High Spin Polarization Research Articles

Simultaneous observation of anti-damping and the inverse spin Hall effect in the La0.67Sr0.33MnO3/Pt bilayer system.

Manganites have shown potential in spintronics because they exhibit high spin polarization. Here, by ferromagnetic resonance we have studied the damping properties of La0.67Sr0.33MnO3/Pt bilayers which are prepared by oxide molecular beam epitaxy. The damping coefficient (α) of a La0.67Sr0.33MnO3 (LSMO) single layer is found to be 0.0104. However the LSMO/Pt bilayers exhibit a decrease in α with an increase in Pt thickness. This decrease in the value of α is probably due to high anti-damping like torque. Furthermore, we have investigated the angle dependent inverse spin Hall effect (ISHE) to quantify the spin pumping voltage from other spin rectification effects such as the anomalous Hall effect and anisotropic magnetoresistance. We have observed a high spin pumping voltage (∼20 μV). The results indicate that both anti-damping and spin pumping phenomena occur simultaneously.

Direct Magnetic Reinforcement of Electrocatalytic ORR/OER with Electromagnetic Induction of Magnetic Catalysts.

Designing stable and efficient electrocatalysts for both oxygen reduction and evolution reactions (ORR/OER) at low-cost is challenging. Here, a carbon-based bifunctional catalyst of magnetic catalytic nanocages that can direct enhance the oxygen catalytic activity by simply applying a moderate (350 mT) magnetic field is reported. The catalysts, with high porosity of 90% and conductivity of 905 S m-1 , are created by in situ doping metallic cobalt nanodots (≈10nm) into macroporous carbon nanofibers with a facile electrospinning method. An external magnetic field makes the cobalt magnetized into nanomagnets with high spin polarization, which promote the adsorption of oxygen-intermediates and electron transfer, significantly improving the catalytic efficiency. Impressively, the half wave-potential is increased by 20mV for ORR, and the overpotential at 10mA cm-2 is decreased by 15mV for OER. Compared with the commercial Pt/C+IrO2 catalysts, the magnetic catalyzed Zn-air batteries deliver 2.5-fold of capacities and exhibit much longer durability over 155 h. The findings point out a very promising strategy of using electromagnetic induction to boost oxygen catalytic activity.

Chiral-induced spin selectivity: A polaron transport model

Weak hyperfine interactions and spin-orbit coupling (SOC) in organic materials result in long spin lifetimes, which is very promising for spintronics. On the other hand, they also make it challenging to achieve spin polarization, which is of crucial importance for spintronics devices. To overcome this obstacle, we have proposed a physical model for spin-polarized electron transport through a chiral molecule based on the chiral-induced spin selectivity. Because the transport in the chiral molecule is not an isolated one, but rather an electron coupled to its surrounding lattice distortions, namely, a spatial localized polaron, an indispensable polaron effect is incorporated in our model. We show that the polaron transport through the chiral molecule exhibits a spin-momentum-locked feature. Interestingly, no matter what their initial spin state is, all of the polarons could transmit through the molecule with their spins being aligned to the same orientation due to the effective ``inverse Faraday effect.'' The coexistence of the electron-lattice coupling and SOC results in the spin and lattice being coupled, which leads to a strongly enhanced spin coherence and then a very high spin polarization of $70%$. In addition, the effects of the helix pitch, polaron size, and drift velocity on spin polarization are also discussed. Our results open the possibility of using chiral molecules in spintronics applications and offer a paradigm for information processing and transmission.

Spectral properties of heterostructures containing half-metallic ferromagnets in the presence of local many-body correlations

In this work, we investigate models for bulk, bi- and multilayers containing half-metallic ferromagnets (HMFs), at zero and at finite temperature, in order to elucidate the effects of strong electronic correlations on the spectral properties (density of states). Our focus is on the evolution of the finite-temperature many-body induced tails in the half-metallic gap. To this end, the dynamical mean-field theory (DMFT) is employed. For the bulk, a Bethe lattice model is solved using a matrix product states based impurity solver at zero temperature and a continuous-time quantum Monte Carlo (CT-QMC) solver at finite temperature. We demonstrate numerically, in agreement with the analytical result, that the tails vanish at the Fermi level at zero temperature. In order to study multilayers, taken to be square lattices within the layers, we use the real-space DMFT extension with the CT-QMC impurity solver. For bilayers formed by the HMF with a band or correlated insulator, we find that charge fluctuations between the layers enhance the finite temperature tails. In addition, in the presence of inter-layer hopping, a coherent quasiparticle peak forms in the otherwise correlated insulator. In the multilayer heterostructure setup, we find that by suitably choosing the model parameters, the tails at the HMF/Mott insulator interface can be reduced significantly, and that a high spin polarization is conceivable, even in the presence of long-ranged electrostatic interactions.

Visualizing Half-Metallic Bulk Band Structure with Multiple Weyl Cones of the Heusler Ferromagnet.

Using a well-focused soft x-ray synchrotron radiation beam, angle-resolved photoelectron spectroscopy was applied to a full-Heusler-type Co_{2}MnGe alloy to elucidate its bulk band structure. A large parabolic band at the Brillouin zone center and several bands that cross the Fermi level near the Brillouin zone boundary were identified in line with the results from first-principles calculations. These Fermi-level crossings are ascribed to majority spin bands that are responsible for electron transport with extremely high spin polarization especially along the direction perpendicular to the interface of magnetoresistive devices. The spectroscopy confirms there is no contribution of the minority spin bands to the Fermi surface, signifying half-metallicity for the alloy. Furthermore, two topological Weyl cones with band crossing points were identified around the X point, yielding the conclusion that Co_{2}MnGe could exhibit topologically meaningful behavior such as large anomalous Hall and Nernst effects driven by the Berry flux in its half-metallic band structure.

Effects of the atomic order on the half-metallic electronic structure in the Co2Fe(Ga0.5Ge0.5) Heusler alloy thin film

The atomic order and its effect on the electronic structure is a key issue in the application of half-metallic Heusler alloys as spin-polarized electron sources. In this study, we investigated the atomic ordering and electronic structures of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{Fe}(\mathrm{G}{\mathrm{e}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5})$ (CFGG) Heusler alloy thin films at various annealing temperatures using anomalous x-ray diffraction (AXRD) and hard x-ray photoelectron spectroscopy (HAXPES). AXRD measurements clearly showed that the Co-Fe disorder is large in the as-deposited state, which is reduced to almost zero by annealing at ${T}_{\mathrm{an}}=\phantom{\rule{0.16em}{0ex}}500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ for 30 min. The reduction of the Co-Fe disorder explains the observed increase in the spin polarization of currents estimated by ordinary and anisotropic magnetoresistance measurements. The photoelectron spectra of the valence band in CFGG thin films with different ${T}_{\mathrm{an}}$ agree well with the first-principles calculations that considers the atomic disorder. We also found that the characteristic peaks appearing in the valence band HAXPES spectra shifted to higher binding energies, compared to the calculated density of states for stoichiometric $\mathrm{L}{2}_{1}$-ordered CFGG. This indicates that the Fermi level is shifted and located in the vicinity of the conduction band edge in the present CFGG thin film due to electron doping by a mildly Co-rich chemical composition. Our first-principles calculations of partial density of state of $sp$ electron predicted that the high spin-polarization is obtainable in a wider energy region near Fermi level in the $\mathrm{L}{2}_{1}$-ordered structure than the B2 structure, which explains the enhancement of spin-polarization by promoting $\mathrm{L}{2}_{1}$-ordering in the present CFGG film.

Resolving the discrepancy between theoretical and experimental polarization of hyperpolarized 129Xe using numerical simulations and in situ optical spectroscopy

For emerging biomedical applications of hyperpolarized xenon, the ability to obtain reliably high nuclear spin polarization levels is paramount. Yet, experimental nuclear spin polarization levels of xenon are highly variable and, more than often than not, well below what theory predicts. Despite rigorous and well-studied theoretical models for hyperpolarization and continuous-flow spin-exchange optical pumping (SEOP), there remains a substantial discrepancy between the theoretical and experimental polarization of 129Xe; inexplicably, seemingly similar experimental parameters can yield very different polarization values. In this paper, the validity of the assumptions typically made about the thermodynamic state of the Rb vapor inside the optical pumping cell and the gas dynamics are investigated through finite element analysis simulations of realistic optical pumping cell models, while in situ optical and nuclear magnetic resonance spectroscopy measurements are used to validate the results of the simulations. Our results show that shorter xenon gas residence times and lower Rb vapor densities than those predicted by empirical saturated vapor pressure curves, along with incorrect SEOP parameters, are the primary cause of the discrepancy between theoretical and experimental polarization values reported in the literature.

Oblique Nuclear Quadrupole Interaction in Self-Assembled Structures Based on Semiconductor Quantum Dots

Dynamic nuclear polarization (DNP) is well recognized as being important in spintronics and quantum-information processing. DNP gives rise to high nuclear spin polarization that not only can prolong electron-spin lifetime by generating an Overhauser field (OHF), but also has fertilized the idea to explore nuclear spin qubits. In strained quantum-dot structures (QDSs), a nuclear spin is coupled to a strain field via its quadrupole moment. It has been shown that such nuclear quadrupole interaction (NQI) can be used to achieve appreciable DNP and hence electron-spin polarization. Here, we uncover magneto-optical anomalies from a series of laterally arranged $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ QDSs that arise from the NQI and DNP in these nanostructures. We find that the principal axis of NQI in symmetry-lowered QDSs significantly deviates from the growth direction, resulting in tilting of OHF with an angle exceeding 37\ifmmode^\circ\else\textdegree\fi{}. The resulting transverse component of OHF is probed with respect to the crystallographic orientations and its influence on the DNP and ensemble spin dephasing is analyzed. We show that a high-symmetry electronic confinement potential for excitons does not guarantee a high-symmetry NQI for nuclei within the same nano-object, thereby calling for correlated optimization in the symmetry of the electronic confinement potential and that of the nuclear spin bath. Our results underline the role of oblique NQI in electron-spin decoherence and depolarization, which has so far largely been overlooked. This work thus sheds light on design rules for engineering the electronic and spin landscape of QDSs for better performance of DNP desirable for applications in spintronics and quantum computation.

Room temperature near unity spin polarization in 2D Van der Waals heterostructures

The generation and manipulation of spin polarization at room temperature are essential for 2D van der Waals (vdW) materials-based spin-photonic and spintronic applications. However, most of the high degree polarization is achieved at cryogenic temperatures, where the spin-valley polarization lifetime is increased. Here, we report on room temperature high-spin polarization in 2D layers by reducing its carrier lifetime via the construction of vdW heterostructures. A near unity degree of polarization is observed in PbI2 layers with the formation of type-I and type-II band aligned vdW heterostructures with monolayer WS2 and WSe2. We demonstrate that the spin polarization is related to the carrier lifetime and can be manipulated by the layer thickness, temperature, and excitation wavelength. We further elucidate the carrier dynamics and measure the polarization lifetime in these heterostructures. Our work provides a promising approach to achieve room temperature high-spin polarizations, which contribute to spin-photonics applications.

Publisher's Note: “High and reversible spin polarization in a collinear antiferromagnet” [Appl. Phys. Rev. 7, 031405 (2020)

Publisher's Note: “High and reversible spin polarization in a collinear antiferromagnet” [Appl. Phys. Rev. <b>7</b>, 031405 (2020)

Perpendicular magnetic tunnel junctions based on half-metallic NiCo2O4

Spin polarization and magnetic anisotropy are key properties that determine the performance of magnetic tunnel junctions (MTJs), which are utilized in various spintronic devices. Although materials that have both high spin polarization and sufficient perpendicular magnetic anisotropy are desirable for further developments of MTJs, such materials are rare because electronic structures necessary for achieving these two properties are distinct. Here, we experimentally show the inverse spinel ferrimagnet NiCo2O4 (NCO), whose band structure is theoretically predicted to be half-metallic, has both high spin polarization and perpendicular magnetic anisotropy. Perpendicular MTJs with NCO magnetic electrodes exhibit magnetoresistance up to 230%, indicating that the spin polarization of perpendicularly magnetized NCO is as high as −73%. Our experimental results demonstrate the potential of NCO as a half-metal with perpendicular magnetic anisotropy, which will lead to new paradigms for designing and developing all-oxide spintronic devices.

High and reversible spin polarization in a collinear antiferromagnet

Antiferromagnetic materials are expected to trigger the revolution of spintronic applications because they are robust against the perturbation of magnetic fields, produce no stray fields, and display ultrafast dynamics. To design antiferromagnetic devices, the generation and manipulation of spin-polarized current in antiferromagnetic materials are vital. Unfortunately, it has long been believed that spin-polarized current could not be achieved in collinear antiferromagnetic materials because of the symmetry of spin sublattices. Inspired by the possibility of breaking the symmetry using a bias voltage, a special Au/NiO/Au junction is proposed and constructed along the [111] direction of NiO using the collinear NiO antiferromagnetic insulator as a prototype. It was found that the symmetry of spin sublattices was broken by an external bias, which induces a nonequivalent transport for spin-up and spin-down currents, that is, the appearance of spin-polarized current. Unexpectedly, this study found that the magnitude and sign of spin polarization could be adjusted by varying the bias. A positive spin polarization up to 80% was obtained at ∼ 0.9 V, and the spin polarization turned negative when the bias voltage was beyond 1.6 V. The related physical mechanisms were explored and clarified by analyzing spin-resolved transmission, kǁ-resolved density of states, and layer-resolved local density of states of Au/NiO/Au, etc. The effects of NiO layer thicknesses, different electrodes, and interfacial disorders on the spin polarization were also analyzed and found to offer promising applications. This study provides new physical insight into spin-polarized current transport in collinear antiferromagnetic materials and greatly contributes to the development of novel antiferromagnetic spintronic devices.

High Spin Polarization in Co2FeSn Heusler Nanowires for Spintronics

Cylindrical nanowires made of Co2FeSn Heusler alloy with high spin polarization have been synthetized via template-assisted electrochemical deposition in nanoporous anodic alumina membranes. Their ...

Tuning magnetic anisotropy and magnetization switching in FeN ferromagnetic films by crystal regulation

Tailoring the magnetic anisotropy and magnetization switching of a ferromagnetic film is crucial in constructing an energy-efficient magnetic random access memory (MRAM). Fe16N2 alloy was predicted to bear giant magnetic anisotropy and high spin polarization ratio in theory and is a promising material to be applied in the magnetic memory devices. However, the related research work was scarcely reported. In this work, we devised a Fe/Cr/FeN/Cr heterostructure to introduce a controllable strain on the FeN layer by a lattice mismatch. The strain is variable with the thickness of FeN layer and can modify the phase constituent in the FeN layer effectively. By using the strain engineering, we achieved a broad tunability of magnetic anisotropy energy ranging 0.1—0.7 × 106 J/m3. Based on the anisotropy modulation, we achieved electrical control of partial magnetization switching in the Fe/Cr/FeN/Pt heterostructure by a spin-orbit-toque (SOT) effect under a current density ~1010 A/m2. These findings may provide a promising ferromagnetic material for realizing energy-efficient SOT-MARM.

Direct observation of ferrimagnetic ordering in inverse Heusler alloy Mn2CoAl

Compensated ferrimagnetic Heusler compounds with high spin polarization and a low net magnetic moment are strategically important materials for spin-logic and further energy-efficient spintronic applications. However, the element-resolved magnetic ordering of these compensated ferrimagnets remains an open issue. Here, we report a direct observation of the spin and orbital moments of the B2 phase Mn2CoAl thin film using the synchrotron-based x-ray magnetic circular dichroism technique. An ferrimagnetic ordering between Mn and Co elements and a compensated-ferrimagnet-like small net magnetic moment of only 0.34 μB/f.u. were observed unambiguously in B2 Mn2CoAl. Antiparallel coupling between Mn and Co is attributed to the mixture of the Mn(B) and Al occupation in the B2 phase Mn2CoAl lattice. This work demonstrates great potential of the compensated ferrimagnetic half-metallic inverse Heusler compounds Mn2CoAl for spintronic applications.

2D Mn2C6Se12 and Mn2C6S6Se6: Intrinsic Room-Temperature Dirac Spin Gapless Semiconductors and Perfect Spin Transport Properties

2D intrinsic ferromagnets with high spin polarization and high Curie temperature are rare and very important for developing novel nanoscale spintronic materials and devices. In this letter, by usin...

Local strain-dependent electronic structure and perpendicular magnetic anisotropy of a MnGaN 2D magnetic monolayer

Local strain-dependent spin-polarized electronic structure of a two-dimensional (2D) magnetic layer is an exciting property for practical applications. For example, it holds the promise for advanced ultrathin spintronic nanodevices with customized electronic and magnetic properties by local strain engineering. Here, we demonstrate that the spin-polarized electronic structure of a 2D manganese gallium nitride (MnGaN-2D) magnetic monolayer is sensitive to intrinsic local lattice strain, as proven by first-principles calculations and indicated by scanning tunneling spectroscopy measurements. Atomic resolution images reveal a highly non-Gaussian lattice spacing/strain distribution, while the spectroscopy reveals variations in the electronic density of states. Simulations of the MnGaN-2D monolayer based on first-principles calculations, including both isotropic and anisotropic strains, confirm a highly strain-dependent manganese partial density of states. Spin-orbit coupling is included which indicates either out-of-plane perpendicular magnetic anisotropy (PMA) or in-plane magnetic anisotropy, dependent on the type of strain whether compressive or tensile, suggesting that MnGaN-2D is magnetoelastic. The MnGaN-2D PMA is further supported by superconducting quantum interference device magnetometry measurements which reveal a high spin polarization of $\ensuremath{\sim}79$% at room temperature.

New equiatomic quaternary Heusler compounds without transition metals KCaBX (X = S and Se): Robust half-metallicity and optical properties

It is known that high spin-polarization and magnetism can be found even in materials with neither transition metals nor rare earths. In this paper, we report results of the structural design, electronic structure, magnetic and optical properties of new equiatomic quaternary Heusler (EQH) KCaBX (X = S and Se) compounds. Electron exchangecorrelation interactions are described by the Wu-Cohen (WC) functional and Tran-Blaha modified Becke-Johnson exchange (mBJ) potential. Ferromagnetic ordering is stable for the cubic structure of space group F43 m in which the K, Ca, B and X atoms are located at 4c, 4d, 4a and 4b Wyckoff positions, respectively. Quaternaries at hand exhibit a perfect spin-polarization around the Fermi level, which is a result of the half-metallicity with metallic spin-up channel and semiconductor spin-dn channel. The ferromagnetic half-metallic and spin-flip band gaps are 2.648(2.470) and 0.673(0.526), respectively, for KCaBS(KCaBSe). Both studied compounds have a total magnetic moment of 2.000 μB. Additionally, the strain effect on the electronic and magnetic properties is also examined. Finally, the optical properties of the KCaBX alloys are investigated for energies up to 25 eV. Optical spectra show the metallic behavior at extremely low energies and semiconductor nature at higher energies. Interestingly, KCaBS and KCaBSe exhibit prospective absorption properties with a quite large absorption coefficient in the ultraviolet regime.

Half-metallic nature of the low-temperature grown Co2MnSi films on SrTiO3

We experimentally show that half-metallic Co2MnSi films can be grown on SrTiO3(001) at around 350 °C by molecular beam epitaxy. Despite relatively low-temperature grown Co2MnSi films, L21-ordered structures, high saturation magnetic moments of ∼5.0 μB/f.u., and the negative sign of the anisotropic magnetoresistance (AMR) indicating half-metallic nature are obtained. For the 350°C-grown Co2MnSi films, a low Gilbert damping constant of ∼0.0035, implying the high spin polarization, is evaluated. This study will open a way for developing novel spintronic applications consisting of Co2MnSi and perovskite oxides such as high-performance interfacial multiferroic systems and efficient spin-to-charge conversion systems.

Spin injection through energy-band symmetry matching with high spin polarization in atomically controlled ferromagnet/ferromagnet/semiconductor structures

Electrical injection of spin-polarized electrons from ferromagnets into semiconductors has been generally demonstrated through a tunneling process with insulator barrier layers that can dominate the device performance, including the electric power at the electrodes. Here, we show an efficient spin injection technique for a semiconductor using an atomically controlled ferromagnet/ferromagnet/semiconductor heterostructure with low-resistive Schottky-tunnel barriers. On the basis of symmetry matching of the electronic bands between the top highly spin-polarized ferromagnet and the semiconductor, the magnitude of the spin signals in lateral spin-valve devices can be enhanced by up to one order of magnitude compared to those obtained with conventional ferromagnet/semiconductor structures. This approach provides a new solution for the simultaneous achievement of highly efficient spin injection and low electric power at the electrodes in semiconductor devices, leading to novel semiconductor spintronic architectures at room temperature.