Direct Exchange Mechanism Research Articles

Magnetism of Co doped graphitic ZnO layers adsorbed on Si and Ag surfaces

The effect of the substrate on Co doped graphitic ZnO layers adsorbed on metallic and semiconductor surfaces are investigated. The first principles results reveal that the magnetic interactions are strongly affected by the substrate. While graphitic layers of ZnO:Co on the well controlled metallic Ag(111) surface present weak ferromagnetic stability, on the semiconductor Si(111) surface they are strongly ferromagnetic coupled. The presence of metallic states in ZnO:Co adsorbed on Ag(111) surface favors a superexchange mechanism, weakening the ferromagnetic interaction. On the other hand, the magnetism of few layers of ZnO:Co on Si(111) surface is governed by a direct exchange mechanism, favoring a ferromagnetic coupling. This confined II–VI system doped with transition metal on a semiconductor surface presents higher magnetic stability than III–V nanostructures, and it is desired for nanostructured oxide/semiconductor room temperature ferromagnetism, using silicon technology.

Exploring exchange mechanisms with a cold-atom gas

Fermionic atoms trapped in a double-well potential are an ideal setting to study fundamental exchange mechanisms. We use exact diagonalization and complementary analytic calculations to demonstrate that two trapped fermions deliver a minimal model of the direct-exchange mechanism. This is an ideal quantum simulator of the Heisenberg antiferromagnet, exposes the competition between covalent and ionic bonding, and can create, manipulate, and detect quantum entanglement. Three trapped atoms form a faithful simulator of the double-exchange mechanism that is the fundamental building block behind many Heisenberg ferromagnets.

Structures and magnetism of multinuclear vanadium-pentacene sandwich clusters and their 1D molecular wires

Two types of multinuclear sandwich clusters, (V(3))(n)Pen(n+1), (V(4))(n)Pen(n+1) (Pen = Pentacene; n = 1, 2), and their corresponding infinite one-dimensional (1D) molecular wires ([V(3)Pen](∞), [V(4)Pen](∞)) are investigated theoretically, especially on their magnetic coupling mechanism. These sandwich clusters and molecular wires are found to be of high stability and exhibit intriguing magnetic properties. The intra-layered V atoms in (V(3))(n)Pen(n+1) clusters prefer antiferromagnetic (AFM) coupling, while they can be either ferromagnetic (FM) or AFM coupling in (V(4))(n)Pen(n+1) depending on the intra-layered V-V distances via direct exchange or superexchange mechanism. The inter-layered V atoms favor FM coupling in (V(3))(2)Pen(3), whereas they are AFM coupled in (V(4))(2)Pen(3). Such magnetic behaviors are the consequence of the competition between direct exchange and superexchange interactions among inter-layered V atoms. In contrast, the 1D molecular wires, [V(3)Pen](∞) and [V(4)Pen](∞), appear to be FM metallic with ultra high magnetic moments of 6.8 and 4.0 μ(B) per unit cell respectively, suggesting that they can be served as good candidates for molecular magnets.

Valence bond crystal and possible orbital pinball liquid in at2gorbital model

We study a model for orbitally degenerate Mott insulators, where localized electrons possess t_2g degrees of freedom coupled by several, competing, exchange mechanisms. We provide evidence for two distinct strongly fluctuating regimes, depending on whether superexchange or direct exchange mechanism predominates. In the superexchange-dominated regime, the ground state is dimerized, with nearest neighbor orbital singlets covering the lattice. By deriving an effective quantum dimer model and analyzing it numerically, we characterize this dimerized phase as a valence bond crystal stabilized by singlet resonances within a large unit cell. In the opposite regime, with predominant direct exchange, the combined analysis of the original model and another effective model adapted to the local constraints, shows that subleading perturbations select a highly resonating ground state, with coexisting diagonal and off-diagonal long-range orbital orders.

Kirkendall Effect and Mechanism of Self-Diffusion in B2 Intermetallic Compounds

Single-phase interdiffusion studies in the B2 intermetallic compounds NiAl and FeAl have demonstrated the incidence of Kirkendall effect in these systems. The implication of this observation on the operating mode of self-diffusion in these systems is discussed. The direct exchange and ring diffusion mechanisms were excluded as the possible mode of substitutional diffusion in metals and alloys on account of the incidence of Kirkendall effect during single-phase interdiffusion studies. It is suggested that on similar phenomenological considerations, the six-jump vacancy cycle mechanism, triple-defect mechanism, and antistructure bridge mechanism are precluded as possible mode of diffusion in B2 intermetallic compounds.

Ionized zinc vacancy mediated ferromagnetism in copper doped ZnO thin films

This paper reports the origin of ferromagnetism in Cu-doped ZnO thin films. Room-temperature ferromagnetism is obtained in all the thin films when deposited at different oxygen partial pressure. An obviously enhanced peak corresponding to zinc vacancy is observed in the photoluminescence spectra, while the electrical spin resonance measurement implies the zinc vacancy is negative charged. After excluding the possibility of direct exchange mechanisms (via free carriers), we tentatively propose a quasi-indirect exchange model (via ionized zinc vacancy) for Cu-doped ZnO system.

First-principles study of magnetic exchange force microscopy with ferromagnetic and antiferromagnetic tips

Using first-principles calculations, we investigate the application of ferromagnetic and antiferromagnetic tips in magnetic exchange force microscopy, a novel technique which allows imaging the magnetic structure of insulating and conducting surfaces on the atomic scale by detecting the exchange forces between magnetic tip and sample. We apply density functional theory using the full-potential linearized augmented plane-wave method to calculate the short-ranged exchange forces between Fe and Cr tips with the antiferromagnetic Fe monolayer on W(001). We include structural relaxations of the tip and sample due to their interaction and consider pure Fe and Cr cluster tips consisting of 14 atoms as well as alloyed Fe-Cr tips. For single atom tips we observe enhanced exchange energies and forces for Cr atoms with respect to Fe due to the larger extent of the $3d$ orbitals of Cr. Surprisingly, the exchange forces for unrelaxed cluster tips of Fe and Cr are of very similar magnitude and in the noncontact regime the Fe tip exhibits a slightly larger exchange force than the Cr tip. This effect stems from the competition of Cr apex atom and base atoms which contribute with opposite sign to the exchange force due to the antiferromagnetic coupling within the tip. For the ferromagnetic Fe cluster tips, the base atoms contribute less to the total exchange force and with the same sign. The Cr cluster tips display a characteristic sign change of the exchange interaction from ferromagnetic at large tip-sample distances to antiferromagnetic at short separations due to the transition from an indirect double exchange mediated by conduction electrons to a direct exchange mechanism due to overlap of $d$ orbitals. Structural relaxations turn out to be more important for Cr-terminated tips as a consequence of larger exchange forces on the apex atom. The magnitude of the exchange energy rises more quickly due to the relaxations and the exchanges forces are significantly enhanced. At intermediate tip-sample separations, relaxed Fe and Cr cluster tips result in similar exchange forces, while at short distances, Fe tips exhibit larger forces. Fe-based tips can be further improved by adsorbing single Cr atoms at their apex. The analysis of spin-resolved charge density difference plots confirms the competing exchange mechanisms and reveals a significant contribution from base atoms to the exchange interaction at small tip-sample separations.

Ferromagnetic mechanism in Ni-doped anatase TiO2

The ferromagnetism in Ni-doped anatase TiO2 is studied by a series of supercell density functional calculations. Our results show that O vacancy (VO) plays a key role of ferromagnetism in Ni-doped TiO2, and the magnetic origin is of Ni3+ ion. In the stoichiometric configuration of 2 Ni with one VO, the system shows the ferromagnetic insulating ground state. A vacancy-mediated direct-exchange mechanism is proposed and it reasonably interprets the ferromagnetism in such structure without using the concept of charge carriers.

An Apparent Violation of Microscopic Reversibility: Mechanisms for Ligand Substitution Reactions of Oxorhenium(V) Dithiolate Complexes

The first theoretical investigation on the mechanism for ligand substitution in five-coordinate square pyramidal oxorhenium dithiolate complexes, CH3ReO(SCH2C6H4S)-X, has been carried out with the Perdew-Burke-Ernzerhof density functional and the Stuttgart relativistic effective-core-potential basis sets. In the mechanism proposed in the experimental kinetic studies, the entering ligand Y attacks the vacant lower axial coordinate site trans to O, and the resulting six-coordinate intermediate rearranges through a turnstile twist (a trigonal prismatic intermediate or transition state) or pentagonal pyramid to allow the leaving ligand X to exit from the same site. These workers proposed this rearrangement to avoid a violation of microscopic reversibility. The computed energy barriers in this reaction pathway show that the turnstile or pentagonal pyramidal transition states are too high in energy to make this pathway accessible. Although the vacant lower axial site in the rhenium complex is the site most easily attacked by Y, transition states for X leaving from the site cis to the attack have quite low energy barriers. Although this direct-exchange mechanism was thought to lead to a violation of microscopic reversibility, we show that these direct-exchange pathways provide low-energy routes for ligand exchange and clarify this apparent violation of microscopic reversibility. Furthermore, computed results of different entering and leaving ligand pairs are analyzed for their effect on the choice of reaction pathways. In the direct-exchange mechanism, the replacement of 4-tBu-pyridine by PPhMe2 is monophasic without an intermediate, but the replacement of PPh3 by PPhMe2 is biphasic (proceeds by a two-stage pathway) and generates the observed intermediate, an isomer of product. These predictions are completely consistent with the observed experimental phenomena. The accuracy of the particular functional/basis set used for the study is compared to 10 functionals in 19 basis sets and to large basis set coupled cluster calculations on model systems.

Mechanism of the MeOH/H2O Substitution in a Series of Biomimetic Bimetallo Zinc‐Based H3O2 Complexes

AbstractSmall bimetallo biomimetic zinc‐based compounds containing a bridging μ‐OH functionality can be easily hydrated to yield complexes containing an intramolecular Zn–H3O2–Zn bridge. Because such bridges are intrinsically more nucleophilic than the corresponding μ‐OH functionality, this effectively activates the complex. The Zn–H3O2–Zn unit is labile, and one can successively substitute the “water” and the “hydroxide” in the bridge for methanol and methoxide to form metal‐bound HO–H–OMe and MeO–H–OMe functionalities. We have performed an extensive density functional investigation of this substitution mechanism for a series of bimetallo zinc complexes which have been synthesised and investigated by F. Meyer et al. There are two mechanisms for water/methanol exchange – an addition–elimination and a direct exchange mechanism. If the metal ions are coordinatively unsaturated, and their ligand sphere does not sterically hinder the approach of the methanol, both pathways compete with each other. For coordinatively saturated bimetallo complexes with larger ligand spheres, only the direct exchange mechanism is possible. Regardless of the mechanism operating, the exchange is facile and is controlled by the relative thermodynamic stability of the intermediates. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Intramolecular energy transfer through charge transfer state in lanthanide compounds: A theoretical approach

A theoretical approach for the intramolecular energy transfer process involving the ligand-to-metal charge transfer (LMCT) state in lanthanide compounds is developed. Considering a two-electron interaction, both the direct Coulomb and exchange interactions are taken into account, leading to expressions from which selection rules may be derived and transfer rates may be calculated. These selection rules show that the direct Coulomb and exchange mechanisms are complementary, in the same way as obtained in previous works for the case of ligand-lanthanide ion energy transfer processes. An important result from numerical estimates is that the channel ligand-LMCT state is by far the dominant case, leading to transfer rates higher than for the channel lanthanide ion-LMCT state by several orders of magnitude. The analysis of the emission quantum yield as a function of the relative energy position of the LMCT state in a typical Eu(3+) compound allows the identification of two quenching regions, the most pronounced one occurring close to the lower ligand triplet level.

Photoinduced Hole Transfer from the Triplet State in a Porphyrin-Based Donor−Bridge−Acceptor System

The triplet excited-state deactivation of a gold porphyrin (AuP) in porphyrin-based donor−bridge−acceptor (D−B−A) systems has been studied. The results from room temperature and 80 K measurements are presented. The primary objectives have been to investigate whether electrons/electron holes or excitation energy could be transferred from 3AuP to the appended zinc porphyrin (ZnP) in the dimers. As the bridging chromophores in our D−B−A systems separate the ZnP and AuP moieties by 19 Å edge-to-edge, we do not expect a significant contribution to either electron or energy transfer from a direct (through space) exchange mechanism. This gives us the opportunity to scrutinize how the bridging chromophores influence the transfer reactions. The results show that quenching of 3AuP occurs with high efficiency in the dimers that are connected by fully conjugated bridging chromophores, whereas no quenching is observed when the conjugation of the bridge is broken. We also observed that the decay of 3AuP is complex at temperatures below 110 K. In addition to the two previously published lifetimes on the order of some 10−100 μs, we have found a third lifetime on the nanosecond time scale.

Temperature and viscosity dependence of the triplet energy transfer process in porphyrin dimers.

The temperature and viscosity dependence of the triplet energy transfer (TET) process in porphyrin dimers has been studied. A zinc porphyrin (donor) and a free base porphyrin (acceptor) are covalently linked together by rigid bridging chromophores at a center-center distance of 25 A. Due to the large donor-acceptor distance and the weakness of the spin forbidden transitions involved, neither direct (through space) electron exchange nor Coulombic mechanisms are expected to contribute to the observed TET process. The results from transient absorption measurements at temperatures between room temperature and 80 K show that TET occurs with unexpectedly high efficiency in the systems connected by fully conjugated bridges and a pronounced temperature dependence of the process is observed. Comparison of the TET efficiencies in dimers connected by different bridging chromophores correlates well with a transfer reaction governed by a through bond exchange (superexchange) interaction. However, in high viscosity media the TET process is dramatically slowed down. This is attributed to a conformational gating of the TET process where the electronic coupling varies strongly with the relative orientation of the donor and the bridging chromophore. Further, the zinc porphyrin donor offers two distinct donor species, T(1A) and T(1B). At room temperature, the TET rate constant of the T(1A) species is about two orders of magnitude larger than for the T(1B) species. The dimers studied are well suited model systems for materials where the rate of the transfer reactions can be changed by external stimuli.

Diffusion parameters of B in Cu determined by β-radiation-detected NMR

Using \ensuremath{\beta}-radiation-detected nuclear magnetic resonance (\ensuremath{\beta}-NMR) in the special form of cross-relaxation spectroscopy we studied the diffusional behavior of implanted ${}^{12}\mathrm{B}$ nuclei in Cu single crystals at temperatures $T=115--750\mathrm{K}.$ An extended and completely revised dynamic theory of nuclear cross relaxation is developed to analyze the experimental data. Especially the incorporation of (hitherto neglected) spin-lattice relaxation effects turned out to be an important improvement. Applying this formalism we obtain activation energies of ${E}_{a}=0.57(5)\mathrm{eV}$ for the migration of interstitial B, the dominating fraction for $T\ensuremath{\lesssim}400\mathrm{K},$ and of ${E}_{a}=1.15(10)\mathrm{eV}$ for substitutional B formed at higher temperatures. The previously found direct-exchange mechanism for the diffusion of substitutional B in Cu is confirmed by our analysis.

Interaction of oxygen with oxides:: How to interpret measured effective rate constants?

Taking oxygen incorporation as an example into oxides, it is shown how effective interfacial rate constants measured under various experimental conditions have to be interpreted. The treatment in terms of chemical kinetics allows us to correlate them with microscopic rate constants and carrier concentrations. Close to equilibrium the situation is also considered from the standpoint of linear irreversible thermodynamics. After a brief discussion of bulk transport, interfacial steps are treated. As in the bulk different effective interfacial rate constants have to be considered depending on whether one performs a chemical experiment ( k ̄ δ , stoichiometric change), a tracer experiment ( k ̄ ∗ , change in the tracer distribution) or a (stationary) conductivity experiment ( k ̄ Q ). Besides the fact that the three rate constants may differ due to different experimental conditions (presence of electrodes etc.), they may be characterised by different mechanisms (different chemical resistances due to the different roles of the electrons) and are finally conceptually different (different chemical capacitance). Owing to the special role of the electrons, it proves worthwhile to distinguish between electron-rich and electron-poor compounds. As far as the first category is concerned, the mechanism, i.e. the rate determining step, is expected to be the same and the rate constants are then shown to scale in a manner analogous to the bulk diffusion coefficients ( k ̄ δ/w O = k ̄ * , and k ̄ *≃ k ̄ Q provided that the experimental conditions are comparable). This is no longer true in the case of an electron-poor material, in which mechanistic differences appear. In the case of free surfaces, ionisation steps are difficult in the chemical experiment, while in the tracer experiment this step can be by-passed by a direct tracer exchange mechanism ( k ̄ δ/w O ≠ k ̄ *) . A metal coating enhances the electron transfer rate for the chemical experiment; in addition anomalous surface Haven ratios are possible ( k ̄ Q≠ k ̄ *) . The treatment shows that the k̄s are determined by the product of the exchange rate of the rate determining step (inverse effective resistance) and the bulk chemical capacitance. The scaling term reduces to the inverse ionic defect concentration in the case of electron-rich materials. It is also shown that the latter result is valid even far from equilibrium for a variety of mechanisms (though not for all) while proximity to equilibrium must be assumed if changes in the electron concentration become important. Processes at internal interfaces are also considered with special emphasis on space charge effects. Finally, the fact is stressed that, similarly as in the conductivity experiment, in the case of tracer and chemical experiments flux constriction phenomena lead to effective surface resistances which could be erroneously interpreted as a proper blocking mechanism. The interfacial treatment is applied to experimental examples, in particular to acceptor-doped SrTiO 3.

Energy-transfer processes induced by exchange interactions

A study is made of energy-transfer processes by conventional direct exchange in condensed matter within the conceptual framework of the crystal model. The study is presented in two stages. As a first approximation, a simple model is obtained developed along an isotropic interaction approximation that depends exponentially on the distance of the donor and acceptor ions, and in which the angular factors are considered as averages. In that case, the statistical analysis of the crystal model leads to a formalism reminding one of the master equation obtained previously for electric dipole--electric dipole interactions, and showing the need for this analysis in order to know how many and which of the acceptor ions actually contribute to the rate of energy transfer. In a second stage, the problem is worked out using the approach originally proposed by Levy [Phys. Rev. 177, 509 (1969)] to calculate the matrix elements of the interaction, in which the bipolar system is referred to one of the centers. The formalism is completely general and rigorous, analyzing in detail the anisotropy factors of the conventional direct exchange mechanism for a pair of interacting ions, and their consequences on the global rates of energy-transfer processes. Work has been focused on the general derivation of the equations and on the analysis of its formal, rather than specific, calculations. However, for the sake of completeness, the ${\mathrm{LnCl}}_{3}$ and ${\mathrm{LnF}}_{3}$ systems are used as examples.

Direct exchange and interaction of 3d impurities on the (0 0 1) surface of iron

We present a detailed ab-initio study of direct exchange and interaction processes of 3d atoms on the Fe(0 0 1) surface. The calculations are based on local density functional theory and apply a KKR-Green's function method for impurities on surfaces. For practically all 3d transition metal impurities on Fe(0 0 1) we find a strong tendency for a direct exchange mechanism into the first surface layer. The early 3d impurities V, Cr and Mn strongly repel each other on neighbouring positions within the first layer, while Ni and Cu atoms show a moderate repulsion. The ab-initio results are in good agreement with STM studies for Cr/Fe(0 0 1) by the NIST group and present valuable predictions for all 3d/Fe(0 0 1) systems.

Energetics of3dImpurities on the (001) Surface of Iron

We present a detailed ab initio study of the alloying process in the dilute limit for $3d$ atoms on the Fe(001) surface. The calculations are based on local density functional theory and apply a KKR-Green's function method for impurities on surfaces. For practically all $3d$ transition metal impurities on Fe(001) we find a strong tendency for a direct exchange mechanism into the first surface layer. The early $3d$ impurities V, Cr, and Mn strongly repel each other on neighboring positions within the first layer, while Ni and Cu atoms show a moderate repulsion. The ab initio results are in good agreement with STM studies for Cr/Fe(001) and present valuable predictions for all $3d$/Fe(001) systems.

The discovery and acceptance of the Kirkendall Effect: The result of a short research career

In the 1940s, it was a common belief that atomic diffusion took place via a direct exchange or ring mechanism that indicated the equality of diffusion of binary elements in metals and alloys. However, Ernest Kirkendall first observed inequality in the diffusion of copper and zinc in interdiffusion between brass and copper. This article reports how Kirkendall discovered the effect, now known as the Kirkendall Effect, in his short research career.

Diffusion of Boron in Copper by Direct-Exchange Mechanism.

The behavior of B impurities implanted into Cu single crystals has been investigated by means of b radiation detected nuclear magnetic resonance and cross-relaxation spectroscopy. Diffusion of substitutional B (Bs) in Cu is observed in the temperature range of T ­ 600 750 K. By combining information from new and formerly published data it is shown that this diffusion is not mediated by any other defect; it rather takes place by a direct site exchange between Bs and neighboring Cu atoms. To our knowledge Bs in Cu is the first system for which this long discussed diffusion mechanism has been established experimentally. [S0031-9007(96)01798-X]