Anomalous Hall effect in highly Mn-Doped silicon films

The transport and magnetic properties of Mn _x Si_{1 ? x} films with a high (x ≈ 0.35) content of Mn produced by laser deposition at growth temperatures of 300–350°C have been studied in a temperature range of 5–300 K in magnetic fields of up to 2.5 T. The films exhibit a hole-type metallic conductivity and a relatively weak change of magnetization in a temperature range of 50–200 K. An anomalous Hall effect with an essentially hysteretic behavior from 50 K up to ≈230 K has been discovered. The properties of the films are explained by the two-phase model, in which ferromagnetic clusters containing interstitial Mn ions with a localized magnetic moment are embedded in the matrix of a weak band MnSi_{2 ? x} (x ≈ 0.3) type ferromagnet with delocalized spin density.     

Magnetostriction of Hexagonal HoMnO<Subscript>3</Subscript> and YMnO<Subscript>3</Subscript> Single Crystals

We report on the magnetostriction of hexagonal HoMnO₃ and YMnO₃ single crystals in a wide range of applied magnetic fields (up to H = 14 T) at all possible combinations of the mutual orientations of magnetic field H and magnetostriction ΔL/L. The measured ΔL/L(H, T) data agree well with the magnetic phase diagram of the HoMnO₃ single crystal reported previously by other authors. It is shown that the nonmonotonic behavior of magnetostriction of the HoMnO₃ crystal is caused by the Ho³⁺ ion; the magnetic moment of the Mn³⁺ ion parallel to the hexagonal crystal axis. The anomalies established from the magnetostriction measurements of HoMnO₃ are consistent with the phase diagram of these compounds. For the isostructural YMnO₃ single crystal with a nonmagnetic rare-earth ion, the ΔL/L(H, T) dependences are described well by a conventional quadratic law in a wide temperature range (4–100 K). In addition, the magnetostriction effect is qualitatively estimated with regard to the effect of the crystal electric field on the holmium ion.     

Energy disorder in polysilanes

The absorption spectra (T=295 K), photoluminescence spectra (T=5–295 K), and thermally stimulated luminescence curves (T=5–295 K) of poly(di-n-hexylsilane) (PDHS), poly(methyl-n-hexylsilane) (PMHS), poly(methylcyclohexylsilane) (PMCHS), and poly(methylphenylsilane) (PMPS) films are measured. The results obtained are analyzed within the model of random hoppings of excitons and charge carriers over sites with a Gaussian distribution of the density of states. It is established that the variance parameters of the density-of-state functions of excitons and charge carriers characterize the energy disorder and depend on the chemical nature of side groups of the polymer, the conformation of segments of the main chain of the polymer macromolecule, and the temperature. At room temperature, the energy disorder in crystalline regions of the PDHS film is explained in terms of fluctuations in the number of monomer units in chain segments. In polymers with nonsymmetric side groups (such as PMHS, PMCHS, and PMPS), the disorder is more pronounced due to the formation of conformers in which silicon atoms occupy different positions in the chain. In the PMPS polymer, the disorder occurs through one more mechanism associated with fluctuations of the angle between the plane of the phenyl ring and the axis of the polymer segment due to mixing of σ-and π-electron states.     

Luminescence of Tl<Superscript>+</Superscript> ions in a KZnF<Subscript>3</Subscript> crystal

The luminescence spectra of a KZnF₃: Tl⁺ crystal are investigated in the energy range from 4.75 to 5.9 eV at temperatures of 10–300 K upon excitation into the A absorption band (5.7–6.3 eV). At T=300 K, the luminescence spectra exhibit an intense band with a maximum at 5.45 eV, which is attributed to single Tl⁺ ions substituted for K⁺ ions. The 5.723-eV intense narrow band observed at T<20 K is assigned to the ³Γ_1u-¹Γ_1g zero-phonon transition, which is weakly allowed by the hyperfine interaction. The luminescence decay is studied as a function of temperature. The main characteristics of the luminescence spectra are adequately described in terms of the semiclassical theory based on the Franck-Condon principle and the Jahn-Teller effect for an excited sp configuration of the Tl⁺ ion with the use of the parameters obtained earlier from analyzing the absorption spectra of the system under investigation.     

EPR and photoluminescence spectra of smooth CD<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films from T-10 tokamak: The effect of iron impurity

The electron and spin structure of thick smooth hydrocarbon CD _x films (“flakes”) with a high relative deuterium concentration of x ~ 0.5, redeposited from deuterium plasma discharge onto the walls of the vacuum chamber of the T-10 tokamak and containing ~1 at % of 3d-metal impurities due to erosion of the chamber walls, are studied using electron paramagnetic resonance (EPR) and photoluminescence (PL). The resulting spectra are compared for the first time to the EPR and photoluminescence spectra of polymer (soft) a-C:H(D) films (H(D)/C ~ 0.5), which are considered model analogues of smooth CD _x films. A certain similarity of the CD _x films with a-C:H films was found in the electronic structure of the valence band. At the same time, the differences in the EPR and photoluminescence spectra were observed due to the presence of 3d-metal impurities in the CD _x samples, contributing to the conversion of sp ³ → sp ² in the formation of films in the tokamak and upon heating and thermal desorption. An impurity of, presumably, 3d metals was detected for the first time by EPR in the a-C:H films in an amount of approximately 0.2 ppm, related to the evaporation of graphite.     

Influence of electromagnetic radiation on the phase composition of clustered CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The effect of white and UV radiation on the phase composition of amorphous CN _x films are studied by X-ray diffraction analysis and visible-range spectroscopy. The films have variable-range atomic order and consist of amorphous graphite clusters (30 Å) crystalline clusters (50–100 Å) of graphite, diamond, and carbon nitride phases; and intercluster medium with long-range (1–2 Å) atomic order. It is shown that irradiation of the films by white light facilitates the growth of fine graphite clusters. Irradiation by UV light suppresses the growth of the graphite and carbon nitride phases, favoring the growth of the diamond phase (1.5%). It is demonstrated that a change in the mesoscopic phase composition of the CN _x films causes a change in the energy gap width in the visible range from E _g = 0.75 eV for the films irradiated by white light to E _g = 1.75 eV for those exposed to UV radiation.     

Thermal quenching of luminescence of BaY<Subscript>2</Subscript>F<Subscript>8</Subscript> crystals activated with Er<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er³⁺ and Tm³⁺ ions in BaY₂F₈ crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er³⁺ and Tm³⁺, respectively. It is shown that quenching of 5d-4f luminescence of Tm³⁺ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.     

Multiferroic properties and structural features of <Emphasis Type="Italic">M</Emphasis>-type Al-substituted barium hexaferrites

Precise studies of the crystal and magnetic structures of M-type substituted barium hexaferrites BaFe_12–x Al _x O₁₉ (0.1 ≤ x ≤ 1.2) have been performed by powder neutron diffraction in the temperature range 300–730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in M-type substituted barium hexaferrites BaFe_12–x Al _x O₁₉ (x ≤ 1.2) with a collinear ferromagnetic structure.     

Magnetic and transport properties of colossal magnetoresistance compound EuB<Subscript>6</Subscript>

The galvanomagnetic and magnetic properties of EuB₆ single crystal have been measured over wide temperature (1.8–300 K) and magnetic-field (up to 70 kOe) ranges, and the parameters of charge carriers and the characteristics of the magnetic subsystem are estimated in the paramagnetic and ferromagnetic (T < T _C ≈ 13.9 K) phases of this compound with strong electron correlations. In the temperature range T < T* ≈ 80 K, a magnetoresistance hysteresis Δρ(H)/ρ(0) is detected; it reaches a maximum amplitude of about 5% at T ≈ 12 K. The anomalies of charge transport observed in the temperature range T _C < T < T* are shown to be related to the magnetic scattering of charge carriers (m _eff = (15–30)m ₀, where m ₀ is the free-electron mass) that results from a short-range magnetic order appearing upon the formation of ferromagnetic nanoregions (ferrons).     

Electronic excitation energy transfer and nonstationary processes in KH<Subscript>2</Subscript>PO<Subscript>4</Subscript>:Tl crystals

We report the results of our experimental study and numerical simulation of the electronic excitation energy transfer to impurity centers under conditions where nonstationary processes take place in the hydrogen sublattice of potassium dihydrogen phosphate (KH₂PO₄) single crystals doped with mercury-like Tl⁺ ions (KDP:Tl). We present the experimental results of our investigation of the decay kinetics of the transient optical absorption (100 ns–50 s) of intrinsic defects in the hydrogen sublattice of KDP:Tl obtained by pulsed absorption spectroscopy and the results of our study of the dynamics of the change in steady-state luminescence intensity with irradiation time (1–5000 s). To explain the transfer of the energy being released during electron recombination involving intrinsic KDP:Tl lattice defects, we formulate a mathematical model for the transfer of this energy to impurity Tl⁺ luminescence centers. Within the model being developed, we present the systems of differential balance equations describing the nonstationary processes in the electron subsystem and the hydrogen sublattice; provide a technique for calculating the pair correlation functions Y(r, t) of dissimilar defects based on the solution of the Smoluchowski equation for the system of mobile hydrogen sublattice defects; calculate the time-dependent reaction rate constants K(t) for various experimental conditions; and outline the peculiarities and results of the model parametrization based on our experimental data. Based on our investigation, the dramatic and significant effect of a gradual inertial increase by a factor of 50–100 in steady-state luminescence intensity in the 4.5-eV band in KDP:Tl crystals due to the luminescence of mercury-like Tl⁺ ions has been explained qualitatively and quantitatively.     

Magnetic capacitance of the Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>FeO<Subscript>3</Subscript> thin films

The capacitance, inductance, and dissipation factor of the Gd _x Bi_1–x FeO₃ films were measured in the temperature range of 100 K < T < 800 K in magnetic fields of up to 8 kOe at frequencies of 0.1–100 kHz. The magnetic susceptibility maxima in the low-temperature region and dependences of the relaxation time and inductance on prehistory of the films cooled in zero and nonzero magnetic fields are established. The giant increase in magnetic capacitance in the external bias electric field is found. The results obtained are explained by the domain structure transformation in external electric and magnetic fields.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Effect of the magnetic anisotropy energy distribution of MnSb clusters on spontaneous magnetization reversal of GaMnSb thin films

Temperature m(T) and time m(t) dependences of the magnetic moment of GaMnSb thin films with MnSb clusters have been measured. The m(t) dependences are straightened in semilogarithmic coordinates m(lnt). The temperature dependences of magnetic viscosity S(T) corresponding to the slope of straight lines m(lnt) have been studied. It have been demonstrated that the behavior of dependences S(T) is governed by the lognormal distribution of the magnetic anisotropy energy of MnSb clusters. It have been found that the behavior of dependences m(T) measured after the films were cooled in zero magnetic field and in magnetic field H = 10 kOe is also governed by the lognormal distribution of the magnetic anisotropy energy of MnSb clusters.     

Charge-transfer bands in crystals of alkaline earth fluorides with Eu<Superscript>3+</Superscript> and Yb<Superscript>3 + 1</Superscript>

The absorption, luminescence, and excitation spectra of CaF₂, SrF₂, and BaF₂ crystals with EuF₃ or YbF₃ impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT₁) were observed at energies below the 4f ⁿ -4f ^{n ? 1}5d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT₂) with energies 1.5–2 eV lower than those of the CT₁ bands have been found in the spectra of CaF₂ and SrF₂ crystals with EuF₃ or YbF₃ impurities. The fine structure of the luminescence spectra of CaF₂ crystals with EuF₃ impurities has been investigated under excitation in the CT bands. Under excitation in the CT₁ band, several Eu centers were observed in the following luminescence spectra: C _4v , O _h , and R aggregates. Excitation in the CT₂ bands revealed luminescence of only C _4v defects.     

Deactivation pathways of the electronic excitation of ions of lanthanide complexes in polymers with functional groups

Complexes Eu(TTA)₃phen and Eu(MBTA)₃phen, as well as complexes Tb(MBTA)₃phen and Tb(TTA)₃phen, which do not luminesce in solutions, are shown to luminesce in polymer films (TTA is thenoyltrifluoroacetone, MBTA is n-methoxybenzoyltrifluoroacetone, and phen is o-phenanthroline). Luminescence of complexes of Eu and Tb in films of a polymer, poly(methylene-bis-anthranilamide) 1,6-hexamethylenedicarboxylic acid (PAA-5), having a high concentration of functional anthranilate groups, is studied. From the behavior of the luminescence intensity (I _lum), the luminescence decay time, and the luminescence spectra of complexes of these lanthanides in polymer films, the following regular features were revealed. (i) During the film preparation at 90°C, Ln complexes are attached to PAA-5 via anthranilate groups. (ii) Irradiation of these films in the range of the absorption band of ligands (TTA or MBTA) leads to deactivation of the electronic excitation of ions according to the diketone detachment mechanism and to further binding of complexes to polymers. In this case, I _lum(Eu(III)) decreases because the introduction of anthranilate groups of the polymer into the first coordination sphere of Eu(III) complexes enhances the nonradiative deactivation of these ions, whereas I _lum(Tb(III)) increases since the introduction of these groups suppresses the nonradiative deactivation of Tb complexes through triplet states of ligands (TTA and MBTA). (iii) Upon storage of films in the dark (20°C), complexes detach themselves from the polymer and return to their initial structure. In PAA-5 films into which Eu and Tb complexes were simultaneously introduced, the color of the emission from the irradiation spot changes from red to green.     

Critical magnetic field of the vortex-free state in NbC thin films and prospects for its observation in MgB<Subscript>2</Subscript>

The critical magnetic fields H _c‖ and H _c2 are measured for thin films of the isotropic superconductor NbC. It is revealed that the critical fields exhibit strong anisotropy due to the vortex-free state of the film in a magnetic field aligned parallel to its surface. The H _c‖/H _c2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence H _c‖(T) agrees quantitatively with the concepts of microscopic theory on the vortex-free state of a thin film of a clean superconductor in the temperature range below T _c . As the electron mean free path decreases under irradiation of the film with a low dose of He⁺ ions, the critical field H _c‖ remains unchanged near T _c but increases significantly at lower temperatures. The well-known theoretical models are used to estimate the electronic parameters and thicknesses of MgB₂ films for which the specific features associated with the vortex-free state of the two-gap superconductor can manifest themselves in the temperature dependence of the critical magnetic field H _c‖(T).     

Effect of structural defects on the magnetic properties of the EuBaCo<Subscript>1.90</Subscript>O<Subscript>5.36</Subscript> single crystal

The effect of structural defects in cobalt and oxygen sublattices with the constant average oxidation level 3+ of all cobalt ions on the magnetic properties of the EuBaCo_1.90O_5.36 single crystal has been studied. The magnetic properties of the single crystal and the polycrystalline sample of the corresponding composition are compared in the range T = 200–650 K. The results show that the cobalt-deficient EuBaCo_2–xO_5.5–δ samples demonstrate a three-dimensional XY ferromagnetic ordering of magnetic sublattices. The values of the effective magnetic moment at T > 480 K indicate the existence of the IS and HS states of Co³⁺ ions. The large difference of values of μ_eff of the EuBaCo_1.90O_5.36 single crystal and polycrystal can be due to that the magnetic ion spins lie in plane ab. The magnetic field directed along plane ab substantially influences the magnetic ordering at T < 300 K.     

Raman scattering spectroscopy of inclusions of carbon in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> films and its solid solutions with HfO<Subscript>2</Subscript>

We apply Raman scattering spectroscopy to study the nature of carbon inclusions in Al₂O₃ and (HfO₂) _x (Al₂O₃)_{1 ? x} films deposited using volatile complex compounds. Raman spectra of the films under investigation contain D and G vibrational modes, which indicate that carbon clusters of the sp ² configuration tend to form in the films. We estimate the size of clusters from the integrated intensity ratio I _D /I _G and find it to be in the range of 14–20 Å. The content of hydrogen in carbon clusters is calculated from the height of the photoluminescence pedestal and is found to vary from 14 to 30 at % depending on the regime of the film’s synthesis.     

Transport properties of Co<Subscript>2</Subscript>CrAl Heusler alloy films

The effect of atomic disorder on the electron transport and the magnetoresistance (MR) of Co₂CrAl Heusler alloy (HA) films has been investigated. We show that Co₂CrAl films with L2₁ order exhibit a negative value for the temperature coefficient of resistivity (TCR) in a temperature range of 10 < T < 290 K, and the temperature dependence of electric conductivity varies as T ^3/2 similarly to that of the zero-gap semiconductors. The atomic or the site disorder on the way of L2₁ → B2 → A2 → amorphous state in Co₂CrAl HA films causes the deviation from this dependence: reduction in the absolute value of TCR as well as decrease in the resistivity down to ?(T = 293 K) ～ 200 μΩ cm in comparison to ?(T = 293 K) ～ 230 μΩ cm typical for the Co₂CrAl films with L2₁ order. The magnetic-field dependence of MR of the Co₂CrAl films with L2₁ order is determined by two competing contributions: a positive Lorentz scattering and a negative s-d scattering. The atomic disorder in Co₂CrAl films drastically changes MR behavior due to its strong influence on the magnetic properties.     

Magnetic properties of single crystals of a new cobaltite TbBaCo<Subscript>4</Subscript>O<Subscript>7+<Emphasis Type="Italic">x</Emphasis></Subscript>

The temperature and field dependences of the magnetization of a single crystal of a new class of layered cobaltites, TbBaCo₄O_7+x , with a structure containing a Kagomé lattice and a triangular lattice were measured. The measurements were performed on a SQUID magnetometer at temperatures in the range 2–300 K and in magnetic fields of up to 55 kOe for two field orientations. The anisotropy of the magnetization was studied, and the presence of antiferromagnetic ordering in fields H < H _c and a weak magnetic-field-induced (H > H _c ) ferromagnetic component in the low-temperature range was demonstrated. The magnetic characteristics of the initial TbBaCo₄O_7+x single crystal and the single crystal annealed in an O₂ atmosphere were compared.