room–temperature (300 K) were observed for LCSMO:xAg2O composites. The peak TCR increased from 3.7% K−1 (299.3 K, for x = 0) to 14.8% K−1 (301.0 K, for x = 0.2), and MR rose from 16.9% (303.0 K, for x = 0) to 34.1% (300.6 K, for x = 0.2). These improvements in peak TCR were attributed to increased grain size and formation of metallic silver at GBs or within the composites. Moreover, peak MR was higher in Ag2O doped composites, which was mainly due to large spin polarization effect and strong coupling ferromagnetic domains under external magnetic field. Overall, the large room-temperature TCR and MR values of LCSMO:0.2Ag2O composites look promising for future advanced uncooling infrared bolometers and magnetic sensors.

Spin-dependent transport in Normal/Ferromagnetic/Normal monolayer zigzag molybdenum disulfide nanoribbon junction

In this paper, with the three-band tight-binding model, we investigate spin transport in Normal/Ferromagnetic/Normal monolayer zigzag molybdenum disulfide nanoribbon junction with double electric barriers using Green’s function technique. We discuss the influence of the height of the double electric barrier on spin transport. The results demonstrate that spin conductance has relatively large dependence on electric field modulation. We can obtain large spin polarization in low energy region which may be applied as a spin filter. In addition, we study the effect of magnetic field, and we find that we can obtain a larger spin polarization area which may provide a theoretical basis for spin filter application.

Efficient full spin\u2013orbit torque switching in a single layer of a perpendicularly magnetized single-crystalline ferromagnet

Spin–orbit torque (SOT), which is induced by an in-plane electric current via large spin-orbit coupling, enables an innovative method of manipulating the magnetization of ferromagnets by means of current injection. In conventional SOT bilayer systems, the magnetization switching efficiency strongly depends on the interface quality and the strength of the intrinsic spin Hall Effect. Here, we demonstrate highly efficient full SOT switching achieved by applying a current in a single layer of perpendicularly magnetized ferromagnetic semiconductor GaMnAs with an extremely small current density of ∼3.4 × 105 A cm−2, which is two orders of magnitude smaller than that needed in typical metal bilayer systems. This low required current density is attributed to the intrinsic bulk inversion asymmetry of GaMnAs as well as its high-quality single crystallinity and large spin polarization. Our findings will contribute to advancements in the electrical control of magnetism and its practical application in semiconductor devices.

Intrinsic Half‐Metallicity in 2D Ternary Chalcogenides with High Critical Temperature and Controllable Magnetization Direction

AbstractSearching for 2D ferromagnetic materials with a high critical temperature, large spin polarization, and controllable magnetization direction is a key challenge for their broad applications in spintronics. Here, through a systematic study on a series of 2D ternary chalcogenides with first‐principles calculations, it is demonstrated that a family of experimentally available 2D CoGa2X4 (X = S, Se, or Te) are half‐metallic ferromagnets, and they exhibit high critical temperature, fully polarized spin state, and strain‐dependent magnetization direction simultaneously. Following the Goodenough–Kanamori rules, the half‐metallic ferromagnetism of CoGa2X4 family is caused by superexchange interaction mediated by CoXCo bonds. The half‐metal gaps are large enough (>0.5 eV) to ensure that the half‐metallicity is stable against the spin flipping at room temperature. Magnetocrystalline anisotropy energy calculations indicate that CoGa2X4 favor easy plane magnetization. Under achievable biaxial tensile strain (2–6%), the magnetization directions of CoGa2X4 can change from in‐plane to out‐of‐plane, providing a route to control the efficiency of spin injection/detection. Further, the critical temperatures Tc of ferromagnetic phase transition for CoGa2X4 are close to room temperature. Belonging to the big family of layered AB2X4 compounds, the proposed CoGa2X4 systems will enrich the available 2D candidates and their heterojunctions for various applications.

Half-metallic, Magnetic, and Optical Properties for the (001) Surface of Binary Heusler Alloy MgCl3

The half-metallic (HM), magnetic, and optical properties for the (001) surface of binary Heusler alloy MgCl3 were investigated by the first-principles method. The Mg*Mg*-term and Mg*Mg-term had the lower surface energies among the ClCl-term, Mg*Mg*-term, ClMg-term, ClCl*-term, and Mg*Mg-term (001) surfaces, thus, they were the more stable surfaces. The band decomposed charge density showed clearly the valence band maximum was contributed not only by the ClB-3p states but also the ClA-3p and ClC-3p states. The HM character was destroyed in all the five terminations of the (001) surface. The stable Mg*Mg*-term (001) surface had the largest spin polarization − 83.0%. The hydrogen-termination effect showed an effective method to enlarge the spin polarizations for the ClCl-term and ClCl*-term surfaces to produce thin films which had potential applications in spintronics. There were great changes of atomic magnetic moments for Cl atoms in any layer. In energy range of 1.6–3.1 eV for the visible light, the Mg*Mg*-term had superiority in the absorption for purple light.

Role of antisite disorder, electron-electron correlations, and a surface valence transition in the electronic structure of CeMnNi4

CeMnNi$_4$ exhibits an unusually large spin polarization, but its origin has baffled researchers for more than a decade. We use bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory based on the Green's function technique to demonstrate the importance of electron-electron correlations of both the Ni 3$d$ ($U_{Ni}$) and Mn 3$d$ ($U_{Mn}$) electrons in explaining the valence band of this multiply correlated material. We show that Mn-Ni anti-site disorder as well as $U_{Ni}$ play crucial role in enhancing its spin polarization: anti-site disorder broadens a Ni 3$d$ minority-spin peak close to the Fermi level ($E_F$), while an increase in $U_{Ni}$ shifts it towards $E_F$, both leading to a significant increase of minority-spin states at $E_F$. Furthermore, rare occurrence of a valence state transition between the bulk and the surface is demonstrated highlighting the importance of HAXPES in resolving the electronic structure of materials unhindered by surface effects.

Enhanced stability and stacking dependent magnetic/electronic properties of 2D monolayer FeTiO3 on a Ti2CO2 substrate

With large magnetic moments, complete spin-polarization, high TC, and tunable magnetism characters, 2D FeTiO3/Ti2CO2 is proposed as an ideal material for high-performance spintronics.

Gate-tunable large spin polarization in a few-layer black phosphorus-based spintronic device.

We report a first principles study of the gate tunable spin transport properties in trilayer black phosphorus (BP) placed on a Ni(100) electrode. In the proposed spintronic device, the spin-dependent conductance and the corresponding spin polarization can be largely modulated by the gate voltage. Instead of resorting to the gate tunable spin orbital interaction (SOI) mechanism in the traditional spintronic devices, the control scheme here is achieved by the giant Stark effect induced semiconducting-metallic phase transition of the few-layer BP. It is found that the quantum tunneling effect is gradually reduced as the gate voltage increases. During this process, the conductances of spin up and spin down components increase differently and hence the spin polarization is largely varied from 7% to 77% in parallel configuration (PC) and from 1% to 54% in anti-parallel configuration (APC). More importantly, the maximum magnetoresistance (MR) value can reach as large as 80%. Finally, we find that the non-equilibrium spin-polarized current can also be controlled by the external bias voltage. For instance, a large turn on/off ratio can be realized by the gate voltage (4239 for spin down current and 1809 for spin up current) when the bias voltage is fixed at 0.01 V. Our theoretical findings pave a feasible way for the novel application of few-layer BP in spintronic nanodevices.