V. A. Sirenko

Spin-reorientational transitions in low-doped Nd1?xCaxMnO3 manganites: the evidence of an inhomogeneous magnetic state

As t udy of crystal structure, elastic, and magnetic properties of low-doped Nd1−x Cax MnO3 (x 0.15) perovskites has been carried out. The ferromagnetic component is shown t oi ncrease under hole doping and, simultaneously, the temperature of the orbital order–disorder phase transition decreases. The mechanism of the concentrational transition from a weak ferromagnetic state (x = 0) to a ferromagnetic one (x > 0.15) is discussed using a two-phase model, according t ow hich the samples consist of weak ferromagnetic and ferromagnetic phase se xchange coupled at their boundary. It is found that interaction between different magnetic phases leads to spin reorientation which takes place for 0.06 x 0. 1c ompounds around Teff ∼ 9K . In the temperatur er ange from 5 to 20 K, metamagnetic behaviour is revealed for the Nd0.92Ca0.08MnO2.98 sample. H versus T as well as T versus x magnetic phase diagrams, which are characterized by the missing of a canted phase, are proposed. The appearance of orientational transitions is explained on the basis of a magnetic analogue of the Jahn–Teller effect taking into account that the magnetic moment so f Nd ions are ordered parallel to the moments of Mn ions in the ferromagnetic phase, and opposite to the direction of the weak ferromagnetic vector at T > Teff in the weak ferromagnetic phase.

Smart materials for ranging systems

List of Contributors.- Preface.- Magnetic dynamics of co nanospheres: origin of the enhanced anisotropy J. Bartolome et al.- Nanoparticles in oxide glasses: magnetic resonance studies J. Kliava, R. Berger.- Effect of magnetic field on the magnetic state of copper metaborate G. Petrakovskii et al.- Intrinsically inhomogeneous magnetic states in antiferromagnets in the vicinity of phase transitions induced by magnetic field V. Eremenko, V. Sirenko.- NMR study of the successive phase transitions in CuB2O4 M. Chiba et al.- Magnetic properties of cuprate perovskites A. Sherman.- Quantum critical phenomena and the anomalous low-temperature properties of the U(Pt, Pd)3 system J.J.M. Franse.- The periodic Anderson model in the generating functional approach Yu.A. Izyumov, D.S. Alexeev.- Orbital ordering and phase separation phenomena in lightly doped manganites I.O. Troyanchuk et al.- Interface and magnetic characterization of FM/AF/FM multilayers V.R. Shah et al.- Squid-based magnetic microscope S. Bondarenko, N. Nakagawa.- Femtosecond ellipsometry: a new tool for the measurement of hot electron momentum relaxation times in metals V.V. Kruglyakl, R.J. Hicken.- Ferromagnetic shape memory alloys Ni2+xMn1 A.N. Vasiliev et al.- Magnetostriction of manganites and cobaltites H. Szymczak, R. Szymczak.- Structural study of magnetostrictive superconducting compounds V.V. Eremenko et al.- Subject index.

Antiferromagnet-ferromagnet phase transition in lightly doped manganites

Magnetic and structural phase diagrams of the La0.88MnOx, La1−xSrx(Mn1−x/2Nbx/2)O3, Nd1−xCaxMnO3, and Bi1−xCaxMnO3 series, constructed on the basis of x-ray, neutron powder diffraction, Young’s modulus, magnetization and resistivity measurements, are presented. It is shown that the main factor controlling the antiferromagnet-ferromagnet phase transition in the manganites is a type of an orbital state. The results are discussed in the framework of structurally driven magnetic phase separation model.

Role of acoustic phonons in the negative thermal expansion of layered structures and nanotubes based on them

Calculations on a microscopic level are used to explain the experimentally observed negative linear thermal expansion along some directions in a number of crystalline compounds with complicated lattices and anisotropic interactions between atoms. Anomalies in the temperature dependence of the coefficient of linear thermal expansion are analyzed in layered crystals made up of monatomic layers (graphite and graphene nanofilms) and multilayer “sandwiches” (transition metal dichalcogenides), in multilayered crystal structures such as high-temperature superconductors where the anisotropy of the interatomic interactions is not conserved in the long-range order, and in graphene nanotubes. The theoretical calculations are compared with data from x-ray, neutron diffraction, and dilatometric measurements.

AC response of 2H–NbSe2 single crystals with electron-irradiation-induced defects

The generation of defects in NbSe(2) single crystals by electron irradiation has been investigated by a combination of ac susceptibility and structural measurements. Remarkably, thanks to the layered structure of NbSe(2), we show that electronic irradiation cannot only create point defects but also in-plane extended defects, which modify anisotropically the ac response. Indeed, the analysis of the onset of the nonlinear susceptibility response, H(ac)(l)(T), as a function of irradiation dose and field orientation shows a correlated increase in the density of anisotropic defects induced by electron irradiation. Also, we measured a decrease in the strength of the pinning (Labusch) constant α(L) accounting for elastic vortex oscillations within the linear Campbell regime for high-dose-irradiated samples in a transverse field, again compatible with the presence of planar defects hindering vortex pinning. X-ray powder diffraction and TEM electron diffraction measurements suggest these in-plane defects may result from the rupture of Se-Se bonds and the formation of nanorods and nanowires by NbSe(2) sheet rolling.

Effect of step-edge on spectral properties and planar stability of metallic bigraphene

Phonon and electron spectra of metallic bigraphene are analyzed in the presence of step-edge crystal imperfection. Different geometries of step-edge are considered. The dynamic planar stability of the considered structure is proved for temperatures above the ambient. The number of phonon states is shown to grow near the K-point of the first Brillouin zone, compared to pristine graphene. It is found, that this type of defects causes substantially nonuniform distribution of electron states and the pronounced increase in the number of states with energies close to Fermi energy can be expected in electron spectrum of the graphene-based compounds. The performed calculations are in good agreement with inelastic neutron, x-ray and Raman measurements.

Magnetic Anisotropy in 2H-NbSe2 Electron Irradiated Single Crystals

The anisotropic magnetic properties of the metallic layered compound with hexagonal crystal structure 2H-NbSe2 are investigated as a function of their dependence on high-energy electron irradiation. Pauli paramagnetism of free electrons is shown to dominate the magnetic susceptibility, P. The anisotropy is related to spin-orbit effects on the hybridized electronic states. Irradiation affects the density of states at the Fermi surface, increasing both P and the anisotropy. Below a threshold temperature, TS=54 K, the paramagnetic contribution, which increases with the dose, is ascribed to dangling bonds, nanotubes and nanorods generated by irradiation.

On the character of the scattering of wide-band x-ray radiation in the low-temperature phase of 2H‐NbSe2

The intensity of x-ray scattering in the layered metal 2H‐NbSe2 has been investigated experimentally in the temperature range below the temperature of the transition into the charge-density wave state. It is shown that the relation I∼(TC−T)−γ with TC∼30K and γ=1.6±0.2, as is characteristic for quasi-elastic (quasi-Bragg) scattering, describes the scattering intensity. Such scattering is strong when the width ΔeXR of the spectral distribution of the x-ray radiation is greater than the spectral width of the fluctuations Δef of the order parameter. The inequality Δef<Δef holds for scattering of slow neutrons, for which quasi-elastic scattering is immeasurably weak.

On the magnetic susceptibility of niobium diselenide

The magnetic properties of 2H‐NbSe2, a layered metal with a hexagonal structure of the crystal lattice, are measured, and the change of those properties after irradiation with high-energy electrons is investigated. It is shown that the main part of the magnetic susceptibility comes from the Pauli paramagnetism of free charge carriers. The low-temperature deviations from Pauli paramagnetism are explained by dangling interatomic bonds, which are present in small numbers even in the unirradiated samples.

Anisotropy of the temperature dependence of the lattice parameters of EuBa2Cu3O7−d in the quasiharmonic limit

An x-ray diffraction analysis of the compound EuBa2Cu3O7−δ is carried out in the temperature range 20–300K. At all temperatures of measurement the structure is described in the orthorhombic system. Nonmonotonic growth of the crystal lattice parameter c with decreasing temperature is observed in the interval 150–20K, and substantial features of the temperature dependence of the coefficient of linear thermal expansion appear below the Debye temperature. The results of the experiment are in complete agreement with the conclusions of a microscopic treatment of the vibrations of the atoms of the BaO and europium layers and also the Cu–O chains.