Electron-bottleneck mode for paramagnetic impurities in metals in the case of anisotropic exchange interaction

Equations of the Bloch-Hasegawa type are derived in the case of anisotropic exchange interaction between the subsystems of the magnetic moments of paramagnetic impurities and conduction electrons under conditions of their collective motion. Expressions describing the effective linewidth of electron paramagnetic resonance and the effective g factor are obtained.     

Paramagnetic resonance of gadolinium in intermetallic compounds YAl2 and Y1−w Th w Al2

The paramagnetic resonance of Gd in YAl₂ was measured in the temperature range from 5 °K to 70 °K. The experimental results are discussed with Hasegawa's theory. The dependence of linewidth on temperature and Gd-concentration indicates the existence of a “bottleneck” in the relaxation between the conduction electrons and the lattice. If nonmagnetic thorium impurities are added an additional relaxation becomes effective, caused by spin-flip scattering of the conduction electrons at these impurities. In this case the bottleneck is removed.     

Magnetic resonance of paramagnetic ion-nuclei in metals and superconductors

The magnetic resonance lineshape of paramagnetic ion-nuclei in metals is calculated using the temperature Green functions method and is analyzed for limiting cases of fast and slow spin lattice relaxation of localized moments. The longitudinal spin lattice relaxation rate for paramagnetic ion-nuclei in type II superconductors due to the hyperfine coupling with local moments is calculated. The influence of the fluctuation coupling of electrons on relaxation of paramagnetic ion-nuclei in “dirty” type II superconductors is investigated in magnetic field slightly above the upper critical field H_c2.     

Evidence for the spin-dependent scattering of conduction electrons on Mn2+ ions in Hg1−xMnxTe and Cd1−xMnxSe mixed crystals

Changes in the resistivity of Hg_1?xMn_xTe and Cd_1?xMn_xSe mixed crystals associated with paramagnetic resonance of the Mn²⁺ ions have been observed at liquid helium temperature in a strong magnetic field. The effect was recorded by monitoring the submillimeter radiation induced photoconductivity in a swept magnetic field. An increase in the resistivity associated with EPR of the Mn²⁺ ions is interpreted in terms of the spin- dependent scattering of electrons on magnetic impurities, the spins of which are selectively depolarised by means of paramagnetic resonance. Some additional effects influencing the experiments are also discussed.     

Enhanced superconductivity by correlations between two Kondo impurities

We study the interplay between magnetic correlations of two Kondo impurities and superconducting singlet pairing. Performing a Schrieffer-Wolff transformation in the zero-bandwidth limit of the two-impurity Anderson model we obtain the Hamiltonian of two magnetic impurities and we add a superconducting term to the conduction electrons. The model allows us to study the effect of the magnetic correlation between the impurities on the superconducting ground state. At zero temperature, different superconducting ground states can be obtained depending on the magnitude of magnetic coupling between S₁ and S₂. For increasing coupling, the superconducting region is enlarged showing an interesting result: in the strong coupling limit, where the impurities are in a very strong ferromagnetic correlation state, half of the conduction electrons are decoupled from the local moments of the impurities and take advantage of the superconducting pairing lowering the ground state energy. On the contrary, when the coupling between S₁and S₂ decreases, the scenario of the two independent Kondo impurities in presence of superconductivity emerges and all the conduction electrons are involved in the pair breaking physics. At finite temperature, we obtain the phase diagram and we observe a region of parameters where the re-entrance phenomenon occurs.     

Electronic structure and spatial distribution of the spin density of shallow nitrogen donors in the SiC lattice

The discovery of unique magnetooptical properties of paramagnetic centers in silicon carbide, which make it possible to control spins of small arrays of centers of atomic sizes to single centers at room temperatures, using the techniques of optical detection of the magnetic resonance, posed a number of problems, among which one of the main ones is the creation of conditions under which spin relaxation effects are minimized. As studies of properties of spin nitrogen-vacancy centers in diamond showed, the main contribution to spin relaxation is made by the interaction with nitrogen donors, being a major impurity in diamond. A similar problem exists for silicon carbide, since nitrogen donors are also basic background impurities. The objective of this work is to study the spatial distribution of the spin density of nitrogen donors in two basic silicon carbide polytypes, i.e., 4H-SiC and 6H-SiC, to use this information for minimizing the interaction of nitrogen donors with paramagnetic centers in silicon carbide. The results of the study are analyzed by magnetic resonance methods; the spin density distribution on the nearest coordination spheres of nitrogen donors occupying carbon sites in silicon carbide is determined. It is concluded that paramagnetic centers in the 4H-SiC polytype, including silicon vacancies, can be more stable to the interactions with unpaired donor electrons, since electrons are not localized on the coordination sphere closest to the paramagnetic center in this case.     

Optical orientation of nuclei in nitrogen alloys GaAsN at room temperature

The intensity and giant circular polarization of edge luminescence in a longitudinal magnetic field have been measured in nitrogen alloys GaAsN under circularly polarized pumping. It has been found that these dependences are shifted with respect to zero field by a value B _eff. The magnitude of the internal field B _eff increases with the pumping intensity and reaches saturation (≈250 G) at large excitation densities. The saturation of the B _eff field with growth of pumping indicates that this is a field of nuclei, polarized dynamically due to hyperfine interaction with optically oriented deep paramagnetic centers, rather than a field of exchange interaction created on the center by spin-polarized photo-excited conduction electrons. The short time of nuclear polarization by electrons (<15 μs), measured under modulation of circular polarization of the exciting light with high frequency, points to a small number of nuclei undergoing hyperfine interaction with an electron localized at a center.     

The fluctuation freeze-out of electrons in Cdx Hg1−x Te

The model was developed on the basis of experimental data, explaining pecularities of galvanomagnetic properties of Cd_x Hg_1?xTe by the “fluctuation freeze-out” of electrons - i.e. by their localization in fluctuation potential wells. In semimetallic samples this process is associated with a sharp increase in the fluctuation potential amplitude when the acceptor level goes out from the conduction band with the growth of magnetic field.     

Fir magnetospectroscopy on (CuIn)1?xMn2xTe2

The paramagnetic resonance of Mn²⁺ in the diluted magnetic semiconductor (CuIn)_1–xMn_2xTe₂ was observed in a pulsed magnetic field up to 15 T at temperatures ranging from 4.2 K to 45 K. The temperature dependence of the line width of the paramagnetic resonance in (CuIn)_1–xMn_2xTe₂ resembles the behaviour of other diluted magnetic semiconductors. A Dzyaloshinski-Moriya exchange constant of approximately 0.62 K was derived. This value fits well with the values reported for II–VI based diluted magnetic semiconductors [1], if we consider the larger degree hybridization of the 3d electrons with the band electrons in chalcopyrite semiconductors.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—I: Thermal equilibrium case

The far IR cyclotron resonance of conduction electrons is investigated in n-type indium antimonide in the quantum regimes, ?ω_ck_BT and ?ω_c?k_BT. The resonance peak position, half width, and the degree asymmetry in the line shape are studied as a function of temperature. In analyzing the experimental data, the three band model has been employed together with modern theoretical results of electron scattering by ionized impurities in the presence of a strong magnetic field. It is found that, for example for an experiment at 84 μm, the Une width depends very little on temperature between 4.2 and 45 K where the ionized impurity scattering is dominant, and increases rapidly with temperature above 45 K where the onset of phonon scattering is expected. Further details of the ionized impurity scattering were investigated by using three different laser wavelengths 84, 119 and 172μm. The line width at the phonon-limited temperature region depends very little on magnetic field and sample. The temperature dependence of the band gap was also determined by analysis of the resonance peak shift.     

ESR Investigations on Polyethylene–Fluorine Functionalized Single Wall Carbon Nanotubes Composites

Electron spin resonance investigations on nanocomposites obtained by dispersing fluorinated single walled carbon nanotubes within polyethylene are reported. Three resonance lines assigned to uncoupled electronic spins confined within magnetic impurities, amorphous carbon, and single wall carbon nanotubes have been observed. The temperature dependence of these lines is analyzed in detail and used to assign each component of the as-recorded ESR spectrum to a precise component of the nanocomposite. Magnetic impurities are originating from catalysts residues (in our case, Fe impurities). Surprisingly, the narrowest line is due to paramagnetic defects (amorphous carbon) while the broad line originates from electrons delocalized over conducting nanotubes. The broadening of this line reflects a bottleneck in the relaxation mechanism, triggered by the interaction of the uncoupled electrons localized on carbon nanotubes with the magnetic impurities.     

Spectroscopic Studies on Pure and Histidine-Functionalized Multiwalled Carbon Nanotubes

Electron paramagnetic resonance, Fourier transform infrared, and ultraviolet-visible studies were carried out for pure and histidine-functionalized multiwalled carbon nanotubes. Electron paramagnetic resonance absorption spectral data were found to be the best fit for the Gaussian lineshape. The g-values indicate the presence of magnetic impurities in the samples and the interaction between the localized electrons and delocalized electrons in the nanotubes trapped at defects or magnetic ion sites. The electron spin concentration decreases with increasing concentration of histidine, which implies that the unpaired electrons undergo a reduction process in the histidine-functionalized multiwalled carbon nanotubes. Fourier transform infrared study confirms the presence of functional groups in pure and histidine-functionalized multiwalled carbon nanotubes. Ultraviolet-visible study reveals the formation of a charge transfer complex in histidine-functionalized multiwalled carbon nanotubes.     

Polarized neutron study of TbNi2

Neutron diffraction experiments have been carried out on a TbNi₂ single crystal. Below the Curie temperature, 42 K, a magnetic contribution is observed only on nuclear scattering peaks. Therefore, the terbium atoms form a ferromagnetic structure. Polarized neutron measurements performed in the paramagnetic state, in an applied magnetic field of 57 kOe, reveal a non-uniform polarization of the conduction band. Within the experimental accuracy, no 3d magnetic moment is observed on nickel atoms. This result is consistent with the assumption of rare earth magnetic ordering occurring through the polarization of conduction electrons.     

Hysteresis and memory of the magnetic resonance of conduction electrons in solids. From bistability to stochastic resonance

The magnetic resonance line of conduction electrons in solids may exhibit bistable hysteresis if several conditions are fulfilled. Its mechanism is presented and the manifestation of bistability in the ESR of conduction electrons in single crystal and polycrystalline samples is discussed. The characteristics of the dynamics of the bistability show that bistable resonance can be assimilated to one-dimensional overdamped motion of the spin system in the nuclear field space, driven by a bistable potential. It is shown for the first time that noise acting on this bistable resonance can create order, by the phenomenon of stochastic resonance.     

The anion and cation effects in the magnetic anisotropy of rare-earth compounds: Charge screening by conduction electrons

The formation of magnetic anisotropy (MA) in rare-earth compounds with transition metals has been analyzed. The screening of the charges creating the crystal field by conduction electrons has been shown to play an important role. The calculations took into account the Friedel charge-density oscillations. The model used for RCo₅ is the point-charge crystal field including nonuniform screening by conduction electrons with an anisotropic Fermi surface. The mechanisms of strong MA due to light-element impurities (hydrogen and nitrogen) are considered. The effective charge of an impurity can heavily depend on its ionic radius and the characteristics of the Fermi surface (in particular, on the Fermi momentum k _F ) of the screening electrons. The screening of the cation and anion charge in hydrides and nitrides based on the R₂Fe₁₇ and RFe₁₁Ti intermetallic compounds is discussed.     

Formation andstability of magnetic moment on isolated Rh impurities in dilute PtFe alloys

The local susceptibilities (T) of isolated Rh impurities in paramagnetic dilute Pt(Fe) alloys have been measured using the TDPAC method. The results show existence of 4d magnetic moment for Rh with reduced spin fluctuation temperatures (T _sf). It is suggested that moment stability scaled byT _sf is governed mainly by the interatomic d-d exchange interaction caused by induced ferromagnetic polarization of host d band electrons.     

Some crystalline field effects in metals

We discuss different physical effects which are caused by the crystalline electric field splitting of rare earth ions in metals. Thus, the rare earth ions may be impurities dissolved in a metallic matrix or they may form a regular lattice. In the former case we distinguish between the cases of a normal conducting and a superconducting matrix. The influence of the crystalline field splitting on the properties of the conduction electrons is calculated. In the case of a normal matrix anomalous behaviour of the thermoelectric power is found due to the impurity levels. If the matrix is superconducting large deviations result from the theory of Abrikosov and Gorkov which describes the influence of non-split magnetic impurities on superconductivity. A comparison of the theory with available experiments is presented.

For the case that the rare earth ions form a regular lattice we discuss various aspects of the collective excitations in the paramagnetic state. Special attention is paid to the soft mode problem of exchange induced ferromagnets. Furthermore we discuss the influence of impurities on the excitation spectrum (localized modes, resonant modes etc.) and on the magnetic ordering temperature.     

Superconductivity and spectroscopy of heterofullerides Rb<Subscript>2</Subscript>MC<Subscript>60</Subscript>, K<Subscript>2</Subscript>MC<Subscript>60</Subscript>, and KM<Subscript>2</Subscript>C<Subscript>60</Subscript> (M = Mg,Be)

A series of new heterofullerides with compositions Rb₂MC₆₀, K₂MC₆₀, and KM₂C₆₀ (M = Mg, Be) have been synthesized. Measurements of the temperature dependences of the magnetic susceptibility in the temperature interval from 4.2 to 300 K reveal a superconducting transition in heterofullerides K₂MgC₆₀, KMg₂C₆₀, K₂BeC₆₀, and Rb₂BeC₆₀ at temperatures T _c = 13–22 K. The electron states with uncompensated spin are studied by the electron paramagnetic resonance technique. The contributions of conduction electrons and localized electrons to the paramagnetic susceptibility are extracted.     

Nuclear and ion spins in semiconductor nanostructures

Spin interactions are studied between conduction band electrons in GaAs heterostructures and local moments, specifically the spins of constituent lattice nuclei and of partially filled electronic shells of impurity atoms. Nuclear spin polarizations are addressed through the contact hyperfine interaction resulting in the development of a method for high-field optically detected nuclear magnetic resonance sensitive to 10⁸ nuclei. This interaction is then used to generate nuclear spin polarization profiles within a single parabolic quantum well; the position of these nanometer-scale sheets of polarized nuclei can be shifted along the growth direction using an externally applied electric field. In order to directly investigate ion spin dynamics, doped GaMnAs quantum wells are fabricated in the regime of very low Mn concentrations. Measurements of coherent electron spin dynamics show an antiferromagnetic exchange between s-like conduction band electrons and electrons localized in the d-shell of the Mn impurities, which varies as a function of well width.     

Electric field-induced quasi-elastic inter-landau level scattering in the space-charge-limited magnetoconductivity of n+n−n+ InP structures

Magnetophonon resonance and quasi-elastic inter-Landau level scattering (QUILLS) are investigated in the transverse magnetoconductivity, σ_xx, of short (1 μm) n⁺n⁻n⁺ InP structures under conditions of space-charge-limited conduction. The damping of the magnetophonon resonance amplitudes at large E is also investigated. The electric field dependence of the QUILLS process in InP is in excellent agreement with a model that we have previously proposed to describe the process in n⁺nn⁺ GaAs layers. The observed form of the space-charge-limited current-voltage characteristics (I ∝ V²) is discussed in terms of the behaviour of electrons in short, lightly doped semiconductor layers under the influence of large, mutually perpendicular electric and magnetic fields.