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 magne tic frustration with bending of the Mn-O-Mn bonds. The relative streng th 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 th e electronegativity of the divalent cation. The magnetoresistance of t hese 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 c urve T-C(R(0)) is not universal but it is sensitive to the alkaline io ns.