Magnetic and Electronic Properties of Ln$_{1-x}$Sr$_x$CoO$_3$

The magnetic and electronic properties of bulk, polycrystalline, Ln$_{1-x}$Sr$_{x}$CoO$_{3}$ (Ln=La, Nd and Pr, 0$\le $x$\le $0.6) were investigated. The La and Nd systems show a crossover from insulating glassy behavior to metallic ferromagnetism at x=0.18. The only differences between the two systems are the lower transition temperatures for Nd (due to reduced bandwidth) and the antiferromagnetic alignment of Nd and Co, inducing ferrimagnetism. Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ however, is radically different. A similar cluster glass to ferromagnet crossover occurs at low doping but at higher doping the system exhibits a second magnetic transition [1], well below $T_{C}$. Through a systematic investigation as a function of composition we have discovered that this effect is maximized at half-doping, vanishing completely at x$<$0.40 and x$>$0.60. Possible explanations (including ferrimagnetism, spin-state transitions, phase competition, and charge/orbital ordering) are discussed in light of the neutron diffraction, transport and magnetometry results. We acknowledge support from the ACS Petroleum Research Fund. [1] R. Mahendiran and P. Schiffer, Phys. Rev. B. \textbf{68} 024427 (2003).