Showing papers on "Exchange interaction published in 1975"

Spin-orbit coupling, Fermi surface, and optical conductivity of ferromagnetic iron

Abstract: A previous self-consistent linear-combination-of-atomic-orbitals calculation of energy bands in iron has been extended through the inclusion of spin-orbit coupling. The exchange interaction is incorporated according to the $X\ensuremath{\alpha}$ method. The Fermi surface is described in detail and compared with the results of measurements of the de Haas-van Alphen effect, and of magnetoresistance anisotropy. The interband contribution to the optical-conductivity tensor was computed using matrix elements determined from wave functions including spin-orbit coupling. Both diagonal and off-diagonal elements of this tensor have been obtained.

A calculation of the interaction between pairs of rare-gas atoms

Abstract: A recent theory of intermolecular forces is modified by the exclusion of part of the previously estimated exchange energy and the inclusion of the long-range dispersive interaction. This procedure is justified by comparison with the theory of plasma oscillations. Results have been obtained which are in good agreement with experiment in the case of He-He, Ne-Ne, Ar-Ar and Kr-Kr interactions and are in reasonable accord in the case of the unlike-atom pairs.

The specific heat of magnetic linear chains

Abstract: Energy spectra and thermodynamical quantities of finite linear chains and polygons of axially symmetrically interacting spins have been numerically calculated. Extrapolation of the molar heat capacities versus the number of spins yielded estimates for infinite systems. Ground-state energies were also obtained. Data are presented for both ferromagnetic and antiferromagnetic chains. The influence of axially symmetric anisotropies in the exchange interaction was studied for S = 1 2 and 1. This influence proved to be qualitatively the same in both cases. For S = 1 to 5 2 , data are included for isotropic exchange in combination with several crystal field anisotropy parameter (D) values, both positive and negative. Further, the effect of the simultaneous presence of a D term and exchange anisotropy is studied for several cases. Agreement of the extrapolated data is found with some existing experimental and theoretical results.

Antisymmetric exchange in the trinuclear clusters of copper (II)

Abstract: The magnetic susceptibility of Cu3(C6H5N2O)3(OH)SO4 · 10.5 H2O crystals was measured in the temperature interval 4.2–11 K and magnetic field range 0–50 kOe. The theoretical analysis of the effects of antisymmetric exchange was performed. An adequate interpretation of the experimental results may be done in terms of an antisymmetric exchange which is about 12 cm−1 in the crystals discussed. The rhombic distortions of the trigonal cluster were shown to reduce the antisymmetric exchange.

The Proof of a Triplet Ground State in the N,N′-Di-t-butyl-m-phenylenebinitroxide Biradical

Abstract: The magnetic susceptibility of the N,N′-di-t-butyl-m-phenylenebinitroxide biradical has been measured in the temperature range from 60 to 300 K. The susceptibility follows the Curie-Weiss law, with a paramagnetic Curie constant of 0.90±0.04 K·-emu/mol and a Weiss constant of −19±2 K. This susceptibility suggests that the ground state of the biradical is a triplet and that an intramolecular pure triplet approximation, 2J>>kT, is reliable in the temperature region below 300 K. The negative Weiss constant is due to the negative exchange interaction, J′<0, between the triplets.

Magnetization of amorphous Gd‐Co‐Ni films

Abstract: The temperature dependence of magnetization from 4.2 to 500 °K for amorphous Gd‐Co‐Ni films obtained by evaporation is found to be satisfactorily described by a two‐sublattice model in wihch the two 3d transition metals form one sublattice interacting antiferromagnetically with the Gd sublattice. The unique magnetization behavior observed is attributed to an intersublattice exchange interaction which is small in comparison to the transition‐metal intrasublattice exchange interaction.

Specific Heat Due to Spin Fluctuations in Nearly and Weakly Antiferromagnetic Metals

Abstract: The specific heat of nearly and weakly antiferromagnetic metals is discussed by using the renormalized spin fluctuation theory, which has been previously applied to the study of the effect of spin fluctuations on the magnetic susceptibility in antiferromagnetic metals. The coefficient of the linear specific heat enhanced by the effect of spin fluctuations, Δγ, is given as a function of α by Δγ(α)-Δγ(1)∝-|α-1| 1/2 for both paramagnetic and antiferromagnetic phases near the critical boundary (α=2 I χ S 0 , I is the exchange interaction, χ S 0 the staggered susceptibility for I =0). Although the enhanced linear specific heat is predominant at low temperatures, the effect is considerably suppressed because of the renormalization of spin fluctuations as the temperature increases. Numerical calculations are presented for the free electron model with Umklapp processes.

Magnetically controlled electron localization in Eu-rich EuO

Abstract: A calculation of the linear-response properties of an electron gas, coupled by the s-f exchange interaction to a lattice of localized spins, is presented. Coupled-charge and spin-density-response functions are determined in the random-phase approximation and used to compute a Mott-type instability of metallic Eu-rich EuO toward formation of magnetic polarons bound to oxygen vacancies. Critical carrier densities are obtained as a function of temperature and phase diagrams of the metallic and insulating states are presented and compared with existing theories and experimental results.

Magnetism in praseodymium-neodymium single-crystal alloys

Abstract: Pr-Nd alloys represent an almost ideal model system for a mixture of singlet-doublet ions (Seff=1) and Kramers' doublet ions (Seff=1/2). Neutron scattering and magnetization measurements have been made on single crystals of Pr-3.0% Nd, Pr-5.5% Nd and Pr-26.3% Nd. These alloys order antiferromagnetically (on the hexagonal sites) at 6.3K, 6.5K and 11.4K, respectively. Measurements of the magnetic-field dependence of the magnetization were also made. The concentration dependence of TN and the temperature dependence and magnitude of the induced moments can be understood in terms of a simple molecular-field theory. The authors conclude that pure Pr has a close-to-critical ratio between exchange interaction and the crystal field (=0.95). The crystal field acting on the Nd ion is smaller than that acting on Pr. The exchange interactions between Pr-Pr, Nd-Nd, and Pr-Nd are found. The relative magnitudes agree with the de Gennes scaling factor.

A study of structurally disordered single-element magnetic materials beyond the homogeneous molecular field approximation

Abstract: A usual assumption in the theory of amorphous magnetic materials is 〈Si〉 = S. It is shown that in amorphous materials this is a very crude approximation. The magnetic properties of small random clusters of localised spins were studied beyond the above simplification. It is found that in this case the fluctuations of the exchange interaction around its crystalline mean value cause an increase of the Curie temperature. In the light of this conclusion most of the experimental results in this direction should be reinterpreted. It is argued that the magnetic properties inherently depend on higher spatial correlations than pair–pair ones. [Russian Text Ignored].

Magneto-Optical Studies of Direct Exciton State in Thallous Chloride

Abstract: The magneto-optical properties of the ls exciton in TIC! associated with the direct transition at the X point in Brillouin zone are .studied theoretically and experimentally. It is found that the triplet exciton state as well as the longitudinal exciton state becomes observable in a certain configuration of the light and the magnetic field. These results are analyzed by the theory based on the exciton exchange interaction. The electron-hole intravalley exchange energies for the two exciton polarizations are obtained as 2. 7 and 1.5 me V. The effective g value of exciton parallel to the valley direction is estimated to be 3.4.

A Self-Consistent Calculation of Magnetic Properties of Single-Element Amorphous Materials beyond the Homogeneous Molecular Field Approximation

Abstract: A self-consistent calculation of magnetic properties beyond the homogeneous molecular field approximation is given. It was found that the Curie temperature, due to fluctuations of the exchange interaction only, increases. It is also shown that the magnetic properties inherently depend on higher spatial correlations of spins then pair-pair ones. [Russian Text Ignored].

Domain wall calculations for SmCo 5

Abstract: The structure of domain walls may be studied theoretically by considering the wall either to be a continuous magnetization distribution (micromagnetic-type approach) or a discrete moment distribution. It is shown that the discrete moment approach becomes necessary in materials of high magnetocrystalline continuous such as SmCo 5 , where the domain walls are only of the order of 10-20 atomic distances wide. The structure of such walls is found to be very sensitive to short range variations in exchange and anisotropy and the presence of an anomalous exchange interaction between two spins produces pinning fields which are of the same order as values of coercivity in SmCo 5 . It is concluded that exchange anomalies may play an important role in domain wall pinning in the rare-earth/cobalt materials and that further information on the exchange mechanisms could help to explain their high coercivities.

1s and 2s states of direct exciton in a magnetic field

Abstract: The behaviour of the direct exciton 1s and 2s states in cubic semiconductors in a magnetic field is investigated. The degeneracy and warping of valence bands, the diamagnetic term and the electron-hole exchange interaction are taken into account. The energy splittings and relative intensities of the Zeeman components are expressed in terms of the Luttinger-band edge parameters up to the second order of the perturbation theory. The results are obtained purely analytically by use of the reduced Coulomb Green's function. The selection rules for the electric-dipole transitions at magnetic field along the principal crystal directions are deduced with the help of group-theoretical technique.

Is CTS a One-Dimensional Magnet?

Abstract: The exchange interaction in a famous antiferromagnetic linear chain system of CTS (copper tetra-amine sulfate, Cu(NH 3 ) 4 SO 4 H 2 O) was investigated using millimeter and newly developed submillimeter spin resonance techniques. The “intrachain” exchange interaction J was determined to J / k = 0.24±0.08 K which is less than one tenth of the exchange interaction to be expected from the linear chain model. The result shows that CTS can not be considered as a typical example of a linear chain antiferromagnet.

Effective Nuclear Spin Hamiltonian for Electronic Singlet Ground State System

Abstract: Along the same line as in the previous paper, an effective nuclear spin Hamiltonian is derived for the system where magnetic ions have the electronic singlet ground state and are interacting each other by the exchange interaction. All crystal-field levels of electrons are taken into account so that the Hamiltonian can be used for any realistic case in the nuclear regime. The exchange enhancement appears in the form of (1-η) -1 for the electronic Zeeman energy, the I - I coupling, the pseudo nuclear quadrupole interaction, and the pseudo nuclear Zeeman effect, η being proportional to the exchange constant. Examples are shown for rare earth ions and the resulting I - I coupling has the form of the Heisenberg model in the cubic case but of the X - Y model in the uniaxial case.

Spin susceptibility of exchange enhanced metallic films

Abstract: The spin susceptibility of an exchange enhanced metallic film is studied with the objective of determining how the paramagnetic properties are modified by the surfaces and the finite size of the system. In contrast to the usual treatments, an exchange interaction of finite range λ, comparable to the Fermi wavelength λF, is used. The exchange interactions are taken into account within a mean field theory approximation.The spin susceptibility is explicitly evaluated for a model in which the boundaries are treated as infinitely high potential barriers. It is found that the results depend sensitively on the value of λ. In particular, the large quantum oscillations in the surface magnetization found by earlier workers are significantly reduced in magnitude when a realistic value of the range is used. A generalized Stoner criterion determining the ferromagnetic instability in a film of thickness L is also derived from the spin susceptibility. The critical temperature TcF for a film is found to be depressed belo...

Phase Transition of Nearly Two-Dimensional Heisenberg Ferromagnets with Weak Perturbations

Abstract: The double-time Green's function method incorporated with Tyablikov's decoupling is applied to the nearly two-dimensional ferromagnets of layer-structure copper compounds. Besides the strong intralayer ferromagnetic exchange interaction we take account of weak perturbations, i.e., interlayer exchange interaction, intralayer exchange anisotropy, and dipolar interaction. We obtain the leading two in the inverse transition temperature T c -1 , i.e., terms logarithmically singular and constant with respect to the weak perturbation. Spin contraction at zero temperature is very small. Outside the very vicinity of zero temperature the equation for spontaneous magnetization σ as a function of k B T /2 J is reduced to a much simpler one containing only one parameter k B T c c a l c /2 J . Comparison of the calculated T c and temperature dependence of σ with experimental deta seems to show the soundness of applying the RPA theory to these systems. The contributions of dipolar interaction are examined.

Magnetic excitations of mixed CoF2/ZnF2 crystals

Abstract: The magnetic excitations in mixed crystals of Co1-cZncF2 with c=0.14+or-0.02, 0.31+or-0.03 and 0.77+or-0.10 have been measured using neutron inelastic scattering techniques. The results for the first two crystals show that the excitations decrease in frequency with increasing zinc concentration and also have a finite lifetime. For the crystal containing the largest concentration of ZnF2 well defined magnetic excitations were not observed. These results are very similar to the predictions of the virtual lattice model in which each exchange interaction of CoF2 is reduced by the concentration of CoF2 in the mixed crystal. Reasonable agreement is also found with the results of calculations based on the single-site CPA approximation but the results do not clearly show that any one of the particular variants of this model gives a markedly better result than any of the other variants.

A Theory for Self-Trapped Magnetic Polaron in Ferromagnetic Semiconductor with a Narrow Band

Abstract: In a ferromagnetic semiconductor with a narrow band the mechanism which causes magnetization of localized spins around a conduction electron is similar to that of the double exchange interaction, and the potential which traps the conduction electron is a pseudopotential. It is shown that the spin flip processes of the conduction electron make the magnetic polaron unstable, and that even in the case of the narrow band limit the radius of the magnetic polaron is large extending over many lattice sites. These properties are different from those of the usual lattice polaron in narrow band.

Crystal field and exchange interactions in ErAg

Abstract: Neutron inelastic scattering experiments have been performed on a polycrystalline sample of ErAG to obtain information on the crystal field level structure and on the exchange interaction. The measurements in the paramagnetic phase yield the crystal field parameters W=-0.040+or-0.002 meV and chi =0.42+or-0.02 which correspond to a Gamma 8(3) ground state with an overall splitting of 190K. The paramagnetic molecular field parameter is lambda P=(-1.4+or-0.2)*1023 Oe2 erg-1. From the measurements in the ordered states we obtain the molecular field parameters lambda c=(1.07+or-0.03)*1024 Oe2 erg-1 and lambda i(1.11+or-0.06)*1024 Oe2 erg-1 in the commensurate and incommensurate region, respectively. These parameters give an excellent description of the magnetic properties of ErAg. From the measured molecular field parameters three exchange integrals have been calculated.

ELECTRON SPIN RESONANCE STUDY OF INTERACTING DONOR CLUSTERS IN PHOSPHORUS-DOPED SILICON AT 100 GHz AND LOW TEMPERATURES

Abstract: ESR experiments have been performed on phosphorus-doped silicon with concentrations ranging from 19 x 10 17 cm-3 to 28 ×1018 cm-3 at 14-42 K and 100 GHz The peak shift of the central ESR line with respect to the center of the two hyperfine lines has been investigated as a function of donor concentration, temperature, and microwave power In general, it consists of a microwave power-dependent part and of a microwave power-independent one The former part is interpreted in terms of the Overhauser effect ; the latter one is due to an asymmetry of the spectrum, understood on the basis of a new model proposed by one of the authors, which takes into account the clustering of nearby donors according to the exchange interaction between their electronic spins, and the hyperfine interaction with 31P nuclei

Exchange interaction between orbitally degenerate ions Case of two 2 P ions

Abstract: The problem of the exchange interaction between two 2 P ions is studied. As a first step, from the true electrostatic hamiltonian, the symmetry and the energy of each term arising from the interaction between two orbitally degenerate ions are determined. As a second step, a phenomenological model, invariant with respect to a unitary transformation between the atomic orbital basis functions leading to the same terms as the true electrostatic perturbation, is proposed.