Spin Polarization Coupling Research Articles

Spin Selectivity-Based Enantiomeric Discrimination by Chiral Nanostructured Au Films

Photomagnetic-chiral anisotropy (PM-ChAC–E) and surface-enhanced Raman scattering-chiral anisotropy (SERS-ChAC–E) are novel enantiomeric discrimination techniques generated by chiroptical response due to the interactions between chiral nanostructured metal films (C) and enantiomers (E). The normal Gauss meter and the Raman scattering spectrometer can be used in these versatile techniques. Here, we report the discrimination mechanism of chiral nanostructured Au films (CNAFs) based on the spin polarization coupling (SPC) between the enantiomers and CNAFs. The CNAFs represent Boerdijk-Coxeter-Bernal helical Au nanofibers with different lengths and widths perpendicularly grown on substrates. The optical activities, magnetic circular dichroism, and magnetic tip conducting atomic force microscopy indicated that the spin selectivity of CNAFs depends on the length of CNAFs. The spin selectivity-dependent selective enhancement of photomagnetic field and Raman scattering suggested that the chirality-induced SPC between the enantiomers and CNAFs gives rise to the phenomena of PM-ChAC–E and consequently SERS-ChAC–E. These findings provide possible mechanism based on spin selectivity in chiral structures and make it a universal theory for enantiomeric discrimination.

Chiral Nanostructured Bimetallic Au–Ag Films for Enantiomeric Discrimination

AbstractHerein, the chiral nanostructured bimetallic Au–Ag films (CNAAFs) with both photomagnetic‐chiral anisotropy to enantiomers (PM‐ChAC‐E) and surface‐enhanced Raman scattering‐chiral anisotropy to enantiomers (SERS‐ChAC‐E) for efficient enantiomeric discrimination are reported. The CNAAFs are synthesized by electrodeposition on n‐type Si (n‐Si) substrates with the assistance of N‐acetyl‐L/D‐cysteine (S/R‐NAC) as symmetry‐breaking agents, which are consisted of polycrystalline Boerdijk–Coxeter–Bernal (BCB) tetrahelical structured Au–Ag alloy nanowires grown on n‐Si substrates perpendicularly. The anisotropic factors of PM‐ChAC‐E (gPM‐ChA) and SERS‐ChAC‐E (gSERS‐ChA) of CNAAFs in the range of 0.68–1.12 and 1.45–1.89 are obtained, respectively. Considering the increase of magnetic fields and SERS intensities with enantiomeric excess (ee) values of adsorbed enantiomers on the CNAAFs are higher than that of pure chiral nanostructure Au films, the enhancement of PM‐ChAC‐E and SERS‐ChAC‐E effects of CNAAFs can be attributed to the enhanced selective spin polarization coupling between CNAAFs and enantiomers. It is predicted to be due to the strong surface plasmon resonance generated by the active electrons of Ag under laser irradiation.

Ultrafast chiral peptides purification via surface plasmon enhanced spin selectivity

By D-arginine and L-arginine chiral peptides induced spin selectivity and Au NPs enhanced spin polarization, chiral peptides purification has been effectively simplified and the purification performance has raised from a mixture system. The angular momentums of light are operated by the polarizer and wave plates. Au NPs decorated ZnO nanorods electrodes are utilized to modulate the polarization of spintronic. Seed growth methods are for synthesizing spherical Au NPs. UV light reduction methods are for urchin-liked Au NPs. Au NPs are decorated on ZnO nanorods electrodes for rising photon to electron conversion efficiency and enhancing spin polarization rates by surface plasmon effect. From our results, photon to the electron conversion efficiency of ZnO nanorods electrodes has effectively enhanced by urchin-liked Au NPs decorating. Ultrahigh localized plasmon conversion efficiency as high as 60% was also obtained. Besides, density functional theory (DFT) calculations simulated the force on spintronic. Since the D-arginine and L-arginine are on Au substrate, DFT results demonstrate different angular momentum and spin polarization coupling. Along with urchin-liked Au NPs rising chiral induced spin polarization by surface plasmon resonance, the sensitivity of chiral arginine has been raised around 5000% from bare ZnO nanorods electrodes. The purification and separation time of a specific chiral arginine only needs 5 min.

Selection rules and dynamic magnetoelectric effect of the spin waves in multiferroic BiFeO3

We report the magnetic field dependence of the THz absorption and non-reciprocal directional dichroism spectra of BiFeO$_3$ measured on the three principal crystal cuts for fields applied along the three principal directions of each cut. From the systematic study of the light polarization dependence we deduced the optical selection rules of the spin-wave excitations. Our THz data, combined with small-angle neutron scattering results showed that i) an in-plane magnetic field rotates the $\mathbf{q}$ vectors of the cycloids perpendicular to the magnetic field, and ii) the selection rules are mostly determined by the orientation of the $\mathbf{q}$ vector with respect to the electromagnetic fields. We observed a magnetic field history dependent change in the strength and the frequency of the spin-wave modes, which we attributed to the change of the orientation and the length of the cycloidal $\mathbf{q}$ vector, respectively. Finally, we compared our experimental data with the results of linear spin-wave theory that reproduces the magnetic field dependence of the spin-wave frequencies and most of the selection rules, from which we identified the spin-polarization coupling terms relevant for the optical magnetoelectric effect.

Photomagnetic-chiral anisotropy of chiral nanostructured gold films

•PM-ChA was created by CNAFs under laser irradiation •The mechanism is attributed the directional alignment of specific spins •PM-ChA can be applied for universal enantiomeric discrimination •Enantiomeric discrimination is associated with selective spin polarization coupling The photomagnetic effect is known to be related only to transition metal atoms with empty orbitals and spin-state changes. In general, noble metals with high electron density cannot be used as photomagnetic materials because of the absence of spin polarization. However, here, we found that photomagnetic-chiral anisotropy (PM-ChA) was generated in chiral nanostructured Au films (CNAFs), and opposite photomagnetic fields (PMFs) were created in the antipodal CNAFs under unpolarized irradiation. The PM-ChA was speculated to arise from directional alignment of opposite spins polarized by the asymmetric spin-orbit coupling due to the opposite effective magnetic fields induced by electron motion in the antipodal helical structure. PM-ChA can be applied in enantiomeric quantification, probably because of the spin polarization coupling between CNAFs and enantiomers. PM-ChA of noble metals generated by directional spin control in helical nanostructures could provide new strategies in quantum technology and theories in magnetophysics. The photomagnetic effect is known to be related only to transition metal atoms with empty orbitals and spin-state changes. In general, noble metals with high electron density cannot be used as photomagnetic materials because of the absence of spin polarization. However, here, we found that photomagnetic-chiral anisotropy (PM-ChA) was generated in chiral nanostructured Au films (CNAFs), and opposite photomagnetic fields (PMFs) were created in the antipodal CNAFs under unpolarized irradiation. The PM-ChA was speculated to arise from directional alignment of opposite spins polarized by the asymmetric spin-orbit coupling due to the opposite effective magnetic fields induced by electron motion in the antipodal helical structure. PM-ChA can be applied in enantiomeric quantification, probably because of the spin polarization coupling between CNAFs and enantiomers. PM-ChA of noble metals generated by directional spin control in helical nanostructures could provide new strategies in quantum technology and theories in magnetophysics. Photomagnetic-chiral anisotropy of chiral nanostructured gold filmsLiu et al.ChemFebruary 10, 2022In Brief(Chem 8, 186–196, January 13, 2022) Full-Text PDF

Spin-coupling-induced Improper Polarizations and Latent Magnetization in Multiferroic BiFeO3

Multiferroic BiFeO3 (BFO) that exhibits a gigantic off-centering polarization (OCP) is the most extensively studied material among all multiferroics. In addition to this gigantic OCP, the BFO having R3c structural symmetry is expected to exhibit a couple of parasitic improper polarizations owing to coexisting spin-polarization coupling mechanisms. However, these improper polarizations are not yet theoretically quantified. Herein, we show that there exist two distinct spin-coupling-induced improper polarizations in the R3c BFO on the basis of the Landau-Lifshitz-Ginzburg theory: ΔPLF arising from the Lifshitz gradient coupling in a cycloidal spin-density wave, and ΔPms originating from the biquadratic magnetostrictive interaction. With the help of ab initio calculations, we have numerically evaluated magnitudes of these improper polarizations, in addition to the estimate of all three relevant coupling constants. We further predict that the magnetic susceptibility increases substantially upon the transition from the bulk R3c BFO to the homogeneous canted spin state in a constrained epitaxial film, which satisfactorily accounts for the experimental observation. The present study will help us understand the magnetoelectric coupling and shed light on design of BFO-based materials with improved multiferroic properties.

Magnetic structures and excitations in a multiferroic Y-type hexaferrite BaSrCo2Fe11AlO22

Multiferroic hexaferrites have been extensively investigated because they exhibit ferroelectricity associated with noncollinear magnetic orders in relatively low magnetic field and high temperature regions. To understand the static and dynamical spin-polarization coupling in such systems, it is necessary to determine the magnetic structures and spin-wave modes. This is impossible for polycrystalline samples. Here, the authors have successfully grown single crystals of BaSrCo${}_{2}$Fe${}_{12}$AlO${}_{22}$ by means of a laser floating zone technique. They perform magnetization, electric polarization, and neutron diffraction measurements to establish the magnetic phase diagram. Several interesting features are seen and the new understanding should provide a useful guide for developing multiferroic-based functional devices working at room temperature.

Electric polarization memory effect in a magnetoelectric multiferroic CuFe1−xGaxO2

Magnetic oxide CuFeO2 is a magnetoelectric multiferroic with new type of spin–polarization coupling different from that in the spin-current mechanism, where magnetic field-induced or nonmagnetic impurity-induced proper helical magnetic ordering generates a spontaneous electric polarization parallel to the helical axis. Using a CuFe1−xGaxO2 sample with x=0.035, in which the single ferroelectric phase is realized below TN∼8 K in zero magnetic field unlike CuFe1−xAlxO2, we have performed pyroelectric current, thermally stimulated current (TSC) and polarized neutron diffraction measurements to study a memory effect that the electric polarization is partially preserved even for 2nd-cooling from above TN without poling electric field. It was found that the charge trapped during 1st-cooling with poling electric field, which is released as TSC on heating, plays an important role in the memory effect.

Microscopic theory of spin-polarization coupling in multiferroic transition metal oxides

A systematic microscopic theory of magnetically induced ferroelectricity and lattice modulation is presented for all electron configurations of Mott-insulating transition-metal oxides. Various mechanisms of polarization are identified in terms of a strong-coupling perturbation theory. Especially, the spin-orbit interaction acting on the ligand p orbitals is shown to give the ferroelectric polarization of the spin-current form, which plays a crucial role particularly in eg systems. Semiquantitative agreements with the multiferroic TbMnO3 are obtained. Predictions for X-ray and neutron scattering experiments are proposed to clarify the microscopic mechanism of the spin-polarization coupling in different materials.