Showing papers on "Exchange interaction published in 1981"
TL;DR: In this paper, the authors used a canonical transformation to derive an effective extended Hubbard Hamiltonian with intra-atomic repulsion and Ising-type interatomic exchange interaction in an effective external magnetic field from the extended Hubbard model with intra atomic attraction.
Abstract: We have used a canonical transformation to derive an effective extended Hubbard Hamiltonian with intra-atomic repulsion and Ising-type interatomic exchange interaction in an effective external magnetic field from the extended Hubbard model with intra-atomic attraction. The absence of magnetic ordering in the original Hamiltonian is transformed into the condition of one quasiparticle per atom. In the strong---intra-atomic---attraction limit, we can further derive from the transformed Hamiltonian an antiferromagnetic anisotropic Heisenberg Hamiltonian in an external field and with a given value of magnetization. A phase diagram of the charge-ordered, the singlet-superconducting, the mixed, and the nonordered phase is then obtained with the mean-field approximation. We continue to investigate the collective excitations in the MFA states. For various special cases our results reduce to the known results or the exact solutions.
115 citations
TL;DR: In this paper, the strong cubic anisotropy of a single UAs was analyzed by writing down a Hamiltonian and then obtaining the q-dependent susceptibility via a mean-field calculation.
Abstract: A number of neutron elastic scattering experiments are reported on single crystals of the compound UAs. This has the NaCl crystal structure, orders antiferromagnetically with the type-I structure (AF-I) at T/sub N/approx.126 K, and exhibits a first-order phase transition to the type-IA structure at 63.5 K. The most interesting behavior can be studied via the critical (diffuse) scattering near T/sub N/. Above T/sub N/ the critical scattering is anisotropic and centered about a wave vector that suggests a tendency to order with an incommensurate structure, but then at T/sub N/ suddenly orders with the commensurate type-I ordering. We present a detailed analysis by writing down a Hamiltonian in which the strong cubic anisotropy is included, and then obtain the q-dependent susceptibility via a mean-field calculation. The resulting parameters show that the z-z exchange interaction within the (001) planes is approx.40 as strong as the x-x or y-y coupling between the spins. Moreover, the competition within the interactions leads to the system experiencing ''frustration'' in deciding between AF-I or a sinusoidal spin arrangement, so that the system is in the vicinity of a Lifshitz point. The strong cubic anisotropy is interpreted as arising from bonding effects between the 5f wave functionsmore » and the anion p orbitals.« less
59 citations
TL;DR: In this article, the relative stability between the ferromagnetic state (FMS) and the helical spin density wave (HSDW) is discussed phenomenologically, and the arguments are made to the spin systems of which the spin structures are described by the spin density S ( r ) varying slowly with the position r and the SEI is favorable to the FMS.
Abstract: For the spin systems with both the symmetric exchange interaction (SEI) and the antisymmetric exchange interaction (AEI) (the Dzyaloshinsky-Moriya interaction), the relative stability between the ferromagnetic state (FMS) and the helical spin density wave (HSDW) is discussed phenomenologically. The arguments are made to the spin systems of which the spin structures are described by the spin density S ( r ) varying slowly with the position r and the SEI is favorable to the FMS. For the 21 types of crystal symmetry with the isogonal point groups having no inversion symmetry, the characteristics of the instability of the FMS are investigated. By use of the Landau free energy obtained for crystals with the isogonal point groups T and O , the magnetization process, the magnetic phase diagram, and the intensity of the neutron diffuse scattering are calculated. A comparison between the calculated results and the experiments on MnSi is also given.
53 citations
TL;DR: In this article, the effects of electronic correlation on the ground state energy, effective local moment and charge fluctuations for a model of transition metals was discussed, where the model consists of a bcc or fcc-canonicald-band with intraatomic Coulomb and exchange interaction.
Abstract: We discuss the effects of electronic correlation on the ground state energy, effective local moment and charge fluctuations for a model of transition metals. The model consists of a bcc or fcc-canonicald-band with intraatomic Coulomb and exchange interaction. We find that for realistic interaction parameters the correlated state shows enhanced spinfluctuations indicating the formation of local moments and strongly reduced charge fluctuations compared to the HF state. We also find that in generale
g
andt
2g
-states can show a different extent of correlation which suggests the use of anisotropic exchange correlation potentials in transition metals. The application of our model to bcc iron and fcc nickel is also considered.
38 citations
TL;DR: In this paper, the compound tropylium bis[1,2−dicyanoethylenedithiolato] nickelate (III) has been prepared and its crystal structure determined; the space group is P21/c, Z=2, the unit cell axes are a=6.364, b=7.458, c=17.991 A, and β=91.22
Abstract: The compound tropylium bis[1,2‐dicyanoethylenedithiolato] nickelate (III), i.e., (C7H7)+ [Ni(mnt)2]−, has been prepared and its crystal structure determined; the space group is P21/c, Z=2, the unit cell axes are a=6.364, b=7.458, c=17.991 A, and β=91.22 °. The Ni(mnt)2− and C7H7+ ions form segregated stacks in the direction of the b axis. This is in contrast with the structure of the analogous thiete complex (C7H7)+ [NiS2C2 (CF3)2]−, in which crystals alternating stacks of cations and anions are present. The susceptibility appears to be dominated by a strong one‐dimensional, isotropic, antiferromagnetic exchange coupling (J0=−4.4 cm−1). However, the angular dependence of the EPR linewidth at room temperature has to be explained with an additional anisotropic exchange interaction of 0.26 cm−1. Furthermore, Q‐band EPR measurements in the ab plane reveal a very small interstack exchange interaction (0.015 cm−1 at room temperature) which increases by a factor of 4 upon lowering the temperature to 8 K and whic...
37 citations
TL;DR: In this article, the magnetic phase diagram of (Fe c Ni 85−c )Mn 15, (Fe cm Ni 80−c ), (fe c Ni 80 −c )mn 20 alloys with f.c.
35 citations
TL;DR: In this article, a Pidgeon-Brown model modified by exchange interaction was applied to explain the experimental data and band-structure parameters and exchange constants were determined as a function of composition and temperature.
Abstract: Interband magneto-optical transitions were studied in Hg1-xMnxTe mixed crystals for x ranging from 0.001 to 0.155 and in the temperature range 10K-65K. A Pidgeon-Brown model modified by exchange interaction was applied to explain the experimental data. Band-structure parameters and exchange constants were determined as a function of composition and temperature. The values of the magnetisation obtained by fitting the theory of the authors' experimental data differs from those obtained by the usual method. An acceptor state degenerate with the conduction band was observed for zero-gap Hg1-xMnxTe crystals.
33 citations
33 citations
TL;DR: In this paper, the authors measured the susceptibility of single crystals of CsCuCl 3 in 4.2 K < T <500 K for H ⊥ c and H // c, and found a discontinuity in the susceptibility arising from a Jahn-Teller cooperative phase transition at T t ∼ 420 K.
Abstract: Susceptibilities of single crystals of CsCuCl 3 are measured in 4.2 K< T <500 K for H ⊥ c and H // c . Behaviors characteristic to a one-dimensional Heisenberg ferromagnet with weak interchain interactions are observed. The behaviors are analyzed by the high temperature expansion approximation, and intrachain and interchain exchange interaction constants, J 0 and J 1 , are estimated. The results are, J 0 / k =(24 ±3) K and J 1 / J 0 =-0.16 ±0.04. A discontinuity in the susceptibility arising from a Jahn-Teller cooperative phase transition are observed at T t ∼420 K. The discontinuity is due to a change in J 0 at T t . Relation between the phase transition and the change of J 0 is discussed.
30 citations
TL;DR: In this article, the development of magnetic ordering in single-crystal praseodymium has been studied over the temperature range 0.03-4.2K using thermal neutron scattering techniques.
Abstract: Using thermal neutron scattering techniques, the development of magnetic ordering in single-crystal DHCP praseodymium has been studied over the temperature range 0.03-4.2K. The intensity of the broad elastic peak around the wavevector 0.11 tau 100 (which has been observed in previous studies of Pr) increased steadily as the temperature was reduced. In addition, new satellite reflections originating from a sinusoidally modulated magnetic structure with wavevector 0.13 tau 100 were observed at temperatures well below 1K. The magnetic transition is believed to be driven by an enhancement of the exchange interaction via the hyperfine interaction. No temperature dependence of the magnetic excitation energies between 4.2K and 0.03K was detected.
30 citations
TL;DR: In this article, the variational method of Stollhoff and Fulde is applied to a model Hamiltonian describing the $d$ band of a transition metal to obtain the correlated ground state.
Abstract: The variational method of Stollhoff and Fulde is applied to a model Hamiltonian describing the $d$ band of a transition metal to obtain the correlated ground state. The model used includes the Coulomb and exchange intra-atomic energies and is rotationally symmetric in orbital and spin space. The calculated cohesion energy has a minimum for a half-filled $d$ band, which is connected with gradual localization of a magnetic moment in the presence of the Hund's-rule exchange interaction. At the same time the charge fluctuations are strongly suppressed and spin fluctuations enhanced. We have found a corrected Stoner criterion which gives a ferromagnetic ground state for realistic values of the model parameters for $3d$ metals, and is in agreement with the exact result of Kanamori at very low electron and hole concentrations.
TL;DR: In this paper, the Pidgeon-Brown model modified by exchange interaction was applied to Hg1−xMnxSe alloys at temperatures from 10 to 40 K and compositions x up to 11.5%.
Abstract: Interband magnetooptical studies are performed on Hg1−xMnxSe alloys at temperatures from 10 to 40 K and compositions x up to 11.5%. The obtained spectra are analysed using the Pidgeon-Brown model modified by exchange interaction. Band parameters, including exchange integrals, are determined by a fitting procedure of the interband transition energies.
TL;DR: The spin wave excitations of an 153EuS single crystalline sample have been measured by inelastic neutron scattering through the entire 1. Brillouin zone with the momentum Q parallel to the main symmetry directions as discussed by the authors.
Abstract: The spin wave excitations of an 153EuS single crystalline sample have been measured by inelastic neutron scattering through the entire 1. Brillouin zone with the momentum Q parallel to the main symmetry directions 〈100〉, 〈110〉, and 〈111〉, and for temperatures ranging from well below (1.3 K) to well above (36 K) the Curie temperature (Tc = 16.6 K). Least square fits of the low‐temperature spin wave dispersion curves to the Holstein‐Primakoff theory yielded the exchange energy constants Jr of the Heisenberg operator up to the fifth (r = 5) Eu‐Eu‐neighbour interactions as follows: J1/kB = 0.22(1±3)K, J2/kB = −0.10(0±4)K, J3/kB = 0.00(6±2)K, J4/kB = −0.00(7±2)K, and J5/kB = −0.00(4±2)K. They correspond with Θp = 21.1±0.2 K and also with the theoretically expected ratio of Tc/Θp. The temperature dependence of the spin wave energies, which has been measured for large‐q spin‐waves up to T/Tc = 1.5, does not agree with the spin‐wave renormalization theory. This EuS‐result differs from EuO, where the spin‐wave ren...
TL;DR: In this article, the spin dependence of the absorption of a polarized electron beam in the amorphous ferromagnet Ni40Fe40B20 and a W(100) single crystal was investigated.
Abstract: The exchange interaction and the spin‐orbit interaction are observed to cause a spin dependence of the absorption of a polarized electron beam in the amorphous ferromagnet Ni40Fe40B20 and a W(100) single crystal respectively. The enhancement of the spin dependence, near the energy where the secondary electron yield is unity, is shown to provide a simple efficient detector of spin polarization.
TL;DR: In this paper, the authors presented magnetic and neutron diffraction measurements made on the new series Pd2−xCuxMnIn, in which the conduction electron concentration is varied via the X rather than the Z sites.
Abstract: The Heusler alloys have the L21 structure and occur at the compoistion X2MnZ. The majority, such as Cu2MnIn, are ferromagnetic with a moment of about 4 μB at the Mn sites but some, such as Pd2MnIn, are antiferromagnetic. The Pd2MnIn1−xSnx and Pd2MnIn1−ySby [1,2] series have magnetic structures that change from antiferromagnetic fcc type 2 to type 3A to ferromagnetism as the proportions of the Z site atoms In/Sn/Sb are varied. Recently two models have been proposed to explain the 3d exchange interactions in Heusler alloys. Stearns [3] suggests a model with three interactions and the X atoms playing a dominant role. Price [4] proposes a single interaction dependent only upon the average conduction electron concentration. Results are presented of magnetic and neutron diffraction measurements made on the new series Pd2−xCuxMnIn, in which the conduction electron concentration is varied via the X rather than the Z sites. The changes in magnetic structure observed are similar to those that occur in the series Pd...
TL;DR: In this paper, the random anisotropy model of magnetoelastic interaction is considered from both classical and quantum point of view, and the exchange interaction is treated in the molecular field approximation.
Abstract: Random anisotropy model of magnetoelastic interaction is considered from both classical and quantum point of view. In both cases the exchange interaction is treated in the molecular field approximation. It was shown that the temperature dependence of the effective magnetoelastic tensor components has a complicated character, often different from the one predicted by the single-ion or two-ion theories.
TL;DR: In this paper, the first single crystal susceptibility and magnetization studies on DMSO have been conducted on powders and the results showed that the exchange interaction J to be relatively large (J/k = 45 K) with rather large interchain interactions (J′/J = −0.04).
Abstract: CuCl2⋅DMSO is one of a series of linear chain spin 1/2 compounds recently shown to have predominantly ferromagnetic interactions in the paramagnetic state. Initial studies1 conducted on powders showed the exchange interaction J to be relatively large (J/k = 45 K) with rather large interchain interactions (J′/J = −0.04). The present paper reports the first single crystal susceptibility and magnetization studies on this compound. The monoclinic compound has been found to order antiferromagnetically at 4.80 K with the easy axis the a axis. The two χ⊥′s are nearly equal, indicating uniaxial anisotropy. A spin‐flop transition has been observed when an external field was applied along the easy axis. The magnetic field phase diagram for this transition has been measured down to 2 K in fields up to 9.5 kOe. Using a molecular field model, effective fields have been estimated for the antiferromagnetic coupling between chains and for the spin anisotropy. The antiferromagnetic exchange field is 23 kOe, 3.6% of the effective ferromagnetic exchange field within the chains (Hfm). The anisotropy field was found to be only 6×10−4Hfm, indicating nearly perfectly Heisenberg‐type intrachain exchange.
TL;DR: In this paper, the mechanism by which the magnetic order is suppressed in intermediate valence compounds was studied by a diagrammatic perturbation theory in the hybridization interaction, and the effective inter-site exchange interaction was derived for the periodic Anderson model.
Abstract: We study the mechanism by which the magnetic order is suppressed in intermediate valence compounds. By a diagrammatic perturbation theory in the hybridization interaction, the effective inter-site exchange interaction is derived for the periodic Anderson model. The result includes the effect of charge fluctuations and is valid for arbitrary position of the local electron level relative to the Fermi level. It is shown that the intersite interaction is destructive to magnetic fluctuations with any wavenumber in the case of intermediate valence.
TL;DR: In this paper, an effective exchange interaction between rare-earth magnetic moments near the surface of ferromagnetic superconductors is calculated under various surface boundary conditions both for the persistent current and the original exchange coupling in the normal state.
Abstract: An effective exchange interaction between rare-earth magnetic moments near the surface of ferromagnetic superconductors is calculated under various surface boundary conditions both for the persistent current and the original exchange coupling in the normal state. In the bulk system, the persistent current screens the exchange interaction through the electromagnetic interaction, but the screening effect is weakened in the vicinity of the surface. The behavior of the penetrated magnetic field is calculated by using the obtained effective exchange interaction. Some discussions are given on the comparison between the present calculation and our previous calculation made by the boson transformation method.
TL;DR: In this article, a doubly degenerate Hubbard model is used to explain the phase diagram of the mixed compounds NiS 2-x Se x and the metal-nonmetal transition in the antiferromagnetic phase.
Abstract: The explanation of the phase diagram of the mixed compounds NiS 2- x Se x is given with the use of the doubly degenerate Hubbard model. The metal-nonmetal transition in the antiferromagnetic phase can be explained by introducing the intra-atomic exchange interaction.
TL;DR: In this article, the angular dependence of the linewidth is analyzed, namely in the a, c plane, where the interchain contribution Δ H 12 is extracted from the total linearewidth Δ H, from which they conclude that the single crystal EPR study at Q-band provides a direct and precise method to determine the exchange interaction between inequivalent copper ions.
Abstract: Room temperature electron paramagnetic resonance (EPR) measurements were carried out on single crystals of CuCl 2 ·2H 2 O around their three crystallographic axes at X-band (9.5 GHz) and Q-band (35 GHz). Owing to the interchain exchange interaction which is stronger than the difference in Zeeman energies of the dissimilar ions, the EPR spectrum is always reduced to a Lorentzian singlet, not only at X-band, but even at Q-band. There is no frequency dependence of the g factors from X-band to Q-band. The sinusoidal angular dependence of g 2 in any plane is related to the fact that the relative anisotropy Δ g / g is moderate in the a , c plane. The angular dependence of the linewidth is analyzed, namely in the a , c plane, where the interchain contribution Δ H 12 is extracted from the total linewidth Δ H . This permits us to determine the following value of the interchain exchange interaction: J ′/ k = (0.40 ± 0.04)K, from which we conclude that the single crystal EPR study at Q-band provides a direct and precise method to determine the exchange interaction J ′ between inequivalent copper ions.
TL;DR: The magnetic susceptibility of 1,5-diphenyl-3-(4-chlorophenyl)verdazyl (Cl-TPV) has been measured on a powder sample in the temperature region between 1.8 and 80 K.
Abstract: The magnetic susceptibility of 1,5-diphenyl-3-(4-chlorophenyl)verdazyl (Cl-TPV) has been measured on a powder sample in the temperature region between 1.8 and 80 K. It exhibited a round maximum at 21.0 K; this temperature is the highest among those observed for verdazyl monoradicals. The susceptibility can be well described by the Heisenberg linear chain model; the estimated exchange interaction parameter is J⁄k=−16.4 K. The crystal structure of Cl-TPV has been determined by means of X-ray diffraction. The crystal is orthorhombic, with the space group of Pbca and with a=22.041(5), b=7.166(4), c=21.379(5) A, and Z=8. The molecules are arranged in a column along the b axis. The exchange interaction has been discussed based on the crystal structure and the spin density distribution.
TL;DR: In this article, a single isotropic line havingg=1.992±0.00024 was observed in contrast to the seven line spectrum reported by Abraham, which was nearly one hundred times less than the calculated dipolar linewidth and analysis verified that the lineshape was Lorentzian indicating exchange narrowing.
Abstract: Electron spin resonance studies have been made at 9.1 GHz on Gd3+/MgO single crystals grown by electrofusion and containing low gadolinium concentrations. A single isotropic line havingg=1.992±0.00024 was observed in contrast to the seven line spectrum reported by Abraham. The experimental peak-to-peak linewidth for the −1/2↔+1/2 transition at 293 K was 0.3 mT and was independent of polar angle. This was nearly one hundred times less than the calculated dipolar linewidth and analysis verified that the lineshape was Lorentzian indicating exchange narrowing. The linewidth was independent of temperature from 4.2 to 293 K and the exchange energy derived for a gadolinium concentration of 310 ppm was 15 GHz. A discussion is given of the cubic field splittings of Gd3+ in oxide crystals and a comparison made of the exchange energies of transition group ions in the MgO lattice.
TL;DR: In this article, the effect of spin fluctuation is considered by the same procedure established in the Kondo problem and the exchange interaction between magnetic moments is approximated by the molecular fields, and the experimental results of the Knight shift and the anomalous transport properties of α-Mn are found to be qualitatively well understood by this phenomenological model.
Abstract: Different temperature dependence of the 55 Mn Knight shift has been observed for each Mn atom at four inequivalent crystallographic sites in α-Mn metal. These characteristic properties are tried to understand by the phenomenological localized spin fluctuation model, where the effect of spin fluctuation is considered by the same procedure established in the Kondo problem and the exchange interaction between magnetic moments is approximated by the molecular fields. The experimental results of the Knight shift and the anomalous transport properties of α-Mn are found to be qualitatively well understood by this phenomenological model. The existence of the localized spin fluctuations in α-Mn, postulated previously by the XPS measurements, is confirmed.
01 Jan 1981
TL;DR: In this article, the collective features of the interacting boson model are described as a result of coherent admixtures of proton and neutron s- and d-bosons, which are obtained by diagonalizing the Hamiltonian which contains a strong and attractive quadrupole-quadrupole interaction.
Abstract: The interacting boson model has been very successful in the description of collective bands in many nucleil). The various limits of collective spectra2) as well as the transitions between them3) are very well accounted for. In its detailed version (IBM), collective states of nuclei with valence protons and neutrons outside closed shells are obtained from a boson Hamiltonian with proton s-bosons (with l=0) and d-bosons (l=2) and neutron s- and d-bosons4). The collective features (in particular enhanced E2 electromagnetic transitions) emerge as a result of coherent admixtures of proton and neutron s- and d-bosons. These admixtures are obtained by diagonalizing the Hamiltonian which contains a strong and attractive quadrupole-quadrupole interaction between proton and neutron bosons. On the other hand, the eigenstates of the proton part of the boson Hamiltonian have definite (generalized) seniorities and are characterized by the numbers of d-bosons. A similar situation holds for the neutrons.
TL;DR: In this paper, temperature dependence of luminescence spectra is measured of the Heisenberg-type two dimensional antiferromagnets: (C n H 2n+1 NH 3 ) 2 MnCl 4 with n=1,2, and 3 in the temperature range of 4-300K.
TL;DR: In this paper, the authors investigated the spin-lattice relaxation of localized states in evaporated amorphous silicon films and measured the temperature and concentration dependences of the relaxation rate in the temperature range 4.2-290 K.
Abstract: 2014 We have investigated the spin-lattice relaxation of localized states in evaporated amorphous silicon films. Temperature and concentration dependences of the relaxation rate have been measured in the temperature range 4.2-290 K. Below 10 K a phonon bottleneck is present. Between 10 and 100 K, a single phonon process predominates. It can be accounted for assuming a significant exchange interaction. Above 100 K, spin-lattice relaxation is governed by hopping. J. Physique LETTRES 42 ( 1981 ) L-21 L-24 1 er JANVIER 1981,
TL;DR: In this paper, the role of non-linear c-f exchange interaction effect on the magnetic properties of rare-earth metals was investigated using a simple band model and obtained the magnetic phase diagram at zero temperature.
Abstract: The authors have investigated the role of non-linear c-f exchange interaction effect on the magnetic properties of the rare-earth metals. Using a simple band model they obtained the magnetic phase diagram at zero temperature. The non-linear effect stabilised the cone structure without the crystal field and caused the ferromagnetic transition. The magnitude of the helical wavevector Q decreased as the helical component mod (S(Q)) mod increased. When (S) was regarded as the observed magnetisation, the temperature dependence of Q agreed with the experimental results. The magnetic properties of Gd and its alloys were considered on this phase diagram. The effective lifetimes of the c electrons were also important for the phase diagram. The crystal field made the phase diagram complicated. For the easy plane anisotropy, the fan appeared instead of the cone. For the easy axis anisotropy, the helix was squared by the higher harmonics and became the antiphase domain structure in infinitesimal c-f exchange.
TL;DR: In this paper, the ability of existing nonadjustable local exchange approximations to represent interelectronic exchange is tested by comparing valence-electron energies and average radii in Rb and Ag as determined in two different approaches: a relativistic-modelpotential approach and one employing the polarizable frozen-core relatival Hartree-Fock method.
Abstract: The ability of existing nonadjustable local-exchange approximations to represent interelectronic exchange is tested by comparing valence-electron energies and average radii in Rb and Ag as determined in two different approaches: a relativistic-model-potential approach and one employing the polarizable frozen-core relativistic Hartree-Fock method. The calculations are identical in these two approaches except for the treatment of exchange. Two approximations originally developed for electron-atom-scattering processes are found to be the most accurate. A new approximation based on the free-electron-gas model is also discussed.