Magnetically inhomogeneous ground state below the first-order valence transition in (Pr1−yYy)0.7Ca0.3CoO3−δ

Abstract

Pr-based perovskite cobaltites, such as ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ and (${\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{)}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ have recently been discovered to undergo a first-order metal-insulator transition on cooling, thought to arise from an unusual shift in electron occupancy from Pr to hybridized Co-O orbitals. While this transition is known to generally suppress long-range ferromagnetic ordering, the true nature of the magnetic ground state remains unclear. In this work, we have performed structural, magnetic, transport, magnetotransport, and small-angle neutron scattering measurements on (${\mathrm{Pr}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{)}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($0.000\ensuremath{\le}y\ensuremath{\le}0.200$) in order to develop a complete microscopic picture of the ground state and a full appreciation of the evolution of the first-order transition with Y doping. Our magnetization and zero-field resistivity measurements confirm the presence of an abrupt temperature-dependent metal-insulator transition on cooling, accompanied by a sharp drop in magnetization, setting in between $y$ = 0.050 and 0.075. Small-angle neutron scattering measurements suggest the presence of critical ferromagnetic fluctuations above the metal-insulator transition for $y$ = 0.075, indicating that the system is poised to order ferromagnetically. This magnetic scattering is suppressed at the metal-insulator transition but reemerges at lower temperatures (below 40--50 K) due to the formation of a ferromagnetic cluster state. The clusters have a mean correlation length of \ensuremath{\sim}50 \AA{} at 4 K, although magnetic inhomogeneity occurs across a broad spectrum of length scales, evidencing a highly inhomogeneous ground state, which we relate to the numerous sources of chemical and structural disorder. Interestingly, the magnetically inhomogeneous state manifests an intercluster magnetoresistance effect and a strong field-cooling effect on the low-temperature transport. We interpret these results, quite generally, in terms of the electronic shift from Pr to Co driving the system from the ferromagnetic side of the generic perovskite cobaltite magnetic phase diagram to the insulating side (where ferromagnetic clusters are well known to exist). These results thus shed significant light on the formation of the magnetically inhomogenous ground state of Pr-based cobaltites undergoing a first-order metal-insulator transition and indeed provide clear and direct evidence of such.