arXiv subjects

B. I. Belevtsev

Publications and source records attributed to B. I. Belevtsev.

35 records · Page 2Linked to original sources

arXiv2003

Magnetoresistive study of antiferromagnetic--weak ferromagnetic transition in single-crystal La$_{2}$CuO$_{4+δ}$

The resistive measurements were made to study the magnetic field-induced antiferromagnetic (AF) - weak ferromagnetic (WF) transition in La$_2$CuO$_4$ single-crystal. The magnetic field (DC or pulsed) was applied normally to the CuO$_2$ layers. The transition manifested itself in a drastic decrease of the resistance in critical fields of ~5-7 T. The study is the first to display the effect of the AF -WF transition on the conductivity of the La$_2$CuO$_4$ single-crystal in the parallel - to - CuO$_2$ layers direction. The results provide support for the 3-dimensional nature of the hopping conduction of this layered oxide.

cond-mat.str-el↗

arXiv2003

Intrinsic and extrinsic inhomogeneities in mixed-valence manganites

It is suggested that extrinsic inhomogeneities in mixed-valence manganites deserve more attention and they should be taking into account on equal footing with hypothetical phase separation while examinating experimental data and developing the theoretical models of influence of stoichiometric and other types of inhomogeneities on properties of these and other transition-metal oxides.

cond-mat.mtrl-sci↗

arXiv2003

Spin states and phase separation in La_{1-x}Sr_{x}CoO_3 (x=0.15, 0.25, 0.35) films: optical, magneto-optical and magneto-transport studies

Optical absorption and transverse Kerr effect spectra, resistivity and magnetoresistance of La$_{1-x}$Sr$_{x}$CoO$_3$ ($x=0.15, 0.25, 0.35$) films have been studied. The temperature dependencies of the optical and magneto-optical properties of the films exhibit features, which can be attributed to the transition of the Co$^{3+}$ ions from the low-spin state (S=0) to the intermediate-spin state (S=1) and to orbital ordering of the Co$^{3+}$ ions in the latter state. The evolution of the properties influenced by doping with Sr is interpreted on the basis of the phase separation model.

cond-mat.str-el↗

arXiv2003

Anisotropic magnetoresistive and magnetic properties of La_{0.5}Sr_{0.5}CoO_{3-δ} film

The magnetic and transport properties of La_{0.5}Sr_{0.5}CoO_{3-δ} film grown on a LaAlO_3 substrate by pulsed-laser deposition are studied. The properties are found to be influenced by the magnetic anisotropy and inhomogeneity. Magnetoresistance anisotropy is determined by the shape anisotropy of the magnetization and the strain-induced magnetic anisotropy due to the film-substrate lattice interaction. Indications of the temperature-driven spin reorientation transition from an out-of plane orderded state at low temperatures to an in-plane ordered state at high temperatures as a result of competition between the mentioned sources of magnetic anisotropy are found.

cond-mat.mtrl-sci↗

arXiv2003

Giant radio-frequency magnetoabsorption effect in the cobaltite ceramic La_{0.5}Sr_{0.5}CoO_3

The DC transport properties of and the radio-frequency (RF) wave absorption (at 1.33 MHz) in a ceramic sample of La_{0.5}Sr_{0.5}CoO_{3-δ} are measured. The Curie temperature, T_{c}, of the sample is about 250 K. A giant negative magnetoabsorption effect is found. In the vicinity of T_{c}, the absolute value of the magnetoabsorption is about 38 % in the rather low magnetic field 2.1 kOe. This differs drastically from the measured DC magnetoresistance (MR) δ(H) =[R(0)-R(H)]/R(0) which is a mere 0.26 % near T_{c}in the same field and increases to about 2.15 % in H=20 kOe. The phenomenon can be understood taking into account that the magnetoabsorption is determined by influence of magnetic field on the conductivity and the magnetic permeability, while the MR is determined solely by the former. The magnetoabsorption effect can be used to develop RF devices controlled by magnetic field and temperature.

cond-mat.mtrl-sci↗

arXiv2003

Thermal Conductivity in Magnetic Superconductors

We present here an overview of thermal conductivity behavior in magnetic superconductors: rare-earth nickel borocarbide intermetallic compounds, RNi_{2}B_{2}C, and the 2122 rare-earth rutheno-cuprates. The borocarbide family of intermetallics exhibits the large variety of phenomena - only superconductivity (R = Y, Lu), coexistence of superconductivity and magnetic order (R = Tm, Er, Ho, Lu), only magnetic order (R = Tb, Gd), and heavy fermion behavior (R = Yb) - within the same crystal structure. The rutheno-cuprate Eu_{1.5}Ce_{0.5}RuSr_2Cu_{2}O_{10-δ} exhibits coexistence of weak ferromagnetism and superconductivity at a surprisingly high temperature (\approx 45 K) and onset of antiferromagnetic order near 200 K. We demonstrate how coexistence of superconductivity and magnetic order in these compounds is releaved by the thermal conductivity studies.

cond-mat.supr-con↗

arXiv2002

Magnetoresistance and magnetic anisotropy in La$_{0.5}$Sr$_{0.5}$CoO$_{3-δ}$ film

The magnetic and transport properties of La$_{0.5}$Sr$_{0.5}$CoO$_{3-δ}$ film grown on a LaAlO$_3$ substrate by pulsed-laser deposition are studied. The properties are found to be influenced by a combined influence of the magnetic anisotropy and inhomogeneity. Magnetoresistance anisotropy is determined by the shape anisotropy and the strain-induced magnetic anisotropy due to the film-substrate lattice interaction. Indications of the temperature-driven spin reorientation transition from an out-of plane orderded state at low temperatures to an in-plane ordered state at high temperatures as a result of competition between the mentioned anisotropy sources are found.

cond-mat.mtrl-sci↗

arXiv2002

Giant change in IR light transmission in La_{0.67}Ca_{0.33}MnO_{3} film near the Curie temperature: promising application in optical devices

Transport, magnetic, magneto-optical (Kerr effect) and optical (light absorption) properties have been studied in an oriented polycrystalline La_{0.67}Ca_{0.33}MnO_{3} film which shows colossal magneto-resistance. The correlations between these properties are presented. A giant change in IR light transmission (more than a 1000-fold decrease) is observed on crossing the Curie temperature (about 270 K) from high to low temperature. Large changes in transmittance in a magnetic field were observed as well. The giant changes in transmittance and the large magneto-transmittance can be used for development of IR optoelectronic devices controlled by thermal and magnetic fields. Required material characteristics of doped manganites for these devices are discussed.

cond-mat.mtrl-sci↗

arXiv2002

DC transport properties and microwave absorption in bulk ceramic sample and film of La_{0.5}Sr_{0.5}CoO_{3-δ}: magnetic inhomogeneity effects

The DC transport properties and microwave absorption (at 41 GHz) are measured in a bulk ceramic sample and a film (220 nm thick) of La$_{0.5}$Sr$_{0.5}$CoO$_{3-δ}$. The samples are polycrystalline. The microwave conductivity, which is related mainly to the intragrain conductivity, increases by the order of the magnitude at the transition to the ferromagnetic state. The increase is far greater than that of found in the known DC measurements in doped cobaltates of the best crystal perfection. The found microwave effect is attributed to inherent magnetically inhomogeneous state of the doped cobaltates. Relaying on the results obtained it can be suggested that rather low magnetoresistance in the doped cobaltates as compared with that of the manganites is attributable to their more inhomogenenous magnetic state.

cond-mat.mtrl-sci↗

arXiv2001

Transport and magnetic anisotropy in CMR thin film La_{1-x}Ca_{x}MnO_{3} (x \approx 1/3) induced by a film-substrate interaction

We present a study of anisotropy of transport and magnetic properties in La_{1-x}Ca_{x}MnO_{3} (x \approx 1/3) film prepared by pulsed-laser deposition onto a LaAlO_{3} sunstrate. We found a non-monotonic dependence of magnetoresistance (MR) on magnetic field H for both H perpendicular and parallel to the film plane but perpendicular to the current. This rather complex behavior of MR manifests itself at low temperatures, far below the Curie temperature. Two main sources of MR anisotropy have been considered in the explanantion of the results: (1) the existence of preferential directions of magnetization (due to strains stemming from the lattice film-substrate mismatch or other reasons); (2) dependence of resistance on the angle between current and the magnetization, which is inherent in ferromagnets.

cond-mat.mtrl-sci↗

arXiv2000

Effect of γ-irradiation on superconductivity in polycrystalline YBa_{2}Cu_{3}O_{7-δ}

A bulk polycrystalline sample of YBa_{2}Cu_{3}O_{7-δ} (δ\approx 0.1) has been irradiated by γ-rays with ^{60}Co source. Non-monotonic behavior of T_c (defined as the temperature at which normal resistance is halved) with increasing irradiation dose Φ(up to about 220 MR) is observed: T_c decreases at low doses (Φ\leq 50 MR) from initial value (\approx 93 K) by about 2 K and then rises, forming minimum. At highest doses (Φ\geq 120 MR) T_c goes down again. The temperature width, δT_c, of resistive transition increases rather sharp with dose below 75 MR and somewhat drops at higher dose. We believe that this effect is revealed for the first time at γ-irradiation of high-T_c superconductor. The cross section for the displacement of lattice atoms in YBCO by γ-rays due to the Compton process were calculated, and possible dpa values were estimated. The results obtained are discussed taking into account that the sample is granular superconductor and, hence, the observed variations of superconducting properties should be connected primarily with the influence of γ-rays on intergrain Josephson coupling.

cond-mat.supr-con↗

arXiv2000

Effect of γ-irradiation on superconducting transition temperature and resistive transition in polycrystalline YBa_{2}Cu_{3}O_(7-δ)

A bulk polycrystalline sample of YBa_(2)Cu_(3)O_(7-δ) (δ\approx 0.1) has been irradiated by γ-rays with ^{60}Co source. Non-monotonic behavior of T_{c} with increasing irradiation dose Φ(up to 220 MR) is observed: T_{c} decreases at low doses (Φ< 50 MR) from initial value (\approx 93 K) by about 2 K and then rises, forming a minimum. At higher doses (Φ> 120 MR) T_{c} goes down again. The temperature width of resistive transition increases rather sharply with dose below 75 MR and drops somewhat at higher dose. The results observed are discussed, taking into account the granular structure of sample studied and the influence of γ-rays on intergrain Josephson coupling.

cond-mat.supr-con↗

arXiv2000

Influence of high-energy electron irradiation on the transport properties of La_{1-x}Ca_{x}MnO_{3} films (x \approx 1/3)

The effect of crystal lattice disorder on the conductivity and colossal magnetoresistance in La_{1-x}Ca_{x}MnO_{3} (x \approx 0.33) films has been examined. The lattice defects are introduced by irradiating the film with high-energy (\simeq 6 MeV) electrons with a maximal fluence of about 2\times 10^{17} cm^{-2}. This comparatively low dose of irradiation produces rather small radiation damage in the films. The number of displacements per atom (dpa) in the irradiated sample is about 10^{-5}. Nethertheless, this results in an appreciable increase in the film resistivity. The percentage of resistivity increase in the ferromagnetic metallic state (below the Curie tempetature T_{c}) was much greater than that observed in the insulating state (above T_{c}). At the same time irradiation has much less effect on T_{c} or on the magnitude of the colossal magnetoresistance. A possible explanation of such behavior is proposed.

cond-mat.mtrl-sci↗

arXiv1999

Anisotropic magnetoresistive properties of La_{1-x}Ca_{x}MnO_{3} (x \approx 1/3) film at temperatures far below the Curie temperature

A sharp distinction between magnetoresistance (MR) behavior for the magnetic fields applied perpendicular (H_{perp}) and parallel (H_{par}) to the film plane is found in colossal-magnetoresistance film La_{1-x}Ca_{x}MnO_{3} (x \approx 3). At increasing of H_{perp} the MR is first negative (at H_{perp} < 4 kOe), then positive (4 kOe < H_{perp} < 12 kOe), and then negative again (H_{perp} > 12 kOe). At increasing of H_{par} the MR is positive below H_{par} \simeq 6 kOe and negative above it. In both cases the magnetic field was perpendicular to the current. The anisotropic behavior of this kind occurs only at low temperatures (T < 18 K) and is quite different from the results of previous studies.

cond-mat.mtrl-sci↗

arXiv1998

Negative magnetoresistance in the nearest-neighbour hopping conduction in granular gold film

The low temperature (0.5-55 K) conduction of semicontinuous gold film vacuum deposited at T \approx 50 K is studied. The film is near the percolation threshold (thickness 3.25 nm). Its resistance is extremely sensitive to the applied voltage U. At low enough U the film behaves as an insulator (two-dimensional granular metal). In this state the dependences R(T) \propto \exp (1/T) (for T \leq 20 K) and R(U) \propto \exp (1/U)) (for T \leq 1 K and U > 0.1 V) are observed. Magnetoresistance (MR) is negative and can be described by ΔR(H)/R(0) \propto -H^2/T. This negative MR which manifests itself for nearest-neighbour hopping is rather uncommon and, up to now, has not been clarified. The possible mechanisms of such case of negative MR are discussed.

cond-mat.dis-nn↗

arXiv1998

Electron relaxation in disordered gold films

The analysis of quantum corrections to magnetoconductivity of thin Au films responsible for by the effect of weak electron localization has made it possible to determine the temperature dependences of electron phase relaxation time in the temperature range 0.5--50 K for different degrees of crystal lattice disorder. The disorder was enhanced by irradiating the films in vacuum with 3.5 keV Ar ions. The experimental data clearly demonstrate that the contribution of electron-electron interaction to electron phase relaxation increases with disorder and support the theoretical prediction that the frequency of electron-phonon scattering tends to diminish upon a decrease in electron mean free path. It is found that the spin-orbit scattering rate decreases with disorder. In our opinion, such unusual behavior can take place for thin films at decreasing the electron mean free path provided, that the surface electron scattering contributes significantly to the total spin-orbit scattering.

cond-mat.dis-nn↗

arXiv1998

Non-linear effects in hopping conduction of single-crystal La_{2}CuO_{4 + δ}

The unusual non-linear effects in hopping conduction of single-crystal La_{2}CuO_{4 + δ} with excess oxygen has been observed. The resistance is measured as a function of applied voltage U (10^{-3} V - 25 V) in the temperature range 5 K < T < 300 K. At relatively high voltage (U > 0.1 V) the conduction of sample investigated corresponds well to Mott's variable-range hopping (VRH). An unusual conduction behavior is found, however, in low voltage range (approximately below 0.1 V), where the influence of electric field and (or) electron heating effect on VRH ought to be neglected. Here we have observed strong increase in resistance at increasing U at T < 20 K, whereas at T > 20 K the resistance decreases with increasing U. The magnetoresistance of the sample below 20 K has been positive at low voltage and negative at high voltage. The observed non-Ohmic behavior is attributable to inhomogeneity of the sample, and namely, to the enrichment of sample surface with oxygen during the course of the heat treatment of the sample in helium and air atmosphere before measurements. At low enough temperature (below 20 K) the surface layer with increased oxygen concentration is presumed to consist of disconnected superconducting regions (with T_{c} about 20 K) in poor-conducting matrix. The results obtained demonstrate that transport properties of cuprate oxides may be determined in essential degree by structural or stoichimetric inhomogeneities. This should be taken into account at evaluation of "quality" of high-temperature superconductors on the basis of transport properties measurements.

cond-mat.supr-con↗