Electron-magnon Interaction Research Articles

Electrical transport in below 60 K

The electrical transport properties of have been investigated using complex-plane impedance analysis and dielectric measurements with measurements of four-probe dc conductivities and magnetic susceptibilities. The temperature dependencies of the bulk conductivities obtained by impedance analysis, and dielectric relaxation processes provide evidence of the hopping conduction below 60 K, which is remarkably different from the hopping conduction in , despite the degree of doping with Sr being very small. The features in the low-doping range are: (i) the emergence of the hopping conduction at markedly lower temperatures in comparison with the case for non-doped ; (ii) the abrupt reduction of the activation energy required for the conduction, 0.042 eV at x = 0.03 to 0.021 eV at x = 0.07, from 0.34 eV for ; and (iii) a progressive decrease in the activation energy with increasing x. The difference in conduction behaviour between and is due to the alternation of the spin state of caused by the Sr doping. The results have been discussed in terms of a process of hopping of small polarons localized by the electron-lattice interaction in tandem with the electron-magnon interaction.

TJ-MODEL IN TERMS OF EQUATIONS WITH VARIATIONAL DERIVATIVES

An s-d exchange model of a magnetically ordered metal is formulated in terms of a generating functional that describes the interaction of the system with fluctuating fields. The electron and spin Green's functions are represented by variational derivatives of this functional with respect to fields. Such an approach, which was recently successfully applied by the authors to the isotropic Heisenberg model, now is extended to the case where localized spins interact with conduction electrons. Exact equations were obtained for the electron and magnon Green's functions and the structure of the self-energy part of electrons and magnons was investigated. The iteration of these equations generates a perturbation theory in s-d interaction in the form of a diagrammatic technique for spin and Fermi operators. Exact graphical representations for the electron and magnon Green's functions through vertex parts of the electron-magnon interaction are given. Several approaches are suggested for approximately solving the equations for vertex parts. As limiting cases, all earlier known results concerning the spectrum and damping of spin waves in a ferromagnet are obtained, as well as those concerning the magnetic polaron and dynamic magnetic susceptibility. In the limit of a strong s-d exchange interaction, the spectrum of magnon for the ferromagnetic state of the system has been calculated.

Magnetotransport properties of NiFe–Ag granular alloys: Origin of the thermal behavior

The effect of the temperature and magnetic field on the giant magnetoresistivity (GMR) of two FeNi–Ag granular alloys of composition Fe11.4Ni6.4Ag82.2 and Fe7.6Ni16.4Ag76.0 is discussed. Both samples were prepared by rf magnetron sputtering. Parts of them were rapidly annealed at 600, 650, and 750 °C. All samples displayed giant magnetoresistivity which decays from its maximum value with a Tm behavior, with m≈0.8–0.9, suggesting that the decrease in the maximum magnetoresistivity is due to the reduction in the particle magnetization associated with the spin wave excitation, which is a different mechanism to the electron-magnon interaction responsible for the T dependence of GMR in magnetic multilayers. Magnetoresistivity ρM decreases with temperature sharing essentially the same temperature decrease as the square of the macroscopic magnetization M in the whole magnetic field range studied, which is due to the reduction in the particle magnetization and to superparamagnetic effects. The effect of the width of the particle size distribution and interparticle interactions on the linear relation ρM vs M2 are discussed. Care should be taken when representing ρM/ρ(T,H=0) vs (M/Ms)2 because the strong temperature-dependent slope shown in these plots is mainly due to the temperature dependence of both the resistivity ρ(T,H=0) and Ms, and it is not an intrinsic T dependence of GMR in granular alloys. Experimental results suggest that in granular materials, magnetoresistivity is dominated by magnetic moments at the surface of the particles, which also play a very important role in the demagnetization processes, and small magnetic particles.

Heating of magnons by hot charge carriers and electrical properties of HgCr2Se4

Strong variations of the conductivity and the Hall coefficient as a function of the magnetic and electric field strengths are discovered in the ferromagnetic semiconductor HgCr2Se4. The nature of these phenomena is discussed in connection with its electronic structure and the strong interaction between the electric and magnetic subsystems in this magnetic semiconductor. The results can be interpreted within a model of ordinary semiconductors with consideration of the strong electron-magnon interaction specific to magnetic semiconductors, the heating of magnons by hot charge carriers, and the trapping of charge carriers (the formation of ferrons) due to the s-d exchange interaction.

On competition between s- and d-symmetry types of the order parameter in high-temperature superconductors

It is shown that strong anisotropy of the quasi-two-dimensional electron spectrum in high-temperature superconductors and the competition between attraction and repulsion in the electron–electron interaction can lead either to anisotropic s-, or d-type of Cooper pairing depending on the system parameters. In the case when attraction prevails on the entire Fermi surface (e.g., due to electron–phonon interaction), the sxy- or s*-symmetry in the superconducting order parameter is advantageous from the energy point of view. At the same time, the dx2−y2-symmetry of the gap is observed in the case of repulsion in the entire Brillouin zone (which is typical, for example, of the electron–magnon interaction) as well as in the case of effective attraction on the regions of Fermi surface with the maximum density of states. In the latter case, the superconducting transition temperature Tc is always higher than in the former case, indicating the important role of additional attraction in the mechanism of high-temperature superconductivity.

Influence of ferromagnetic spin waves on persistent currents in one-dimensional mesoscopic rings.

The influence of the electron-magnon and the electron-phonon interactions on the persistent current in a one-dimensional mesoscopic ring is studied. We show that, due to the electron-magnon interaction, the amplitude of the persistent current is exponentially reduced compared to the free case. Two features occur in the presence of an electron-phonon interaction. For the normal state of electrons, the persistent current is weakened by the Debye-Waller factor. Considering the so-called Peierls distortions, we show that the effect of the Peierls instability on the amplitude of the persistent current (i.e., the oscillation with respect to the flux) is suppressed significantly and the persistent current will be practically undetectable in the case of a wide-gap Peierls material. \textcopyright{} 1996 The American Physical Society.

Origin of linear background measured on YBaCuO-Au point contact junctions

The differential conductance of YBaCuO-Au point contacts was measured after formation of contact by high bias voltage (up to ±2V). Changes of the differential conductance from a linear dependence to a quadratic one and vice versa with respect to the bias voltage polarity aplied were observed. These features were described in terms of inelastic tunneling due to strong electron-magnon interactions in the barrier. A good coincidence of theoretical and experimental characteristics indicates that the linear background of differential conductance is not an intrinsic property of high-Tc superconductors

Many-particle theory of electron transport processes in transition liquid and amorphous metals

We consider electron-ion, electron-electron, electron-phonon and electron-magnon interactions in the theory of electroconductivity and thermal conductivity of transition liquid and amorphous metals. Expressions for relaxation times are decomposed into series by powers of the weak pseudopotential of the electron-ion interaction, hybridization potential and exchange interaction of the electrons of different subsystems. Other interactions in metal are taken into account in arbitrary order. The temperature dependence of the magnetic contributions to the electrical resistivity and thermal resistivity of ferromagnetic and antiferromagnetic liquids and amorphous metals is studied over a wide temperature range.

A THEORY OF FERROMAGNETISM IN THE HUBBARD MODEL WITH INFINITE COULOMB INTERACTION

On the basis of a regular perturbation theory in the form of diagrammatic technique with X-operators, a ferromagnetic state in the Hubbard model with infinite on-site Coulomb interaction U is studied for a wide range of electron concentration n. Treating the kinetic energy of electrons as perturbation, we get the exact graphic representations for three Green’s functions [Formula: see text] and [Formula: see text] for electrons with spin ↑ and ↓ (spontaneous magnetic moment is in ↑ direction) and [Formula: see text] for spin waves. All of them are expressed through the unique system of four-point and three-point vertex parts for effective electron-magnon and electron-electron interactions. These vertex parts are calculated in two approximations: a low-density approximation for n≪1 or nh=1–n≫1, and the generalized random-phase approximation (GRPA), which was suggested earlier by us for the description of paramagnetic phase in this model. An important result in both cases is the understanding of essential difference of electron states with spin ↑ and ↓. The state for ↑ has coherent character in all region of electron concentration (n>nc) where a ferromagnetic state exists, while the state for spin ↓ has mixed characters including both coherent and incoherent contribution. For the saturated ferromagnetism (n>ns) when nh≪1, [Formula: see text] is an entirely incoherent (nonquasiparticle) state. Appearance of incoherent state is probably a general property of strongly correlated systems distinguishing its behavior from the Fermi liquid. We show also that the Hubbard-I approximation has no region of applicability for the electron with spin ↓ in a ferromagnet and we found a new factor describing the correlation-narrowing of ↓ spin electron band. Green’s function [Formula: see text] calculated in two different limits n≪1 and nh≪1, are jointed together to allow us to calculate ns which lies in the intermediate concentration regime. For simple cubic lattice we found ns=0.87. In the limit nh≪1 our results reduce to the earlier known ones, including Nagaoka’s result for the spin wave energy. We point out the way to construct a rigorous theory for ferromagnetic state in the intermediate range of electron concentration. In the conclusion several important problems are discussed in connection to the continuation of the present work.

Temperature-dependent electronic structure and magnetic behavior of Mott insulators.

We investigate the temperature-dependent electronic and magnetic properties of transition-metal monoxides by use of a theoretical model that takes into account the 3d subbands of the transition metal and the oxygen 2p subbands. Characteristic properties of these Mott insulators are caused by strong 3d-electron correlations, described by intra-atomic Coulomb interactions and characterized by three different coupling constants: U, U\ifmmode\bar\else\textasciimacron\fi{}, and J. The intraband coupling U turns out to be decisive for the appearance of spontaneous antiferromagnetism. U\ifmmode\bar\else\textasciimacron\fi{} is a direct interband Coulomb interaction responsible for a nonuniform occupation of the 3d subbands, and therewith decisive for the insulator properties of these materials. The interband exchange J describes an effective electron-magnon interaction and polarizes magnetically inactive subbands. The self-consistent solution of our model yields a quasiparticle density of states that contains three nonconnected energy regions with predominantly 3d character: two below, and one above, the chemical potential \ensuremath{\mu}. The insulator gap is practically temperature independent, persisting for arbitrary T\ensuremath{\gg}${\mathit{T}}_{\mathit{N}}$. A necessary condition for the insulating behavior is an integral number ${\mathit{n}}_{3\mathit{d}}$\ensuremath{\ge}5 of 3d electrons. The antiferromagnetism is caused by 10-${\mathit{n}}_{3\mathit{d}}$ exactly half-filled 3d subbands, while ${\mathit{n}}_{3\mathit{d}}$-5 subbands are completely filled. The magnetic moment remains stable for arbitrarily high temperatures. The temperature dependence of the sublattice magnetization is caused by a ``kinetic Heisenberg exchange.'' The 2p-3d hybridization plays only a minor role with respect to the magnetic and insulating properties of Mott insulators, but becomes decisive for the nature of the band gap and the interpretation of photoemission data. Our model calculation correctly predicts MnO, FeO, CoO, NiO, and CuO to be antiferromagnetic insulators, while VO turns out to be a nonmagnetic metal.

De Haas-van Alphen Effect in the Antiferromagnetic Compound SmIn3

We have measured the de Haas-van Alphen effect in the antiferromagnetic compound GdIn 3 . The Fermi surface of GdIn 3 is almost the same as that of the non- f reference compound LaIn 3 . The cyclotron masses of GdIn 3 are also close to those of LaIn 3 . These results indicate that the 4 f electrons in GdIn 3 are well localized and that mass enhancement due to the electron-magnon interaction is extremely small.

Polarons in thet-J model

A convenient form of the Peierls-Hubbard Hamiltonian is obtained for the case when the Hubbard repulsion is the largest energy parameter. It allows to consider in the spin-wave approximation the properties of the one-hole low-lying excitations of a 2d lattice. For the parameters approximately corresponding to La2CuO4 it is shown that the hole polarons in the CuO2 planes of lightly doped samples are of large size with a solitonlike-shaped highly asymmetric wave function oriented along the diagonals of the planes or of small size depending on the value of the electron-phonon coupling. In both cases the cooperative effect of the electron-phonon and electron-magnon interactions leads to a large effective mass and to hopping transport of the excitations, with preferential jumps along the diagonals in the former case and rotationally symmetric in the latter. For hoping matrix elements which are small in comparison with a phonon quantum the competition between the interactions leads to the decrease of the total spin in the ground state with increasing electron-phonon coupling.

The transport properties of ferromagnetic metals: I. Conductivity

In the framework of s-d(f) exchange model the conductivity of pure ferromagnetic metals is considered. The calculations are based on the system of quantum kinetic equations, derived diagrammatically using the linear response concept. For solving the system of integral equations for the vertex function associated with the deviation of electron distribution function from the equilibrium value, the Chapman-Enskog method is used. In the frequency range 1 τ ≲ω⪡T the results for the conductivity σ(ω) differ from the usual Drude-Zener formula due to renormalization effects caused by electron-magnon interactions. The temperature dependence of the static resistivity ϱ = 1 σ (0) is described by the following scenario: ϱ≈T 9 2 in the low temperature region T⪡ T 0, ϱ≈ T 2 in the range T 0⪡ T⪡ T c and ϱ≈ T at elevated temperatures T⪡ T c .

PECULIARITY OF SUPERCONDUCTIVITY IN A METAL WITH A SPIRAL MAGNETIC STRUCTURE

Superconductivity states in a metal with existing spiral magnetic structure is studied. s-d exchange interaction of the conduction electrons with localized spins forming the magnetic ordering is treated in the framework of the strong coupling theory of superconductivity. Two separate factors are taken into account which influence the possibility of coexistence of superconductivity and magnetic ordering: electron-magnon interaction, as well as a modification of the electron spectrum due to its interaction with the static magnetic structure. The gap function is a 2 × 2 matrix with respect to the band indexes of hybridized electron states with up and down spins. The linearized Eliasberg's type equations for diagonal and off-diagonal elements are separated. Off-diagonal and diagonal elements correspond to quasisinglet and quasitriplet pairings respectively. As a direct consequence of violation of the time reversal invariance due to magnetic ordering, the diagonal gap function does not have a definite parity with respect to frequency. For this situation a moment method is developed for a calculation of the superconducting transition temperature in the limit of weak s-d-exchange coupling.

Temperature dependence of sound attenuation in impure metals.

Temperature dependence of the sound-attenuation coefficient in an impure metal arising from inelastic electron scattering is investigated. We consider the electron-phonon and electron-electron interactions, as well as the electron-magnon interaction in a ferromagnetic metal, as processes of inelastic electron scattering. The interference effects between inelastic electron scattering and the elastic electron-impurity scattering, together with the weak-localization effect, are taken into account.

Influence of Electron–Magnon Interaction on Electronic and Magnetic Properties of Ferromagnetic 4f Systems

AbstractA fully selfconsistent approach is proposed to the s‐f model of a ferromagnetic 4f‐system. A Green's function method is used for studying the mutual influence of itinerant conduction electrons and localized 4f‐moments. The striking temperature‐ and carrier concentration‐dependence is inspected of the conduction band structure, the electron spin polarization, and the Curie‐temperature Tc. A drastic Tc‐enhancement is observed with increasing band occupation, n, as a consequence of the s‐f exchange and the Coulomb interaction between the conduction electrons.

On the photostimulated amplification of magnons in magnetic semiconductors

We analyse the conditions for spin wave amplification in non degenerate magnetic semiconductors via electron-magnon interaction in a laser field. By considering the full electron-photon-magnon scattering processes we show that magnons may be amplified only in one range of magnon wavenumber.

Density-of-states and tunneling phenomena in degenerate ferromagnetic semiconductors

The peculiarities of the density-of-states near the Fermi energy of the degenerate ferromagnetic semiconductor (FMS) due to the electron-magnon interaction are considered. They lead to asymmetric anomalies of the current-voltage characteristic σ( U) of the metal-insulator-degenerate FMS tunnel junction at small bias. It is shown that the measurement of the quantity dσ( U)/d U enables one to investigate the magnon density-of-states.

Low-temperature Peltier heat of an itinerant electron in a ferromagnetic semiconductor.

The Peltier heat of a wide-band itinerant carrier in a ferromagnetic semiconductor has been calculated for temperatures below the Curie temperature. In this regime we treat the spin fluctuations within the spin-wave approximation. The coupling of the charge carrier to the local moments is via local intra-atomic (e.g., s-f or s-d) exchange. Taking the strength of the intra-atomic exchange interaction to be small compared with the carrier's electronic bandwidth, we treat the interaction between the carrier and the local moments perturbatively through second order. We use the perturbed energy to compute the free energy of the coupled electron-magnon system. From the carrier-induced change of the system's free energy we directly obtain the carrier's Peltier heat. The Peltier heat contains two terms of opposite sign which both increase in magnitude with increasing temperature. These two terms arise from the first- and second-order contributions to the energy of the coupled system. Except at very low temperatures, the first-order contribution dominates. Then the electron-magnon interaction provides a negative contribution to the Peltier heat of a ferromagnetic semiconductor. The magnitude of this contribution varies as ${T}^{3/2}$.

The damping of spin waves in dirty conducting ferromagnets due to the electron-magnon interaction

The Green's function of a magnon interacting with conduction electrons which move in an arbitrary non-periodic potential field is calculated within thes-d(f) exchange model. The contribution of the lowest order damping to the spin-wave resonance linewidth is discussed. The temperature and wavevector dependences of the second order damping due to the electron-magnon scattering are investigated. The local and optical magnon modes are considered.