Structure, Magnetization and Resistivity of La_(1-x)M_xCoO₃ (M = Ca, Sr, and Ba)

We present an investigation of the influence of structural distortions in charge-carrier doped \lmco by substituting La$^{3+}$ with alkaline earth metals of strongly different ionic sizes, that is M = Ca$^{2+}$, Sr$^{2+}$, and Ba$^{2+}$, respectively. We find that both, the magnetic properties and the resistivity change non-monotonously as a function of the ionic size of M. Doping \lmco with M = Sr$^{2+}$ yields higher transition temperatures to the ferromagnetically ordered states and lower resistivities than doping with either Ca$^{2+}$ or Ba$^{2+}$ having a smaller or larger ionic size than Sr$^{2+}$, respectively. From this observation we conclude that the different transition temperatures and resistivities of \lmco for different M (of the same concentration $x$) do not only depend on the varying chemical pressures. The local disorder due to the different ionic sizes of La$^{3+}$ and M$^{2+}$ play an important role, too.