A.R. Kaul

Magnetocaloric effect in La1?xAgyMnO3 (y ? x): direct and indirect measurements

For the first time the magnetocaloric properties of La0.9Ag0.1MnO3, La0.8Ag0.2MnO3, La0.85Ag0.15MnO3, La0.8Ag0.15MnO3 and La0.8Ag0.1MnO3 manganites have been investigated by direct and indirect measurement techniques. All samples showed almost the same relative cooling power (RCP). Temperatures of maxima of the magnetocaloric effect (MCE) are between a few degrees below freezing and the room temperature region. The compounds showed RCP values of about 100 J kg−1 at a field change of 2.6 T, which is about half the RCP of gadolinium. Because of considerable MCE and the Curie temperatures ranging from 269 to 303 K, these materials could be used as magnetic refrigerants for magnetic refrigeration in the sub-room and room temperature range.

Metal-insulator transition induced by oxygen isotope exchange in the magnetoresistive perovskite manganites

Perovskite manganites derived from LaMnO3 have recently become the subject of intensive study following the discovery of ‘colossal’ magnetoresistance (a magnetically induced change in electrical resistance of up to several orders of magnitude) in several members of this family of compounds. The manganites exhibit a broad range of electronic and magnetic phases, ranging from low-resistance ferromagnetic metals to high-resistance insulators, which are extremely sensitive to variation of composition, temperature and pressure. A recent study showed that such sensitivity also extends to oxygen isotope exchange: replacing 16O with 18O in La0.8Ca0.2MnO3 produces an unusually large change in the magnetic properties (a 21-kelvin decrease in the Curie temperature). The magnitude of this isotope shift is evidence for the essential role played by electron–phonon coupling in determining the transport properties of these materials. Here we show that this sensitivity to oxygen isotope exchange can be even more extreme. In its normal state, the compound La0.175Pr0.525Ca0.3MnO3 undergoes an insulator-to-metal transition as it is cooled below ∼95u2009K. But we find that, after substituting 18O for 16O, the compound remains an insulator down to 4.2u2009K, so providing a vivid demonstration of the importance of lattice vibrations in these materials.

Perovskite rare-earth nickelates in the thin-film epitaxial state

We have succeeded in the preparation of thin films of rare-earth nickelates RNiO3u200a(R=Pr, Nd, Sm, and Gd) under reduced oxygen pressure <0.02u200abar by metalorganic chemical-vapor deposition owing to their epitaxial stabilization on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3 phases. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The epitaxial films of nickelates were strained and consisted of 90° domains with the orthorhombic Pnma structure. The transport properties of the strained films on LaAlO3 were similar to those of the bulk material of the same composition under applied pressure of 9 kbar but they were different from the properties of the bulk material under ambient pressure. The result implies that transport properties of RNiO3 films with sharp metal-to-insulator transition can be effectively tuned by the control of the lattice strain.

Modification of the ground state in Sm-Sr manganites by oxygen isotope substitution

The effect of O-16-->O-18 isotope substitution on electrical resistivity and magnetic susceptibility of Sm1-xSrxMnO3 manganites is analyzed. It is shown that the oxygen isotope substitution drastically affects the phase diagram at the crossover region between the ferromagnetic metal state and that of antiferromagnetic insulator (0.4

Continuous YBCO deposition onto moved tapes in liquid single source MOCVD systems

Abstract In a small scale MOCVD system YBCO films with j c(77 K ) > 1 MA were deposited onto substrates, which were continuously moved through the deposition chamber. Two routes for the buffer layer deposition onto textured Ni tapes were chosen: a) CeO 2 , LaMnO 3 and LaCrO 3 films were deposited in hydrogen atmosphere by MOCVD. On LaAlO 3 and R-Al 2 O 3 crystals epitaxial films were obtained b) textured NiO layers were grown by oxidation of textured Ni tapes. Based on the small scale system an enlarged MOCVD reactor was constructed.

Continuous YBCO deposition by MOCVD for coated conductors

Abstract MOCVD is an economic method for the production of coated conductors. A single source MOCVD-system for continuous deposition on metal tapes was developed. Single crystals, IBAD buffered tapes and textured Ni-tapes were transported through the deposition zone and coated with YBCO. The velocity for the substrate transport was 4 m/h. On single crystals and IBAD buffered substrates the best critical current density of the 300–350 nm thick YBCO films were 2.5 and 4.8 MAxa0cm −2 at 77 K. On CeO 2 /YSZ/CeO 2 buffered textured nickel tapes the j c of the 500 μm thick YBCO film was 0.25 MAxa0cm −2 at 77 K. Textured oxide buffer layers were deposited by MOCVD on single crystals and textured Ni in reducing atmosphere.

Resistivity, specific heat, and magnetocaloric effect of La0.8Ag0.1MnO3: Effect of isotopic substitution of 16O → 18O

Results of the experimental studies of resistivity, specific heat, and magnetocaloric effect in lanthanum deficient manganite of La0.8Ag0.1MnO3 are presented. The influence of isotopic substitution of oxygen 16Оu2009→u200918О on the resistivity, specific heat, and magnetocaloric effect in La0.8Ag0.1MnO3 is estimated. The magnetocaloric effect has been studied by two different methods, which agree well enough. The entropy change as a result of the magnetocaloric effect varies between 2 and 4.1u2009J/kg K in the magnetic field of 26 kOe in the study samples.

Behavior of the atomic and magnetic structure of La0.35Pr0.35Ca0.30MnO3 at a metal-insulator phase transition

The evolution of the structural and magnetic properties of the CMR (colossal-magnetoresistance) compound La0.35Pr0.35Ca0.30MnO3 as the temperature changes from 10 to 293 K is investigated by means of neutron diffraction. It is shown that the changes in the transport and magnetic properties are directly related with the rearrangement of the atomic structure. A phase transition to the metallic state occurs together with simultaneous ferromagnetic ordering of the manganese moments and is accompanied by a jump in volume. The static distortions of the oxygen octahedra which are observed to occur prior to the magnetic phase transition and which are practically absent at room temperature and in the FM phase attest to the orbital ordering of oxygen atoms on the bonds, with freezing-in of the Jahn-Teller modes.

Epitaxial perovskite rare-earth nickelates and their heterostructures with CMR manganites

Thin films of rare-earth nickelates RNiO3 (R=Pr, Nd, Sm, Gd) were prepared under a reduced oxygen pressure of <0.02 bar by MOCVD on perovskite substrates. The film–substrate lattice mismatch is critical for the epitaxial stabilization of RNiO3. Increase of the lattice mismatch or film thickness results in the deposition of rare-earth oxides and NiO instead of RNiO3. The transport properties of the films on LaAlO3 were similar to those of the bulk material of the same composition under an applied pressure of 9 kbar. The properties of RNiO3 films with a sharp metal-to-insulator transition can be effectively tuned by the lattice strain.

Anomalous frequency behavior of temperature dependent impedance spectra of the LuFe2O4 multiferroic

We have measured temperature dependent impedance spectra of LuFe2O4 multiferroic polycrystalline samples in the temperature range of 100–400u2009K, in a wide frequency range of measuring AC voltage: from 20u2009Hz to 120u2009MHz. In spectra measured at 30–70u2009MHz and at 200–260 К, we observed anomalous positive temperature coefficient of resistance. We explain the anomaly on the basis of the generalized barrier model.We have measured temperature dependent impedance spectra of LuFe2O4 multiferroic polycrystalline samples in the temperature range of 100–400u2009K, in a wide frequency range of measuring AC voltage: from 20u2009Hz to 120u2009MHz. In spectra measured at 30–70u2009MHz and at 200–260 К, we observed anomalous positive temperature coefficient of resistance. We explain the anomaly on the basis of the generalized barrier model.