J. Salminen

Variable-range hopping conductivity in La1-xCaxMn1-yFeyO3: evidence of a complex gap in density of states near the Fermi level

The resistivity, ρ, of ceramic La1−xCaxMn1−yFeyO3 with x = 0.3 and y = 0.0–0.09 is found to obey, between a temperature Tv ≈ 310–330 K and the ferromagnetic-to-paramagnetic transition temperature, TC = 259–119 K (decreasing with y), the Shklovskii–Efros-type variable-range hopping conductivity law, ρ(T) = ρ0 (T) exp [(T0 /T)1/2 ]. This behaviour is governed by generation of a soft Coulomb gap Δ ≈ 0.42 eV in the density of localized states and a rigid gap δ(T) ≈ δ(Tv)(T/Tv)1/2 with δ(Tv) ≈ 0.16, 0.13 and 0.12 eV at y = 0.03, 0.07 and 0.09, respectively. Deviations from the square root dependence of δ(T), decreasing when y is increased, are observed as T → TC. The prefactor of the resistivity follows the law ρ0 (T) ~ Tm, where m changes from 9/2 at y = 0 to 5/2 in the investigated samples with y = 0.03, 0.07 and 0.09, which is connected to introduction of an additional fluctuating short-range potential by doping with Fe.

Non-universal low-field magnetic scaling and variable-range hopping conductivity as a consequence of disorder in La1−xCaxMn1−yFeyO3

Abstract Low-field (B=2−80 G ) dc magnetic susceptibility, χ , investigated in hole-doped La 1− x Ca x Mn 1− y Fe y O 3 (LCMFO) with x =0.3 and y =0.05−0.10, exhibits non-uniform critical behavior near the paramagnetic (PM)-to-ferromagnetic (FM) transition temperature, T C . We observe the scaling law, χ −1 ( T )− χ −1 ( T C )∼( T / T C −1) γ ≡ τ γ , with γ ≈1.4 corresponding to a 3D Heisenberg spin system below τ cr ∼0.1–0.2, and with γ ≈1.7 characterizing a 3D percolation system, above τ cr . The non-universal scaling results from strongly inhomogeneous distribution of holes leading to formation of percolation clusters from the hole-rich nanoscale FM particles, embedded in the host PM matrix. The resistivity, ρ , of LCMFO exhibits between a temperature T v ≈310–330 K and T C the Shklovskii–Efros-type variable-range hopping conductivity law, ρ ( T )= ρ 0 ( T )exp[( T 0 / T ) 1/2 ], governed by generation of a complex gap in the density of states. The prefactor follows the law ρ 0 ( T )∼ T m , where m changes from 9/2 at y =0 to 5/2 at y =0.03, 0.07 and 0.09, reflecting an additional fluctuating short-range potential induced by doping with Fe.

Unconventional critical behavior of magnetic susceptibility as a consequence of phase separation and cluster formation in La0.7Ca0.3MnO3 thin films

Nonuniversal critical behavior of the dc magnetic susceptibility χ−1(T)−χ−1(TC)∼(T/TC−1)γ≡τγ is observed near the paramagnetic (PM) to ferromagnetic (FM) transition in La0.7Ca0.3MnO3 film samples. The value of γ≈1.4 obtained for 0<τ<τ1 with τ1≈0.05–0.09 corresponds to a three-dimensional Heisenberg spin system and that of γ≈2.4 for τ1<τ<τ2 with τ2≈0.2–0.3 to a two-dimensional percolation system. This behavior is attributed to phase separation by presence of FM clusters with temperature dependent correlation length in the PM matrix of the film.

Variable-range hopping conductivity and absence of a true metal–insulator transition in La0.7−δ Ca0.3Mn1−yFeyO3

Resistivity of La0.7−δ Ca0.3Mn1−yFeyO3 (LCMFO) with y=0–0.05, δ=0 and 0.017 is investigated between 4.2–350 K, showing that La vacancies increase the effect of Fe doping by influencing electronic states through additional microscope disorder. The Shklovskii–Efros variable-range hopping conductivity, governed by generation of a soft parabolic gap in the spectrum of localized states due to Coulomb interaction between charge carriers, is observed both above and well below the ferromagnetic-to-paramagnetic transition temperature. Our results give evidence for absence of a true metal–insulator transition both in undoped material and in LCMFO.

Influence of sequential etching on YBCO films deposited by PLD from a nanostructured target

Thin YBa/sub 2/Cu/sub 3/O/sub 6+x/ (YBCO) films with T/sub c/ = 90 K and high critical current in the order of 10/sup 7/ A/cm/sup 2/ at 77 K are prepared with pulsed laser deposition from a nanostructured target. Changes in the superconducting and structural properties of the films are investigated after repeated thinning by Ar-ion sputtering or by electrochemical etching. As shown by Auger depth profiling, the chemical composition is preserved through the film during Ar-ion sputtering while electrochemical etching changes the atomic concentrations and surface structures. Investigations by SQUID magnetometry and by conventional resistivity measurements give evidence that the dependence of T/sub c/ on the film thickness is similar to as-prepared ultrathin films. The thickness dependence of J/sub c/ indicates an excess of pinning centers on the surface of the as-deposited film.

Laser deposition of thin films from La0.7Ca0.3MnO3 targets prepared by sol–gel and solid-state methods

The sol–gel (SG) method is used to synthesize La0.7Ca0.3MnO3 nanopowders having mean particle thickness of 1.3 nm. Targets for pulsed laser deposition of thin films are made from this material and compared with targets sintered from micron-size powder prepared by a typical solid-state (SS) method. Emission line intensity ratios observed from the laser plume give evidence that the paths of atomic species deposited from the SS target are clearly longer. The sizes of the particles in the SG plume are smaller than in the SS plume and agree with the particle sizes in the corresponding targets. According to x-ray photoelectron spectroscopy the deposited films have on the surface some excess of Ca and deficiency in Mn. The composition of the films prepared from the SG target is closer to ideal than the composition of the films made from the SS target. The SG films have a steeper magnetic transition and smoother surface than the SS films. Therefore the SG material is better suited for deposition of smooth thin films.