V. A. Komashko

Transport properties of La0.8Ca0.2MnO3 epitaxial films prepared by rf magnetron sputtering using soft targets

Epitaxial La0.8Ca0.2MnO3 films have been prepared by the rf magnetron sputtering method using so-called ”soft” (or powder) targets instead of solid-state ones. The as-grown films have a highly oriented perovskite-like structure, a sharp metal–insulator transition in zero magnetic field at a temperature (Tp) near 220 K, and a temperature coefficient of resistance (TCR) of 900%. We found that a reduction in the film thickness leads to a shift of Tp in the low-temperature range and a significant suppression in the TCR value. The obtained films also exhibited an anisotropic temperature behavior of the magnetoresistance in a magnetic field of 1.5 T.

Oxygen-deficiency-activated phase transition in a long-aged La0.8Ca0.2MnO3 film

The magnetic and transport properties of as-deposited and long-aged La0.8Ca0.2MnO3−δ films have been investigated in a wide temperature range. The x-ray diffraction data have shown separation of the film into two crystalline phases of the cubic and rhombohedral symmetry with different oxygen contents of δ≃0.08 and 0.16, respectively, after a half-year aging at room temperature in air. Both phases testify two different electronic (metal–insulator) and magnetic transitions with similar maximum values of magnetoresistance. The possible mechanism for such a type of structural transition in the aged manganite thin films is discussed.

Lattice-strain-driven ferromagnetic ordering in La0.8Sr0.2MnO3 thin films

The effects of lattice strain on the magnetic and the transport properties of La0.8Sr0.2MnO3 films grown on an (001) LaAlO3 substrate and on a La0.8Ca0.2MnO3 layer have been studied. It was observed that the metal-insulator and the ferromagnetic transitions turn out to be at higher temperatures for the film deposited on La0.8Ca0.2MnO3 layer with respect to that on LaAlO3. The dependence of Curie temperature on the bulk and the Jahn–Teller strains has also been determined.

Features of the angular dependence of the critical current in thin epitaxial films of HTSC YBa2Cu3O7−δ in a magnetic field

The dependence of the critical current density jc on the magnitude and direction of the magnetic field H is investigated in thin epitaxial films of YBa2Cu3O7−δ having a high value of jc in the absence of field (∼106u2009A/cm2 at 77 K) and a thickness d less than twice the magnetic field penetration depth λ. It is found that the jc(H) curves have a low-field plateau both for fields perpendicular and parallel to the film. In a magnetic field perpendicular to the film, the “effective pinning” plateau extends to a field corresponding to a density of Abrikosov vortices threading the film at which it is no longer favorable for them all to be pinned at edge dislocations in the interblock walls, and a fraction of them become unpinned. In contrast, in a field parallel to the film the end of the plateau is unrelated to depinning of threading vortices parallel to the film; instead, jc(H) decreases after the plateau region because the magnetic field parallel to the film weakens the pinning of vortices perpendicular to th...

Magnetic and transport properties of La0.8Sr0.2MnO3/La0.8Ca0.2MnO3 bilayer

The effects of lattice strain on the magnetic and the transport properties of La0.8Sr0.2MnO3 films grown on an (001) LaAlO3 substrate and on a La0.8Ca0.2MnO3 layer were studied. It was observed that the metal-insulator and the ferromagnetic transitions occur at higher temperatures for the film deposited on La0.8Ca0.2MnO3 layer than on LaAlO3. The dependence of Curie temperature on the bulk and the Jahn—Teller strains were also determined.

Giant resistance switching effect in nano-scale twinned La0.65Ca0.35MnO3 film

The magnetic and transport properties of a 20 nm-scale twinned La0.65Ca0.35MnO3 film are investigated in a temperature range of 77–300 K. The coexistence of ferromagnetic metallic and charge-ordered insulating phases is suggested by analyzing the temperature and current dependences of the resistance at low temperatures. It is shown that thermocycling leads to the formation of a nonequilibrium state in the ensemble of charge-ordered domains and to the appearance of a giant switching in resistance of up to 100%. The experimental results are discussed on the basis of phase separation.

Magnetic phase diagram and structural separation of La0.7(Ca1−ySry)0.3MnO3 thin films

The structural, magnetic, and transport properties of La0.7(Ca1?ySry)0.3MnO3 films, deposited on a LaAlO3?(001) single crystalline substrate by rf-magnetron sputtering using “soft” (or powder) targets, have been investigated. It was found that at 0.3 ? y ? 0.5 both the rhombohedral (R3c) and the orthorhombic (Pnma) crystal phases in the form of nanoscale clusters are coexistent at room temperature. The observed structural clustering is accompanied by two-stage magnetic and electronic transitions, and governed by a nonuniform distribution of the lattice strain through the film. It was shown that for the films with 0 ? y ? 0.5 the nonlinear (almost parabolic) MR(H) dependence is typical while, for 0.65 ? y ? 1.0, the linear MR(H) behavior is observed at room temperature. The magnetotransport properties of films are explained within the framework of field-dependent activation-energy model. The magnetic phase diagram for La0.7(Ca1?ySry)0.3MnO3 thin-film system is also presented.

Observation of resistance switching between insulating and metallic states in nano-crystalline La0.65Ca0.35MnO3 film

Abstract The magnetic and transport properties of a 20-nm-scale twinned La0.65Ca0.35MnO3 film have been investigated in a temperature range of 77–300xa0K. The coexistence of ferromagnetic metallic (FM) and charge-ordered insulating (COI) phases is suggested based on analysis of resistance temperature and current dependencies at low temperatures. It is shown that thermocycling leads to formation of a non-equilibrium state in the ensemble of the charge-ordered domains and to appearance of a giant (up to 100%) resistance switching between MF and COI states. The experimental results are discussed from the viewpoint of the phase separation phenomenon.

Observation of the strain-driven charge-ordered state in a La0.7Ca0.3MnO3−δ thin film with oxygen deficiency

The magnetic and transport properties of La0.7Ca0.3MnO3−δ films with an oxygen deficiency (δ≃0.1) and a La0.9Ca0.1MnO3 film with the stoichiometric oxygen content are investigated in a wide temperature range. It is shown that the charge-ordered insulating (COI) state is observed for a La0.7Ca0.3MnO2.9 film with thickness d⩽30u2009nm, which manifests mainly a cubic crystal structure with an anomalously small lattice parameter for this composition. An increase in the film thickness (d≃60u2009nm) leads to a structural transition from the lattice-strained cubic to the relaxed rhombohedral phase and is accompanied by a shift of the Curie point (TC) to lower temperature and a frustration of the COI state. The magnetic and transport properties of the La0.7Ca0.3MnO2.9 film with d≃60u2009nm are similar to those exhibited by the optimally oxygen-doped La0.9Ca0.1MnO3 film. It is concluded that the formation of the COI state in the La0.7Ca0.3MnO3−δ compound is governed by a compression of the crystal lattice rather than accumula...

Vortex pinning anisotropy in perfect YBCO films studied by transport current measurements

Abstract Critical current densities Jc are studied for YBa2Cu3O7−δ films with J c (77 K )>3 MA / cm 2 in a range of applied fields, temperatures, angles between the field vector and the C-axis of YBCO films. Two-peaked angular Jc(H,T) dependencies are detected. Opposite temperature behavior of Jc(θ)-peaks for H || C and H || ab is found. The J c (H || C) -peak first rises with T and then diminishes while T approaches Tirr(H). In contrast, the J c (H || ab) -peak becomes more remarkable with temperature increase. Jc(H,T)-curves for H || C -orientation exhibit two distinct crossovers, corresponding to transitions from Jc(H)∼const at low fields to Jc∝H−0.5 at intermediate fields and to Jc∝H−q (q∼1–1.5) at higher fields. Vortex interaction with linear pin arrays is assumed to be the reason for the phenomena.