Magnetic frustration in mixed valence manganites

Investigation of structural and magnetotransport properties of ${\mathrm{L}}_{2\mathrm{/}3}$ ${\mathrm{A}}_{1\mathrm{/}3}$ ${\mathrm{MnO}}_3$ (A=Ca,Sr) oxides reveals a gradual increase of the magnetic frustration with bending of the Mn-O-Mn bonds. The relative strength of competing magnetic interactions is controlled not only by ${\mathrm{R}}_0$ [${\mathrm{R}}_0$ is the mean radius of the ions at the (L, A) site] but also by the electronegativity of the divalent cation. The magnetoresistance of these oxides results mainly from spin disorder and the narrowing of the σ(${\mathrm{e}}_{\mathrm{g}}$) band has only a minor effect. We have observed that the curve ${\mathrm{T}}_{\mathrm{C}}$(${\mathrm{R}}_0$) is not universal but it is sensitive to the alkaline ions.