Antiparallel Spin Configuration Research Articles

Modeling spin interactions in carbon peapods using a hybrid density functional theory

The $\text{Sc}@{\text{C}}_{82}$ endohedral fullerenes inside a single-wall semiconducting or metallic nanotube form a well-defined chain of antiferromagnetically coupled spins. Using hybrid density functional theory (DFT), we find that the spin resides mainly on the fullerene cage, whether or not the fullerenes are in a nanotube. The spin interactions decay exponentially with fullerene separation and the system can be described by a simple antiferromagnetic Heisenberg spin chain. Energy parameters for a generalized Hubbard-Anderson model are deduced from the DFT calculations and yield a second-order Heisenberg exchange energy, which is in good agreement with total-energy calculations for parallel and antiparallel spin configurations. Within the accuracy of the calculations, neither semiconducting nor metallic nanotubes affect the interactions between the fullerene electron spins.

Electronic structure of GaN codoped with Mn and Cr

We have studied the spin-polarized electronic structure of GaN codoped with transition metal (TM) atoms Mn and Cr, with different possible geometrical sites for the dopants, to determine the most favored geometry. The variations in the band structure, density of states, and TM site magnetic moment are analyzed for these geometries. Parallel and antiparallel spin configurations are energetically compared for all the geometries and it is observed that the parallel spin configuration with near neighbor substitution of TM atoms is the most favorable. Our results indicate that Ga(MnCr)N is a prospective candidate as a spintronics material to work at room temperature.

Intermediate resonance excitation in the [formula omitted] reaction

The helicity dependence of the total cross section for the γ→p→→pπ0π0 reaction has been measured for the first time at incident photon energies from 400 to 800 MeV. The measurement, performed at the tagged photon beam facility of the MAMI accelerator in Mainz, used the large acceptance detector DAPHNE and a longitudinally polarized frozen-spin target. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, most likely due to the intermediate production of the D13(1520) resonance. However, the contribution of the antiparallel spin configuration, arising from other reaction mechanisms, is also not negligible. This result gives important new information to resolve the existing model discrepancies in the identification of the nucleon resonances contributing to this channel.

First-principles investigation of spin-polarized conductance in atomic carbon wires

We analyze spin-dependent energetics and conductance for one-dimensional (1D) atomic carbon wires consisting of terminal magnetic (Co) and interior nonmagnetic (C) atoms sandwiched between gold electrodes, obtained by employing first-principles gradient-corrected density functional theory and Landauer's formalism for conductance. Wires containing an even number of carbon atoms are found to be acetylenic with $\ensuremath{\sigma}\text{\ensuremath{-}}\ensuremath{\pi}$ bonding patterns, while cumulene structures are seen in wires containing an odd number of carbon atoms as a result of strong $\ensuremath{\pi}$ conjugation. In the case of even C-atom wires, the antiparallel Co spin state remains the ground state up to 12 carbon atoms. For odd C-wire systems, the antiparallel spin configuration between the two terminal Co atoms remains the ground state irrespective of the number of C atoms in the wire. The 14-carbon atom wire is seen to have a parallel Co spin configuration in the ground state. The stability of the antiferromagnetic state in the wires is ascribed to the superexchange effect. For the cumulenic wires, this effect is constant for all wire lengths. For the acetylenic wires, the superexchange effect diminishes as the wire length increases, with the exception of the wire containing two carbon atoms. The superexchange characteristic length in acetylenic C wires is found to be $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Conductance calculations at the zero-bias limit show spin-valve behavior, the parallel Co spin-configuration state giving higher conductance than the corresponding antiparallel state, and a nonmonotonic variation of conductance with the length of the wires for both spin configurations.

Structural defects in Sr2FeMoO6 double perovskite: Experimental versus theoretical approach

The lower than expected magnetization of imperfect Sr2FeMoO6 (SFMO) double perovskites is usually attributed to the presence of Fe at antisite positions that would be antiferromagnetically coupled to their regular neighbors. However, ab initio calculations suggest strongly that such defective Fe sites would be ferromagnetically coupled and, consequently, the magnetization reduction would originate from other kinds of defects. The magnetic, hyperfine, and structural properties of SFMO perovskites prepared by solid-state reaction under a variety of conditions are reported and correlated with ab initio calculations of the magnetic moments and hyperfine fields of Mo and Fe ions in different local environments (antisites, antisite neighbors, and neighbors of an oxygen vacancy). When plotted against the order parameter the experimental magnetization is found to decrease at a rate of about −7.6μB per Mo–Fe antisite pair as in other previous experiments, where the theoretical calculation predicts −6.56μB per antisite pair if the moments of Fe antisites are antiparallel to the regular Fe moments. Unfortunately, the energy of this configuration is found to be 0.7eV higher than that of the parallel configuration for which the magnetization reduction is only −0.19μB per antisite pair. Sources for the supplementary reduction of magnetization have then been considered. The presence of spurious phases cannot account for the observation. Oxygen vacancies do reduce significantly the magnetization (−2.00μB∕vacancy), but no significant sign of their effect is found in the Fe Mössbauer and Mo nuclear magnetic resonance spectra. Moreover, the position of the spectral lines of defects are compatible with the theoretical findings for Fe antisites in the antiparallel spin configuration.

Magnetism of Co<tex>$_1 - rm x$</tex>Fe<tex>$_rm x$</tex>-NOL in Specular Spin-Valves

We investigated the magnetic properties of the Co/sub 1-x/Fe/sub x/-natural oxidized nano-oxide layer (NOL) in the specular spin-valve system by precise measurement of magnetization at 77 K and M-T curves after field cooling at /spl plusmn/5 kOe. The result suggests the antiferromagnetic component behavior of the NOL and its Ne/spl acute/el temperature (T/sub N/) shifts to the higher temperature with increasing Fe composition. Below the T/sub N/, the antiparallel spin configuration at the pinned layers is stabilized by the exchange bias field induced from antiferromagnetic component of NOL by the field cooling in negative field direction.

Oscillatory spin-polarized conductance in carbon atom wires

Zero temperature spin-polarized transport in atomic wires consisting of magnetic (Co) and nonmagnetic (C) atoms sandwiched between gold electrodes is investigated using gradient-corrected density functional theory and Landauer's formalism. Our calculation shows a spin valve behavior with the parallel magnetization state between the two Co atoms giving higher conductance than the respective antiparallel magnetization state and a nonmonotonic variation of magnetoconductance with wire length. We term the more conductive parallel magnetization state the on state and the antiparallel magnetization state the off state. The ground state of wires containing up to five carbon atoms has antiparallel (off) spin configurations between the Co. The additional stability of the antiferromagnetic state in wires containing an even number of carbon atoms is ascribed to an enhanced superexchange mechanism facilitated by \ensuremath{\sigma}-\ensuremath{\pi}-conjugation present in the systems.

Antiferromagnetism in the exact ground state of the half-filled Hubbard model on the complete bipartite graph

As a prototype model of antiferromagnetism, we propose a repulsive Hubbard Hamiltonian defined on a graph $\L={\cal A}\cup{\cal B}$ with ${\cal A}\cap {\cal B}=\emptyset$ and bonds connecting any element of ${\cal A}$ with all the elements of ${\cal B}$. Since all the hopping matrix elements associated with each bond are equal, the model is invariant under an arbitrary permutation of the ${\cal A}$-sites and/or of the ${\cal B}$-sites. This is the Hubbard model defined on the so called $(N_{A},N_{B})$-complete-bipartite graph, $N_{A}$ ($N_{B}$) being the number of elements in ${\cal A}$ (${\cal B}$). In this paper we analytically find the {\it exact} ground state for $N_{A}=N_{B}=N$ at half filling for any $N$; the repulsion has a maximum at a critical $N$-dependent value of the on-site Hubbard $U$. The wave function and the energy of the unique, singlet ground state assume a particularly elegant form for $N \ra \inf$. We also calculate the spin-spin correlation function and show that the ground state exhibits an antiferromagnetic order for any non-zero $U$ even in the thermodynamic limit. We are aware of no previous explicit analytic example of an antiferromagnetic ground state in a Hubbard-like model of itinerant electrons. The kinetic term induces non-trivial correlations among the particles and an antiparallel spin configuration in the two sublattices comes to be energetically favoured at zero Temperature. On the other hand, if the thermodynamic limit is taken and then zero Temperature is approached, a paramagnetic behavior results. The thermodynamic limit does not commute with the zero-Temperature limit, and this fact can be made explicit by the analytic solutions.

Distribution of Eigenvalues of Non-Hermitian Random XXZ Model

To consider interaction effects in a directed random system, we have studied a directed random XXZ model numerically. The exact diagonalizaton method is utilized to obtain distribution of complex eigenvalues. Starting from a strong non-Hermiticity limit with J z = 0, where the system becomes a non-interacting one-way model, inclusion of antiferromagnetic J z makes eigenvalues distributed on many concentric circles grouped by the number of neighboring anti-parallel spin configurations in an Ising limit. In this transition, an Ising gap opens as a result of continual recombination of spectral flows into smaller circles in the complex plane.

Para- and Ortho-Trions on a Ring: A Simple Model

Motivated by recent experiments on luminescence from semiconductor nanorings, we study theoretically the quantized states of a charged exciton (trion) in a ring geometry. In particular, we compare the energies of the states with antiparallel (para-trion) and parallel (ortho-trion) spin configurations of two electrons, that, together with a hole, constitute a trion. In order to assess the experimentally relevant domain of parameters, when the size quantization energy is comparable to the exciton binding energy, we adopt a simple model of an infinitely narrow ring and short-range electron–electron and electron–hole interactions.

Influence of spin-dependent scattering at rough interface on the giant magnetoresistance in magnetic multilayers

Spin-dependent scattering in Co/Cu and Co/Ru multilayers with interface roughness is estimated. The electronic band structures of Co/Cu and Co/Ru multilayers with smooth interfaces for parallel and antiparallel spin configurations are calculated self-consistently using linear muffin-tin orbital atomic sphere approximation (LMTO-ASA) band structure calculations based on density functional theory. The spin-dependent scattering caused by interface roughness is calculated from the transition probabilities obtained from self-consistent wave functions and potential differences found from these band structure calculations. Our results indicate that spin-dependent scattering caused by interface roughness plays an important role in the giant magnetoresistance (GMR) effect for magnetic multilayers, as predicted by calculations using a simple model. Furthermore, it is concluded that the scattering depends on differences in the band structure for different spin configurations, as well as for interface roughness. The GMR difference between Co/Cu and Co/Ru can be explained qualitatively by difference in spin-dependent scattering due to interface roughness, which is caused by the band structure with different spin configuration.

Measurement of exchange effects in (e,2e) collisions on lithium

We report on our triple differential electron-atom ionization experiment with resolution of the spin degree of freedom. We use polarized electrons and polarized, low-Z atoms and obtain asymmetries for antiparallel and parallel spin configurations which can either relate to singlet versus triplet scattering or to direct and exchange processes. The spin asymmetry we measure is sensitive to exchange effects between the projectile electron and the active atomic electron. We present: 1) Results for an angular distribution at 54.4 eV incident energy with fixed ratio of shared energies (E A /E B = 2.4) which we compare with a DWBA and with the CCC theory. 2) Results for an energy sharing spectrum at 25.4 eV with fixed angles of escape, θ A = -θ B = 45°, which we compare with a DS3C calculation. Agreement between theory and experiment is in both cases good.

Magnetic properties of Fe/Cu(001) superlattices

The electronic and magnetic structure of Fe/Cu(001) superlattices are investigated with the first-principles all-electron linearized augmented plane wave method in the local spin-density functional approximation. The results show that the magnetic moment of the interface Fe layer is stabilized at the high spin state of the fcc Fe crystals. When the thickness of Fe layers is larger than two monolayers, the interior layers always exhibit an antiparallel spin configuration in its lowest energy phases.

Temperature dependence of the hyperfine parameters of maghemite and al-substituted maghemites

Synthetic aluminum-substituted maghemite samples, γ-(AlyFe1-y)2O3 with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mossbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: δA=δB-0.12 mm/s. From the temperature dependence of δB it was possible to determine the characteristic Mossbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing Al content, at least up to 10 mole%. The temperature dependence of the A-and B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: JAB=-25 K; JAA=-18 K and JBB= -3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the constancy of this quantity for the B sites, suggest that the Al cations occupy the B sites.