Electrical and magnetic properties of La_(0.67)Ba_(0.33)Mn_(1-x)(Me)_xO_3 perovskite manganites: case of manganese substituted by trivalent (Me = Cr) and tetravalent (Me = Ti) elements

摘要

The effects of non-magnetic Ti~(4+) substitution on the structural, electrical and magnetic properties of La_(0.67)Ba_(0.33)Mn_(1-x)(Me)_xO_3 (0 ＜ x ＜ 0.1) are investigated and compared to those existing in La_(0.67)Ba_(0.33)Mn_(1-x)Cr_xO_3 (magnetic Cr~(3+)). The structural refinement by the Rietveld method revealed that Ti-doped samples crystallize in the cubic lattice with space group Pm3m, while samples with Cr crystallize in the hexagonal setting of the rhombohedral R3C space group for identical contents of dopant. The most relevant structural features are an increase of the lattice parameters, of the cell volume and of the inter-ionic distances with increasing Ti doping level. Both series of samples show a decrease of the paramagnetic-ferromagnetic transition temperature when the amount of chromium or titanium increases. Transport measurements show that when increasing the metal doping, the resistivity increases whereas the metallic behavior of the parent compound La_(0.67)Ba_(0.33)MnO_3 is destroyed. For a substitution higher than 5 at.% of Ti and 10 at.% of Cr, the samples exhibit a semiconducting behavior in the whole range of temperature, for which the electronic transport can be explained by variable range hopping and/or small polaron hopping models.