Spin Splitting Energy Research Articles

Ferromagnetic properties of GdN thin films studied by temperature dependent circular polarized spectroscopy

Magneto-optical properties of GdN thin films have been investigated by employing temperature dependent optical circular dichroism spectroscopy and angular dependent resonance field measurement. Spin-splitting in the band structure of GdN thin films has been evaluated by optical circular dichroism absorbance spectra, and the manifested spin-splitting energy ascribed to the difference between majority and minority spin band states in GdN. The plot of left circular polarization and right circular polarization bandgap reflects the half-hysteresis loop (positive side) trend, which is evidenced by magnetization measurements. The angular dependent resonance field measurements showed strong magnetic anisotropy along in-plane of GdN, which is attributed to the disturbance in the spin alignment in GdN. We demonstrate that the ferromagnetic properties depend on the film thickness. These results provide a pathway to control the spin ordering using circularly polarized light and the magnetic anisotropy.

Spin-polarized Wide Electron Slabs in Functionally Graded Polar Oxide Heterostructures

We report on the high mobility wide electron slabs with enhanced correlation effects by tailoring the polarization effects in a functionally graded ZnMgO/ZnO heterostructures. The characteristics of three-dimensional (3D) spreading electrons are evidenced by the capacitance-voltage profiling and the quantization of 3D Fermi surface in magneto-transport measurements. Despite the weak spin-orbit interaction, such electron slabs are spin-polarized with a large zero-field spin splitting energy, which is induced by the carrier-mediated ferromagnetism. Our results suggest that the vast majority of electrons are localized at the surface magnetic moment which does not allow spin manipulations, and only in the region visited by the itinerant carriers that the ferromagnetic exchange interactions via coupling to the surface local moments contribute to the spin transport. The host ferromagnetism is likely due to the formation of Zn cation vacancies on the surface regime induced by the stabilization mechanism and strain-relaxation in ZnMgO polar ionic surface.

Giant spin Seebeck effect in a non-magnetic material

The spin Seebeck effect is observed when a thermal gradient applied to a spin-polarized material leads to a spatially varying transverse spin current in an adjacent non-spin-polarized material, where it gets converted into a measurable voltage. It has been previously observed with a magnitude of microvolts per kelvin in magnetically ordered materials, ferromagnetic metals, semiconductors and insulators. Here we describe a signal in a non-magnetic semiconductor (InSb) that has the hallmarks of being produced by the spin Seebeck effect, but is three orders of magnitude larger (millivolts per kelvin). We refer to the phenomenon that produces it as the giant spin Seebeck effect. Quantizing magnetic fields spin-polarize conduction electrons in semiconductors by means of Zeeman splitting, which spin-orbit coupling amplifies by a factor of ∼25 in InSb. We propose that the giant spin Seebeck effect is mediated by phonon-electron drag, which changes the electrons' momentum and directly modifies the spin-splitting energy through spin-orbit interactions. Owing to the simultaneously strong phonon-electron drag and spin-orbit coupling in InSb, the magnitude of the giant spin Seebeck voltage is comparable to the largest known classical thermopower values.

Electronic and Spin Structure of Metal Phthalocyanines

We present the first-principles calculations of MPc molecules having the magnetic moment. Some of these molecules contain the magnetic 3d-metal atom (MnPc, FePc, and CoPc), whereas others have the non-magnetic metal atom with an odd number of electrons (CuPc, AgPc, and GaPc). Calculations show that the density functional theory greatly decreases the HOMO-LUMO gap and the spin-splitting energy, comparing with hybrid functional results closely related to the many-electron theory. It is found that the HOMO-LUMO gap of MPc molecules shows moderate changes, while their spin-splitting energy is very sensitive to the localization of spin density and varies very significantly.

Spin splitting in bulk wurtzite AlN under biaxial strain

The spin-splitting energies in biaxially strained bulk wurtzite material AlN are calculated using the linear combination of atomic orbital (LCAO) method, and the equi-spin-splitting distributions in k-space near the minimum-spin-splitting (MSS) surfaces are illustrated. These data are compared with those derived analytically by two-band k · p (2KP) model. It is found that the results from these two methods are in good agreement for small k. However, the ellipsoidal MSS surface under biaxial compressive strain does not exist in the 2KP model, because the data points are far from the Γ point. Instead, three basic shapes of the MSS surface occur in the wurtzite Brillouin zone: a hyperboloid of two sheets, a hexagonal cone, and a hyperboloid of one sheet, evaluated from the LCAO method across the range of biaxial strains from compressive to tensile.

Spin-polarized two-dimensional electron gas in undoped MgxZn1−xO/ZnO heterostructures

Quantum transport properties of two-dimensional electron gas (2DEG) in undoped MgxZn1−xO/ZnO heterostructures grown by metal organic vapor phase epitaxy have been investigated. A large zero-field spin-splitting energy more than 15 meV in the 2DEG is determined at 1.6 K. Meanwhile, ferromagnetism is observed in the heterostructures. The findings reveal that the 2DEG is spin polarized at zero magnetic fields. It is believed that the exchange interaction between the itinerant electrons in the two-dimensional channel and the magnetic polarons in the MgxZn1−xO barrier around the interface results in the spin polarization of the 2DEG.

Anomalous spin–orbit coupling in high-density two-dimensional electron gas confined in InGaAs/InAlAs quantum well

We study the magnetotransport property of a high-density two-dimensional electron gas confined in InGaAs/InAlAs quantum well. Both beating pattern in the Shubnikov–de Hass oscillation of resistivity and weak antilocalization effect are observed. From these two effects, Rashba spin-splitting energy is extracted. The extracted Rashba spin-splitting energy shows a nonmonotonic dependence on Fermi wave vector, contrary to the prevailing linear Rashba model. This anomalous behavior can be attributed to the nonlinear Rashba spin-splitting mechanism [Yang et al., Phys. Rev. B 74 (2006) 193314].

Electronic and magnetic properties of semiconducting nanoclusters and large organic molecules: Features interesting for spintronics

Spin properties of single-doped and single-electron charged nano-systems having an odd number of electrons are studied. Starting from an expression for quasiparticle energies in the GW approximation, a simple analytical expression for the spin-splitting of an electron spectrum in such system is derived. First-principles calculations by the DFT–GGA, Hartree-Fock, GW- and hybrid functional methods, which were performed for the silicon clusters and metal phthalocyanine molecules of 1nm diameter, support this analytical consideration. They show that the spin-splitting energy calculated by the DFT–GGA method is about one order lower, than the results obtained with the methods based on the many-electron theory. A large value of spin-splitting in investigated nano-systems, which is typically of several eV, has an origin in strong localization of electrons and weak screening of exchange interaction. A possible use of this effect in spintronic applications is discussed.

Application of block diagonal technique to a Hamiltonian matrix in performing spin-splitting calculations for GaN wurtzite materials

The bulk inversion asymmetry (Dresselhaus) terms (i.e., B2, B1, and B′1 terms) of wurtzite materials are determined. The 2 × 2 conduction band, 2 × 2 heavy-hole band, 2 × 2 light-hole band, and 2 × 2 crystal-field split-off hole band matrices of wurtzite semiconductors are developed and decoupled by using a block diagonal technique. Importantly, those 2 × 2 block diagonal matrices incorporate not only the interband coupling effect but also the bulk inversion asymmetry effect. Analytical expressions for the conduction and the valence band spin-splitting parameters and energies of GaN wurtzite materials are formulated by solving the block diagonal matrices. The presence of these terms is shown to include the spin-splitting phenomenon.

Magnetotransport property of HgCdTe inversion layer

HgCdTe-based metal-insulator-semiconductor field effect transistor is fabricated by low-cost liquid phase epitaxy technique. Clear SdH oscillation in xx and quantum Hall plateaus of xy are observed, indicating that it is a good transistor. By measuring the magnetoresistance near zero field, we observe the weak antilocalization effect in our sample, suggesting a relatively strong spin-orbit coupling. The experimental data can be well fitted by the ILP theory. The fitting-obtained spin-splitting energy increases with increasing electron concentration, and the maximum reaches up to 9.06 meV. From the obtained spin-splitting energy, we calculate the spin-orbit coupling parameter and find that it increases with increasing electron concentration, which is contrary to the observations in a wide quantum well.

Zero-field spin splitting in a two-dimensional electron gas with the spin–orbit interaction revisited

We consider a two-dimensional electron gas (2DEG) with the Rashba spin–orbit interaction (SOI) in the presence of a perpendicular magnetic field. We derive analytical expressions of the density of states (DOS) of a 2DEG with the Rashba SOI in the presence of a magnetic field by using the Green’s function technique. The DOS allows us to obtain the analytical expressions of the magnetoconductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and give rise to the beating patterns in the amplitude of the Shubnikov–de Haas (SdH) oscillations. We find a simple equation which determines the zero-field spin splitting energy if the magnetic field corresponding to any beat node is known from the experiment. Our analytical results reproduce well the experimentally observed non-periodic beating patterns, number of oscillations between two successive nodes and the measured zero-field spin splitting energy.

Anisotropic in-plane spin splitting in an asymmetric (001) GaAs/AlGaAs quantum well

The in-plane spin splitting of conduction-band electron has been investigated in an asymmetric (001) GaAs/AlxGa1-xAs quantum well by time-resolved Kerr rotation technique under a transverse magnetic field. The distinctive anisotropy of the spin splitting was observed while the temperature is below approximately 200 K. This anisotropy emerges from the combined effect of Dresselhaus spin-orbit coupling plus asymmetric potential gradients. We also exploit the temperature dependence of spin-splitting energy. Both the anisotropy of spin splitting and the in-plane effective g-factor decrease with increasing temperature.PACS: 78.47.jm, 71.70.Ej, 75.75.+a, 72.25.Fe,

Enhancement of Rashba coupling in vertical In0.05Ga0.95As/GaAs quantum dots

We study the spin-splitting energies in low-potential-barrier quantum dots, finding splitting energies that are orbital state dependent. The theoretical analysis is done with a generalization of the Fock-Darwin states in the presence of spin-orbit interactions. We discuss experimental evidence indicating that the Rashba interaction strength in vertical In${}_{x}$Ga${}_{1\ensuremath{-}x}$As/GaAs quantum dots is in the range 80 meV \AA{} $\ensuremath{\leqslant}{\ensuremath{\lambda}}_{R}\ensuremath{\leqslant}$ 120 meV \AA{}. This enhanced spin-orbit interaction can be understood from the high penetration of the electron wave function into the quantum well with low-potential barrier.

Enhanced magneto-optical oscillations from two-dimensional hole-gases in the presence of Mn ions

We investigated the effects of nearby Mn ions on the optical properties of two-dimensional hole-gases confined in InGaAs/GaAs quantum wells. We observed energy oscillations on both the averaged emission and the spin-splitting energies, whereas the first one presents maxima at all integer filling factors, and the second one, solely at odd-filling factors. The strength of the oscillations clearly increases with the Mn concentration. Furthermore, considering the relatively low-mobility of our structures, the oscillations are surprisingly strong and robust, persisting up to relatively high temperatures and excitation intensities.

SPIN SPLITTING PROPERTIES IN GaSb Γ AND L/X-VALLEYS

The band structures of GaSb in the Γ-valley as well as L/X-valley are excellently reproduced with sp3d5s* nearest-neighbor tight-binding model. The electron spin splitting energy and the spin–orbit interaction coefficients in the Γ, L and X multi-valleys are calculated. In the Γ-valley, the results obtained are in good agreement with experimental report. We then further extend our calculations to the GaSb L- and X-valleys. We also present the results of the top three valence bands in the Γ-valley. Due to the small band offset and large difference of the density of states between Γ- and L-valleys, it is shown that even at zero electric field, there is still an explicit electron population in the L-valley. These results are important to the realization of spintronic device and the investigation of spin dynamics far away from equilibrium.

Spin-dependent tunneling through double-barrier quantum wells with random corrugation interfacial roughness

We use the transfer matrix method to study spin-dependent tunneling transmission through the NM/EuS/NM/EuS/NM junction which affected by random corrugation interfacial roughness. The temperature dependence of spin-splitting energy is calculated for EuS magnetic barrier within the mean-field approximation. We perform several calculations to study the effect of random nonplanar interfacial on transmission probability for several widths of quantum well at different temperatures. Our results show that the scattering process due to random roughness open the new conduction channels (the dip and peak resonance energies). The results may be useful in designing spintronic devices.

비대칭 자기터널접합에서의 수직 스핀 전달 토크: 물질 변수에 대한 의존성

스핀전달토크는 나노구조에서 자성상태를 제어하는데 유용한 수단이다. 자기터널접합에서 스핀전달토크는 자성물질층의 자화가 이루는 평면에 평행한 성분과 수직인 성분으로 나눌 수 있다. 이중 평행한 성분의 스핀전달토크의 성질은 상당히 잘 알려져 있으나, 수직인 성분의 스핀전달토크의 성질에 대해서는 여전히 이견이 많다. 비대칭 자기터널접합에서의 최근 실험에서, 수직전달토크의 전압 의존성이 전압의 이차항 성분뿐만 아니라 일차항 성분도 가짐을 보고하였다. 하지만 물질 변수에 대한 의존성은 여전히 잘 알려지지 않았다. 이 논문에서는 비대칭 자기터널접합에서의 스핀전달토크의 전압의존성을, 강자성층의 스핀 갈라짐 에너지와 일함수의 차이, 그리고 페르미 에너지를 변화시켜 가면서 체계적인 조사를 하였다. Spin-transfer torque is a useful tool to control the magnetic state in nanostructures. In magnetic tunnel junctions, the spin-transfer torque has two components, the in-plane spin torque and the perpendicular spin torque. While properties of the in-plane spin-transfer torque are relatively well understood, properties of the perpendicular spin-transfer torque still remain controversial. A recent experiment demonstrated that in asymmetric magnetic tunnel junctions, the bias voltage dependence of the perpendicular spin-transfer torque contains both linear and quadratic terms in the bias. However it still remains unexplored how the bias voltage dependence changes as a function of material parameters. In this paper, we systematically investigate the perpendicular spin-transfer torque in asymmetric magnetic tunnel junction by varying spin splitting energy, work function difference, and Fermi energy of the ferromagnetic metal leads.

Subband Structure Engineering in Silicon-On-Insulator FinFETs Using Confinement

Splitting between equivalent valleys larger than the spin splitting energy is observed in confined electron systems, e.g. Si films grown either on SiGe substrate or Si dioxide and Si/SiGe quantum dots. Understanding the contribution of different factors in the valley degeneracy lifting is of key importance for the development of spin-based devices in Si. We demonstrate that the splitting between equivalent valleys strongly depends on the confinement direction and that it is orientation dependent. To explain the effect we use a simple but accurate two-band k·p model for the conduction band in silicon. Our data is in good agreement with recent results obtained by first-principle calculations.

Magnetotransport investigation of conducting channels and spin splitting in high-density AlGaN/AlN/GaN two-dimensional electron gas

Magnetotransport properties of a high-density AlGaN/AlN/GaN two-dimensional electron gas are reported. A quantitative model of amplitude and phase modulation of Shubnikov-de Haas oscillations was developed. This analysis allows the extraction of the spin splitting energy even if nodes and double peaks in the Fourier spectrum are not clearly observed. In this way the presence of two conducting channels was clearly identified. A spin splitting of 1.2 meV was found.

Determination of spin-polarized quantum well states and spin-split energy dispersions of Co ultrathin films grown on Mo(110)

Co thin films were epitaxially grown on Mo(110) and investigated by spin-polarized low-energy electron microscopy. We find that the spin asymmetry of the electron reflectivity from the Co film alternates its sign as a function of both the electron energy and the Co film thickness as a result of spin-polarized quantum well states in the Co film. By measuring spin-dependent quantum well states, we are able to resolve the spin-split energy dispersions of the Co film precisely. The determined spin-resolved energy bands are further tested by fitting the quantum well states using the phase accumulation model, and the result shows an excellent agreement between the fitting and the experimental data.