Showing papers on "Exchange interaction published in 1982"

The Jahn-Teller effect and magnetism: transition metal compounds

Abstract: The properties of magnetic insulators containing orbitally degenerate transition metal ions (Jahn-Teller ions) are discussed. The Jahn-Teller effect in these insulators causes structural phase transitions, lowers the lattice symmetry, and gives rise to an orbital ordering. Various interactions responsible for these effects are discussed: the electron-lattice, quadrupole-quadrupole, and exchange interactions. The mutual effects of the orbital ordering and the magnetic properties of corresponding compounds are discussed. The exchange interaction in the cases of twofold and threefold orbital degeneracy is discussed. The effect of a magnetic field on the orbital and magnetic structure and the temperature dependence of the exchange interaction are studied. The properties of several representative compounds containing Jahn-Teller ions are discussed.

Theory of Electronic Structures and Lattice Distortions in Polyacetylene and Itinerant Peierls Systems. I ---UHF Transfer Matrix Method Adapted to the Long Range Coulomb Interaction and UHF States in Regular Bond Alternated Lattice---

Abstract: We develop in this series a theory of itinerant Peierls systems that mainly aims to study polyacetylene. In order to investigate roles of the long range Coulomb interaction in electronic structures and lattice distortions in a Peierls system, we develop a transfer matrix technique adapted to the. unrestricted Hartree·Fock (UHF) approximation. The method makes it pos­ sible to calculate UHF states and equilibrium lattice geometries in a lattice with any aperiodic structure due to the presence of solitons. Applying the method, we obtain the UHF states in the case of regular lattice that may become the HF ground state. There are at least eight such UHF states which are distinguished by the long range orders in the spin and charge densities and the bond orders. The nearest neighbour exchange interaction is responsible to produce such plentiful UHF states. We calculate the bond alternation potential and the band gap in trans polyacetylene using paramet­ rizations of the Hamiltonian and the elastic potential that are able to well reproduce spectra and equilibrium geometries of small conjugated hydrocarbons. The bond alternation potential is very sensitive to the strength of the nearest neighbour Coulomb interaction. The Coulomb potential with fast damping prevents the bond alternation. The band gap is much larger than the energy 2 eV of the lowest absorption band as long as the Coulomb interaction in poly­ acetylene is similar to that in small conjugated hydrocarbons, suggesting the excitonic nature of the absorption band. We make also an illustrative calculation for an itinerant spin Peierls system with an antiferromagnetic spin order as well as a lattice dimerization.

Electrical, optical, and magnetic properties of Hg1−xMnxTe

Abstract: We discuss the band structure and the optical, electrical, and magnetic properties of the compound semiconductor Hg1−xMnxTe, formed by substitution of the magnetic Mn ions for Hg in the HgTe lattice. In the absence of a magnetic field, this alloy is very similar to its sister compound Hg1−xCdxTe. Although Mn is not a group II element, Hg1−xMnxTe crystallizes in the zincblende structure, forming good quality crystals up to x≊0.35. Its energy gap and related band parameters vary with x, but at a rate about twice as fast as in Hg1−xCdxTe. The electrical properties of Hg1−xMnxTe are again similar to those of Hg1−xCdxTe, exhibiting, e.g., electron mobilities in excess of 106cm2/V⋅s. Because Mn is a magnetic ion, Hg1−xMnxTe differs from Hg1−xCdxTe in its magnetic properties, as well as in the behavior of its electrical and optical properties in the presence of a magnetic field. For example, for x≳0.17 Hg1−xMnxTe exhibits a transition to the spin glass phase at low temperatures. Furthermore, the presence of Mn ions leads to an exchange interaction between the localized magnetic moments and the band electrons, which in turn affects the band parameters and leads to new and spectacular effects in the transport and optical properties.

Relation between Structural and Magnetic Properties of Compound MnAs1-xPx (0≦x≦0.275)

Abstract: The crystallographic parameters of compounds MnAs 1- x P x (0≦ x ≦0.275) are determined as a function of temperature between about 100 and 600 K. It is found that these crystals except for MnAs take a MnP type structure in low temperature side and transform to a NiAs type at critical temperatures. These structural transitions are of second kind and accompanied with very large dilation (∼10%) of crystal volume. In order to see the relationship between the structural and magnetic properties, the temperature variation of magnetic susceptibilities of the compounds is also measured. Simple analysis of the experimental results, based on the Curie-Weiss law, reveals the way in which the spin value of Mn atoms and the exchange interaction between them depend on the crystal voluem (or lattice parameter).

Magnetism in transition metals at finite temperatures: I. Computational model

Abstract: Considerable evidence indicates that in Fe above the Curie temperature the electron system forms local magnetic moments which are relatively stable in magnitude and rotate to destroy long-range order. The local moments arise from exchange polarisation of the electron gas in a similar way to the conventional picture of the ferromagnetic ground state of the metal. The authors set up a one-electron-like Hartree-Fock-like model with a Stoner-like intra-atomic exchange interaction for calculating the electronic structure of such a system with arbitrary directions for the atomic moments. The directions for a cluster of 500-1000 atoms are assumed and fed into the calculation, from which the magnitudes of the moments are computed self-consistently using Haydock's recursion method (1980). The total energies are also obtainable. Results are presented for two simple types of magnetic configuration: spin spirals represent a smooth variation of the direction of magnetisation whereas alternating tilts form a rough variation back and forth from atom to atom.

Magnetic ordering in EuxSr1−xS, a diluted Heisenberg system with competing interactions (invited)

Abstract: Presence of additional antiferromagnet bonds in a ferromagnet changes magnetic ordering in a dilution series drastically. An ideal system to study these effects is the insulating EuxSr1−xS, based on EUS, the well‐known model substance for a Heisenberg ferromagnet. The competition between ferromagnetic nearest neighbor exchange J1 and antiferromagnetic next‐nearest neighbor exchange interaction J2 with J2/J1 = −0.5 is shown to cause a breakdown of ferromagnetism in EuxSr1−xS at x = 0.51, far above the percolation threshold xp = 0.13. In the intermediate concentration regime, 0.13⩽x⩽0.51, spin‐glass behavior is observed very similar to the features in metallic spin glasses. Results on (Eu, Sr)S provide clear evidence for a cooperative phenomenon at the spin‐glass transition, as distinguished from ordinary thermal blocking of superparamagnetic clusters. By increasing the Eu concentration just above xc, long‐range ferromagnetic order is found to coexist with a high amount of wrong aligned spins, which again i...

Anomalous magnetic structures and phase transitions in non-Heisenberg magnetic materials

Abstract: This review is primarily devoted to a discussion of isotropic magnetic materials with localized magnetic moments between which the exchange interaction is more complex than in the usual Heisenberg model. They include insulators for which the biquadratic or multispin exchange mechanisms are comparable with the bilinear exchange, conductors which do not satisfy the conditions of validity of the RKKY indirect exchange theory, and the nuclear magnetic material solid helium. Additionally, an analysis is made of high-anisotropy magnetic materials behaving similarly to isotropic non-Heisenberg materials. These anisotropic materials have anomalous properties, compared with the Heisenberg case. Experimental data on these materials and their theoretical interpretation are given. The following topics are discussed: 1) the conditions for a strong non-Heisenberg exchange and non-Heisenberg Hamiltonians; 2) quadrupole ordering and order-proper disorder phase transitions; 3) order-order and order-improper disorder phase transitions; 4) metamagnetism of isotropic materials; 5) normal sequences and "devil's ladders" of phase transitions between commensurable structures; 6) canted antiferromagnetism of high-symmetry crystals which do not obey the condition for the existence of relativistic Dzyaloshinskiĩ-type interactions.

On the electronic structure of transition-transition metal glasses

Abstract: By solving the Schrodinger equation exactly for representative clusters of nonoverlapping spherically symmetric potential wells, designed to describe one-electron potentials in metallic glasses, the authors have studied the electronic structure of PdcZr1-c, FecZr1-c, CocZr1-c, NicZr1-c and CucZr1-c with c approximately 0.3. The authors' clusters were selected to display the significance of both positional and chemical order. Their results suggest that the latter is the more important effect as far as the electronic states are concerned. They found that the nonselfconsistent potentials, constructed by overlapping atomic charge densities and taking the exchange interaction into account by using the Slater approximation ( alpha =1), gave rise to a 'band structure' consistent with the measured photoemission and soft X-ray spectra.

Dynamical critical behavior of isotropic ferromagnets

Abstract: Experiments measuring the dynamical exponent $z$ in the isotropic ferromagnets EuS, EuO, Ni, Fe, and Co are reviewed. Our recent hyperfine interaction experiments demonstrating crossover in $z$ are described in detail. In contrast to early neutron experiments on isotropic ferromagnetis it is found that with the exception of Co, pure Heisenberg behavior, i.e., $z=2.5$, is not observed in any of the materials surveyed. Instead, when either the wave number or the reduced temperature is sufficiently small, isotropic ferromagnets exhibit asymptotic behavior characterized by $z=2.0$. The most likely theoretical explanation is that significant spin-nonconserving forces are perturbing the Heisenberg exchange interaction. For ESR and neutron studies of EuS and EuO it has been shown that the observed behavior can be explained by dipolar forces. For hyperfine interaction experiments on Ni and Fe, the observed crossover to $z=2.0$ must be attributed to other, stronger spin-nonconserving forces. In the exceptional case of Co, where crossover to $z=2.0$ has not yet been observed, it is expected that future, more nearly asymptotic experiments, will detect crossover.

The asymptotic theory of resonance charge exchange between diatomics

Abstract: The asymptotic theory of resonance charge exchange between a ground-state diatomic molecular ion and its neutral parent is presented The parameters of the valence electron wavefunction and asymptotically precise exchange interaction potential are calculated The role of rotational transitions is discussed The vibrational excitation transfer is taken into account and the coupled equations, describing the charge exchange process between diatomics, are solved both in limiting cases and numerically The total charge transfer cross sections are calculated for many diatomic systems and the results are compared with experimental data

4 f Spin Dynamics of Isolated Nd, Pr, and Ce Ions in Simple Metals

Abstract: The $4f$ spin dynamics of extremely dilute Nd, Pr, and Ce ions in liquid metals are microscopically observed by the time-differential perturbed $\ensuremath{\gamma}$-ray distribution method. The $4f$ spin rates are dominated by strong mixing exchange interaction in all systems investigated, which is, e.g., reflected by a Kondo-like behavior for Nd systems. The pressure-driven valence transition in dilute $\mathrm{La}\mathrm{Pr}$ alloys is probably of the $4{f}^{2}\ensuremath{\rightarrow}4{f}^{1}$ type. One has to expect that $4f$ instabilities might occur for all light rare-earth ions in appropriate systems.

Heat Capacity of PrNi2, YNi2 and Pseudobinary PrcLa1-cNi2 System

Abstract: Heat capacity data are presented for intermetallic compounds PrNi 2 , YNi 2 and the pseudobinary system Pr c La 1- c Ni 2 . The temperature range covered is 2 to 80 K. The magnetic heat capacity deduced under certain assumptions shows that in PrNi 2 the singlet \(\varGamma_{1}\) is the ground level and that the first excited level lies about 60 K above the ground level. An excess contribution is found around 10 K. The excess heat capacity is understood if the excited level is assumed to have appreciable broadening due to the exchange interaction acting between Pr atoms.

Magnetic properties of the quasi 1d Ising ferromagnet Co[(CH3)3NH]Cl3·2H2O

Abstract: A.c. susceptibility measurements on the 1d Ising ferromagnet Co[(CH 3 ) 3 NH]Cl 3 ·2H 2 O (CoTAC) are reported. At zero field these measurements were performed at temperatures between 1.2 and 150 K, with the a.c. field h parallel to the three orthogonal crystal axes. The data show the 3d AFM ordering along c to occur at T c = 4.179(5) K, but the moments are canted over 22° towards the a axis. The results were satisfactorily described with an intrachain exchange interaction J b / k = 13.8(5) K and interchain interactions z c J c / k = 0.28(5) K and z a J a / k = -0.0320(5) K. From the field-dependent susceptibilities the metamagnetic phase diagram was obtained. The tricritical point occurred at an internal field of 29(1) Oe and T c = 4.125(5) K. Along the crystal a axis CoTAC behaves as a weak ferromagnet. From the frequency dependence of χ it was possible to determine the domain-wall relaxation times. These time constants were found to vary exponentially with T . from which it was suggested that the spin reorientation arises due to impurity relaxation. The normalized susceptibilities as a function of reduced temperature demonstrated the occurrence of two lattice dimensionality crossovers. The spontaneous (weak) ferromagnetic moment parallel to a varies with T in a way similar as calculted for a 3d Ising ferromagnet.

Relation between Exchange Interaction and Crystal Structure of Verdazyl Radicals. As Studied by Means of McConnell’s Spin Density Hamiltonian

Abstract: The exchange interactions in four kinds of verdazyl crystals, 2,4,6-triphenylverdazyl (TPV), 2,4,6-triphenyl-3-methylverdazyl (MeTPV), 2,4,6-tri-p-tolylverdazyl (TTV), and 2,6-di-p-tolyl-4-phenylverdazyl (DTPV), have been studied by means of McConnell’s spin density Hamiltonian. The calculated exchange parameters, J, have been compared with the experimental values derived from the magnetic susceptibilities. The observed order of J’s J⁄k=−5.4 (TPV), −9.1 (MeTPV), −11.1 (DTPV), and 0–−0.3 K (TTV), can be qualitatively explained by the following equation:J∝\undersetijΣ (−1⁄rij)ρilρjl+1,where ρil and ρjl+1 are π-spin densities on atoms i and j of the l-th and (l+1)-th molecules in the crystallographic column and rij is the interatomic distance between atoms i and j. The magnetic susceptibility data of MeTPV and TTV between 1.6 and 300 K are presented and interpreted with the Heisenberg linear chain model. The susceptibility of DTPV is re-interpreted in the same framework as above.

Ab initio calculations of exchange repulsion between two Ar atoms

Abstract: The first order exchange energy for the Ar-Ar interaction has been calculated using an SCF wavefunction for Ar in a large gaussian basis set. It is shown that in the region of the van der Waals minimum more than 95 per cent of the first order exchange originates from the interaction of M-shell electrons and that three-orbital terms dominate this interaction. The terms of the order S 4 prove to be negligible. Interaction energies from SCF supermolecule calculations are also given.

Evidence of the Conduction-Valence Band Overlap Induced by a Magnetic Field in Zero-Gap Hg1-xMnxTe

Abstract: Experimental evidence and theoretical results are given of the conduction–valence bands overlap induced by an exchange interaction in zero-gap Hg1–xMnxTe. The Fermi level as well as the concentration of electrons and holes strongly depend on the magnetic field in the region where the overlap exists. Experimenteller Nachweis und theoretische Ergebnisse der durch Austauschwechselwirkung induzierten Leitungsband–Valenzband-Uberlappung in gaplosem Hg1–xMnxTe werden angegeben. Sowohl das Fermi-Niveau als auch die Konzentration der Elektronen und Locher hangen vom Magnetfeld in dem Bereich ab, wo Uberlappung existiert.

High‐frequency sound as a probe of exchange energy in nickel

Abstract: The experiments performed at 9.37 GHz involve excitation of sound at ferromagnetic resonance via magnetoelastic coupling, its propagation through thick Ni samples, and reconversion to microwaves. In the ω‐k plane there is a substantial angle between the ω axis and the phonon dispersion relation. The intersection of the phonon and spin wave dispersion occurs at a wave vector for which a large part of the spin wave energy is due to exchange. It is for this reason that sound is particularly appropriate for measurement of exchange energy.

Wave-Number Dependent Oscillator Strength and Longitudinal-Transverse Splitting of Exciton in CuCl

Abstract: Wave-number dependence of exciton longitudinal-transverse splitting was considered in a previous paper, where a microscopic calculation of the electron-hole exchange interaction was shown to account for experiment on CuCl. In this paper, macroscopic approach, based on considerations on the dielectric function, is taken to the same problem. For exciton with finite wave number, it is necessary to distinguish between longitudinal and transverse oscillator strength; the former is proportional to the longitudinal-transverse splitting of exciton. Equivalence of the two approaches is proved.