Showing papers on "Exchange interaction published in 1974"
TL;DR: An exact expression for the exchange and correlation energy of an inhomogeneous electron gas, as defined by Hohenberg, Kohn and Sham (1966), is derived in this article, and a formula linking the surface exchange energy of a half space to the Kohn-Sham one electron potential follows without approximation.
Abstract: An exact expression for the exchange and correlation energy of an inhomogeneous electron gas, as defined by Hohenberg, Kohn and Sham (1966), is derived. This expression is separated into exchange and correlation terms and a formula linking the surface exchange energy of a half space to the Kohn-Sham one electron potential follows without approximation. For an infinite barrier model, the local density approximation gives a surface exchange energy 50% greater than the exact value, a large and previously unsuspected error. An exact evaluation of the surface correlation energy is not feasible, but the authors argue that the dominant contribution, arising from the difference in zero point energy between bounded and unbounded systems, can be estimated using a simple model. Numerical results, not dependent on the introduction of arbitrary plasmon wavevector cutoffs, give surface correlation energies larger than Lang and Kohn (1970), who work from a local formula, by a factor of six.
384 citations
TL;DR: In this paper, magneto-optical properties of EuO single crystals, doped with up to 5-at.% Gd, were investigated and the exchange interaction between free electrons and $4f$ spins was found to decrease with increasing doping.
Abstract: This paper is concerned with magneto-optical properties of EuO single crystals, doped with up to 5-at.% Gd. The optical absorption and the Faraday rotation have been measured above and below the magnetic-ordering temperatures and in various magnetic fields. In addition the reflectivity between 13 eV and 250 \ensuremath{\mu}m has been observed and the Curie temperature has been determined. The measurements permit the evaluation of the magnetic short-and long-range order. The exchange interaction between free electrons and $4f$ spins can be determined and is found to decrease with increasing doping. The analysis of the optical data permits the derivation of the carrier concentration, their mobility, and temperature dependence. The carrier concentration shows an exchange-induced ionization near ${T}_{C}$ for nondegenerate samples and the mobility has a sharp minimum at ${T}_{C}$. The transport parameters also reflect the fact that the electron-$4f$-spin interaction decreases with increasing Gd concentration, probably owing to electrostatic-shielding effects.
113 citations
TL;DR: In this paper, the various contributions to the hyperfine interactions as measured with the Mossbauer Effect technique, namely the isomer shift, the magnetic hyperfine interaction and the quadrupole interaction are summarized.
86 citations
TL;DR: In this article, the magnetization and the related magnetocrystalline anisotropy in terms of a cubic crystalline electric field and an isotropic exchange interaction are described.
Abstract: Magnetization measurements are reported for single crystals of PrAl2 in the range from 4.2K to 30K for magnetic fields up to 150 kOe applied in the (100), (110) and (111) directions. For these measurements, together with the magnetization results obtained earlier for TbAl2 the authors give a quantitative quantum mechanical description of the magnetization and the related magnetocrystalline anisotropy in terms of a cubic crystalline electric field and an isotropic exchange interaction. The parameters used in this description can be unified to good approximation to all REAl2 intermetallic compounds where detailed experiments are available.
52 citations
TL;DR: In this article, it was shown that the higher order spin coupling can be derived from the minimum condition of the free energy of the cluster system and that the magneto-striction is an important origin for the appearance of the higher-order spin coupling.
Abstract: From the analyses of the magnetic susceptibility and/or the magnetic heat capacity of the polynuclear complex compounds of iron group ions, it has been suggested that there exists an additional higher order spin coupling besides the usual Heisenberg exchange interaction. It is shown that the higher order spin coupling can be derived from the minimum condition of the free energy of the cluster system. For that purpose, a magnetoelastic term is introduced in the free energy and a dependence of the exchange coupling on the distance between interacting ions is assumed. In an alternative way, it is also shown that the higher order spin coupling can be derived from the perturbation procedure in which the spin-orbit interaction and the usual exchange interaction are taken as the perturbation. A quantitative comparison of the present theory with the values obtained from data analyses makes it clear that the magneto-striction is an important origin for the appearance of the higher order spin coupling.
42 citations
TL;DR: In this paper, a simple model for a nondilute alloy containing giant spin clusters is used to calculate the spin contribution to the resistivity, the neutron cross section, and the specific heat, which is expected to be applicable to Ni-Cu alloys near the critical concentration for ferromagnetism.
Abstract: A simple model for a nondilute alloy containing giant spin clusters is used to calculate the spin contribution to the resistivity, the neutron cross section, and the specific heat ${C}_{V}$. The model is expected to be applicable to Ni-Cu alloys near the critical concentration for ferromagnetism. Intracluster interactions are treated exactly using a near-neighbor Heisenberg exchange interaction. Intercluster interactions are treated within the molecular-field approximation. It is assumed that in paramagnets there is a weak local field, which derives from the magnetic-anisotropy energy. Reasonable semiquantitative agreement with resistivity, elastic neutron scattering, and low-temperature specific-heat measurements on Ni-Cu for a range of concentrations and temperatures is obtained if the average cluster contains 50 Ni spins and if a Ni atom has a spin when eight or more of its near neighbors are also Ni. It is found that the anomalous temperature dependence of the resistivity, which behavior is common to a variety of alloy systems, can be accounted for using the present theory if the Fermi wave vector times the lattice spacing is less than \ensuremath{\cong}2. The previously unexplained behavior of the elastic neutron cross section in paramagnetic alloys can also be understood within this theoretical framework. In contrast to earlier discussions, it is shown that the spin contribution to ${C}_{V}$ is not temperature independent; in ferromagnetic alloys this contribution is found to increase with increasing temperature and in paramagnetic alloys it decreases with temperature. It is believed that this temperature dependence has, in the past, been incorrectly attributed to the electronic contribution to the specific heat. The validity of previous suggestions that the specific-heat and neutron-scattering measurements on these alloys probe different spin clusters is also questioned.
37 citations
TL;DR: In this article, the temperature variation of magnetization in the quadratic layer ferromagnet K 2 CuF 4 has been investigated by measuring the NMR frequency of Cu 63 or out-of-layer F 19 nuclei at zero and applied fields in the temperature region of 0.67.
Abstract: The temperature variation of magnetization in the quadratic layer ferromagnet K 2 CuF 4 has been investigated by measuring the NMR frequency of Cu 63 or out-of-layer F 19 nuclei at zero and applied fields in the temperature region of 0.27≦ T / T c ≦0.67. The results are analyzed by a two-dimensional spin wave theory. Under the external field the dipole interaction is approximated in a form of the effective field. The X Y -like anisotropy of exchange interaction is taken into consideration. Good agreement between the theory and the experimental results is obtained. The X Y -like anisotropy itself is not effective to the magnetization, but its contribution to the magnetization cannot be neglected when it cooperates with the external field. The parameter of the exchange interaction J within the layer is obtained to be 12.37 K. The NMR result at zero applied field is also discussed.
34 citations
TL;DR: In this article, the magnetic properties of NdAl2 are calculated using a Hamiltonian including crystal-field and isotropic exchange interaction terms, and a two-dimensional mean-field theory is evaluated to calculate single-crystal magnetization curves.
Abstract: The magnetic properties of NdAl2 are calculated using a Hamiltonian including crystal-field and isotropic exchange interaction terms. A two-dimensional mean-field theory is evaluated to calculate single-crystal magnetization curves. It is shown that the magnetic properties can be understood using the crystal-field parameters derived from the magnetic exciton spectrum measured by Houmann et al. by means of inelastic neutron scattering. The combined lambda -Schottky anomaly in the heat capacity is explained. No additional parameters are introduced.
30 citations
TL;DR: The average magnetic susceptibilities of six binuclear complexes of cobalt(II) between 80 and 400 K were reported in this article, and it was concluded that the t2g spin of Co2+ probably makes a ferromagnetic contribution.
Abstract: The average magnetic susceptibilities of six binuclear complexes of cobalt(II) between 80 and 400 K are reported. The compounds studied are Co2(dhph)2X4,nH2O (dhph = 1,4-dihydrazinophthalazine, X = Cl or Br), Co2(dppn)2X4,nH2O [dppn = 3,6-di-(2-pyridyl)pyridazine, X = NO3 or ClO4], Co2(dppn)(SO4)2,5H2O, and Co2(Me2dppn)(NO3)4,2CH3OH [Me2dppn = 3,6-di-(6-methyl-2-pyridyl)pyridazine]. These compounds exhibit weak antiferromagnetic exchange, and the values of the effective isotropic exchange parameter are estimated, making allowance for spin–orbit coupling and the effects of orbital reduction and axial distortion parameters. The values of are compared with those of the analogous nickel complexes reported earlier, and it is concluded that the t2g spin of Co2+ probably makes a ferromagnetic contribution. The exchange interaction is analysed in a one-electron orbital basis, leading to an anisotropic form for the exchange parameter. The average susceptibility data do not distinguish between the isotropic and anisotropic models, however, and the precise identity of the orbitals responsible for the ferromagnetic contribution remains uncertain.
26 citations
TL;DR: In this article, the local approximation of Lang and Kohn is shown to grossly underestimate the correlation constribution to the surface energy, and it is shown that this failure is substantially compensated by an over-estimate of the exchange energy.
25 citations
TL;DR: In this article, the low temperature magnetic properties of antiferromagnetic UCrO 4 and NdCrTiO 5 have been studied using magnetic and magnetoelectric powder susceptibility measurements.
TL;DR: In this article, the trigonal field components of Ug ′(2 T 1g, 2 H) in dilute crystals were identified as the trigonal field component of trigonal phonon lines, and the preferred direction of spin alignment in both states is parallel to the c-axis.
Abstract: Low temperature polarized absorption and magnetic circular dichroism spectra are reported for crystals of CdBr2 doped with varying concentrations of Co2+ in the region near 17 700 cm-1. Sharp zero phonon lines are identified as the trigonal field components of Ug ′(2 T 1g , 2 H) in dilute crystals, and as states arising from ferromagnetically coupled in-plane pairs in the concentrated ones. Measurements of the temperature dependence of the lines, and Zeeman spectra recorded with applied fields up to 50 kG parallel and perpendicular to the crystal c-axis, lead to estimates of the anisotropy of the exchange interaction in the ground state, and to both isotropic and anisotropic exchange in the excited state. The preferred direction of spin alignment in both states is parallel to the c-axis.
TL;DR: In this paper, the magnetic excitation spectrum in NdAl2 has been measured by means of inelastic neutron scattering at 5.3K for momentum transfer in the (110) direction.
Abstract: The magnetic excitation spectrum in NdAl2 has been measured by means of inelastic neutron scattering at 5.3K for momentum transfer in the (110) direction. Several excitation branches are observed and discussed in terms of a hamiltonian including an isotropic Heisenberg exchange interaction and a cubic crystalline electric field. The spectrum is interpreted as comprising of six excitation modes four of which are transverse spin-wave-like excitations (two acoustic and two optical). The remaining two excitations are longitudinal in character (acoustic and optical). The authors obtain values for both the exchange interaction and the crystal-field parameters.
TL;DR: In this article, the temperature dependence of the conductivity of ferromagnetic semiconductors was investigated for both the non-degenerate and degenerate case with allowance of the exchange interaction between moveable electrons and localized magnetic moments.
Abstract: The temperature dependence of the conductivity of ferromagnetic semiconductors is theoretically investigated for both the non-degenerate and degenerate case with allowance of the exchange interaction between moveable electrons and localized magnetic moments. For the former the temperaturedependence of the charge carrier density is found.
[Russian Text Ignored].
TL;DR: In this paper, the magnetization of amorphous antiferromagnetic manganese phosphate glass has been studied as a function of the concentration, temperature, and magnetic field.
TL;DR: In this paper, an alternative kind of anisotropic coupling between the total angular moments (Ji and Jj) on the sites i and j is introduced which is proportional to the following combination of Racah operators: O2, -2(Ji), O2-1(Jj), -2, −2(jj), expressed with respect to a coordinate system with the z axis along the c direction.
Abstract: From a general two ion spin Hamiltonian, an expression is deduced for the energies of spin waves propagating in a hexagonal solid in which the magnetic moments are ordered in a conical or helical structure. The spin wave dispersion relation in the c direction of Er in its conical magnetic phase at 4.5K, which has been studied by Nicklow et al (1971) is reanalysed. In this analysis an alternative kind of anisotropic coupling between the total angular moments (Ji and Jj) on the sites i and j is introduced which is proportional to the following combination of Racah operators: O2, -2(Ji), O2, -2(Jj), expressed with respect to a coordinate system with the z axis along the c direction. The resulting anisotropy (both the constant and the q dependent part) is reduced by an order of magnitude in comparison with that deduced by Nicklow et al (1971). The constant anisotropy is found to be equal to about 20 meV (a rough estimate from magnetization measurements gives 5-10 meV). The anisotropic part of the exchange interaction is found to be of the same order of magnitude as the isotropic part.
TL;DR: In this paper, the kinetic exchange mechanism in Anderson's theory of super-exchange is formulated for the case of Co 2+ in CoO, and the exchange interaction depends on the orbital augular-momentum operators as well as on the spin operators.
Abstract: The kinetic exchange mechanism in Anderson's theory of super-exchange is formulated for the case of Co 2+ in CoO. The exchange interaction depends on the orbital augular-momentum operators as well as on the spin operators. The nearest neighbor interaction is shown to be largely anisotropic. The coeffcients of the exchange interaction are estimated with the following parameters: Mattheiss'APW result and its modification for the transfer integrals, Anderson's value for the effective Coulomb integral, and the experimental values for the intra-ionic exchange integral and the crystalline field splitting. The estimated values should be correct at least in order of magnitude on account of the fact that the came procedure in the cases of NiO and MnO give result in fairly good agreement with the experimental result. The potential exchange in CoO is also discussed.
TL;DR: In this paper, the treatment of exchange energy in inversion layers is extended to take the layer thickness into account when only the lowest subband is occupied by electrons, and the exchange energy of electrons in Si is close to the zero-thickness limit obtained previously.
Abstract: The treatment of exchange energy in inversion layers is extended to take the layer thickness into account when only the lowest subband is occupied by electrons. The exchange energy of electrons in Si is close to the zero-thickness limit obtained previously. Wave functions are only slightly contracted because of exchange. The exchange energy in excited subbands is smaller in magnitude, and has a smaller variation with wavevector, than in the lowest subband. Estimates based on a three-dimensional local density approximation give the approximate magnitude of the exchange energy but do not give its dependence on the spatial extent of the wave function correctly and cannot account for image effects.
TL;DR: In this article, the self-diffusion constant in bcc solid was calculated in the temperature regime where an atom changes place with a neighboring vacancy more often than flip-flops are induced through the Pauli exchange interaction.
Abstract: The self-diffusion constant in bcc solid
3
He is calculated in the temperature regime where an atom changes place with a neighboring vacancy more often than flip-flops are induced through the Pauli exchange interaction. The calculation is based on the method of moments and describes vacancies with a model of fictitious spinS=1. The scattering of phonons by vacancies impedes the quantum tunneling of vacancies: the quantitative discussion of this effect shows that its relation with the lifetime of vacancy waves is not simple.
TL;DR: In this paper, the authors compare the ESR spectra of insulating EuS with those of metallic GdS and prove the existence of an exchange interaction between conduction electrons and localized 4 f moments in these sulphides.
TL;DR: In this paper, the authors analyzed the coefficients occurring in the polynomial in the scalar product S A · S B, representing an effective spin hamiltonian for the direct first-order exchange interaction energy of two weakly-coupled atoms A and B in the state of highest multiplicity.
TL;DR: In this article, the space and time-dependent spin correlation function for a linear chain of classical magnetic moments coupled by a purely isotropic exchange interaction was investigated and the correlation functions were given in terms of relative variables which at low temperatures turn out to be the normal coordinates appropriate to a system without long-range-order.
Abstract: We investigate the space- and time-dependent spin-correlation-function for a linear chain of classical magnetic moments coupled by a purely isotropic exchange interaction. The correlation functions are given in terms of relative variables which at low temperatures turn out to be the normal coordinates appropriate to a system without long-range-order. Analytic expressions for the correlation functions, including correctly the fluctuations responsible for the absence of long-range-order, are given at low temperatures. Well-defined spin wave oscillations exist for wavelengths smaller than the correlation length. In this region our results are consistent with experimental data for the isotropic Heisenberg antiferromagnet TMMC.
TL;DR: In this paper, a theory for the critical behaviour of the Ising model in a transverse field is presented, where the existence of a central component in the spectral density of the longitudinal spin relaxation function is shown to result selfconsistently from decay into itself and energy-fluctuation diffusion mode.
Abstract: A theory for the critical behaviour of the Ising model in a transverse field is presented. The existence of a central component in the spectral density of the longitudinal spin relaxation function is shown to result self-consistently from decay into itself and energy-fluctuation diffusion mode. Estimates of the various quantities are made forT nearT c , the damping is seen to be temperature dependent. Our theory is appropriate for the caseΓ ∼J whereΓ andJ are the magnitudes of the transverse field and the exchange interaction between the spins respectively. Its relevance to neutron scattering data on Van Vleck paramagnets is discussed.
Patent•
23 Dec 1974
TL;DR: In this article, a process for the manufacture of magnetic materials containing a phase A with a ferromagnetic spin system, of composition Co x Ni 1 -x and a phase B with an antiferromagnetic spins system of composition (CoO) x (NiO) 1 -X, with values of x from 0.05 to 0.96, the phases A and B showing exchange interaction.
Abstract: A process for the manufacture of magnetic materials containing a phase A with a ferromagnetic spin system, of composition Co x Ni 1 -x and a phase B with an antiferromagnetic spin system of composition (CoO) x (NiO) 1 -x , with values of x from 0.05 to 0.96, the phases A and B showing exchange interaction. The materials are manufactured by surface oxidation of starting materials having a ferromagnetic layer, at least on the surface but optionally extending deeper, of composition A, to produce the antiferromagnetic phase B, nitrous oxide being used as the oxidant. Magnetic materials thus obtained have an asymmetrical hysteresis loop, which manifests itself at different temperatures, namely the particular Neel temperature of the phase B, which depends on the composition of the phases which varies in accordance with the values of x.
TL;DR: In this paper, the influence of the conduction band on phonon dispersion relations in lead was investigated and core exchange interaction was found to have a significant influence on the dispersion.
TL;DR: In this article, the resistivities of a series of dilute Mo Mn, Mo Fe and Mo Co alloys were measured between 1.5 K and room temperature in order to understand the nature of the s-d exchange interaction in these alloys.
Abstract: The resistivities of a series of dilute Mo Mn, Mo Fe and Mo Co alloys were measured between 1.5 K and room temperature in order to understand the nature of the s-d exchange interaction in these alloys. It was found that the resistivity of Mo Co and Mo Fe shows a normal Kondo effect with a Kondo temperature of 46±2 K and about 0.3 K, respectively, while the resistivity of Mo Mn shows a positive temperature coefficient with a characteristic temperature of 17±4 K, which is considered as the Kondo temperature of this alloy. In addition to these anomalies due to magnetic scattering, a peak was observed at about 50 K in the resistivity of all the alloys studied. This peak was attributed to the differing anisotropies of the relaxation times for impurity and phonon scattering.
TL;DR: The concept of exchange energy as it arises in Thomas-Fermi theory is examined in this paper, where it is found that the imposition of Hermitian symmetry upon the single particle Thomas Fermi density matrix changes the single-particle exchange energy density operator in a drastic way.
Abstract: The concept of exchange energy as it arises in Thomas‐Fermi theory is examined. It is found that the imposition of Hermitian symmetry upon the single particle Thomas‐Fermi density matrix changes the single particle exchange energy density operator in a drastic way.
TL;DR: In this article, the temperature dependence of spin cluster resonance signals can be used to calculate the dominant exchange interaction in chainlike structures and it is shown that impurities in the sample can result in a drastic change of the calculated value of the exchange.
Abstract: The temperature dependence of spin cluster resonance signals can be used to calculate the dominant exchange interaction in chainlike structures. It is shown that impurities in the sample can result in a drastic change of the calculated value of the exchange. The results are applied to the case of CsCoCl 3 ·2H 2 O.
TL;DR: In this paper, the effect of random exchange interaction on the low temperature behavior of these thermodynamic quantities is compared with those for a periodic chain and the dependence of spin-spin correlation function on distance and temperature is discussed.
Abstract: exchange interaction J are studied exactly in the case of distributions of the type P(J) =AIJI' for Jm!n'S:.J'S:.Jmax and P(J) =0, otherwise. The results are compared with those for a periodic chain. We find that the specific heat C~T•+l for Jm1n=O and kT