Enhancement of the Curie temperature in NdBaCo ₂O _5.5 by A-site Ca substitution

Layered cobaltites RBaCo ₂O _5.5 (R = rare earth and Y) undergo a sequence of magnetic and electronic transitions between antiferromagnetic/ferrimagnetic/paramagnetic and insulating/paramagnetic metallic states with respective transition temperatures T _N (∼230-260K) < T _C (∼250-290K) < T _MI (∼360K). We have synthesized a Nd _1-xCa _xBaCo ₂O _5.5 series (0 ≤ x ≤ 0.2) of cation-[(Nd,Ca)/Ba]- ordered and oxygen vacancy-ordered materials and investigated them by neutron diffraction, magnetization, and 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 ∼340K. This is the largest enhancement of T _C observed for these cobaltites. Unlike hole doping by adding oxygen, Ca doping does not disrupt the cation and oxygen vacancy orderings up to x = 0.20.