Electronic structures of magnetic ultrathin films Co/Au(1 1 1) studied by spin-resolved photoelectron spectroscopy

We have measured the spin-resolved photoemission spectra of the Co thin films grown epitaxially on Au(1 1 1) substrate in order to investigate their valence band structures. It is proved that the electronic structures of Co thin films are pretty different from that of bulk hcp-Co. It is observed that as the films grow thicker, the electronic structures become closer to those of the bulk Co with the magnetic anisotropy turning into in-plane magnetization from out-of-plane magnetization.     

Thermodynamic Stability,Half-Metallic and Optical Properties of Sc2CoSi [001] Film:a DFT Study

The electronic and optical characteristics of the Sc2 CoSi Heusler with L21 structure and also the surface effect on electronic and optical properties, and the ?lms thermodynamic stability of the [001] direction in four cases including:Sc-Sc, Sc-Co, Sc-Si and Co-Si terminations are studied using the ?rst principles calculations(FPLAPW) within the framework of the density functional theory(DFT). The band structure calculations represent the ferromagnetic halfmetallic properties with 100% spin polarization and 0.54 e V indirect gap in spin down for Sc2 CoSi bulk with optimized lattice parameters of 6.25 A?. The total magnetic moment obtained for this compound is-1.0 μB, which is in accordance with Slater-Pauling rule. The half-metallic(HM) behavior by 100% spin polarization at Fermi level is occurred in the Sc-Si termination with a 0.32 eV gap in down spin. The optical responses have been calculated for the bulk and ScSi termination by a red shift in these parameters and the metallic treatments have been increased. According to the thermodynamic phase diagrams, it is shown the Sc-Si and Sc-Sc terminations are more stable than other terminations.     

MoSi2薄膜电子性质的第一性原理研究

采用第一性原理计算方法, 研究了四方MoSi₂薄膜的电子性质. 计算结果表明, 各种厚度的薄膜都是金属性的, 并且随着厚度的增加, 其态密度与能带结构都逐渐趋近于MoSi₂块体的特性. 通过对MoSi₂薄膜磁性的分析, 发现三个原子层厚的薄膜具有磁性, 其原胞净磁矩为0.33 μ_B; 而当薄膜的厚度大于三个原子层时, 薄膜不具有磁性. 此外, 进一步对单侧加氢饱和以及双侧加氢饱和结构下三原子层MoSi₂薄膜的电子性质进行了研究, 发现单侧加氢饱和的三原子层MoSi₂薄膜具有磁性, 其原胞净磁矩为0.26 μ_B, 而双侧加氢饱和三原子层MoSi₂薄膜是非磁性的. 双侧未饱和与单侧加氢饱和的三原子层MoSi₂薄膜的自旋极化率分别为30%和33%. 这些研究结果表明, 三原子层厚的MoSi₂ 超薄薄膜在悬空或者生长于基底之上时具有金属磁性, 预示着它在纳米电子学和自旋电子学器件等方面都有潜在的应用前景.     

Extrinsic electronic transport in La0.7(Sr,Ca)0.3MnO3 films deposited on step edges and bicrystal substrates

In this paper, the electronic transport of La(0.7)(Sr,Ca)(0.3)MnO(3) films grown by pulsed laser deposition on a LaAlO(3)(001) substrate with deep parallel structured steps and a 30° symmetric bicrystal SrTiO(3)(001) substrate have been discussed. The electronic transport properties have been related to the well-known extrinsic transport of bulk manganite compounds. The spin-glass-like behavior with a characteristic peak at 20 K and domain formation at the grain boundary is observed. Further, it has been quantified from the resonant tunneling model that mixed magnetic interactions play a significant role in the manganite films deposited on step edges.     

Evolution of ferromagnetism with sputtering gas in Mn:ZnO films

The effect of introducing nitrogen and oxygen in the sputtering working gas on the magnetic properties of Mn:ZnO thin films has been investigated. A set of films has been characterized by X-ray diffraction, X-ray absorption near edge structure (XANES) and optical absorption spectroscopy to correlate its magnetic properties with Mn electronic characteristics. Mn²⁺ substituting Zn²⁺ in the wurtzite structure has been obtained for the films presenting considerably high saturation magnetization values. The change in the magnetic behaviour seems to be associated with the electronic carrier density in the films.     

Electronic structure and phase separation in La<Subscript>x</Subscript>MnO<Subscript>3</Subscript> films (0.83≤<Emphasis Type="Italic">x</Emphasis>≤1.10): Optical and magnetooptical data

The variations in the electronic structure and the evolution of phase separation as a result of creation of vacancies or excess of lanthanum in lanthanum manganites are studied on the basis of analysis of optical and magnetic properties of La_xMnO₃ epitaxial films (0.83≤x≤1.10) in the fundamental absorption range. The Kerr effect, the temperature dependences of resistivity, optical density, and magnetoabsorption of light indicate the charge and magnetic phase separation in the films. The fine structure observed in the spectrum is attributed to spectral overlapping of electron transitions with charge transfer and geometrical resonances reflecting an inhomogeneous nanoscopic structure of the films, which strongly depends on stoichiometry and stresses emerging during film deposition. It is shown that, in contrast to bulk polycrystals, the gradient of stresses over the film thickness significantly affects the phase separation in the films.     

Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF_2 substrate by pulsed laser deposition(PLD) method. Measurements on FeSe/CaF_2 samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140-160 K, much higher than ~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF_2 films by PLD method with the FeSe/SrTiO_3 films by molecular beam epitaxy(MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF_2 is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF_2 films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO_3 films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.     

应变对钴铁氧体电子结构和磁性能影响的第一性原理研究

尖晶石型钴铁氧体(CoFe₂O₄)因具有良好的电磁性质, 广泛应用于计算机技术、航空航天及医学生物等领域. 特别是钴铁氧体薄膜在磁电复合材料中具有良好的应用前景. 本文基于密度泛函理论的第一性原理平面波赝势法, 结合广义梯度近似, 通过采用更接近于实验上外延生长的二维应变模型, 研究了钴铁氧体薄膜的结构稳定性、电子结构和磁性能. 结果表明: 在二维应变作用下, 反尖晶石结构的钴铁氧体比正尖晶石结构的稳定, 但是与平衡基态相比, 两者能量差减小, 这表明在应变作用下, 八面体晶格中的Co²⁺离子与四面体晶格中的Fe³⁺离子更容易进行位置交换, 形成混合型结构的钴铁氧体; 同时随着应变的增大, 钴铁氧体的能带带隙减小, 晶格中的原子磁矩发生变化, 但总磁矩变化不明显. 关键词： 尖晶石型钴铁氧体 第一性原理 电子结构 磁性能     

Ab initio hartree-fock investigation of beryllium slabs

An analysis of the electronic structure in beryllium slabs with an ab initio LCAO HF method is presented. Band structure, electronic density and density of states were determined for thin films of 1 to 4 atomic layers of Be. A comparison of the electronic structure in films of different thickness with the situation found for bulk Be and Be clusters shows strong modifications in the surface region and a progressive reconstruction of the bulk bonds.     

First-principles study of FeSe epitaxial films on SrTiO_3

The discovery of high temperature superconductivity in Fe Se films on SrTiO_3 substrate has inspired great experimental and theoretical interests. First-principles density functional theory calculations, which have played an important role in the study of bulk iron-based superconductors, also participate in the investigation of interfacial superconductivity. In this article, we review the calculation results on the electronic and magnetic structures of Fe Se epitaxial films, emphasizing on the interplay between different degrees of freedom, such as charge, spin, and lattice vibrations. Furthermore, the comparison between Fe Se monolayer and bilayer films on SrTiO_3 is discussed.     

Finite size effect and phase diagram of ultra-thin La0.7Sr0.3MnO3

La_0.7Sr_0.3MnO₃ (LSMO) ultra-thin films have been grown by pulsed laser deposition in conditions optimized for cation stoichiometry and bulk-like physical properties in the thick limit. Through electrical transport and magnetic measurements, a phase diagram is constructed as a function of film thickness. With decreasing thickness, the LSMO films cross over at high temperatures from a paramagnetic metal to a paramagnetic insulator, and at low temperatures from a ferromagnetic metal to a ferromagnetic insulator, in close similarity to that observed for varying the electronic bandwidth in bulk manganites.     

Ferromagnetism in epitaxial orthorhombic YMnO3 thin films

Epitaxial orthorhombic YMnO₃ thin films, (0 0 1) oriented, have been grown by pulsed laser deposition on (0 0 1)SrTiO₃ substrates. Their crystal structure and magnetic response have been studied in detail. Although bulk o-YMnO₃ is antiferromagnetic, our magnetic measurements reveal intriguing thermal hysteresis between the zero-field-cooled and field-cooled curves below the onset of the antiferromagnetic ordering temperature, thus signaling a more complex magnetic structure with net ferromagnetic moments. We discuss on the possible origin of this net magnetization and we have found a correlation of the magnetic response with the strain state of the films. We propose that substrate-induced strain modifies the subtle competition of magnetic interactions and leads to a non-collinear magnetic state that can thus be tuned by strain engineering.     

Magnetic and electronic properties of Mn2Sn thin films: First-principles calculations and high temperature series expansions

In this paper, first-principles calculations have been performed in order to study the electronic and magnetic properties of the bulk and the (0 0 1) surface of Mn₂Sn thin films using self-consistent ab initio calculations, based on the density functional theory approach and using the product of the muffin-tin radius and the maximum reciprocal space vector, R_MT. K_max, we full potential linear augmented plane wave methods. Spin-polarized calculations within the framework of density-functional theory are a powerful tool for describing the magnetism of itinerant electrons in solid state materials. The total and partial density of states of Mn₂Sn thin films were calculated. The magnetic moments considered to lie along the c axes are computed. The data obtained from the ab initio calculations are used as an input for the high temperature series expansions calculations used to compute other magnetic parameters. The critical temperature and exchange interactions between the magnetic atoms in the Mn₂Sn thin films are obtained using high temperature series expansions and mean field theory, respectively.     

Structural and optical properties of sputtered gadolinium nitride films

Rare-earth (RE) materials have high magnetic moments and form a wide range of magnetic structures. There has been speculation in the literature that rare-earth nitrides may form half-metallic ferromagnetics. This is surprising because, based on a simple ionic model, trivalent rare-earth nitrides would be expected to be insulators with a similar electronic structure to the divalent rare-earth chalcogenides. However if it is the case that they are half-metallic or narrow gap insulators, then they have potential applications in spin-filtering devices. In the present investigation, We have deposited GdN films on glass substrate at room temperature by Ar/N₂ mixed gas plasma-radio frequency (rf) sputtering method. The structure and the complex optical properties as well as the energy gap of GdN thin films as a function of N₂ partial pressure are determined.     

Ab initio study of electronic and magnetic properties of the Fe3Zn intermetallic

First-principles calculations have been carried out to study the electronic structure and magnetic properties of the Fe₃Zn compound with the full-potential linearized augmented-plane wave (FLAPW) method. The results indicate a lower magnetostriction for Fe₃Zn as compared to Galfenol (Fe-Ga), as a result of a weaker spin-orbit coupling, which is due to a smaller magnetic moment induced on the Zn atom. With the Zn addition to Fe the bulk modulus and the cohesive energy (per atom) decrease, whereas the electronic specific heat coefficient γ has a substantial increase.     

Influence of Size Effects on the Electronic Structure of Hexagonal Gallium Telluride

Using methods of the density functional theory, the electronic band structure of a hexagonal modification of the layered GaTe semiconductor has been calculated. The structural parameters of a bulk crystal with the β-polytype symmetry have been determined taking into account van der Waals interactions and agree with experimental data for polycrystalline films within 2%. Estimates for the position of extrema of the upper valence band and the lower conduction band have been obtained with respect to the vacuum level for bulk β-GaTe and for ultrathin plates with the number of elementary layers ranging from 1 to 10, which corresponds to a thickness range of 0.5–8 nm. The calculations demonstrate that hexagonal GaTe is an indirect band gap semiconductor with a forbidden band width varying from 0.8 eV in the bulk material to 2.3 eV in the monolayer.     

Magnetism in systems with reduced dimensionality and chemical disorder: The local environment effects

We study influence of the local chemical environment, the so-called local environment effects, on the electronic structure and properties of magnetic systems with reduced dimensionality and chemical disorder, and show that they play a crucial role in a vicinity of magnetic instability. As a model, we consider Fe–Ni Invar. We present results obtained from ab initio calculations of the electronic structure, magnetic moments, and exchange interactions in random fcc Fe–Ni alloy, for a single monolayer alloy film on a Cu (0 0 1) substrate as well as in the bulk. We analyze the difference between the film and the bulk magnetization, which is found to be most pronounced for dilute alloys. We also analyze a sensitivity of the individual magnetic moments and effective exchange parameters to the local chemical environment of the atoms.     

Co (III) ions high-spin configuration in nonstoichiometric Co3O4 films

Optical spectra in UV, visible and near IR wave-length range have been measured and the SCF X_∝SW method calculation of Co₃O₄ electronic structure has been carried out. The nature of absorption bands observed has been revealed. As shown, the principal peculiarity of the nonstoichiometric films electronic structure compared with “bulk” samples is a stabilization of Co(III) ions high-spin configuration in Co₃O₄ spinel octahedral sublattice.     

Co掺杂ZnO薄膜的结构和磁学性能

研究了用单束脉冲激光沉积法制备的Co掺杂ZnO薄膜的结构和磁学性能。XRD表征结果表明制备的薄膜是具有沿c轴择优取向的纤锌矿点阵结构。然而,进一步的高分辨电子显微镜结果显示整个样品上的晶体取向并不完全相同。很难说明形成了单晶。结果分析表明Co占据了部分Zn的格点,并对电子结构产生了影响。室温下观察到了磁滞回线,显示掺杂Co可以实现ZnO的磁性翻转,但磁性比较小。该薄膜与我们以前用双束脉冲激光沉积法制备的Co掺杂ZnO薄膜具有相似的性能,提示我们其内部的机制可能相似。     

Electronic and magnetic properties of free and supported transition metal clusters

The spin-polarized relativistic version of the multiple scattering or the Korringa–Kohn–Rostoker method for electronic structure calculations has been used to study the electronic and magnetic properties of free and supported transition metal clusters. Corresponding results are presented for the spin- and spin–orbit-induced orbital magnetic moments in free Fe and FePt clusters. For both systems a pronounced enhancement is found for the spin as well as for the orbital moments compared with the corresponding bulk value which diminishes in an oscillatory fashion with increasing cluster size. Corresponding investigations on small Co clusters deposited on a Pt (111) surface also revealed a strong dependence of the magnetic properties on the cluster size and shape. A comparison of our theoretical results with available experimental data led to rather satisfying agreement.