Enhancement of the Curie temperature in NdBaCo2O5.5byA-site Ca substitution

Abstract

and investigated them by neutron diffraction, magnetization, electronic and thermal transport. We observe that upon Ca doping T N is decreasing to 0 for x=0.1 and T C is increasing and coincides with T MI for x>0.12, which weakly changes with Ca substitution from ~360 to ~340 K. This is the largest enhancement of T C observed for these cobaltites.Unlike hole doping by adding oxygen, the Ca doping does not disrupt the cation and oxygen vacancy orderings up to x=0.20.2

Impact of charge doping, oxygen disorder and hydrostatic pressure on thermoelectric and magnetic properties of NdBa0.94La0.06Co2O5+δ

“…x BaCo (3+x/2)+ 2 O 5.5 system for which a wide single-phase hole doping range c = x/2 = 0-0.1 was achieved, the T MIT was only slightly and linearly suppressed from 345 to 340 K [5]. The T C showed linear increase from 260 to 340 K for c = 0-0.08 (x = 0.16) where it merged with the T MIT .…”

“…Thermal conductivity exhibits characteristic decrease at T MIT due to removal of the electronic contribution and is well correlated with suppression of the Seebeck coefficient. The T MIT and the Neel temperature T N reach maximum values for slightly electron doped sample c = À0.03 with a perfect oxygen ordering d = 0.5 while the Curie temperature T C shows continuous decrease with the increase of d. Comparison of magnetic ac and dc measurements with those for the GdBaCo 2 O 5+d (Taskin et al, 2005) and Ca substituted NdBaCo 2 O 5.5 (Kolesnik et al, 2012) systems shows that perfection of the oxygen ordering is more important in controlling the magnetic properties than the overall charge doping. The hydrostatic pressure was observed to stabilize the antiferromagnetic phase for NdBa 0.94 La 0.06 Co 2 O 5.5 .…”

“…The layered perovskites RBaCo 2 O 5+d (R = lanthanide ion or Y, d = 0.5) are characterized by a sequence of complex magnetic and electronic phase transitions observed with increasing temperature: antiferromagnet-ferromagnet (T N = 200-250 K), ferromagnet-paramagnet (T C = 280-310 K), and insulator-metal (T MIT = 330-360 K), which can be tuned by a substitution at the R-, Ba-and Co-sites or by a change of oxygen content 5+d (d-doping) [1][2][3][4][5]. The perovskite-related structure [3] consists of the layers of RO d -CoO 2 -BaO-CoO 2 aligned along the c-axis.…”

“…Pure charge doping was studied by heterovalent substitutions at the R-site for compounds with d = 0.5, which maintained the oxygen vacancy ordering, i.e., it did not affect the ratio of the pyramidal and octahedral coordinations of cobalt [5]. Rather different phase diagram of electronic and magnetic transitions was established by using cation substitutions than by d-doping.…”

“…The Co ions are coordinated by 5 or 6 oxygen ions, forming pyramids or octahedrons, depending on the oxygen content d. The addition or removal of oxygen ions to/from the RO d planes changes the ratio of the Co octahedral to pyramidal coordination and alters charge doping of the CoO 2 planes. For d = 0, i.e., for the compound with no oxygen in the RO d layer, all Co 2.5+ ions are pyramidally coordinated (CoO 5 ). An increase of d to 1 leads to a transition to octahedral coordination of all Co 3.5+ ions (CoO 6 ).…”