Absence of long-range magnetic order in the La1.4Sr0.8Ca0.8Mn2O7 bilayered manganite.

This paper presents the results of a high-resolution neutron diffraction and magnetometry investigation on the optimally doped (x = 0.3) La(1.4)Sr(0.8)Ca(0.8)Mn(2)O(7) bilayered manganite. In particular, two samples with different oxygen contents have been studied to put in prominence the role of the Mn average valence states at fixed cation concentration. The results show, for the first time, the absence of long-range magnetic order in this optimally doped manganite when the A-site of the structure is doped with equal proportions of isovalent Ca and Sr. This holds for both samples, which present different lattice effects with T, thus suggesting the primary role of cation disorder as the source of the lack of long-range order. The presence, for both samples, of defined insulating- to metallic-like transitions suggests that the transport properties are not linked to the evolution of long-range order and that two-dimensional spin ordering in the layers of the perovskite blocks may be sufficient to "assist" the hole hopping. A possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions.

Doping effects in single-layered La0.5Sr1.5MnO4 manganite.

In this paper we report the results of the synthesis and structural, transport, and magnetic characterization of pure La(0.5)Sr(1.5)MnO(4) and B-site lightly doped samples, i.e. La(0.5)Sr(1.5)Mn(0.95)B(0.05)O(4), where B = Ru, Co, and Ni. The choice was made in order to probe the charge ordering/orbital ordering ground state of the monolayered La(0.5)Sr(1.5)MnO(4) manganite as a consequence of the cation doping. It is shown that even a light doping is successful in suppressing the charge and orbital order found in pure La(0.5)Sr(1.5)MnO(4). No long-range magnetic order has been detected in any of the doped samples but the setup of a spin-glass state with a common freezing temperature ( approximately 22 K). Structural parameters show an anisotropy in the lattice constant variation, with the tetragonal distortion increasing as the cell volume reduces, which may suggest a variation in the orbital character of the e(g) electrons along with the overall cation size.

Influence of Ru doping on the structure, defect chemistry, magnetic interaction, and carrier motion of the La(1-x)Na(x)MnO(3+delta) manganite.

In this work, we report a structural, electrical, and magnetic characterization of the La(1-x)Na(x)Mn(1-y)Ru(y)O(3+delta) (LNMRO) system with x = 0.05 and 0.15 and y = 0, 0.05, and 0.15, also comprising an investigation of the role of the oxygen content on the related redox properties. The experimental investigation has been realized with the aid of X-ray powder diffraction, electron microprobe analysis, thermogravimetry, electrical resistivity and magnetization measurements, and electron paramagnetic resonance. We demonstrate that the effect of ruthenium doping on the studied LNMRO compounds is not only directly related to the Ru/Mn substitution and to the Ru oxidation state but also indirectly connected to the oxygen content in the sample. Our data show that ruthenium addition can "improve" electrical and magnetic properties of nonoptimally (low) cation-doped manganites, causing an increase of the T(C) value and the insurgence of magnetoresistance effect, as observed for the x = 0.05 and y = 0.05 sample (MR congruent with 60% at 7 T and at approximately 260 K).