Solid State Phenomena Vols. 168-169

Abstract: The temperature dependences of the transverse and longitudinal hypersound velocities at the frequencies (0.5-0.7) GHz were measured in the La1-xSrx MnO3 (x = 0.125; 0.15; 0.175) compositions. Structural phase transitions the position of which is confirmed by the electric resistance and magnetic measurements data for the same samples are observed. These results were analyzed within the model of the competing Jahn-Teller distortions and magnetic ordering. Anomalies in the behavior of the longitudinal hypersound velocities were related to the local Jahn-Teller distortions the suppression of which upon magnetic ordering is considered as a possible origin of a colossal magnetoresistance.

Abstract: The results of structural neutron diffraction study and the data on the transport and magnetic properties (the linear and nonlinear (second and third order) susceptibilities) are presented for the La0.88Ca0.22MnO3 single crystal. This compound has the orthorhombic Pbnm space group, showing no structural transformations between 90 K and 300 K. It exhibits the paramagnet-ferromagnet (P-F) (TC  186 K) and insulator – metal (I-M) (ТIM  201 K > TC) phase transitions. The measurements of the second harmonic of the magnetization in the parallel ac and dc H magnetic fields, M2(H,T), showed that below T* 252 K, the FM clusters appear in the PI matrix undergoing the second-order transition. The concentration of the FM clusters increases with cooling so that it is above the percolative threshold value below TIM, and the sample exhibits metallic behavior. The M2(H) hysteresis loops possess a high sensitivity to the I-M transition. Their usual central symmetry is broken in the vicinity of TIM and the loops become asymmetric. This effect is attributed to cluster link nucleation-and-growth process.

Abstract: Based on a simple model, theoretical description of localized spins of hybridized electron states of donor impurities in the conduction band of asemiconductor has been developed. Equations are formulated and solved numerically that describe the concentration dependence of localized spins. It is shown that they can exist and reach maximal values in a certain interval of impurity concentration in the vicinity of its value, at which the Fermi energy of the electronic system coincides with the energy level of the donor impurity. The conclusions obtained are compared with the results of Anderson’s theory.

Abstract: The results of experimental investigations of Fe1-xCoxSi crystals in the impurity limit are presented in this article. We made an attempt to study changes of magnetic properties and conductivity in mixed Fe1-xCoxSi crystals in the impurity limit. Magnetoresistance properties are well described in the framework of Kondo model. The presence of Co-subsystem leads to the occurrence of spin-dependent channel in electron scattering conduction.

Abstract: The low temperature investigations of the magnetic properties of non-twinning LiCu2O2+δ crystals showed that a new spontaneous magnetization phase arises in the compound at temperatures less than T~150 K along with the basic long-range helimagnet phase at T<24 K. The increase of an extra oxygen Оδ concentration intensifies ferromagnetic response of this phase. We suggested a cluster model for the new phase as an antiferromagnet structure with a short-range order with the weak and hidden ferromagnetism on the basis of both the theoretical group analysis of the magnetic and crystal lattice symmetry and the analysis of the temperature, field, hysteresis and anisotropy properties of spontaneous magnetization.

Abstract: The polycrystal samples of Gd1-xCaxCoO3-δ with different content of Ca were synthesized by three different techniques: solution method, solid state reaction and a sol-gel method. A study of magnetic and electric properties have been made. It is shown that the magnetic properties of this compound are in a high degree close to the same of the parent GdCoO3 with Co3+ ions being in a low spin state below the room temperature. The conductivity investigation revealed its increase by alkaline-earth substitution typical in rare-earth cobaltites.

Abstract: Comparative study of magnetic properties of highly oriented pyrolytic graphite irradiated with different particles suggests that ions (H+, He+, C+) with the energy in the range of hundreds keV yield larger values of induced magnetization compared to the protons with the energy of several MeV. These values increase with ion fluence but abruptly decrease when the concentration of interstitial defects becomes so large that the graphite stacking sequence is collapsed.

Abstract: The magnetooptical and optical properties, and giant magnetoresistance (GMR) of MBE-grown Fe(tx, Å)/Cr10 Å (tx=0.3-30 Å) superlattices and nanostructured multilayers are studied. The data obtained are used for characterization of magnetic clusters in structures with ultrathin Fe layers (tFe<6.6 Å) and for estimation of interfacial electron scattering parameters in GMR superlattices.

Abstract: We report on electronic transport in nearly magnetic conductors with strong structural disorder. The initial motivation for this work was a large positive magnetoresistance (MR) found in magnetically ordered ground state of (Y1-xGdx)Co2 alloys. This was a surprising result since a large positive MR is not expected in a system with strong static magnetic or structural disorder. Contemporary theory of magnetotransport and common sense agree that an external magnetic field should suppress magnetic fluctuations, resulting in a negative MR. On the contrary; a positive MR suggests that an external magnetic field enhances static magnetic disorder. It was shown that unusual MR of (Y1-xGdx)Co2 alloys is related to a combination of structural disorder and metamagnetic instability of itinerant Co-3d electrons. The new mechanism of MR is common of a broad class of materials featuring a static magnetic disorder and itinerant metamagnetism. Such systems display a number of unusual properties, among them strong pressure and magnetic field dependencies of resistivity and thermopower, Non-Fermi-Liquid (NFL) behavior of resistivity and, possibly, of thermopower. We review the relevant experimental data, mostly the properties of RCo2-based alloys, and discuss the theoretical model developed for the interpretation of the experimental results. This model includes new mechanism of magnetoresistivity in structurally disordered itinerant magnetic alloys.

Abstract: Faraday rotation (FR) and magnetic circular dichroism (MCD) of nanocomposite structures based on potassium-aluminum-germanium-boron glasses co-doped with Fe and rare earth (RE) or Y+Bi oxides have been studied. Formation of magnetic nanoparticles as a result of the glass heat treatment ensures them magnetic and magneto-optical properties typical of magnetically ordered substances. At the same time, glasses keep transparence in visual spectral range owing to low paramagnetic oxides concentration. FR and MCD spectra of the heat treated glasses are shown to be very close to those of γ-Fe2O3 and practically independent of the RE element nature for the light wave energies lower than 22000 cm-1. For higher energies, the MCD spectrum shape depends strongly on RE what evidences the different origin of magneto-optical effects in two spectral ranges.