V. G. Prokhorov

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.

Evidence for the charge-ordered state and phase separation at room temperature in half-doped La0.5Ca0.5MnO3 films

The microstructure and the magnetotransport properties of La0.5Ca0.5MnO3 films, prepared by rf magnetron sputtering on a LaAlO3 substrate, have been investigated. The electron-diffraction and the high-resolution electron-microscopy studies reveal the coexistence at room temperature of double-period modulated (charge-ordered) and unmodulated (charge-disordered) phases. It is suggested that the high-temperature fragmentation of film results in the magnetic-phase-separated state at low temperatures.

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.

Microstructures and the corresponding magnetic properties of half-doped Nd0.5Sr0.5MnO3 films

The microstructural and magnetic properties of amorphous, nanoclustered, and self-organized bilayer Nd0.5Sr0.5MnO3 films, prepared by rf magnetron sputtering, are investigated. The amorphous film was found to be a typical paramagnet with free motion of the individual Mn spins, and the magnetic properties are well described on the basis of the Curie–Weiss approximation. The nanoclustered film manifests magnetic properties mimicking those of superparamagnetic particles following nonclassical magnetic dynamics. The self-organized bilayer film demonstrates a negative exchange bias typical of a ferromagnet/antiferromagnet hybrid system, in spite of the fact that both layers have a ferromagnetic origin. The magnetic properties of the films are discussed in detail on the basis of modern theoretical models.

Flux pinning and the paramagnetic Meissner effect in MgB2 with TiO2 inclusions

The mixed-state superconducting properties of bulk MgB2+2?at.%TiO2, prepared by the in?situ solid state reaction, have been investigated. The high-resolution transmission electron microscopy study reveals that the sample has combined microstructures, consisting of regions with small-?and large-size MgB2 grains, and the MgO inclusions. The particular grains are separated by thin TiB2 interlayers along the c axis. Analysis on the mixed-state parameters, such as the upper critical field, the coherence length and the Ginzburg?Landau parameter ?, proves that the MgB2+2?at.%?TiO2 belongs to a high-? type-II superconductor in the dirty limit. The non-uniformly scaled microstructure results in an unusual (close to the double peak) magnetic field dependence of the pinning force density. The field-cooled temperature dependence of magnetic moment exhibits a transition of the sample to the paramagnetic state at certain applied magnetic fields, which is treated as a manifestation of the paramagnetic Meissner effect. The experimental results are discussed on the basis of modern theoretical approaches.

Magnetic and transport properties driven by lattice strain in La0.7Ca0.3MnO3∕BaTiO3 and La0.7Sr0.3MnO3∕BaTiO3 bilayer films

The microstructure and the magnetic and transport properties of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 films deposited on a BaTiO3 layer (LCMO/BTO and LSMO/BTO) and on a LaAlO3(001) single crystal (LCMO/LAO and LSMO/LAO) by rf-magnetron sputtering using “soft” (or powder) targets are investigated. The films grown on BTO demonstrate biaxial tensile in-plane and compressive out-of-plane strains, while the films grown on LAO, in contrast, manifest compressive in-plane and tensile out-of-plane strains. The films with biaxial tensile in-plane lattice strain undergo the magnetic transition at a higher temperature than that for the biaxial compressive case. This argues that the Mn–O–Mn bond-angle variation, controlled by the lattice strain, plays a more important role in the formation of the spin ordering than the attendant modification of the Mn–O bond length. It is shown that the magnetic inhomogeneity, expressed by a significant difference between the field-cooled and zero-field-cooled temperature-dependent magnetization, has a metallurgical rather than an electronic nature, and is controlled by the crystal lattice distortion and the microstructure defects. The observed enhancement of the magnetoresistance effect in the LSMO/BTO bilayer at room temperature makes this object greatly beneficial in the development of new hybrid ferromagnetic/ferroelectric devices.The microstructure and the magnetic and transport properties of La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 films deposited on a BaTiO3 layer (LCMO/BTO and LSMO/BTO) and on a LaAlO3(001) single crystal (LCMO/LAO and LSMO/LAO) by rf-magnetron sputtering using “soft” (or powder) targets are investigated. The films grown on BTO demonstrate biaxial tensile in-plane and compressive out-of-plane strains, while the films grown on LAO, in contrast, manifest compressive in-plane and tensile out-of-plane strains. The films with biaxial tensile in-plane lattice strain undergo the magnetic transition at a higher temperature than that for the biaxial compressive case. This argues that the Mn–O–Mn bond-angle variation, controlled by the lattice strain, plays a more important role in the formation of the spin ordering than the attendant modification of the Mn–O bond length. It is shown that the magnetic inhomogeneity, expressed by a significant difference between the field-cooled and zero-field-cooled temperature-dependent magnet...

The controlled charge ordering and evidence of the metallic state in Pr0.65Ca0.35MnO3 films

The resistivity of Pr0.65 Ca0.35 MnO3 films prepared by pulsed laser deposition has been measured in the temperature range 4.2 - 300 K. The formation of metallic phase is suggested by the analysis of the temperature dependence of resistivity at low temperatures under zero-magnetic field. It is shown that the appearance of the charge-ordered state at 205 K can be controlled by the lattice strains accumulated during the film growth. The charge-ordering energy gap was estimated to be 77.5 meV from the experimental data. Experimental results are interpreted on the basis of the phase-separation model.

Evidence for Griffiths phase in La0.4Ca0.6MnO3 film with strip-domain inclusions

The microstructure and the magnetic properties of La0.4Ca0.6MnO3 film, prepared by rf magnetron sputtering on a LaAlO3 substrate, have been investigated. The electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c for the orthorhombic symmetry. The magnetic measurements show that in addition to the expected antiferromagnetic transition at TN≃120 K with decreasing temperature, the film manifests the Griffiths phase behavior in a wide temperature range.

Evidence of non-Dzyaloshinskii–Moriya ferromagnetism in epitaxial BiFeO3 films

X-ray diffraction analysis and high-resolution electron microscopy of BiFeO3 films prepared by dc magnetron sputtering on single-crystal LaAlO3 (001) substrates reveal that the films have a highly c-oriented orthorhombic crystalline structure. The magnetic properties of the BiFeO3 films are typical of ensembles of interacting superparamagnetic clusters, rather than Dzyaloshinskii-Moriya weak ferromagnets. The appearance of extrinsic nanoscale superparamagnetic clusters is explained by an oxygen deficiency in certain regions of the film, where ferromagnetic ordering can be realized through a double-exchange Zener mechanism.