Electron spin polarization and conduction band structure of doped ferromagnetic semiconductors

The spin polarization of the conduction electrons of a doped semiconductor (e.g. EuO + x%Gd) is calculated using a moment method together with an alloy analogy. It is shown to be caused by a complicated temperature- and carrier concentration-dependence of the quasiparticle spectrum of the s-f model. Obtained polarization agrees very well with recent photoemission experimental data.     

Magnetic interactions in europium compounds

The magnetic interactions in EuO, EuS, EuSe and EuTe are usually characterized by exchange constants I₁ and I₂, denoting the interaction between nn and nnn Eu spins, respectively. In this paper, we propose a mechanism for the ferromagnetic interaction I₁. It is an indirect exchange mediated by the spin polarization of the chalcogen p-band electrons via their exchange interactions with the magnetic ?-electrons.     

Influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k-Dresselhaus spin-orbit coupling

The influence of in-plane magnetic field on spin polarization in the presence of the oft-neglected k³-Dresselhaus spin-orbit coupling was investigated. The k³-Dresselhaus term can produce a limited spin polarization. The in-plane magnetic field plays a great role in the tunneling process. It can generate the perfect spin polarization of the electrons and the ideal transmission coefficient for spin up and down simultaneously. In energy scale, complete separation between spin up and down resonance was obtained by a relatively higher in-plane magnetic field while a comparatively lower in-plane magnetic field vanishes the spin separation. On the other hand, the spin relaxation can be suppressed by compensating the oft-neglected k³-Dresselhaus spin orbit coupling using a relatively lower in-plane magnetic field.     

Dresselhaus spin-orbit coupling induced spin-polarization and resonance-split in n-well semiconductor superlattices

Using the transfer matrix method, we investigate the electron transmission over multiple-well semiconductor superlattices with Dresselhaus spin-orbit coupling in the potential-well regions. The superlattice structure enhances the effect of spin polarization in the transmission spectrum. The minibands of multiple-well superlattices for electrons with different spin can be completely separated at the low incident energy, leading to the 100% spin polarization in a broad energy windows, which may be an effective scheme for realizing spin filtering. Moreover, for the transmission over n-quantum-well, it is observed that the resonance peaks in the minibands split into n-folds or (n−1)-folds depending on the well-width and barrier-thickness, which is different from the case of tunneling through n-barrier structure.     

Photoemissionsmessungen an Europiumchalkogeniden

From new photoemission measurements from EuO we show that the existing discrepancy between photoelectric- and optical results can be eliminated. The measurements on cleaved single crystal surfaces show sharper structure than measurements on evaporated films. The energy difference between the center of the 4f-levels and the maximum of the valence band density of states can be read directly from the energy distribution curves. This energy difference decreases with increasing lattice constant from 2.8 eV for EuO to 1.6 eV for EuSe. However the two substances exhibit similar structure in the valence band.     

Paramagnetic resonance linewidth of EuO near the curie temperature

Measurements of the EPR linewidth ΔH of EuO at 9 GHz are reported in the temperature range of 66–300° K, with particular attention to the region near T_c (69.6° K). Comparison with the earlier data of Eastman at 25 GHz in the critical region shows considerable suppression of ΔH at 25 GHz. The temperature-dependent behavior of ΔH in EuO at 9 GHz is similar to the observations in CrBr₃ and it is in qualitative agreement with the zero-field predictions of Huber and Maleev. The quantitative discrepancies are believed to be due to the effect of the resonance magnetic field on EPR spin dynamics near T_c and an inadequate decoupling of the four-spin correlation functions used in the theories.     

Polarization transfer in deuteron stripping reactions

Polarization phenomena involving the spins of a and b in the A(a, b)B reaction are discussed using a complete set of irreducible tensors carrying definite spin transfer. The linear model independent equations relating the cross section and the polarization observables with these tensors are shown to be particularly appropriate for the study of spin dependent interactions, preferentially associated with particular values of spin transfer. The DWBA theory of polarization transfer in deuteron stripping reactions is thoroughly discussed and among the 17 polarization observables we distinguish those likely to be more sensitive to spin dependent distortion, to have stronger deuteron D-state effects, to exhibit the sign-rule j-dependence and other forms of j-dependence. For certain deuteron polarizations, when the spin transfer is pure $\text{s =}\text{1}\text{2}$, it is shown that deuteron stripping reactions are transparent to vector polarization transfer and the outgoing nucléon polarization independent of scattering angle and deuteron energy. DWBA calculations including contributions from spin transfer $\text{3}\text{2}$ through the deuteron D-state and spin-orbit distortion show that polarization transfer in such deuteron polarizations can be explored as a method of producing fast polarized neutrons with known polarization.     

Particular nanowire superlattice as a spin filter

A nanowire superlattice of InAs and GaAs layers with In_0.47Ga_0.53As as the impure layers is proposed. The oft-neglected k³ Dresselhaus spin-orbit coupling causes the spin polarization of the electron but often can produce a limited spin polarization. In this nanowire superlattice, Dresselhaus term produce complete spin filtering by optimizing the distance between the In_0.47Ga_0.53As layers and the Indium (In) in the impure layers. The proposed structure is an optimized nanowire superlattice that can efficiently filter any component of electron spins according to its energy. In fact, this nanowire superlattice is an energy dependent spin filter structure.     

The electron energy loss spectrum of EuO(100)

Energy losses of 200 eV to 2 keV electrons reflected from a disordered EuO(100) crystal show a bulk plasmon loss consistent with just less than six “quasi free” electrons per EuO unit, and 5p → nd resonance losses above the 5p threshold. The ratio of intensity of the 4d¹⁰ 4fⁿ⁰ → 4d⁹ 4fⁿ⁺¹ “giant resonance” loss at 142 eV to the corresponding direct recombination feature varies with energy, while the direct recombination and related Auger channels show similar energy dependence.     

Spontaneous spin polarization in rubrene studied by density functional theory calculations

We theoretically studied the spontaneous spin polarization properties of organic molecule rubrene by using density functional theory calculations. Our investigations show that normally nonmagnetic molecule rubrene could be spin polarized by spinless-hole injection. Magnetic moment of the molecule increases linearly with the extra hole charge amount only when the injected hole charges reach a certain value. The spin density resides predominantly on the carbon atoms in the tetracene backbone of rubrene molecule and also the bond lengths change differently due to the injected charge. Spontaneous spin polarization can be explained as the preferably filling of the spin-splitted carbon p_z orbitals near the Fermi energy for the injected charge.     

Spinpolarisierte Photoelektronen aus EuO

The measurement of the spinpolarization of photoelectrons from ferromagnetic semiconductors with a novel apparatus shows that the surface is magnetically not saturated even at temperatures far below the Curietemperature. In EuO the ionization threshold of the 4f ⁷-states depends on doping; furthermore two types of doping coexist, differing in their spinpolarization and the photoelectric yield spectrum. The surface magnetism is influenced by evaporation of a very thin metal film. EuO is an intense source of highly polarized electrons suitable for use with a pulsed high energy accelerator.     

Exchange narrowing of Mössabauer line

A theoretical investigation is made of the broadening of the Mössbauer line in crystals containing exchange coupled magnetic ions. It is shown that the exchange narrowing of the Mössbauer line takes place near the phase magnetic transition in EuO. The determination of spin correlation function and the critical temperature from the width of the Mössbauer line in EuO is discussed.     

Mössbauer spectroscopy study of the EuO:Fe spintronic material

The states of iron and europium in the EuO:Fe composite spintronic material have been studied by means of room-temperature Mössbauer spectroscopy. In both cases, two sets of lines indicating the ferromagnetism and superparamagnetism of metallic iron nanoparticles in the composite and the appearance of a fraction of europium ions in the Eu³⁺ oxidation state in the matrix in addition to the main Eu²⁺ state have been observed in the respective spectra. In the europium case, the observation implies the possibility of a partial spin polarization of paramagnetic europium ions in Eu-Fe-O clusters and an increased specific magnetization of the composite.     

Effets magnétostrictifs spontanés de quelques composés des terres rares

X-ray measurements as a function of the temperature of the crystal lattice parameters of EuO and EuS reveal a volume reduction of 0.3% and 0.12%, respectively, when these crystals are cooled through the Curie temperature. This contraction can be assigned to the exchange-striction and the relative volume variation is proportional to the spin correlation function. Within the molecular field approximation, this magnetoelastic interaction is described by the relationT _C=T ₀[1+β(V?V ₀)/V ₀]. For EuO and EuS the values of the magnetoelastic interaction parameterβ are ?3.5 and ?6.4, respectively, in agreement with the values derived from measurements of the pressure dependence of the Curie temperature. No anomaly of the expansion coefficient has been detected in the EuSe and EuTe compounds.     

Polarisationsmessungen an Elektronenstrahlen nach der Transmission durch magnetisierte Eisenschichten

Magnetized iron foils of a thickness of 500 Å were transmitted by 60 keV electrons. The spin polarization of the electron beam was investigated by Mott scattering after transmission. No polarization effect was found. Since the lowest detectable polarization degree wasP=0·003 the atomic polarization cross section must be assumed to be smaller than 0·8·10^?20cm². This cross section is the sum of the spin exchange cross section and the spin dependent part of the total scattering cross section. If the electron binding energy is neglected, the spin depentend part of the total scattering cross section can be calculated from the theory of Møller scattering. In the case of our conditions-60 keV and an aperture of 10^?3 radian-the calculated cross section is smaller than the experimental upper limit. In further experiments the electron beam was split into the energy spectrum by an electrostatic analyzer placed between the iron foil and the Mott scattering foil. In these measurements only small parts of the energy spectrum were investigated, however, even here no detectable polarization occured.     

Vertex correction of the anisotropic magnetic impurities to the spin polarization of 2D electron gas

In terms of Kubo's formula and Green's function method, for the two-dimensional electron gas (2DEG) with Rashba spin-orbit coupling (SOC), we study the spin polarization due to the effect from magnetic impurities with anisotropic spin dependent delta type coupling to electrons when an external dc electric field in plane is applied. The vertex correction of impurities in ladder approximation is carried out in the limit of EF?1/τ, Δ. We find that the strength of spin polarization can be significantly modified by vertex correction and the spin polarization is relevant to the anisotropy coefficient γ, but the direction of net spin polarization cannot be changed.     

Effects of adsorbates on electron spin polarization in low energy electron diffraction from tungsten (001): II. Ordered CO overlayer

Electron spin polarization and intensity profiles have been measured in low electron diffraction (LEED) for the (00) beam at θ = 13° and ø = 0° from a W(001) surface exposed to CO and annealed to obtain an ordered c(2 × 2) CO overlayer. The annealed surface with additional CO adsorbed was also studied. The polarization was found to be sensitive to the surface condition and the very distinct P?V profile corresponding to the c(2 × 2) overlayer is believed to be a very sensitive indicator of CO in the β₃ phase. The properties of the annealed surface exposed to further CO suggest the use of this surface as a low energy electron spin polarization analyzer.     

Transverse energy flows in vectorial fields of paraxial beams with singularities

A general study of transverse energy flows (TEF) as physically meaningful and informative characteristics of paraxial light beams’ spatial structure is presented. The total TEF can be decomposed into the spin and orbital contributions giving rise to the spin and orbital angular momentums, correspondingly. Definitions and properties of these constituents are discussed in relation with the optical field representation through linear and circular orthogonal polarization bases. With the help of model examples, the results are applied to investigation of TEF singularities in connection with the usual polarization morphology characteristics of paraxial optical fields. An analysis of TEFs near singular points has been carried out; in particular, the behavior of TEF and its partial contributions near polarization singularities (C-points) has demonstrated the special role of a boundary flow in the origin of the spin angular momentum. The analytical and experimental applicability of the introduced concepts are discussed.     

Magnetic proximity effect in EuO-Cs interface

In the surface of the ferromagnetic insulator EuO is covered with a small amount of cesium, the photoelectric magnetization curves P(H) show magnetic saturation in contrast to those of the pure surface. The proximity of the Cs film causes the paramagnetism of the clean EuO surface to disappear.     

A new scheme for measuring itinerant spin polarizations

An optical method for the measurement of itinerant electron spin polarization is proposed. It is based on the idea that when an itinerant electron is injected into a p-type semiconductor with a valence band spin orbit splitting ? kT, the polarization of the resulting recombination radiation is characteristic of the spin polarization. The feasibility and advantages of this technique are discussed.