Showing papers on "Exchange interaction published in 1997"

Low switching field magnetoresistive tunneling junction for high density arrays

Abstract: A low switching field magnetoresistive tunneling junction memory cell including a first exchange coupled structure having a pair of magnetoresistive layers and an exchange interaction layer sandwiched therebetween so as to pin the magnetic vectors of the pair of layers anti-parallel, a second exchange coupled structure having a pair of magnetoresistive layers and an exchange interaction layer sandwiched therebetween so as to pin the magnetic vectors of the pair of layers anti-parallel, and electrically insulating material sandwiched between the first and second exchange coupled structures to form a magnetoresistive tunneling junction. Each of the first and second exchange coupled structures, and hence the memory cell, has no net magnetic moment.

Spin quantum beats of 2d excitons

Abstract: We report on spin quantum beats of excitonic kind in the time-resolved photoluminescence of quantum wells in a magnetic field. When this field is perpendicular to the growth direction, conditions for the manifestation of the electron or exciton spin precession in the circularly polarized components of the excitonic luminescence are obtained. These results lead to a direct measurement of the electron-hole exchange energy of the 2D exciton and give important insights into the exciton properties.

Spin and spectral signatures of polaron pairs in π-conjugated polymers

Abstract: We have studied polaron pair photoexcitations in a variety of {pi}-conjugated polymer films by photoinduced absorption (PA) and optically detected magnetic resonance (ODMR). Both luminescent and nonluminescent polymers have been studied. An exchange interaction between polarons broadens their magnetic resonance spectrum, permitting the unambiguous identification of the polaron pair species by ODMR. Both isotropic and anisotropic exchange interactions have been observed. An isotropic exchange interaction broadens the spin-(1)/(2) resonance associated with polarons, whereas an anisotropic exchange interaction gives rise to both full-field and half-field powder patterns. We have successfully modeled the observed spin-(1)/(2) ODMR spectra and estimate the strength of the isotropic exchange interaction I. An anisotropic exchange interaction has been observed in C{sub 60}-doped 2,5-dioctyloxy poly(p-phenylene vinylene) and cis-rich polyacetylene. Both polarons and polaron pairs are characterized by two absorption bands. However, the high-energy PA band of polaron pairs is blueshifted with respect to that of isolated polarons and the low-energy PA band of polaron pairs is relatively weak. We have also observed neutral soliton pair excitations in polymers with degenerate ground states. {copyright} {ital 1997} {ital The American Physical Society}

Magnetism and electronic structure of a local moment ferromagnet

Abstract: We propose a self-consistent approximate solution of the s - f model for describing the exchange coupling of a local moment system with a partially filled energy band. Induced electronic correlations account for the characteristic quasiparticle band effects which become manifest via striking temperature dependencies, band deformations and splittings. For weak s - f exchange interactions a `Stoner-like' spin splitting of the conduction band proportional to the f magnetization occurs. As soon as the coupling exceeds a critical value an additional spin splitting of the quasiparticle dispersion sets in, which is due to different elementary excitations. One of these appears as a repeated emission and reabsorption of a magnon by the conduction electron, resulting in an effective electron - magnon attraction. This gives rise to a polaron-like quasiparticle (a `magnetic polaron'). Other elementary processes are connected to magnon emission or absorption by the conduction electron (`scattering states'). The polarization of the conduction band due to the s - f exchange interaction J feeds back to the localized spin system leading to an indirect coupling between the spins. For weak s - f coupling the RKKY mechanism dominates , but with remarkable deviations for intermediate and strong couplings. The Curie temperature saturates with increasing J, where the saturation value is strongly dependent on the band occupation n. The oscillating behaviour of the effective exchange integral connecting the localized spins restricts ferromagnetism to special regions for n. The magnetization curve, the spin polarization of the itinerant electrons, and f - f as well as s - f spin correlation functions are worked out for a simple cubic lattice and discussed in terms of the band occupation n and the s - f exchange coupling J.

Roles of 4 f-5 d Exchange Interactions in Magnetic Circular X-Ray Dichroism at the Rare-Earth L 2,3 Edges

Abstract: The systematic trend in magnetic circular X-ray dichroism (MCXD) at the L 2,3 edges for various rare-earth elements is studied by taking into account the effects of the 4 f -5 d exchange interaction on the electric-dipole transition from the 2 p core state to the outermost 5 d state. The exchange interaction gives rise to, in addition to the usual polarization effect on the 5 d ground state, a novel effect of contraction in the radial part of the 5 d orbits concerned. It is pointed out that, especially the latter effect, which leads to the enhancement of the transition dipole matrix element depending on the spin and the orbital momenta of the 5 d state, plays a crucial role in successfully interpreting the experimental results observed. Quadrupole contributions just below the edges are also discussed.

Analysis of Exchange Interaction and Electron Delocalization as Intramolecular Determinants of Intermolecular Electron-Transfer Kinetics.

Abstract: During the past decades, spectroscopic characterization of exchange interactions and electron delocalization has developed into a powerful tool for the recognition of metal clusters in metalloproteins By contrast, the biological relevance of these interactions has received little attention thus far This paper presents a theoretical study in which this problem is addressed The rate constant for intermolecular electron-transfer reactions which are essential in many biological processes is investigated An expression is derived for the dependence of the rate constant for self-exchange on the delocalization degree of the mixed-valence species This result allows us to rationalize published kinetic data In the simplest case of electron transfer from an exchange-coupled binuclear mixed-valence donor to a diamagnetic acceptor, the rate constant is evaluated, taking into account spin factors and exchange energies in the initial and final state The theoretical analysis indicates that intramolecular spin-dependent electron delocalization (double exchange) and Heisenberg-Dirac-van Vleck (HDvV) exchange have an important impact on the rate constant for intermolecular electron transfer This correlation reveals a novel relationship between magnetochemistry and electrochemistry Contributions to the electron transfer from the ground and excited states of the exchange-coupled dimer have been evaluated For clusters in which these states have different degrees of delocalization, the excited-state contributions to electron transfer may become dominant at potentials which are less reductive than the potential at which the rate constant for the transfer from the ground state is maximum The rate constant shows a steep dependence on HDvV exchange, which suggests that an exchange-coupled cluster can act as a molecular switch for exchange-controlled electron gating The relevance of this result is discussed in the context of substrate specificity of electron-transfer reactions in biology Our theoretical analysis points toward a possible biological role of the spin-state variability in iron-sulfur clusters depending on cluster environment

Ground state of a spin system with two- and four-spin exchange interactions on the triangular lattice

Abstract: We study a spin system with both two- and four-spin exchange interactions on the triangular lattice as a possible model for the nuclear magnetism of solid 3He layers adsorbed on grafoil. The ground state is analyzed by the use of the mean-field approximation. It is shown that the four-sublattice state is favored by introduction of the fourspin exchange interaction. A possible phase transition at a finite temperature into a phase with the scalar chirality is predicted. Application of a magnetic field is shown to cause a variety of phase transitions.

Magnetic properties of oxygen physisorbed in Cu-trans-1,4-cyclohexanedicarboxylic acid

Abstract: Magnetic properties of oxygen physisorbed in Cu-trans-1,4-cyclohexanedicarboxylic acid having one-dimensional micropores are studied. O2 is well known as a magnetic molecule with S=1. In the case of the low adsorption, temperature dependence of the susceptibility shows the Schottky-type broad peak and the high-field magnetization process has a step at around 34T with saturation moment of 2 μ B/O2. These results are interpreted by the dimer model of S=½ with an antiferromagnetic exchange interaction. Broadening of the step of magnetization with increasing the adsorption may be due to the inter-dimer interaction or the distribution of the intra-dimer exchange interaction. Linear magnetization process which is characteristic of one-dimensional Heisenberg antiferromagnet has not been observed even in the full pore region.

Magnetism of a Two-Dimensional Weak-Ferromagnetic Organic Radical Crystal, 1,3,5-Triphenyl-6-Oxoverdazyl

Abstract: A genuine organic radical crystal 1,3,5-triphenyl-6-oxoverdazyl (TOV) has been revealed to be an S =1/2 two-dimensional (2D) weak-ferromagnetic Heisenberg antiferromagnet with the exchange interaction 2 J / k B =-9.0 K by the experiments of magnetic susceptibility, heat capacity, magnetization and electron paramagnetic resonance. The anomalous temperature dependence of the weak ferromagnetism at low temperatures is qualitatively explained by a four-sublattice model with two kinds of Dzyaloshinsky-Moriya (D-M) vectors of opposite sign on the a c -plane. The staggered susceptibility, which is a fictitious but intrinsic quantity for antiferromagnets, is estimated in TOV by making use of the canted weak ferromagnetic moments which dominate below 6 K. Especially the rapid growth of the magnetic susceptibility below 6 K is explained by the staggered susceptibility which makes the crossover effect from 2D Heisenberg to 2D Ising system, triggered by above the D-M anisotropy.

p-d exchange interaction for 3d transition-metal impurities in II-VI semiconductors

Abstract: We have investigated the exchange interaction between the localized $d$ electrons of $3d$ transition-metal impurities and the delocalized host band electrons in II-VI semiconductors based on the configuration-interaction scheme. The exchange constant $N\ensuremath{\beta}$ in the Kondo-type effective Hamiltonian is evaluated from Ti to Ni. For Mn, Fe, and Co, $N\ensuremath{\beta}$ is negative (antiferromagnetic), and is in good agreement with experimental values. The exchange coupling of the Ti impurity is predicted to be positive (ferromagnetic) due to Hund's coupling. For V, Cr, and Ni, in which orbital degrees of freedom is important, the sign of $N\ensuremath{\beta}$ depends on the Jahn-Teller distortion.

Pair Correlations and Exchange Phenomena in the Free Electron Gas

Abstract: We present a rigorous derivation of the formulae of Dirac-Bloch and Wigner-Seitz for the quantum mechanical exchange energy and the ‘exchange hole’ of the free electron gas. More precisely we establish that for arbitrary determinantal ground states of the underlying finite system of N free electrons in a box, subject to periodic or zero boundary conditions, the formulae are accurate to order \(N^{-1/3}\) (per electron) and \(N^{-1/2}\) respectively.

First-principles calculation of the exchange interaction and the exchange force between magnetic fe films

Abstract: First-principles calculations have been performed for the exchange interaction and exchange force between two magnetic Fe(100) thin films. It is shown that the magnitude of the calculated exchange force is of the order of $10^{-9} $N at $d/a < 1$ and of the order of $10^{-10} $N at $1 < d/a < 1.5$, where $d$ denotes the distance between surfaces of the two thin films and $a$ the lattice constant of bulk Fe. The obtained forces are sufficiently larger than the sensitivity of the current atomic force microscopy of $10^{-12} - 10^{-13}$N, which suggests the feasibility of the exchange force microscopy.

Exact results for the optical absorption of strongly correlated electrons in a half-filled Peierls-distorted chain

Abstract: In this, the second of three articles on the optical absorption of electrons in a half-filled Peierls-distorted chain, we present exact results for strongly correlated tight-binding electrons. In the limit of a strong on-site interaction U, we map the Hubbard model onto the Harris-Lange model which can be solved exactly in one dimension in terms of spinless fermions for the charge excitations. The exact solution allows for an interpretation of the charge dynamics in terms of parallel Hubbard bands with a free-electron dispersion of bandwidth W, separated by the Hubbard interaction U. The spin degrees of freedom enter the expressions for the optical absorption only via a momentum-dependent but static ground-state expectation value. The remaining spin problem can be traced out exactly since the eigenstates of the Harris-Lange model are spin degenerate. This corresponds to the Hubbard model at temperatures large compared with the spin exchange energy. Explicit results are given for the optical absor...

Slave-boson mean-field theory of the antiferromagnetic state in the doubly degenerate Hubbard model: The half-filled case

Abstract: The antiferromagnetic ground state of the half-filled Hubbard model with the doubly degenerate orbital has been studied by using the slave-boson mean-field theory, which was previously proposed by the present author. Numerical calculations for the simple cubic model have shown that the metal-insulator transition does not take place except at the vanishing interaction point, in strong contrast with its paramagnetic solution. The energy gap in the density of states of the antiferromagnetic insulator is much reduced by the effect of electron correlation. The exchange interaction $J$ plays an important role in the antiferromagnetism: although for $J=0$ the sublattice magnetic moment $m$ in our theory is fairly smaller than ${m}_{\mathrm{HFA}}$ obtained in the Hartree-Fock approximation, $m$ for $J/Ug0.2$ ($U$ is the Coulomb interaction) is increased to become comparable to ${m}_{\mathrm{HFA}}.$ Surprisingly, the antiferromagnetic state is easily destroyed if a small, negative exchange interaction $(J/Ul\ensuremath{-}0.05)$ is introduced.

Magnetic properties and crystal structures of 1,5-diphenylverdazyls with electron acceptor groups in the 3-position

Abstract: The magnetic properties of four diphenylverdazyls (2−5) with electron acceptor groups in the 3-position were studied by measuring the magnetic susceptibility and heat capacity. The crystal structures of 3 and 4 were determined, and for 2 the crystal structure analysis led to an improved refinement. Verdazyl 2 was found to be an antiferromagnet with a Neel temperature of TN = 1.16 ± 0.04 K. Above the transition temperature TN, 2 behaves as a quasi-one-dimensional Heisenberg ferromagnet with the intrachain exchange interaction of 2J/kB = +7.0 ± 1.0 K. The interchain exchange interaction zJ‘/kB was estimated to be −1.5 ± 0.2 K, where z is the number of interchain bonds per spin. On the other hand, 3 and 5 behave as 1D nonalternating and alternating Heisenberg antiferromagnets with an exchange interaction of 2J/kB = −11.6 K and 2J1/kB = −113 K (alternation parameter α = J2/J1 = 0.4), respectively. The susceptibility of 4 follows the Curie−Weiss law with a negative Weiss constant of −18 K above 100 K. Below 10...

2H NMR Theory of Transition Metal Dihydrides: Coherent and Incoherent Quantum Dynamics

Abstract: In this paper a simple phenomenological description of the effects of coherent quantum and incoherent mutual exchange of two deuteron nuclei in solid state transition metal complexes on their 2 H NMR spectra is given. This description is based on the quantum-mechanical density matrix formalism developed by Alexander and Binsch. Only the nuclear spin system is treated quantum mechanically. The quantum exchange interaction in NMR is included in the nuclear spin Hamiltonian, and the interaction with the surrounding bath and incoherent exchange processes are treated as phenomenological rate processes described by rate constants. The incoherent exchange corresponds formally to 180° rotations or jumps of the D-D vector around an axis perpendicular to this vector and averages the different quadrupole splitting of the two deuterons. In principle the dideuteron pair will exist in several rovibrational states. However, if the interconversion among these states is fast, the dideuteron exchange can be described by an average exchange coupling or tunnel frequency X12 and a single average rate constant k12 of the incoherent exchange. It is shown that the incoherent exchange gives rise to a relaxation of rate -2k12 between coherences created between states of different symmetry. The 2 H NMR line shape of a dideuteron pair in the solid state as a function of tunnel and incoherent exchange rate is studied numerically. For single crystals, the effects of coherent and incoherent exchange are strongly different, in particular if the rate constants are on the order of the quadrupole splitting. The spectra of nonoriented powder samples are more similar to each other. Nevertheless, our calculations show that there are still pronounced differences, which should allow the distinction between coherent and incoherent exchange even in nonoriented samples.

Magnetic order and chemical bonding in the high-TCmolecule-based cyanide magnetsCsM[Cr(CN)6](M=Mn,Ni) from first principles

Abstract: We present the results of first-principles electronic-structure calculations for the molecule-based cyanide high-Curie-temperature magnetic insulators $\mathrm{Cs}M[{\mathrm{Cr}(\mathrm{CN})}_{6}],$ $M=\mathrm{Mn},$ Ni. The calculations are based on density-functional theory and the local-density approximation and employ the augmented-spherical-waves method. The electronic properties are found to be dominated by transition-metal $3d$ and ligand $2p$ orbitals. The chemical bonding is analyzed via an ab initio calculation of the crystal orbital overlap population (COOP). The magnetic properties are explained by the orthogonality/overlap of magnetically active orbitals as studied by the COOP. Possible pathways for the exchange interaction are identified. In particular the sign of the exchange integrals between different effective magnetic sites is deduced. Excellent agreement with experimental data is found. While proving essential for the consistent understanding of electronic, magnetic, and chemical properties of these materials our first-principles approach moreover allows for the prediction as well as the deeper understanding of the type of long-range magnetic order. Thereby it goes beyond the conclusions usually drawn from Goodenough-Kanamori superexchange rules or the model by Kahn and Briat.

Electron-spin relaxation in p-type quantum wells via the electron-hole exchange interaction:The effects of the valence-band spin mixing and of an applied longitudinal electric field

Abstract: The spin-relaxation time of the photogenerated electrons in p-doped quantum wells of GaAs is calculated. The spin-flip mechanism investigated is due to the electron-hole scattering by the exchange interaction. We have shown that the inclusion of the spin mixing of the valence holes has a dual effect. On one hand, it increases the spin-flip scattering rate by enlarging the hole density of states. On the other hand, the exchange strength weakens due to the valence spin mixing. The combined effect is a partial compensation, leaving the spin-relaxation times of the electrons similar to those obtained assuming holes having pure-spin states. We have also investigated the changes on the spin relaxation brought about by an applied electric field along the quantum well growth direction, and we have calculated longer spin relaxation as the field strength increases. Such behavior is characteristic of the exchange spin-flip channel and, if experimentally observed, it will indicate the dominant role played by the type of spin relaxation we have studied in this work.

Multilevel interaction between layers in layered film structures

Abstract: An analysis was carried out of the mechanism underlying magnetic interlayer interaction in film structures. The investigation was based on the assumption that interlayer bonding affects film hysteresis. This was based on experimental data on the coercive force, the domain structure parameters, and the microstructure of Fe19Ni81/Cr/Fe19Ni81 and Fe15Co20Ni65/Cr/Fe15Co20Ni65 films. Theoretical estimates show that, as the thickness of the Cr interlayer increases, the exchange interaction between the ferromagnetic layers can be replaced by the magnetostatic interaction whose effectiveness is determined by surface irregularities and layer ‘magnetization ripples’.

Ground-State Properties of Alternating-Spin XXZ Chains

Abstract: Ground-state properties of ferrimagnetic XXZ chains with alternating spins S A and S B with nearest-neighbor exchange interaction are discussed based upon the results for finite-size systems obtained mainly by the exact diagonalization method and partly by the density matrix renormalization group method. For -1 1, the ground state is doubly degenerate Neel long-range ordered state with a finite energy gap.

Pressure effect on the Curie temperature of CrBr3

Abstract: The pressure dependence of TC of anhydrous CrBr3 was measured with a clamped microbomb. dTC/dP was obtained to be −0.2 K/kbar. This negative sign is discussed in terms of distance dependence of exchange interaction between Cr3+.

Solitary waves in spin chains with biquadratic exchange and dipole-dipole interactions

Abstract: Dynamical theory of solitary wave excitations in spin chains has been studied by a revised Hamiltonian in which the dipole-dipole and biquadratic exchange interactions are taken into account in addition to the Zeeman energy, uniaxial anisotropy and the exchange energy Using the coherent states method combined with the Holstein-Primakoff bosonic representation of the spin operators, we have derived a nonlinear Schrodinger (NLS)-like equation Several analytical solitary wave solutions of NLS-type equation have been presented in detail, with discussion of some of their implications for describing the propagation of spin waves Some of the characteristics of these solitary waves, like their energy and the number of bosonic excitations involved in the solitary wave formation, have also been calculated

Effect of conduction-electron polarization on the magnetism of hcp samarium metal

Abstract: Peculiar magnetic properties of hcp Sm are adequately described by the calculations based on the model with Sm{sup 3+} ions in a metallic matrix, taking into account the lowest three multiplets, the spin-orbit interaction, the crystal fields, and the exchange interaction, and together with the conduction-electron spin polarization. It is found that the effect of the conduction-electron polarization is essential to them. By the analyses, it is concluded that the conduction-electron moment is larger than the localized 4f one and that the polarity of the spin part of the magnetic moment enhanced by the conduction-electron polarization is positive to the total moment. This polarity of the spin moment, which is opposite to the case for the free Sm{sup 3+} ion, is furthermore experimentally verified by the magnetic measurements on the hcp SmNd and SmGd alloys. It is also elucidated through the calculations that the difference between the temperature dependence of the spin moment and that of the orbital one due to the admixture of the J multiplets, in general, results in a variety of temperature dependence of the ordered moment for each Sm{sup 3+} ion, just like the thermomagnetic behavior of ferrimagnets. {copyright} {ital 1997} {ital The American Physical Society}