Bonding, backbonding, and spin-polarized molecular orbitals: basis for magnetism and semiconducting transport in V[TCNE]x approximately 2

X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD) at the V L{2,3} and C and N K edges reveal bonding and backbonding interactions in films of the 400 K magnetic semiconductor V[TCNE]x approximately 2. In V spectra, d{xy}-like orbitals are modeled assuming V2+ in an octahedral ligand field, while d{z{2}} and d{x{2}-y{2}} orbitals involved in strong covalent sigma bonding cannot be modeled by atomic calculations. C and N MCD, and differences in XAS from neutral TCNE molecules, reveal spin-polarized molecular orbitals in V[TCNE]x approximately 2 associated with weaker pi bonding interactions that yield its novel properties.     

Electronic structure of thin film iron-tetracyanoethylene: Fe(TCNE)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Thin film iron-tetracyanoethylene Fe(TCNE) _x , x∼2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organic-based molecular magnets. Photoelectron spectroscopy results show no spurious trace elements in the films, and the iron is of Fe²⁺ valency. The highest occupied molecular orbital of Fe(TCNE) _x is located at ∼1.7 eV vs. Fermi level and is derived mainly from the TCNE⁻ singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results. The Fe(3d)-derived states appear at higher binding energy, ∼4.5 eV, which is in contrast to V(TCNE)₂ where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe²⁺ (3d⁶) configuration with a crystal field parameter 10Dq∼0.6 eV for the Fe(TCNE) _x system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)₂ (10Dq∼2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (T _C ) seen for Fe(TCNE) _x -type magnets.     

Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: In situ X-ray photoemission study

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene [Co(TCNE)_x, x ~ 2] consisting of the transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by using physical vapor deposition method under ultra-high vacuum conditions at room temperature. X-ray photoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic properties of the Co(TCNE)_x thin films. The XPS measurements show that the prepared Co(TCNE)_x films are clean, and oxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~ 1:2 ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt is assigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital (HOMO) of Co(TCNE)_x is found to be at ~ 2.4 eV with respect to the Fermi level, and it is derived either from the TCNE^? singly occupied molecular orbital (SOMO) or Co(3d) states. The peaks located at ~ 6.8 eV and ~ 8.8 eV are due to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)_x, films are compared with those of V(TCNE)_x thin film magnet: a well known system of M(TCNE)_x type of organic magnet, and important points regarding their electronic properties have been brought out.     

Magnetic properties in transition-metal-doped gold clusters: M@Au6 (M = Ti, V, Cr)

The electronic structure and magnetic properties in a series of transition-metal-doped Au clusters, MAu6- (M = Ti, V, Cr), are investigated experimentally using photoelectron spectroscopy (PES) and density functional calculations. PES features due to the impurity atoms and the host are clearly observed. It is found that all the MAu6- and MAu6 clusters possess a planar structure, in which the transition metal atom is located in the center of an Au6 ring and carries large magnetic moments (2, 3, and 4 muB for MAu6, M = Ti, V, and Cr, respectively).     

Multiple photonic responses in films of organic-based magnetic semiconductor V(TCNE)x, x approximately 2

Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)x (x approximately 2; TCNE=tetracyanoethylene; magnetic ordering temperature Tc approximately 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)x are proposed to originate from structural changes induced by internal excitation in (TCNE)- anions, which lead to relaxation to a long-lived metastable state.     

On-line monitoring of Au nanoparticles formation by optical spectroscopy

X-Ray Magnetic Circular Dichroism experiments have been conducted on a vanadium inorganic salt and a vanadium enamino-ketone complex. Measurements at the K edge of oxygen and nitrogen reveal the amount of magnetic moment transferred in the 2p orbitals of the ligand atoms from the magnetic V ion. Measurements at the L edge of vanadium show that the orbital moment is small and that J = L + S, contrary to the expected J = L-S coupling for a 3d metal with less than five d-electrons. This surprising inobservance of Hunds third rule emphasizes the need for more detailed studies and calculations on such hybrid molecules.Received: 19 November 2003, Published online: 20 April 2004PACS: 75.20.-g Diamagnetism, paramagnetism, and superparamagnetism - 75.70.Ak Magnetic properties of monolayers and thin films - 78.70.Dm X-ray absorption spectra     

Influence of the geometric structure on the V L3 near edge X-ray absorption fine structure from vanadium phosphorus oxide catalysts

We present the V L₃ near edge X-ray absorption fine structure (NEXAFS) of a vanadium phosphorus oxide (VPO) catalyst. The spectrum is related to the V3d–O2p hybridised unoccupied states. The overall peak position at the V L₃-absorption edge is determined by the formal oxidation state of the absorbing vanadium atom. Details of the absorption fine structure are influenced by the geometric structure of the compound. Empirically we found a linear relationship between the energy position of several absorption resonances and the V–O bond length of the participating atoms. This allows identification of the contribution of specific V–O bonds to the near edge X-ray absorption fine structure. The bond length/resonance position relationship will be discussed under consideration of relations between geometric structure and NEXAFS features observed in X-ray absorption experiments and theory.     

Structural, magnetic and electronic structure studies of Mn doped TiO2 thin films

We report structural, magnetic and electronic structure study of Mn doped TiO₂ thin films grown using pulsed laser deposition method. The films were characterized using X-ray diffraction (XRD), dc magnetization, X-ray magnetic circular dichroism (XMCD) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. XRD results indicate that films exhibit single phase nature with rutile structure and exclude the secondary phase related to Mn metal cluster or any oxide phase of Mn. Magnetization studies reveal that both the films (3% and 5% Mn doped TiO₂) exhibit room temperature ferromagnetism and saturation magnetization increases with increase in concentration of Mn doping. The spectral features of XMCD at Mn L_3,2 edge show that Mn²⁺ ions contribute to the ferromagnetism. NEXAFS spectra measured at O K edge show a strong hybridization between Mn, Ti 3d and O 2p orbitals. NEXAFS spectra measured at Mn and Ti L_3,2 edge show that Mn exist in +2 valence state, whereas, Ti is in +4 state in Mn doped TiO₂ films.     

钒氧化物薄膜的拉曼散射

用射频磁控溅射以纯金属钒做靶材在氩氧混合气体中制备了钒氧化物 (VO2 (B)、V6O1 3、V2 O5)薄膜。报导了钒氧化物薄膜的拉曼光谱 ,结合这些钒氧化物不同的结构特点 ,对它们的拉曼光谱进行了分类讨论     

Structural,electronic,and magnetic properties of vanadium atom-adsorbed MoSe_2 monolayer

Using the first-principles calculations, we study the structural, electronic, and magnetic properties of vanadium adsorbed MoSe_2 monolayer, and the magnetic couplings between the V adatoms at different adsorption concentrations. The calculations show that the V atom is chemically adsorbed on the MoSe_2 monolayer and prefers the location on the top of an Mo atom surrounded by three nearest-neighbor Se atoms. The interatomic electron transfer from the V to the nearestneighbor Se results in the polarized covalent bond with weak covalency, associated with the hybridizations of V with Se and Mo. The V adatom induces local impurity states in the middle of the band gap of pristine MoSe_2, and the peak of density of states right below the Fermi energy is associated with the V- dz~2 orbital. A single V adatom induces a magnetic moment of 5 μBthat mainly distributes on the V-3d and Mo-4d orbitals. The V adatom is in high-spin state, and its local magnetic moment is associated with the mid-gap impurity states that are mainly from the V-3d orbitals. In addition,the crystal field squashes a part of the V-4s electrons into the V-3d orbitals, which enhances the local magnetic moment.The magnetic ground states at different adsorption concentrations are calculated by generalized gradient approximations(GGA) and GGA+U with enhanced electron localization. In addition, the exchange integrals between the nearest-neighbor V adatoms at different adsorption concentrations are calculated by fitting the first-principle total energies of ferromagnetic(FM) and antiferromagnetic(AFM) states to the Heisenberg model. The calculations with GGA show that there is a transition from ferromagnetic to antiferromagnetic ground state with increasing the distance between the V adatoms. We propose an exchange mechanism based on the on-site exchange on Mo and the hybridization between Mo and V, to explain the strong ferromagnetic coupling at a short distance between the V adatoms. However, the ferromagnetic exchange mechanism is sensitive to both the increased inter-adatom distance at low concentration and the enhanced electron localization by GGA+U, which leads to antiferromagnetic ground state, where the antiferromagnetic superexchange is dominant.     

Revealing the insulating gap in α'-NaV (2)O(5) with resonant inelastic x-ray scattering

We measured the low energy excitation spectrum of α'-NaV (2)O(5) across its charge ordering and crystallographic phase transition with resonant inelastic x-ray scattering (RIXS) at the V L(3) edge. Exploiting the polarization dependence of the RIXS signal and the high resolution of the data, we reveal the excitation across the insulating gap at 1?eV and identify the excitations from occupied 3d(xy) bonding orbitals to unoccupied bonding 3d(xy) and 3d(yz)/3d(xz) orbitals. Furthermore we observe a progressive change of the electronic structure of α'-NaV (2)O(5) induced by soft x-ray irradiation, with the appearance of features characteristic of sodium deficient Na(x)V (2)O(5) (x?相似文献   

Photoinduced magnetism and random magnetic anisotropy in organic-based magnetic semiconductor V(TCNE){x} films, for x approximately 2

The V(TCNE){x}, x approximately 2 is an organic-based amorphous ferrimagnet, whose magnetic behavior is significantly affected in the low field regime by the random magnetic anisotropy. It was determined that this material has thermally reversible persistent change in both magnetization and conductivity driven by the optical excitation. Here, we report results of a ferrimagnetic resonance study of the photoinduced magnetism in V(TCNE){x} film. Upon optical excitation (lambda approximately 457.9 nm), the ferrimagnetic resonance spectra display substantial changes in their linewidths and line shifts, which reflect a substantial increase in the random magnetic anistropy. The results reflect the role of magnetic anisotropy in disordered magnets and suggest a novel mechanism of photoinduced magnetism in V(TCNE){x} induced by the increased structural disorder in the system.     

Observation of electronic ferroelectric polarization in multiferroic YMn2O5

We report the observation of a magnetic polarization of the O 2p states in YMn(2)O(5) through the use of soft x-ray resonant scattering at the oxygen K edge. Remarkably, we find that the temperature dependence of the integrated intensity of this signal closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast with the temperature dependence observed at the Mn L(3) edge, which reflects the Mn magnetic order parameter. First-principles calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O 2p and Mn 3d states results in a purely electronic contribution to the ferroelectric polarization, which can exist in the absence of lattice distortions.     

Local spin density total energy study of surface magnetism: V (100)

Results of self-consistent all-electron local (spin) density functional studies of the electronic and magnetic properties of vanadium (100) 1-, 3-, 5- and 7-layers films are reported using our full-potential linearized augmented plane wave (FLAPW) method. The calculated work function, 4.2 eV, agrees very well with the experimental value of 4.12 eV. From both Stoner factor analyses and spin-polarized total energy calculations, it is concluded that V(100) undergoes a ferromagnetic phase transition only for the monolayer system. The magnetic moment is found to be 3.09μ_B per atom of this monolayer film and to have a total energy 57 mRy below that of the paramagnetic structure. For multilayer V(001) systems, the sharp surface density-of-states peak which is characteristic of the occurrence of surface magnetism in the 3d transition metals is located 0.3 eV above the Fermi level. As a result, the paramagnetic state is stable. In addition, no enhancement of the exchange-correlation integral is found for the surface atoms compared with the bulk value. The lower energy of the paramagnetic structure is further supported by total energy investigations of the multilayer relaxation of V(100) — the calculated interlayer spacings for the paramagnetic surface with a 9% contraction of the topmost interlayer spacing and a 1% expansion of the second interlayer spacing with respect to its bulk value are in good agreement with LEED measurements. It is suggested that the surface magnetism of V(100) may be associated with surface oxygen or caused by impurity induced surface reconstructions.     

Magnetic dichroism and spin structure of antiferromagnetic NiO(001) films

We find that Ni L2 edge x-ray magnetic linear dichroism is fully reversed for NiO(001) films on materials with reversed lattice mismatch. We relate this phenomenon to a preferential stabilization of magnetic S domains with main spin component either in or out of the plane, via dipolar interactions. This suggests a way to selectively control spin structures in 3d systems with small spin-orbit coupling.     

Atomic and Electronic Structure of V/MgO Interface

Thin films of vanadium were deposited on the (001) surface of a MgOsubstrate by molecular beam epitaxy (MBE) and the V/MgO interface wasinvestigated by cross-sectional high resolution electron microscopy(HREM) and electron energy loss spectroscopy (EELS). In order todetermine the location of atoms at the interface, computersimulations were performed for four possible models, and bestmatching between the experimental and simulated images was obtainedfor the model where the V atoms are located directly on top of the Mgatoms at the interface. Interface bonding mechanism was investigatedby a first principles molecular-orbital (MO) calculation using thediscrete-variational (DV)-X method for a modelcluster of the interface, i.e., (Mg₉O₉V₅).The V-3d band was located in between the band-gap of MgO, and nearly empty Mg-3sp orbitals werefound to overlap with the V-3d band. The Mg-3sp and V-3d hybridizedin a bonding manner, thereby generates strong covalent bondingbetween V and Mg. Nearly filled O-2p orbitals were also found tohybridize with the V-3d orbitals in an antibonding manner. The bondoverlap population of the V–O bond was approximately four timessmaller than that of the V–Mg bond when the bond-length was thesame. The near edge structure of EELS specific to the interface wasobtained using a V/MgO multilayer specimen at both Mg-K and O-Kedges. Comparison between the experimental and theoretical spectra bythe present MO calculation clearly found the presence of hybridizedorbitals of V-3d with Mg-3p.     

Magnetic characterization of ZnO doped with vanadium

The magnetic properties of vanadium doped ZnO nanorods formed by different growth methods were examined. The samples under investigation were either grown by metallorganic vapour-phase epitaxy (MOVPE) with in situ incorporation of the V atoms or by aqueous chemical growth (ACG) in combination with an ion-implantation process. The V concentration for all samples was less than 2 at.%.Field-cooled SQUID measurements only reveal a weak ferromagnetic signal still in a very sensitive measuring range. However, the MFM micrographs obtained at room temperature show a clear magnetic contrast and complex structures, the images are in good agreement with the phase images expected for vertically aligned magnetic dipoles. This is a strong indication for a ferromagnetic behaviour at room temperature.     

Modeling the electronic behavior of gamma-LiV2O5: a microscopic study

We determine the electronic structure of the one-dimensional spin- 1 / 2 Heisenberg compound gamma-LiV2O5, which has two inequivalent vanadium ions, V(1) and V(2), via density-functional calculations. We find a relative V(1)-V(2) charge ordering of roughly 70:30. We discuss the influence of the charge ordering on the electronic structure and the magnetic behavior. We give estimates of the basic hopping matrix elements and compare with the most studied (alpha)'-NaV2O5.     

Giant thermopower effects from molecular physisorption on carbon nanotubes

Results are presented of in situ studies of the thermoelectric power and four-probe resistance of single-walled carbon nanotube films during the adsorption of cyclic hydrocarbons C(6)H(2n) (n=3-6). The size of the change in these transport parameters is found to be related to the pi electron population of the molecule, suggesting the coupling between these pi electrons and those in the nanotube wall may be responsible for the observed effects. A transport model for the SWNT film behavior is presented, incorporating the effects of a new scattering channel associated with the adsorbed molecules.     

Depth-dependent magnetism in epitaxial MnSb thin films: effects of surface passivation and cleaning

Depth-dependent magnetism in MnSb(0001) epitaxial films has been studied by combining experimental methods with different surface specificities: polarized neutron reflectivity, x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering and spin-polarized low energy electron microscopy (SPLEEM). A native oxide ～4.5?nm thick covers air-exposed samples which increases the film's coercivity. HCl etching efficiently removes this oxide and in situ surface treatment of etched samples enables surface magnetic contrast to be observed in SPLEEM. A thin Sb capping layer prevents oxidation and preserves ferromagnetism throughout the MnSb film. The interpretation of Mn L(3,2) edge XMCD data is discussed.