Field-induced Spin Polarization Research Articles

Revealing Spin Magnetic Effect of Iron-Group Layered Double Hydroxides with Enhanced Oxygen Catalysis

The oxygen evolution reaction (OER) is the bottleneck limiting the reaction process of water splitting. The OER process involves the recombination of oxygen from diamagnetic singlet state OH or H2O to paramagnetic triplet state O2. The spin conservation for oxygenated intermediates must play an important role in the OER. However, the dynamic mechanism of magnetic field-induced spin polarization is still in its infancy. Herein, based on the spin-coupling interaction of iron group elements, three typical iron group layered double hydroxides (LDHs) were constructed to study the relationship among magnetic field, spin polarization, and OER activity. Combining experimental and theoretical studies, we revealed the spin-magnetic effect of iron group LDHs for enhancing the OER process. There is a positive correlation between the saturation magnetization and OER performance of iron group LDHs under different magnetic fields. The NiCoFe-LDHs (NCFL) endows the strongest OER activity (η10 = 230 mV) and saturation magnetization (Ms = 44 emu mg–1) compared with that of CoFe-LDHs (CFL, η10 = 372 mV, Ms = 21 emu mg–1) and NiFe-LDHs (NFL, η10 = 246 mV, Ms = 29 emu mg–1). The density functional theory calculations show that the Fe sites of NCFL endow a stronger spin-coupling interaction with OH, and Raman spectroscopy further proves the promotion for the formation of the O–O bond of NCFL. Applying an external magnetic field, due to the spin magnetic effect of iron group LDHs, the enhancement amplitude of OER activity is also positively correlated with the magnetism of the catalyst. NCFL has the strongest spin magnetic effect about −34.8 mV T–1 compared with NFL (−27.0 mV T–1) and CFL (−16.7 mV T–1). The overpotential of NCFL is only 206 mV under the condition of 700 mT magnetic field. In conclusion, we demonstrate the mechanism of underlying influence of the spin magnetic effect on the OER performance and provide insights into the relationship between catalysts and oxygenated intermediates. These insights would help to understand and design catalysts at the spintronic level.

Control of chiral orbital currents in a colossal magnetoresistance material.

Colossal magnetoresistance (CMR) is an extraordinary enhancement of the electrical conductivity in the presence of a magnetic field. It is conventionally associated with a field-induced spin polarization that drastically reduces spin scattering and electric resistance. Ferrimagnetic Mn3Si2Te6 is an intriguing exception to this rule: it exhibits a seven-order-of-magnitude reduction in ab plane resistivity that occurs only when a magnetic polarization is avoided1,2. Here, we report an exotic quantum state that is driven by ab plane chiral orbital currents (COC) flowing along edges of MnTe6 octahedra. The c axis orbital moments of ab plane COC couple to the ferrimagnetic Mn spins to drastically increase the ab plane conductivity (CMR) when an external magnetic field is aligned along the magnetic hard c axis. Consequently, COC-driven CMR is highly susceptible to small direct currents exceeding a critical threshold, and can induce a time-dependent, bistable switching that mimics a first-order 'melting transition' that is a hallmark of the COC state. The demonstrated current-control of COC-enabled CMR offers a new paradigm for quantum technologies.

On Guided Remagnetization in Layered Anoheterostructures

Field-guided magnetic dynamics in magnetic multilayer nanostructures involves interconnection of the control field with localized spin states, which can occur directly or indirectly depending on the nature of the field and spin polarization. At the control electromagnetic field, this interconnection can be directly induced by the photon-induced spin-flip processes and indirectly by a bias field during antiferromagnetic exchange relaxation. The control impact of electric field and electric current on the magnetic states occurs indirectly via the spin polarization and spin current in combination with the exchange interaction of these polarized spins with localized magnetic states. The corresponding description of the magnetic dynamics is based on the modified Landau-Lifshitz equation and spin diffusion equations, taking into account the spin Hall and the inverse spin Hall effects for systems with normal metal sublayers. In the case of the magnetic nanostructures with the Rashba spin-orbit interaction in interfaces, the electric field-controlled magnetization is realized via the Rashba field-induced spin polarization, and its exchange interaction with localized magnetic states. Corresponding description is based on a tight-binding model of spin-orbit-coupled electrons exchange coupled to the localized magnetic states.

Magnetic and electronic properties of the ferroelectric-photovoltaic ordered double perovskiteCaMnTi2O6investigated by x-ray absorption spectroscopies

The ferroelectric and magnetic phases of the double perovskite $\mathrm{CaMnT}{\mathrm{i}}_{2}{\mathrm{O}}_{6}$ with $A$-site order have been investigated by soft x-ray absorption and magnetic circular dichroism. All spectra point to a very ionic state of divalent Mn and tetravalent Ti atoms. The effects of the crystal field produced by O ligands around tetravalent titanium and the dissimilar Mn1 and Mn2 sites were investigated. Both the so-called square-planar and the octahedrally coordinated Mn sites spectroscopically contribute in a rather similar way, with little influence by the oxygen environment. Multiplet calculations suggest a small $O\phantom{\rule{0.16em}{0ex}}2p\text{\ensuremath{-}}\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ charge-transfer component in the FE phase. Magnetic symmetry calculations were performed to determine probable configurations of Mn spins compatible with the acentric $P{4}_{2}mc$ structure and, in combination with the computational magnetic results in Inorg. Chem. 56, 11854 (2017), we have identified the $P{4}_{2}^{\ensuremath{'}}{m}^{\ensuremath{'}}c$ as the most likely magnetic space group keeping invariant the unit cell below ${T}_{\mathrm{N}}$. This symmetry forces the sign of the magnetic coupling along the Mn columns parallel to $c$ to reverse with respect to the coupling between neighboring columns. Below ${T}_{\mathrm{N}}$, the dichroic magnetization loops at the $\mathrm{Mn}\phantom{\rule{0.16em}{0ex}}{L}_{3}$ edge confirm the absence of spontaneous ferromagnetism, although a very small field-induced spin polarization was detected in the sample.

Electric field induced spin polarization oscillation in nonmagnetic benzene/Cu(100) interface: First principles calculations

First-principles calculation are presented to study the influences of external electric fields on the spin polarization properties of benzene/Cu(100) system which do not contain any magnetic atom. Our simulations show that an obvious spontaneous spin polarization oscillation occurred in the benzene molecule when the electric fields are applied. The density of states (DOS), spin density distributions, charge transfer properties are also obtained. It is found that the p-d orbital coupling between the benzene molecule and the electrode leads to spin non-degeneration of the DOS near the fermi energy, so the transferred charges from the Cu atoms to the molecule will fill these spin non-degenerate coupled orbitals, and then the benzene molecule becomes spin polarized. The strength of the p-d orbital coupling as well as the transferred charges oscillated with the external electric fields, which induce spin polarization oscillation. The results are favorable for the understanding of spin polarization properties in organic/nonmagnetic metal structures.

Electric field induced spin polarization in graphene nanodots

By employing a configuration interaction approach, we study the spin polarization induced by an electric field applied to graphene nanodots. The evolution of both the ground and excited states of the graphene nanodot with the applied electric field is calculated systematically. Three clearly separated field regimes have been identified for the spin gap in the model system, namely, (a) nearly constant in the nonmagnetic phase, (b) rapidly decreasing in the antiferromagnetic phase, and (c) exponential decay in the ferromagnetic phase. It is therefore demonstrated that ferromagnetism can be induced purely by applying an electric field to a nonmagnetic graphene nanodot. Electron-electron interactions which are greatly modified by the applied electric field are believed to account for the two consecutive magnetic phase transitions.

Experimental investigation of the optical spin-selection rules in bulk Si and Ge/Si quantum dots

We study the relationship between the circular polarization of photoluminescence and the magnetic field-induced spin-polarization of the recombining charge carriers in bulk Si and Ge/Si quantum dots. First, we quantitatively compare experimental results on the degree of circular polarization of photons resulting from phonon-assisted radiative transitions in intrinsic and doped bulk Si with calculations which we adapt from recently predicted spin-dependent phonon-assisted transition probabilities in Si. The excellent agreement of our experiments and calculations quantitatively verifies these spin-dependent transition probabilities and extends their validity to weak magnetic fields. Such magnetic fields can induce a luminescence polarization of up to 3%/T. We then investigate phononless transitions in Ge/Si quantum dots as well as in degenerately doped Si. Our experiments systematically show that the sign of the degree of circular polarization of luminescence resulting from phononless transitions is opposite to the one associated with phonon-assisted transitions in Si and with phononless transitions in direct band gap semiconductors. This observation implies qualitatively different spin-dependent selection rules for phononless transitions, which seem to be related to the confined character of the electron wave function.

Anisotropic dynamical spin-density response in quantum wells with spin-orbit interaction

The electric field-induced spin polarization of a two-dimensional electron gas with spin-orbit interaction is investigated in the frequency domain. We calculate the spin-polarizability tensor using the linear-response theory, taking into account the presence of both Rashba and Dresselhaus spin-orbit couplings. We consider quantum wells grown in the main crystallographic directions and concentrate on the high-frequency response in the collisionless regime. For an anisotropic spin splitting, it is shown that the spin-density response becomes dependent on the direction of the applied electric field and presents characteristic spectral features, in notable contrast to the case of an isotropic spin-orbit coupling. Such behavior is explained in terms of the nonisotropic momentum space available for electric-dipole transitions and the presence of critical points. This anisotropic dynamic response suggests new possibilities of spin manipulation which could find spintronic applications.

Magnetic Nanofilm of Fe3O4 for Highly Efficient Organic Light-Emitting Devices

We have demonstrated enhanced efficiency from organic light-emitting devices (OLEDs) with Fe3O4-coated indium−tin−oxide (ITO) anodes. The current efficiency of the OLEDs with Fe3O4/ITO anode is increased by 10.5%, when an external magnetic field is applied. This improvement is ascribed to the magnetic field-induced spin polarization of holes injected through the spin aligner of the Fe3O4 film, which results in an enhanced formation ratio of singlet excitons. In addition, Fe3O4 is also found acting as an anodic buffer to increase the hole-injection efficiency. The improved luminance and efficiency considered as resulting from the anode interfacial modification of Fe3O4 compared to the devices with bare ITO.

Electric-field induced spin textures in a superlattice with Rashba and Dresselhaus spin-orbit coupling

The DC charge current, field-induced spin polarization and spin textures are studied in the 1D gated superlattice with both fixed and varying Rashba and Dresselhaus contributions to spin-orbit coupling. It is found that a spin component with zero mean value can demonstrate non-vanishing field-induced spin texture in a superlattice cell which can be probed experimentally, with the highest amplitudes achievable in an interval of comparable Rashba and Dresselhaus terms. The consideration of the finite parameters for collision rate and temperature is found to be non-destructive for the calculated current and spin characteristics depending on all states below the Fermi level.

Will zigzag graphene nanoribbon turn to half metal under electric field?

At B3LYP level of theory, we predict that the half-metallicity in zigzag edge graphene nanoribbon (ZGNR) can be realized when an external electric field is applied across the ribbon. The critical electric field decreases with the increase of the ribbon width to induce the half-metallicity. Both the spin polarization and half-metallicity are removed when the edge state electrons fully transferred from one side to the other under very strong electric field. The electric field range under which ZGNR remains half-metallic increases with the ribbon width. Our study demonstrates a rich field-induced spin polarization behavior, which may lead to some important applications in spinstronics.

Effective spin-orbit Hamiltonian and field-induced spin polarization of two-dimensional holes

It is shown that the electric field applied in the plane of two-dimensional hole layers not only excites the intrinsic spin currents, but also leads to a homogeneous spin polarization. Theoretical study of this phenomenon, taking into account both the anisotropy of the Luttinger Hamiltonian and the bulk inversion asymmetry effects for holes, is carried out for the quantum wells grown along [ 0 0 1 ] crystallographic direction. The examples of GaAs and Si layers are considered.

Effect of screening on spin polarization in a two-dimensional electron gas

The experimentally observed amazing dependence of a critical magnetic field B c of a full field-induced spin polarization and a spin susceptibility χ ∗ of a two-dimensional electron gas on the electron density can be explained by screening of the Coulomb potential. The possibility of spontaneous full spin polarization expected at lower electron densities also crucially depends on screening.

Signatures of field induced spin polarization of neutron star matter in seismic vibrations of paramagnetic neutron star

A macroscopic model of the dissipative magneto-elastic dynamics of viscous spin polarized nuclear matter is discussed in the context of seismic activity of a paramagnetic neutron star. The source of the magnetic field of such a star is attributed to Pauli paramagnetism of baryon matter promoted by a seed magnetic field frozen into the star in the process of gravitational collapse of a massive progenitor. Particular attention is given to the effect of shear viscosity of incompressible stellar material on the timing of non-radial torsional magneto-elastic pulsations of the star triggered by starquakes. By accentuating the fact that this kind of vibration is unique to the seismology of a paramagnetic neutron star we show that the high-frequency modes decay faster than the low-frequency modes. The obtained analytic expressions for the period and relaxation time of this mode, in which the magnetic susceptibility and viscosity enter as input parameters, are then quantified by numerical estimates for these parameters taken from early and current works on transport coefficients of dense matter. It is found that the effect of viscosity is crucial for the lifetime of magneto-torsion vibrations but it does not appreciably affect the periods of this seismic mode which fall in the realm of periods of pulsed emission of soft gamma-ray repeaters and anomalous x-ray pulsars—young super-magnetized neutron stars, radiating, according to the magnetar model, at the expense of the magnetic energy release. Finally, we present arguments that the long periodic pulsed emission of these stars in a quiescent regime of radiation can be interpreted as a manifestation of weakly damped seismic magneto-torsion vibrations exhibiting the field induced spin polarization of baryon matter.

In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems.

Using a novel technique, we make quantitative measurements of the spin polarization of dilute [ (3.4-6.8)x10(10) cm(-2)] GaAs (311)A two-dimensional holes as a function of an in-plane magnetic field. As the field is increased the system gradually becomes spin polarized, with the degree of spin polarization depending on the orientation of the field relative to the crystal axes. Moreover, the behavior of the system turns from metallic to insulating before it is fully spin polarized. The minority-spin population at the transition is approximately 8x10(9) cm(-2), close to the density below which the system makes a transition to an insulating state in the absence of a magnetic field.

Several- and Many-Electron Artificial-Atoms at Filling Factors between 2 and 1

We introduce new phenomena that can be studied in an artificial-atom vertical single electron transistor. As we move from the few-electron regime to the several-electron regime, and then the many-electron regime, features in the conductance peaks related to magnetic field induced spin polarization evolve. This allows us to probe the spin-flip region bounded by the last single-particle crossing at low field, and the eventual formation of a maximum density droplet at high field.

Magnetic Field Influence on Electron Tunneling through a Molecular Wire

The magnetic field influence on the elastic and inelastic inter-electrode electron tunneling mediated by a molecular wire is studied theoretically. The wire is supposed to include two paramagnetic ions which should be largely separatel in space and should reduce their electronic ground-state spin from S to S − ½ during the formation of an intermediate bound state with the tunneling electron. A field-induced spin polarization of the outgoing low-temperature tunnel current is demonstrated. In the presence of a single-ion anisotropy, a step-like behavior of the current is obtained. Finally, for the inelastic inter-electrode tunneling, a blocking of the tunnel current appears.

Modulation spectroscopy of Eu-chalcogenide magnetic semiconductors

The critical behaviour of the optical spectra of EuSe is investigated by means of the spin-order modulation spectroscopy including the thermo-reflectance (TR), magneto-thermo-reflectance (MTR) and magnetic field-modulated reflectance (MR) measurements. In the MTR spectra, the exciton-related structure is found to split under an external magnetic field due to the field-induced spin-polarization effect of the Bloch electrons. From the line-shape analysis of the TR and MR spectra, the energy and damping constant of the exciton are quantitatively investigated. The critical change in the exciton relaxation time at T N is ascribed to the dynamical spin-fluctuation effect on the exciton state.