Physical Properties Of La0 Research Articles

Effects of sodium doping on physical properties of La0.75Sr0.25−xNaxCoO3 (0⩽x⩽0.2) cobaltites

The effects of Na doping on the crystallographic, magnetic, electrical and magnetocaloric properties of La0.75Sr0.25−xNaxCoO3 (0⩽x⩽0.2) samples, elaborated using the solid-state reaction method at high temperature, have been investigated. X-ray powder diffraction studies reveal that all our synthesized samples crystallize in the rhombohedral structure (R3¯c space group). Na substitution leads to an enhancement of the Co–O bond length and a reduction in the Co–O–Co bond angle and consequently to a weakening of the double exchange interaction between Co3+ and Co4+ ions. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT show thermomagnetic irreversibility. At low temperatures, with increasing Na amount, the samples change from ferromagnetic-like behavior (x=0, 0.05 and 0.1) to spin-glass one (x=0.15 and 0.2). The ferromagnetic–paramagnetic transition temperature decreases with increasing Na amount from 235 to 200 K (x=0.1). The critical exponent value, associated to the spontaneous magnetization, increases with increasing Na amount from 0.37 for x=0 to 0.57 for x=0.1. The temperature dependence of the electrical resistivity reveals three different regimes as a function of Na content: (i) for x=0, a metallic behavior is observed in the whole temperature range, (ii) for x=0.05, a metal-semiconducting transition is observed around 80 K and (iii) for x⩾0.1, we observe a semiconducting behavior in the whole temperature range. The parent compound (x=0) exhibits a maximum entropy change |ΔSMMAX| of 0.84 J kg−1 K−1 at 220 K in a magnetic applied field of 2 T. |ΔSMMAX| decreases to 0.47 J kg−1 K−1 at 185 K for x=0.1 in the same field magnitude. Well above TC, |ΔSMMAX| shows a linear dependence as a function of H2 indicating the existence of spin fluctuations inside ferromagnetic clusters.

Quantifying Strain Effects on Physical Properties of La0.68Ba0.32MnO3 Epilayers and Heterostructures

epilayers and (LBMO/STO) superlattices were grown epitaxially on STO, –coated STO and single-crystal substrates. Almost pseudomorphic growth of the LBMO/STO superlattice on an STO substrate is observed to a total thickness of . The critical relaxation thickness of LBMO epilayers depends on the lattice misfit. Above the critical thickness, the strain in the epilayers relaxes gradually rather than abruptly to an unstrained state. Both surface and interface roughness of the LBMO epilayer increased as lattice strain decreased. The Jahn–Teller strain in the LBMO lattice was experimentally estimated by measuring the in-plane and out-of-plane X-ray diffractions. The modifications of magnetic and transport properties of manganite epilayers were discussed in relation to changes in the octahedral geometry.

The effect of grain boundaries on magnetic and transport properties in colossal magnetoresistance particle film

La0.67Ca0.33MnO3 grain films have been grown by pulsed electron beam deposition technology,and their magnetic and electronic transport properties investigated in detail.The influence of grain boundaries on the physical properties of La0.67Ca0.33MnO3 grain films were studied.Grain boundaries weaken the coupling between grains,resulting in a cluster spin glass state.The metal-insulator transition temperature (Tp) was shifted far below the ferromagnetic-paramagnetic transition temperature (Tc).At low temperatures,the electron transportation indicates a weak localization behavior,and under low magnetic fields,the intrinsic magnetoresisance effect is screened by the influence of grain boundaries.

Disorder influence on the magnetic properties of La0.55Sr0.45MnO3/SrTiO3 superlattices

The structural and physical properties of La0.55Sr0.45MnO3/SrTiO3 superlattices grown by magnetron sputtering are studied. Two deposition temperatures and different mismatched substrates and buffer layers were used. The structure was determined by refinement through X-ray diffraction pattern fitting. The results indicate 1 unit cell (u.c.) interdiffusion at the interfaces and a 1 u.c. layer thickness fluctuation, i.e. roughness. In-plane hysteresis loops show the expected ferromagnetic behavior, while the perpendicular-to-plane hysteresis loops show anomalies which could evidence magnetic domain size inhomogeneities and stress.

Physical Properties of La0.85K0.15MnO3 in Magnetic Field

We have measured the specific heat, resistance and dc susceptibility of the mixed valence manganite La0.85K0.15MnO3 as a function of temperature. The sample shows colossal magnetoresistance effects with phase transitions at low temperature. The magnetic specific heat of this sample is proportional to the temperature coefficient of the resistance, following the Fisher-Langer relation. This suggests that the sharp drop in the transition region of the sample is due to the magnetic scattering.

Effects of the Oxygen Nonstoichiometry on the Physical Properties of La0.7Sr0.3MnO3−δ□δ Manganites (0≤δ≤0.15)

We present the oxygen deficiency effects on the structural, magnetic, and electrical properties in La0.7Sr0.3MnO3−δ□δ solution where □ is a vacancy and 0≤δ≤0.15. Polycrystalline samples La0.7Sr0.3MnO3−δ□δ were synthesized by a new method. In this series of manganites the Mn3+ content is systematically increased due to the increase in the nonstoichiometry δ. X-ray diffraction analysis shows a phase transition from a rhombohedral to an orthorhombic system at 0.075≤δ≤0.1. The material is ferromagnetic for 0≤δ≤0.1 and antiferromagnetic for 0.125≤δ≤0.15. The Curie temperature TC and saturation magnetization Ms decrease with increasing δ. Resistivity measurements as a function of temperature show a remarkable behavior for the La0.7Sr0.3MnO2.9 compound; it is ferromagnetic metallic for 115≤T≤180 K and becomes ferromagnetic insulator below 115 K, where a charge ordering seems to appear. The difference in the hopping energies in our samples can be related to the existence of two crystallographic structures, one orthorhombic and the other rhombohedral.

Effects of lattice distortion on the physical properties and surface morphology of magnetoresistive perovskite epitaxial films

The effects of lattice distortion on the physical properties of La0.7Ca0.3MnO3 epitaxial films are investigated. Our results suggest that larger substrate-induced lattice distortion gives rise to larger zero-field resistivity and larger negative magnetoresistance. Similar effects are also observed in samples of different thicknesses and on the same substrate material, with larger resistivity and magnetoresistance associated with thinner samples. In addition to x-ray diffraction spectroscopy, the degrees of lattice distortion in different samples are further verified by the surface topography taken with a low-temperature scanning tunneling microscope. Quantitative analyses of the transport properties suggest that the high-temperature (T→TC) colossal magnetoresistance (CMR) in the manganites is consistent with the conduction of lattice polarons induced by the Jahn–Teller coupling, and that the low-temperature (T≪TC) magnetoresistance may be attributed to the magnetic domain wall scattering. In contrast, the absence of the Jahn–Teller coupling and the large conductivity in La0.5Ca0.5CoO3 epitaxial films yield much smaller negative magnetoresistance, which may be attributed to disorder-spin scattering.