Magnetic frustration in mixed valence manganites

Investigation of structural and magnetotransport properties of L(2/3)A(1/3)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 R(0) [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 sigma(e(g)) band has only a minor effect. We have observed that the curve T-C(R(0)) is not universal but it is sensitive to the alkaline ions.