A.V. Lashkul

Room-temperature ferromagnetism and anomalous Hall effect in Si1−xMnx (x ≈ 0.35) alloys

AdetailedstudyofthemagneticandtransportpropertiesofSi1−xMnx (x ≈ 0.35)filmsispresented.Weobserve the anomalous Hall effect in these films up to room temperature. The results of the magnetic measurements and the anomalous Hall effect data are consistent and demonstrate the existence of long-range ferromagnetic order in the systems under investigation. A correlation of the anomalous Hall effect and the magnetic properties of the samples with their conductivity and substrate type is shown. A theoretical model based on the idea of a two-phase magnetic material, in which molecular clusters with localized magnetic moments are embedded in the matrix of a weak itinerant ferromagnet, is discussed and used to explain experimental results. The long-range ferromagnetic order at high temperatures is mainly due to the Stoner enhancement of the exchange coupling between clusters through thermal spin fluctuations (“paramagnons”) in the matrix. Theoretical predictions do not contradict experimental data when model parameters of a plausible order of magnitude are used.

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO2 is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO2 QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO2 QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO2 QDs.

Properties of Zn1 − xCoxO films produced by pulsed laser deposition with fast particle separation

The study is concerned with the structural, optical, magnetic, and transport properties of Zn1 − xCoxO (x = 0.05–0.45) films produced on Al2O3 (0001) substrates at a temperature of Ts = 500°C by pulsed laser deposition with fast particle separation. The film thickness is d = 60–300 nm. It is found that the Zn1 − xCoxO ternary alloy retains its wurtzite-type crystal structure up to x = 0.35, if the films are produced at low buffer-oxygen pressures (∼10−6 Torr). It is established that, in these conditions, the electron concentration is higher than 1020 cm−3 because of the high density of oxygen donor vacancies. In this case, the films start to exhibit ferromagnetism in the magnetization and the anomalous Hall effect at temperatures above 100 K. The sign of the anomalous Hall effect is found to be positive and opposite to the sign of the normal Hall effect, as occurs in Co metal layers. This is indicative of the cluster nature of ferromagnetism of the Zn1 − xCoxO films. For thin Zn1 − xCoxO layers (d = 60 nm, x = 0.2) in a transverse magnetic field, profound hysteresis of the magnetoresistance is observed, which is indicative of the out-of-plain easy axis of magnetization of the films. The magnetic anisotropy is attributed to the structuring of the layers (elongation of magnetic clusters along the growth axis of the films). The structuring can lead to noticeable strengthening of the layer ferromagnetism.

Hopping conductivity of La1-xSrxMn1-yFeyO3

The resistivity, , is investigated in ceramic La1-xSrxMn1-yFeyO3 samples with x = 0.3 and y = 0.03  0.25 at temperatures T ~ 5  310 K in magnetic fields B up to 8 T. The metallic conductivity at y = 0.03 and the variable-range hopping (VRH) conductivity between y = 0.15  0.25 are observed. The Mott VRH conductivity takes place in all investigated samples with y ≥ 0.15 in different temperature intervals. In addition, at y = 0.25 the interval of the Shklovskii- Efros VRH conductivity, characteristic of the Coulomb gap in the density of the localized states (DOS), is observed. Analysis of the VRH conductivity yields the values of the microscopic parameters, including the DOS features and the localization radii of charge carriers.

Magnetic properties of carbon nanoparticles

Magnetization M (T, B) of powder and glassy samples containing carbon nanoparticles is investigated in the interval of temperatures T between ~ 3  300 K and magnetic fields B up to 5 T. Low-field magnetization, M (T), exhibits a strong magnetic irreversibility, which is suppressed above the field of ~ 1 T. The dependence of M (B) saturates at high temperatures above B ~ 2 T, magnetic hysteresis is observed already at 300 K. The values of the saturation magnetization, the coercivity field and the maximum blocking temperature are obtained. Analysis of the experimental data gives evidence for concentration of the magnetization close to the surface of the particles, which is consistent with the origin of magnetism in nanocarbon presumably due to intrinsic disorder and surface defects.

Low-field magnetic properties of La1-xSrxMn1-yFeyO3

Low-field (B = 10 G – 1 kG) magnetic properties are investigated in ceramic samples of La1–xSrxMn1−yFeyO3 (LSMFO) with x = 0.3 and y = 0.15, 0.20 and 0.25. A substantial decrease of the ferromagnetic (FM) Curie temperature, TC, with increasing y is observed, which is connected to breaking of the FM double-exchange interactions by doping with Fe. Strong magnetic irreversibility at B = 10 G gives evidence for a frustrated magnetic state of LSMFO. The asymptotic Curie-Weiss behaviour of the magnetic susceptibility, χ (T), observed well above TC, yields the values of the effective Bohr magneton per magnetic ion, exceeding considerably those of single ions. Critical behaviour of χ (T) ~ (T/TC – 1) −γ is influenced at y = 0.15 and 0.20 by a percolative (γ ≈ 1.8) and non-percolative or Heisenberg (γ ≈ 1.4) spin systems. At y = 0.25 the percolative contribution to χ (T) is not observed. The low-field magnetic properties above can be explained by the phase separation effect or generation of nanosize FM particles in the paramagnetic host matrix of LSMFO.

Quasi-local states of Sn in Bi2Te3 according to the studies of galvanomagnetic effects in classical and quantizing magnetic fields

The magnetic-field dependences of the Hall coefficient and transverse magnetoresistance are studied experimentally and theoretically in p-Bi2Te3 crystals doped heavily with Sn and grown by the Czochralski method in the case of both classical and quantizing magnetic fields as high as 12 T with the magnetic-field orientation along the C3 trigonal axis. The Shubnikov-de Haas effect and quantum oscillations of the Hall coefficient were measured at temperatures of 4.2 and 11 K. The six-ellipsoid Drabble-Wolfe model of the energy spectrum and the magnetic-field dependence of the Hall coefficient are used as the basis for the method for determining the Hall factor and Hall mobility. New evidence is obtained in support of the existence of the narrow band of impurity Sn states occupied partially with electrons against the background of the light-hole band spectrum. The parameters of impurity states are estimated including their energy (ESn ≈ 15 meV), the broadening (Γ « kT), and the radius of localization of the impurity state (R ≈ 30 Å).

Shubnikov-de Haas effect in n-CdSb:In under pressure

Magnetoresistance (MR) of single crystals of the group II–V semiconductor n-CdSb doped with In is investigated in magnetic fields B up to 7 T and temperatures T between 1.2 and 2.4 K under hydrostatic pressure p = 0–10 kbar. Shubnikov–de Haas (SdH) oscillations of MR observed up to B ~ 4 T are characterized by a single period, giving evidence for one participating group of conduction band electrons. Pressure dependence of the SdH period and concentration reveal a second group of charge carriers, connected to a resonance impurity band with delocalized states. The SdH amplitude, A, exhibits non-universal behavior attributable to its sensitivity to scattering mechanism at low quantum numbers. The parameters characterizing the non-universality of A(T) and the pressure dependence of the cyclotron effective mass, mc(p), and of the concentration of conduction electrons, nSdH, are given. The dependences of mc and the Fermi energy on pressure are consistent with Kane-type non-parabolicity of the conduction band.