In order to know if the origin of ferromagnetism induced by the substitution of Co or Ni for Mn in ${\mathrm{LaMnO}}_3$ could be due to their divalent oxidation states, a set of ${\mathrm{LaMn}}_{1-x}{M}_x{\mathrm{O}}_3$ samples has been prepared for $x<~0.2$ and $M=\mathrm{Li},$ Zn, Ni, Co, Ga, and Rh. By combining resistivity, thermopower, magnetization, susceptibility measurements, and room-temperature structural refinements, we show that, similarly to Ni and Co, univalent $\left({\mathrm{Li}}^{+}\right)$ and divalent $\left({\mathrm{Zn}}^{2+}\right)$ $S=0\mathrm{}$ cations induce both ferromagnetism and a more conductive behavior. In contrast, the $S=0\mathrm{}$ trivalent cations $({\mathrm{Rh}}^{3+}$ and ${\mathrm{Ga}}^{3+})$ are found to preserve the orthorhombic $O^{\prime }$ structure, connected with the cooperative Jahn-Teller distortion of ${\mathrm{Mn}}^{3+}$ species in ${\mathrm{LaMnO}}_3.$ Accordingly, ${\mathrm{Rh}}^{3+}$- and-${\mathrm{Ga}}^{3+}$ substituted ${\mathrm{LaMnO}}_3$ manganites are only weak ferromagnets, and the resistivity increases in comparison to ${\mathrm{LaMnO}}_3.$ These results indicate that, for the investigated range of substitution (a maximum of 20%), both cobalt and nickel behave like divalent cations. The observation of ferromagnetism and the resistivity decrease, induced by univalent and divalent cations, ${\mathrm{Li}}^{+}$ and ${\mathrm{Zn}}^{2+},$ is due to the creation of ${\mathrm{Mn}}^{4+}$ species which favor ${\mathrm{Mn}}^{3+}/{\mathrm{Mn}}^{4+}$ double exchange.

• Received 30 July 2001

DOI:https://doi.org/10.1103/PhysRevB.65.104420