Mechanism Of Exchange Interaction Research Articles

Optical characterization of YAl3(BO3)4:Dy3+–Tm3+ phosphors under near UV excitation

Dy3+ and Tm3+ co-doped YAl3(BO3)4 (YAB) phosphors were prepared by solid-state reaction method at 1200°C/3h. The average crystallite size was determined as 52.09nm from the X-ray diffraction measurements. Upon 352 and 359nm near ultra violet excitation, the YAB:Dy3+–Tm3+ phosphors exhibit Dy3+:4F9/2→6HJ (J=15/2, 13/2, 11/2) and Tm3+:1D2→3F4 transitions with different luminescence intensity. The photoluminescence emission and decay measurements revealed the energy transfer from Dy3+ to Tm3+ ions under 359nm excitation only. The energy transfer between Dy3+ and Tm3+ takes place in Dy3+–Tm3+ clusters through exchange interaction mechanism. The Commission International de I’Eclairage chromaticity coordinates of YAB:Tm3+ phosphor (λex=359nm) were found very close to the European Broadcasting Union and National Television Standard Committee illuminants. The emission color of the studied phosphors could be tuned from blue-to-white as a function of excitation wavelength. The YAB:Dy3+–Tm3+ phosphors can be used as potential candidates in display technology.

The energy transfer mechanism in Ce,Tb co-doped LaF3 nanoparticles

LaF3 pure host, LaF3:Ce, LaF3:Tb as well as LaF3:Ce,Tb phosphors were synthesized by the hydrothermal method. X-ray diffraction measurements were in good agreement with the standard data of LaF3 from JCPDS card No. 32-0483 and indicated that the material was nanocrystalline with an average particle size of about 36 nm. Photoluminescence spectra of co-doped samples revealed that the Ce3+ emission was quenched while Tb3+ emission was enhanced, implying that energy was transferred from Ce3+ (the donor) to Tb3+ (the acceptor) in this system. The luminescence intensities and lifetimes of the donor for different concentrations of the acceptor were fitted to theoretical models in order to investigate the energy transfer mechanism. The quadrupole–quadrupole and exchange interaction mechanisms gave the best fit between the experimental data and the theoretical curves. The effective average Bohr radius from the fit to the exchange model is 0.095 nm. Since this is close to the ionic radii of the Ce3+ and Tb3+ ions, it suggests that the exchange interaction mechanism contributes to the energy transfer.

Engineering intriguing electronic and magnetic properties in novel one-dimensional staircase-like metallocene wires

The unprecedented applications of nanoscale materials in spintronics are formidably hampered by the lack of ordered and separately distributed spin structures. Here, on the basis of density functional calculations, we systematically investigate the electronic and magnetic properties of novel one-dimensional staircase-like organometallic wires (MSWNs, M = V, Cr, Mn, Fe, Co, and Ni) constructed with metallocene. Most importantly, we find that the transition-metal atoms in these wires are regularly and separately docked, achieving the long-standing dream of ordered spin arrangement in the studied systems. Moreover, these materials display long-range magnetic coupling: the VSWNs, CrSWNs, MnSWNs, and CoSWNs are ferromagnetic, while NiSWNs are antiferromagnetic and FeSWNs are nonmagnetic. The discrepancy in magnetic coupling for these materials is related to the competition mechanisms of through-bond and through-space exchange interactions. These findings pave a practical route to achieve new electronics and spintronics.

Theory of Mechanism of π–dInteraction in Iron–Phthalocyanine

Transition metal-phtahalocyanine(Pc) compound, TPP[Fe(Pc)(CN)$_2$]$_2$, which is one of molecular conductors of charge transfer type with 3/4-filled conduction band consisting of LUMO of Pc molecules, shows characteristic features in transport and magnetic properties resulting from localized magnetic moments $S=1/2$ associated with Fe$^{+3}$ atoms. We construct an effective tight-binding model of this system and study the mechanism of exchange interaction, $J$, between $d$ and $\pi$ electrons based on both second order perturbation of transfer integrals between $d$ and $\pi$ orbitals and numerical diagonalization. It is found that there is no hybridization between $d$-orbitals and LUMO of $\pi$-orbitals and then super-exchange interaction in the Anderson model does not exist. Instead, processes associated with Hund's rule both on $d$ and $\pi$ orbitals, which may be called "the double Hund's exchange mechanism", turn out to play important roles and the sign of resultant $J$ can be either ferromagnetic or antiferromagnetic depending on model parameters because of competition among various processes. By taking account of magnetic anisotropy due to spin-orbit interactions and comparing with experimental results, it is indicated that $J$ is antiferromagnetic and of the order of 100K.

Theoretical Study of Exchange Coupling in 3d-Gd Complexes: Large Magnetocaloric Effect Systems

Polynuclear 3d transition metal-Gd complexes are good candidates to present large magnetocaloric effect. This effect is favored by the presence of weak ferromagnetic exchange interactions that have been investigated using methods based on Density Functional Theory. The first part of the study is devoted to dinuclear complexes, focusing on the nature and mechanism of such exchange interactions. The presence of two bridging ligands is found more favorable for ferromagnetic coupling than a triple-bridged assembly, especially for complexes with small M-O···O-Gd hinge angles. Our results show the crucial role of the Gd 5d orbitals in the exchange interaction while the 6s orbital seems to have a negligible participation. The analysis of the atomic and orbital spin populations reveals that the presence of spin density in the Gd 5d orbital is mainly due to a spin polarization effect, while a delocalization mechanism from the 3d orbitals of the transition metal can be ruled out. We propose a numerical DFT approach using pseudopotentials to calculate the exchange coupling constants in four polynuclear first-row transition metal-Gd complexes. Despite the complexity of the studied systems, the numerical approach gives coupling constants in excellent agreement with the available experimental data and, in conjunction with exact diagonalization methods (or Monte Carlo simulations), it makes it possible to obtain theoretical estimates of the entropy change due to the magnetization/demagnetization process of the molecule.

The mechanism of anisotropic exchange interaction in superconducting iron arsenides

Using a combination of linear response theory and constrained orbital hybridization approach, we study the mechanism of magnetic exchange interaction of iron-based superconductor. We reproduce the observed highly anisotropic exchange interaction, and our constrained orbital calculation unambiguously identifies that the anisotropic feature of exchange interaction is not sensitive to the unequal dxz/dyz orbital population.

Equilibrium and kinetics studies on the absorption of Cu(II) from the aqueous phase using a β-cyclodextrin-based adsorbent

Abstract A novel β-cyclodextrin-based adsorbent (CDAA) for the removal of Cu 2+ was prepared and characterized. The adsorption capacities of Cu 2+ on CDAA were evaluated under various treatment conditions, including the solution pH, the dosage of the adsorbent, the initial Cu 2+ concentration, and adsorption time. The results indicated that CDAA hydrogel exhibited typically three-dimensional cross-link network structure. There was a significant increase in the adsorption capacity (from 18.93 mg/g to 107.37 mg/g) when the solution pH increased from 2 to 5. The adsorption equilibrium data were fitted and analyzed with Langmuir, Freundlich, Temkin isotherm equations and four adsorption kinetic models. The results suggested that the Freundlich equation model was the best fit with experimental data ( R 2 = 0.995). The kinetic equations showed that the adsorption of Cu 2+ on the adsorbent fit different equations for different concentrations of Cu 2+ . These results indicate that in the present study, Cu 2+ adsorption onto the adsorbent occurred via ion exchange and chemical interaction mechanisms.

Mechanism of quantum dot luminescence excitation within implanted SiO2:Si:C films

Results of the investigation of photoluminescence (PL) mechanisms for silicon dioxide films implanted with ions of silicon (100 keV; 7 × 1016 cm−2) and carbon (50 keV; 7 × 1015–1.5 × 1017 cm−2) are presented. The spectral, kinetic and thermal activation properties of the quantum dots (Si, C and SiC) formed by a subsequent annealing were studied by means of time-resolved luminescence spectroscopy under selective synchrotron radiation excitation. Independent quantum dot PL excitation channels involving energy transfer from the SiO2 matrix point defects and excitons were discovered. A resonant mechanism of the energy transfer from the matrix point defects (E′ and ODC) is shown to provide the fastest PL decay of nanosecond order. The critical distances (6–9 nm) of energy transport between the bulk defects and nanoclusters were determined in terms of the Inokuti–Hirayama model. An exchange interaction mechanism is realized between the surface defects (Es′-centres) and the luminescent nanoparticles. The peculiarities of an anomalous PL temperature dependence are explained in terms of a nonradiative energy transfer from the matrix excitons. It is established that resonant transfer to the luminescence centre triplet state is realized in the case of self-trapped excitons. In contrast, the PL excitation via free excitons includes the stages of energy transfer to the singlet state, thermally activated singlet–triplet conversion and radiative recombination.

Investigating the mechanism of ferromagnetic exchange interaction in non-doped CeO2 with regard to defects and electronic structure

We report a systematic structural, electronic, and magnetic investigation on occurrence of ferromagnetism and its “switch” action in non-doped bulk ceria (CeO2). The magnetization measurements establish that the pristine CeO2 having a paramagnetic ground state can be driven to a ferromagnetic state at room temperature, when hydrogenated at 600°C. The observed H-induced ferromagnetism is closely related to the oxygen vacancies and the Ce valence state. X-ray photoemission results depict that Ce ions reduce from 4+ to 3+ state along with creation of oxygen vacancies during the ferromagnetic transition. A parallel variation of carrier concentration, revealed by resistance measurements, seems to be a secondary effect of the oxygen vacancies creation. The F+ centers, i.e. the electrons in singly occupied oxygen vacancies, seem to play the key role in establishing the ferromagnetism in CeO2, in the framework of bound magnetic polaron model. The exchange mechanism shows a “switch” action such that one could remove the oxygen vacancies through re-heating the H2-treated CeO2 and the ferromagnetism is subsequently vanished.

Magnetic exchange behavior of a long-range Cu II system through the diamagnetic Cu I2 unit bridge: A DFT study

The mechanism of magnetic exchange interaction in an interesting Cu II coordination polymer with the Cu I 2X 2 (X = I or Br) moiety bridge is investigated by the calculations based on density functional theory combined with the broken-symmetry approach (DFT-BS). Surprisingly, different from the experiment results, the calculations reveal that the complex between the magnetic Cu II centers has a fairly weak antiferromagnetic interaction for the long distance (∼10 Å). The analyses show of the molecular orbitals and the spin density distribution favor the very weak antiferromagnetic coupling interactions of the systems.

Electronic structure of C co-doped (Ga, Fe)N-based diluted magnetic semiconductors

We report results of the study on GaN doped with Fe and co-doped with Carbon (C), for acceptors defect, making use of the spin-polarized relativistic Korringa-Kohn-Rostoker coherent-potential approximation method. In order to understand and to explain the half-metallicity and ferromagnetism stability, observed in Ga0.95Fe0.05N with acceptor defects, we calculated the electronic structure and magnetic properties of p-type Ga0.95Fe0.05N. Furthermore, comparison between the electronic structure of the substitutional and interstitial carbon in (Ga, Fe)N is also given. Mechanism of exchange interaction between magnetic ions in Ga0.95Fe0.05N with acceptor defect is investigated. The hyperfine interaction has been calculated for the disordered alloy systems Ga0.95Fe0.05N1-xCx (0.01 ≤ x ≤ 0.08).

Electronic structure of acceptor defects in (Zn,Mn)O and (Zn,Mn)(O,N)

Using first-principles density functional calculations, we study the electronic structure and magnetic properties of Mn-doped ZnO, wurtzite crystal structure, with various defects. This allows to understand and to explain the half-metallicity and the ferromagnetism stability, observed in Mn-doped ZnO with acceptor defects like Zn vacancies. The calculations were performed using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. Hydrogenation effectsin (Zn,Mn)O and (Zn,Mn)(O,N) is also investigated with and without defects. This work presents detailed information about total, atom, and vacancy projected density of states functions, and magnetic moment for different atoms and defects in Mn-doped ZnO and N-codoped (Zn,Mn)O. The Curie temperature TC is evaluated by using the mean field approximation. We show also that higher values of TC are attained for high concentration of vacancy defects sites in (Zn,Mn)O and for small concentration of vacancy defects sites in (Zn,Mn)(O,N). Mechanism of exchange interaction between magnetic ions in Mn-doped ZnO and N-codoped(Zn,Mn)O with and without defects is also investigated. Finally, we propose a model which describes the origin of strong ferromagnetism stability observed in p-type ZnO.

Experimental Evidence for the Participation of 5d GdIIIOrbitals in the Magnetic Interaction in Ni−Gd Complexes

The syntheses, structural determinations, and magnetic studies of two trinuclear Ni-Gd-Ni complexes are described. The structural studies demonstrate that the two complexes present a linear arrangement of the Ni and Gd ions, with Ni ions in slightly distorted square-pyramidal or octahedral environments in complexes 1 and 2, respectively. The Ni and Gd ions are linked by two or three phenoxo bridges, so that complexes 1 and 2 present edge-sharing or face-sharing bridging cores. Ferromagnetic interactions operate in these complexes. While a unique J parameter is able to fit the magnetic data of complex 2, two very different J constants are needed for 1. This result is at first sight surprising, for the structural data of the two Ni-O(2)-Gd cores in complex 1 are quite similar (similar Ni-O and Gd-O bond lengths, similar angles, and dihedral angles), the only difference coming from the angle between the planes defined by the Gd ion and the two bridging phenoxo oxygen atoms of each Ni-O(2)-Gd half core. This latter magnetic behavior can be considered as a signature for the participation of 5d Gd(III) orbitals in the exchange interaction mechanism and can explain why edge-sharing complexes have larger J parameters than face-sharing complexes.

Room-temperature ferromagnetism in dielectric GaN(Gd)

We present an explanation of recently observed giant magnetic moment and room-temperature ferromagnetism in the dielectric GaN doped with Gd. Our approach uses the polarization mechanism of exchange interaction, which occurs if the d-level of Gd appears in the bandgap close to the valence band edge. Calculated ferromagnetic critical temperature and the value of the magnetic moment well correspond to experimental findings.

Electronic structure of p-type (Ga,Fe)N diluted magnetic semiconductors

By ab-initio calculation we show that the (Ga,Fe)N ground state may be changed from anti-ferromagnetic to ferromagnetic by acceptor defect like Ga vacancies. The electronic structures are calculated by using the Korringa–Kohn–Rostoker (KKR) method combined with coherent potential approximation (CPA). We show that we can increase the magnetic moment of Fe in p-type GaN by oxygen co-doping. Mechanism of exchange interactions between magnetic ions in p-type (Ga,Fe)N is also studied. The effect of external magnetic field on the electronic structure of (Ga, Fe)N and p-type (Ga, Fe)N is investigated.

Theoretical study on the magnetic exchange behavior of a hetero-double-bridged binuclear copper(II) complex: orbital complementary effect

The mechanism of magnetic exchange interaction in a μ-chloro and μ-methoxy double-bridged copper(II) complex is investigated by calculations based on density functional theory combined with the broken-symmetry approach (DFT-BS). The calculated results reveal that the complex has a strong antiferromagnetic interaction. By the analyses of magnetic orbital interaction and spin distribution, it is found that there exists orbital complementary effect between the two bridging ligands. In addition, to examine the contribution to the magnetic exchange interaction of each bridging ligand, two single-bridged systems have been studied.

Magnetic Interactions in Two Heterobridged Dinuclear Copper(II) Complexes: Orbital Complementarity or Countercomplementarity?

The mechanisms of magnetic exchange interactions in two heterobridged mu-hydroxyl-mu-X dicopper complexes A and B (X = azaindole for A and X = pyrazole for B) are investigated by the calculations based on density functional theory combined with the broken-symmetry approach (DFT-BS). It is found that although the coordination circumstances of the copper centers in the two complexes are very similar, the magnetic magnitudes and signs are diametrically opposed. By the theoretical analyses of magnetic orbital interaction and spin distribution, it is indicated that the difference between the magnetic properties of the two complexes is due to the distinction of orbital interaction of two bridge ligands. Namely, the weak ferromagnetic coupling for complex A arises from the orbital countercomplementarity of the hydroxo and azaindole bridges while the strong antiferromagnetic coupling for complex B arises from the orbital complementarity of the hydroxo and pyrazolato bridges.

Intraband magneto-optical properties of magnetic quantum dots

The intraband optical properties of quantum dots comprising one electron and up to two Mn ions, subjected to an axial magnetic field, have been investigated theoretically using the configuration-interaction method. We have shown that the electron-Mn and the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ exchange interaction mechanisms induce splitting of the energy levels for zero magnetic field, and lead to ground-state transitions and anticrossing of the states for finite magnetic fields. This results in the presence of interesting effects in the intraband absorption spectrum, such as the appearance of new absorption lines and anticrossing features, absent in the single-particle spectrum. Our analysis predicts that both the electron--Mn ion and the $\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Mn}$ exchange interactions may be probed by means of intraband magneto-optical spectroscopy.

Common origin of ferromagnetism and band edge Zeeman splitting in GaMnAs at low Mn concentrations

We report magnetic circular dichroism measurements on thin GaMnAs films with low Mn concentration, which reveal that exchange-induced spin splitting of the band edges occurs only in samples that show ferromagnetic order, and is not observed in paramagnetic samples. These results lead to the conclusion that Mn ions in the A0 configuration (d5+hole) provide the only mechanism for exchange interaction between Mn spins and band carriers. We also show that there is a linear relation between the observed exchange-induced splitting of the band edges and the Curie temperature, pointing to a common origin of the band edge splitting and ferromagnetism in GaMnAs.

Ferro-antiferromagnetic coupling and unusual transport properties of ferro-antiferromagnetic (100−x) La0.7Pb0.3MnO3+xPr0.63Ca0.37MnO3 (x=–85wt%) composites

We report a low field (H=0.5T) magnetoresistance of colossal magnetoresistive (CMR) composites prepared with ferromagnetic (FM)—antiferromagnetic (AFM) coupled system (100−x) La0.7Pb0.3MnO3(LPMO)+xPr0.63Ca0.37MnO3 with x=0–85wt%. A large increase (∼16% maximum) of magnetoresistance (MR) (∼10% for pure LPMO) is observed at a relatively higher optimum concentration (x=70% AFM). Interesting double peaks are observed for lower AFM concentrations (x⩽35%) in the resistivity-temperature [ρ(T)] curve: one due to the usual intrinsic double exchange interaction (between Mn3+ and Mn4+) mechanism and the other due to the stronger FM-AFM coupling for lower concentration. Unlike the usual ceramic CMR material, thermoelectric power (TEP) in this system is also nonadditive in character, indicating the presence of additional defects and strain in this composites. A two-channel (conducting and semiconductor) model has been used which fit well with the ρ(T) data near the optimum concentration x=70%. Around this highest AFM concentration, showing maximum MR, the corresponding TEP is also higher than those of the individual parent compound, which indicates the presence of maximum grain boundary effect and defects. Positron annihilation lifetime data of the composites also support the presence of such defects and strain in the system with maximum around the critical concentration xC=70%.