Low bandwidth Pr-based cobalt perovskites, such as Pr1-xCa xCoO3-δ, have received significant recent attention as they undergo first-order spin-state transitions with a strong influence on magnetic and transport properties. The unique nature of the Pr-O bond has been implicated as the impetus for these transitions, as it is thought that temperature-dependent charge transfer can occur between Pr and Co ions, i.e., a partial Pr3+→Pr4+ and Co4+→Co3 + valence shift. In the present work, we have studied the related compound Nd1-xCaxCoO3-δ. The Nd3 + ions have very similar ionic radius to Pr3+ but do not induce a temperature-dependent valence shift (at least in the composition range studied here), enabling deconvolution of the intrinsic low bandwidth physics from the unique effects of Pr-O bonding in Pr1-xCa xCoO3-δ. To this end, we have characterized the structural, magnetic, and electronic transport characteristics of Nd1 -xCaxCoO3-δ bulk polycrystals, using neutron diffraction, small-angle neutron scattering, dc and ac magnetometry, and magnetotransport, and have established the Nd1-xCa xCoO3-δ magnetic phase diagram. This phase diagram contains regimes of short-range ferromagnetism and long-range ferromagnetism, in addition to ferrimagnetism. We argue that, with the exception of the valence transition that occurs at high x (e.g., x = 0.5) in Pr1-xCa xCoO3-δ and the low-temperature ordering of Nd3 +moments that results in the ferrimagnetism in Nd1 -xCaxCoO3-δ, the two systems are nearly isostructural and have similar magnetic and transport properties. The low bandwidth physics intrinsic to both systems is summarized as encompassing long-range ferromagnetism with a relatively low Curie temperature due to Co-O-Co bond buckling (<60 K for Nd1-xCaxCoO3 -δ), short-range ferromagnetism that emerges at much higher temperatures (∼270 K for Nd1-xCaxCoO3 -δ), and likely stems from oxygen deficiency, exchange-spring behavior related to magnetoelectronic phase separation, and a doping-driven insulator-metal transition. In addition to elucidating the essential physics of narrow bandwidth perovskite cobaltites, the results thus further confirm the importance of the unique features of the Pr-O bond in driving the abrupt spin-state transition in Pr1-xCaxCoO3-δ.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - Aug 9 2013|