Magnetic polarons and spin-glass behavior in insulating La_{1-x}Sr_{x}CoO_{3} (x=0.125 and 0.15)

The evolution of magnetic polarons in Sr doped LaCoO_{3} (La_{1-x}Sr_{x}CoO_{3}) single crystal and polycrystalline samples are investigated by employing dc and ac magnetic measurement and small angle neutron scattering (SANS) techniques. The effect of magnetic field and temperature on magnetic polarons is experimentally studied for La_{0.875}Sr_{0.125}CoO_{3} and La_{0.85}Sr_{0.15}CoO_{3} compounds that belong to the spin glass insulating regime of the broader compositional phase diagram of this system. Langevin analyses of the isothermal magnetization curves in the notional paramagnetic regime prove the existence of magnetic polarons with large moments. The dc field superimposed ac susceptibility data and the analysis of the glassy dynamics prove that the size of polarons in 15% Sr doped crystal increase as the field is increased while the field effect is not visible in the 12.5% Sr doped crystal. A polycrystalline sample of La_{0.85}Sr_{0.15}CoO_{3} is analyzed by SANS experiments, which confirm nonzero correlation length at temperatures far above the macroscopic ordering temperature and hence the presence of magnetic polarons.