G. G. Kaminsky

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.

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.

Transport properties of La1−xSrxCoO3−δ films (0.15⩽x⩽0.5)

Abstract The magnetic and transport properties of La1−xSrxCoO3 films prepared by laser ablation have been investigated. The metal–insulator transition was observed in the films with decreasing Sr2+ concentration (if x⩽0.25). Analysis of the experimental results revealed that the transport of carriers in the metallic state is governed by electron–electron scattering if the nature of a small energy gap in the spectrum of electron excitations is taken into account. An oxygen deficiency in the aged La0.5Sr0.5CoO3−δ film leads not only to a change in the Co3+/Co4+ ratio, but also to a structural disorder in the Co–O–Co conduction channel due to the accumulated oxygen vacancies. A growth of the resistivity with decreasing temperature for the aged La0.5Sr0.5CoO3−δ film is explained in the framework of the Coulomb-interaction model. Analysis of ρ(T) data shows that the transport of carriers at high temperatures (above the temperature of magnetic transition) in the insulating state (x⩽0.25) is governed by the Mott-like variable-range hopping (VRH) rather than motion of the lattice polarons. However, the VRH model needs to be modified for compounds of this type. The ρ(T) dependence is fairly described based on the two parallel-resistor model in the low-temperature range, where one resistor corresponds to the ferromagnetic metal-cluster network and the other to the insulating matrix (IM). Two mechanisms of magnetoresistance (MR) are involved in the investigated films. In the metallic state (x>0.25) the MR is provided by a change in the electron scattering rate, influenced by the magnetic field. This process is reflected in the MR temperature dependence as a small peak at the Curie temperature. In the insulating state (x⩽0.25) the phase separation into the ferromagnetic metallic clusters and the IM is formed and an applied magnetic field induces a growth of the ferromagnetic metallic phase that can be possibly described by the double-exchange mechanism.

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.

Two-dimensional growth, anisotropic polaron transport, and magnetic phase segregation in epitaxial Nd0.52Sr0.48MnO3 films

Nd0.52Sr0.48MnO3 films have been fabricated by dc magnetron sputtering on single-crystal LaAlO3 (001) and SrTiO3 (011) substrates with additional annealing to relax the lattice strain. Although the Nd0.52Sr0.48MnO3 films were deposited simultaneously on different substrates at the same deposition rate, they differ in thickness by a factor of ? . The observed difference in thickness is explained by the two-dimensional (layer-by-layer) film growth, rather than by a difference in growth rate controlled by the crystalline orientation of the substrate. An analysis of optical and transport properties reveals that the observed anisotropy in the polaron motion is governed by a strong anisotropy in the trapping energy, rather than in polaron formation. It is shown that the deposited Nd0.52Sr0.48MnO3 films exhibit magnetic behavior typical of two-phase magnetic systems and should be regarded as an assembly of interacting magnetic clusters.

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.

Evidence of the Griffiths phase in multiferroic BiMnO3 and BiFe0.5Mn0.5O3 films

Microstructure and magnetic properties of BiMnO3 and BiFe0.5Mn0.5O3 films, prepared by rf magnetron sputtering on LaAlO3 (001) single-crystalline substrate, are investigated. The selected-area electron diffraction analysis allows us to identify the crystal structure of the BiMnO3 film as orthorhombic, while the BiFe0.5Mn0.5O3 film has a hexagonal lattice symmetry. High-resolution electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c in both films. Magnetic measurements show that in addition to the basic paramagnetic phase the films exhibit Griffiths phase behavior in a wide temperature range. We argue that the observed weak ferromagnetism is due to the strip-domain layered inclusions, rather than intrinsic physical origin of the films.

Nonclassical magnetic dynamics and negative exchange bias in Nd0.5Sr0.5MnO3 films

Amorphous, nanocluster, and self-organizing bilayer Nd0.5Sr0.5MnO3 films are prepared by rf magnetron sputtering. The amorphous film turns out to be a typical paramagnet with free motion of the individual Mn spins, the magnetic properties of which are well described by the Curie-Weiss approximation. The nanocluster film manifests magnetic properties mimicking superparamagnetic particles with a nonclassical magnetic dynamics. The unique shape of the hysteresis loops, which have hysteretic lobes at high magnetic field but are nonhysteretic as the field crosses zero, suggests that each particle (nanocluster) is a closure magnetic domain (or magnetic vortex) rather than a single domian. At the same time, a blocked to unblocked transition was observed with increasing temperature, as in the usual superparamagnet. The self-organizing bilayer film demonstrates a negative exchange bias, which is typical for the ferromagnet/antiferromagnet hybrid system in spite of the fact that both layers in our case have a ferro...