Showing papers on "Exchange interaction published in 1987"
TL;DR: In this paper, the exchange parameters of the classical Heisenberg model applied to crystals are obtained using a local spin density functional (LSDF) approach and KKR-Green functions formalism.
1,353 citations
TL;DR: In this article, the authors show that the exchange field of a thin exchange coupled ferromagnetic film reaches a limiting value no matter how large the exchange coupling is, due to domain wall formation in the antiferromagnet.
Abstract: The exchange field Hex transferred from a thick antiferromagnetic substrate to a thin exchange coupled ferromagnetic film is shown to reach a limiting value no matter how large the exchange coupling is. The limit is due to domain‐wall formation in the antiferromagnet. Numerical results based on a simple model for the interface are presented and compared to experimental results.
740 citations
TL;DR: In this paper, ground-state properties for the one-dimensional isotropic spin-1 Heisenberg magnet with antiferromagnetic nearest-neighbor (nn) and next-nearest-NEighbor interactions are studied.
Abstract: Ground-state properties are studied for the one-dimensional isotropic spin-1 Heisenberg magnet with antiferromagnetic nearest-neighbor (nn) and next-nearest-neighbor (nnn) interactions The energy, the singlet-triplet energy gap, the two-spin correlation function, and the den Nijs and Rommelse string correlation function in the ground states of finite-size systems of up to 16 spins are calculated exactly By extrapolating these results, the ground-state properties at the infinite-size limit are discussed It is found, in particular, that as the ratio of the nnn interaction constant to the nn one increases, the string order parameter (the limiting value of the longest-distance string correlation function) as well as the Haldane gap (the limiting value of the singlet-triplet energy gap) divided by the sum of both interaction constants increases This result implies that the competition between the antiferromagnetic nn and nnn interactions stabilizes the Haldane phase
182 citations
TL;DR: In this article, an in situ Kerr rotation measurement is used for the study of antiferromagnetic (AF) ferromagnetic film couples and it is shown that the magnetic anisotropy of MnFe can be estimated from the observed critical thickness to be ∼1.35×105 erg/cm3.
Abstract: We show that an in situ Kerr rotation measurement is a very effective technique for the study of antiferromagnetic (AF) ferromagnetic (F) film couples. Magnetic signals can be obtained even in the case where the (AF) is the top layer up to at least 200 A of AF thickness. We have used this in situ approach combined with ion milling to study the thickness dependence of the magnetic properties of Mn50Fe50/Ni80Fe20 systems. We observe that the exchange bias field has a surprisingly sharp onset at a critical thickness of AF∼50 A. We show that this is consistent with a simple model and that the magnetic anisotropy of MnFe can be estimated from the observed critical thickness to be ∼1.35×105 erg/cm3. The exchange field showed the predicted proportionality to the inverse of the F thickness from ∼50 to 400 A. Auger spectroscopy and spin polarized secondary electron emission have been used to rule out gross artifacts due to ion milling.
153 citations
TL;DR: In this article, a generalized Hubbard model is proposed to account for the unusual ferromagnetic exchange interactions in this system and the results are consistent with a crossover from a dominance of one-dimensional magnetization interaction to a 3D mean-field-like interaction at Tl4.8 K.
Abstract: The temperature and magnetic field dependence of the magnetization and susceptibility of single-crystal decamethylferrocenium tetracyanoethenide (DMeFc TCNE) demonstrate that this material is the first molecular compound with a ferromagnetic ground state. A spontaneous magnetization is observed for Tl4.8 K. The results are consistent with a crossover from a dominance of one-dimensional ferromagnetic exchange interaction to a 3D mean-field-like interaction at \ensuremath{\sim}16 K. The critical exponents are in accord with mean-field behavior. A generalized Hubbard model is proposed to account for the unusual ferromagnetic exchange interactions in this system.
138 citations
TL;DR: In this article, the effect of exchange interaction between bound and continuum electrons was examined for the specific case of the methane molecule in order to study the effect on total cross sections of various local approximations for the exchange interaction.
Abstract: The interaction of slow electrons (collision energies up to about 20 eV) with polyatomic targets is examined for the specific case of the methane molecule in order to study the effect on total cross sections of various local approximations for the exchange interaction between bound and continuum electrons. It is found that the tail of such an interaction, outside the 'hard-core' region of the static potential, plays an important role at very low collision energies and ultimately controls the correct appearance of the experimentally found Ramsauer-Townsend (RT) minimum in the integral elastic cross section. Very good agreement between computed and measured values for the cross sections is found when using the modified semiclassical exchange approximation (MSCE) proposed. The broad resonance, which appears at experimental energies of around 8 eV, is also satisfactorily reproduced by the present methods.
108 citations
TL;DR: In this article, a relativistically extended version of the one-particle formalism is used to calculate the electronic structure of nickel, iron, and palladium metal, and the band structure of iron is discussed in detail.
Abstract: The authors use a relativistically extended version of the one-particle formalism. The electron-electron interaction is assumed to be non-relativistic Coulombic as before. As a consequence, the one-particle Dirac-type equations which now stand in place of the formerly employed Kohn-Sham type equations, contain a diagonal 4*4 matrix whose elements represent the various potential contributions including spin-dependent local potentials describing exchange and correlation as in the non-relativistic case. If asphericity effects of the resulting potential are negligible, these Dirac equations can exactly be reduced to relativistic two-component Pauli-type equations with a diagonal exchange-correlation matrix and a spin-orbit coupling matrix. The latter equations are used to self-consistently calculate the electronic structure of nickel, iron and palladium metal. In addition, the calculation provides magnetic anisotropy energies. The band structure of iron is discussed in detail and compared with other theoretical studies based on different band theoretical methods. The calculations on Pd metal lead without spin-orbit coupling to ferromagnetic spin alignment when the metal is expanded by approximately 5% of the lattice constant. If spin-orbit coupling is included in the self-consistent calculations, the alignment disappears. This effect can only be counteracted by further expanding the lattice up to approximately 10%.
88 citations
TL;DR: In this article, double exchange is generalized to mixed valence (MV) trinuclear clusters and the results obtained by quantum-theoretical calculations of exchange and resonance splittings are compared to the results of the model theory which takes account of Heisenberg and double exchange.
Abstract: The theory of double exchange is generalized to mixed valence (MV) trinuclear clusters. The results obtained by quantum-theoretical calculations of exchange and resonance splittings are compared to the results of the model theory which takes account of Heisenberg and double exchange. The comparison demonstrates unambiguously the coincidence of transfer parameters and the model double exchange parameters. The double exchange parameters of trinuclear MV clusters depend on total and intermediate spins as well as individual ion spin. The common effect of electron (or hole) migration and interion exchange interaction on the magnetic properties of dn -dn -d n+1 (n = 0, 1, 2, 3, 4, 5) and dn -dn -d n-1 (n = 1, 2, 3, 4) MV trimers is considered. Heisenberg and double exchange form exchange-resonance multiplets which are characterized by irreducible representations Γ of D 3h symmetry group of MV clusters. Double exchange splits Heisenberg multiplets but does not mix the states of different S. Level correlation dia...
85 citations
TL;DR: In this article, the authors investigated the properties of the helical spin density wave (HSDW) due to the antisymmetric and symmetric exchange interactions and found that the spin wave spectra for the two types of HSDW are strongly different from each other, especially in the magnetic field dependences.
Abstract: In order to elucidate the different behaviours of the helical spin density waves (HSDW) due to the antisymmetric and symmetric exchange interactions, spin waves in the conical spin density wave (CSDW) states and the induced ferromagnetic states (IFMS) under external magnetic fields are calculated by use of phenomenological Hamiltonians with and without the antisymmetric exchange interaction. It is found that the spin wave spectra for the two types of HSDW are strongly different from each other, especially in the magnetic field dependences, enough to be able to distinguish them. The ESR frequencies are also investigated theoretically for bothtypes. In the light of these results, the observed magnetic field dependencies of the ESR frequencies in MnSi and Fe 1- x Co x Si are discussed to prove the HSDW due to the antisymmetric exchange interaction in these compounds.
82 citations
TL;DR: The magnetic properties of RCo4B compounds (R = Y, Pr, Nd, Gd and Er) were studied in the temperature range 4.2-1000 K as discussed by the authors.
80 citations
TL;DR: In this article, the one-dimensional spin-1/2 Heisenberg antiferromagnet with next-nearest-neighbor exchange interaction and couples to the lattice distortion is investigated.
Abstract: The one-dimensional spin-1/2 Heisenberg antiferromagnet which has next-nearest-, as well as nearest-neighbor exchange interaction and couples to the lattice distortion is investigated. The next-nearest-neighbor interaction is shown to result from the non-adiabaticity of the lattice distortion or the itineracy of electrons. The groundstate of this system is studied by the renormalization group method. In the absence of the coupling to the lattice distortion, the phase diagram consists of three states, i.e., the spin-fluid, the dimer and the Neel state, in essential agreement with Haldane, while in its presence, there are two states, the spin-Peierls and the Neel state. The energy gain due to the lattice distortion is estimated by the self-consistent harmonic approximation.
TL;DR: Self-consistent phonon and Monte Carlo calculations show that the three-body exchange interaction is important in dense helium and stabilizes the hcp structure for pressures greater than about 60 GPa.
Abstract: Self-consistent phonon and Monte Carlo calculations show that the three-body exchange interaction is important in dense helium. Together with a realistic pair potential and the Axilrod-Teller three-body dispersion interaction, it brings calculations into agreement with the experimental equation of state. This interaction stabilizes the hcp structure for pressures greater than about 60 GPa. A speculative phase diagram of high-pressure helium is proposed.
TL;DR: In this paper, a possibility is discussed that d γ-symmetry superconductivity realized by the nearest neighbor exchange interactions can explain the properties of high T c ceramic superconductors recently discovered.
Abstract: Physical properties of d γ-symmetry Cooper pairs were theoretically investigated by assuming the dispersion relation of a simple square lattice with the nearest neighbor transfer integral and attractive interactions between the nearest neighbors. Calculated properties such as the energy gap, the density of states and the NMR relaxation time show highly anisotropic features. A possibility is discussed that d γ-symmetry super-conductivity realized by the nearest neighbor exchange interactions can explain the properties of high- T c ceramic superconductors recently discovered. It is also shown that exchange interactions determined in insulating phases can explain high superconducting transition temperatures in metallic phases.
TL;DR: Premiere theorie quantitative a inclure la nucleation de domaines, the rotation incoherente de grains et les interactions magnetostatiques completes.
Abstract: Premiere theorie quantitative a inclure la nucleation de domaines, la rotation incoherente de grains et les interactions magnetostatiques completes
TL;DR: In this article, the Ising ferromagnet with S = 1, 3/2, 2, 2 and 5/2 was studied and the anomalous behavior of the magnetization curves that is predicted when the ratio of the nearest-neighbor bilinear and biquadratic exchange interactions is negative.
TL;DR: In this article, the low-field electron spin resonance (ESR) spectrum in the superconducting phase of YBa2Cu3O7-x was analyzed and it was shown that the individual spins responsible for superconductivity are formed and dimerized only at Tc, with their intensity sharply increasing below Tc.
Abstract: The recent discovery1–3 of high-temperature superconductivity in the Y(La}–Ba–Cu–O system has generated intense experimental and theoretical activity. On the theoretical side, it has revived the discussion of different modes of pairing in solids4,5. Here we report some important features of the low-field electron spin resonance (ESR) spectrum in the superconducting phase of YBa2Cu3O7–x, giving evidence for the dimerization of electrons on copper pairs. An intense low-field resonance appears below the critical temperature, Tc, and exhibits unresolved hyperfine structure at temperatures below 70 K. The resonance field was found to be dependent on both temperature and microwave frequency. These are well-established features characteristic of copper pairs formed due to an exchange interaction. It appears that the individual spins responsible for superconductivity are formed and dimerized only at Tc, with their intensity sharply increasing below Tc. This observation appears to be a pointer towards the possibility of the disproportionation 2Cu3+ → Cu2+ + Cu4+ in adjacent octahedra, facilitated by coupling to local eg vibrations in the superconducting phase.
TL;DR: In this paper, a representation of the exchange energy functional as the product of a homogeneous gas functional and a rational function approximation in the density gradient is developed and shown to describe accurately both the total exchange energy and the local exchange energy density of atomic systems.
Abstract: A representation of the exchange energy functional as the product of a homogeneous gas functional and a rational function approximation in the density gradient is developed and shown to describe accurately both the total exchange energy and the local exchange energy density of atomic systems. For the atoms H through Kr, the error in the total exchange energy is generally <0.02 hartree except for Cr through Zn where it is <0.21 hartree.
TL;DR: In this article, the effect of temperature on the exchange coupling at the interface of three different bilayered materials was studied with the objective of developing temperature-stable single-domain materials for magnetoresistive readback heads.
Abstract: The effect of temperature on the exchange coupling at the interface of three different bilayered materials was studied with the objective of developing temperature‐stable single‐domain materials for magnetoresistive readback heads. Exchange field (HE) was measured from room temperature to 245 °C for NiFe films coupled to FeMn, αFe2O3, and TbCo. In the permalloy‐FeMn system, the exchange field decreases linearly, and reaches zero at about 150 °C, which is close to the Neel temperature of the antiferromagnet. These results agree well with previous work [C. Tsang and Kenneth Lee, J. Appl. Phys. 53, 2605 (1982)]. HE also decreases linearly in the αFe2O3 system, from 6.8 Oe at room temperature to 1.8 Oe at 245 °C. While the αFe2O3 system offers greater temperature stability, the exchange field produced is not very large, and the coercivity is somewhat high. Changes in HE with variations in the thickness of the αFe2O3 layer were also noted. Results for permalloy coupled to ferrimagnetic amorphous TbCo indicate ...
TL;DR: Specific-heat measurements on amorphous Fe Zr&00 (x 90 and 92) by a thermal relaxation method reveal a temperature decay of the form T(t) To+dTexp[ —(t/r)s] below 20 K, where the alloys are asperomagnetically ordered.
Abstract: Specific-heat measurements on amorphous Fe Zr&00 (x 90 and 92) by a thermal relaxation method reveal a temperature decay of the form T(t) To+dTexp[ —(t/r)s] below 20 K, where the alloys are asperomagnetically ordered. P=0.5 when To 0. The stretched exponential is a manifestation of nonergodic behavior, due to the inability of the spin system to explore the available random magnetic configurations on the 100-ms time scale of the experiment.
TL;DR: In this paper, the magnetoreflectivity and magnetic susceptibility were measured in Zn1−xFexSe (x < 0.07) and large spin splitting was observed due to exchange interaction between band electrons and Fe2+ localized electrons.
TL;DR: In this article, a new local potential for exchange and correlation in ferromagnetic materials is introduced, based on a recent paper by Fritsche (1986) concerned with the alternative foundation of a one-particle theory of many-electron systems.
Abstract: The authors introduce a new local potential for exchange and correlation in ferromagnetic materials. Its derivation rests on a recent paper by Fritsche (1986) concerned with the alternative foundation of a one-particle theory of many-electron systems. The one-particle equations arrived at in that paper contain a 'non-local' exchange-correlation potential which in the present work is approximated by a local expression. On self-consistently solving the associated one-particle equations for nickel metal, they obtain a band structure whose exchange splitting gap of 0.39 eV is considerably closer to the experimental value (namely 0.31 eV) than those which have so far been obtained by using the familiar potential of von Barth and Hedin (1972). The magnetic moment per atom is found to be 0.53 Bohr magnetons and is in fair agreement with the experimental value. Extension of the authors' calculations to finite temperatures leads to a lowering of the magnetic moment. However, due to the neglect of spatially varying excitations they arrive at a hypothetical Curie temperature of approximately 4500 K.
TL;DR: The presence of magnetic and chemical clustering, in Fe65Ni35 Invar, had previously been inferred from various measurements which were reviewed in this paper, where the authors independently confirm both types of clustering and measure the associated supermoment dynamics for the first time.
TL;DR: In this paper, Li2MnF5 was determined by X-ray diffraction (space group C2 c, Z = 4, a = 10.016, b = 4.948, c = 7.408, A, β = 112.19, and Rw = 0.019 for 605 reflections).
TL;DR: In this paper, the mean-field approximation, the cluster variation (CV) method in the tetrahedron approximation and the Monte Carlo (MC) method were applied to a binary alloy, where species A is magnetic while species B is not, and repulsive interactions are assumed between first and second neighbors of the same kind, in addition to a nearest-neighbor ferromagnetic exchange interaction.
Abstract: We treat a model for a binary (AB) alloy, where species A is magnetic (Ising spin σi = ± 1) while species B is not, and repulsive interactions are assumed between first and second neighbors of the same kind, in addition to a nearest-neighbor ferromagnetic exchange interaction. Both the mean-field approximation, the cluster variation (CV) method in the tetrahedron approximation and the Monte Carlo (MC) method are applied; comparing the phase diagrams obtained by the various approximations their accuracy is tested. It is shown that the CV method is in rather close agreement with the MC method for the present problem.
TL;DR: In this article, it was shown that for a new class of polymers, ferromagnetic superexchange may arise, which depends strongly on the energy gap of the delocalized π-electron system.
TL;DR: In this article, a theoretical analysis of the van der Waals vibration-rotation-electron spin states of the O2-O2 dimer in its 3Σ−g −3Σ −g electronic ground state is presented.
Abstract: A theoretical analysis has been made of the van der Waals vibration–rotation–electron spin states of the O2–O2 dimer in its 3Σ−g –3Σ−g electronic ground state. This analysis is based on a Hamiltonian that includes a spin‐dependent O2–O2 interaction potential and it involves also the permutation–inversion symmetry of the system. We have constructed some hindered internal rotor models for the vibrational states of the O2–O2 dimer which correspond with different equilibrium geometries, and for each of these models we have numerically calculated the spin‐rotation fine structure. This fine structure appears to be determined not only by the Heisenberg exchange interaction between the O2 monomer triplet states, but also by intramolecular spin–orbit and spin–spin coupling and, to a smaller extent, by the intermolecular spin–spin (magnetic dipole) interaction and by the Coriolis terms in the kinetic energy. The resulting fine‐structure spectrum is very complex, and very sensitive to the geometry of the O2–O2 dimer, to the nature of its internal motions and to the various magnetic couplings. This implies that detailed measurements of this spectrum, which can be interpreted with the help of the theory presented here, will yield interesting information on the properties of the O2–O2 dimer and, at the same time, verify our assumptions on the magnetic interactions between O2 molecules which have important consequences for the properties of solid oxygen.
TL;DR: In this article, a mean field theory of the state of compositional order of spin polarized binary alloys was developed based on the local spin density (LSD) approximation for describing the electrons, and the usual averages over the statistical mechanical ensemble were carried out with the aid of the selfconsistent Korringe-Kohn-Rostoker coherent potential approximation (SCF•KKR•CPA).
Abstract: We have developed a first‐principles electronic theory of concentration fluctuations in spin polarized binary alloys It is a mean field theory of the state of compositional order and it is based on the local spin density (LSD) approximation for describing the electrons The usual averages over the statistical mechanical ensemble are carried out with the aid of the self‐consistent Korringe–Kohn–Rostoker coherent‐potential approximation (SCF‐KKR‐CPA) To illustrate the main consequences of the theory we study the compositional short‐range order in the NicFe1−c alloy system We find that the ordering energy is almost entirely of magnetic origin
TL;DR: In this article, exchange coupling between ferromagnetic films across nonmagnetic intermediate layers by means of light scattering from spin waves and microwave absorption was studied and it was shown that the effective interlayer exchange decreases when the interlayer thickness d0 increases.
Abstract: We have studied exchange coupling between ferromagnetic films across nonmagnetic intermediate layers by means of light scattering from spin waves and microwave absorption. The ferromagnetic films were Ni0.8Fe0.2 (permalloy) and Fe and we chose Au, Ag, Cu, V, Cr, Pd, Ge, Si, Mn, and Bi as nonmagnetic interlayer materials. As expected the effective interlayer exchange decreases when the interlayer thickness d0 increases. Minimum thickness d0 where the ferromagnetic films are exchange decoupled range between 10 and 40 A. We compare these results with the concentration dependence of the magnetic ordering temperature in alloys of Fe and Ni with other elements, taking this as a measure for the strength of the exchange. There is a good correspondence which indicates that the coupling effects in the layered structures and the alloys have the same origin which is presumably by RKKY interaction.