Chemical pressure effects of cation size variation in layered manganites

We have investigated the electric and magnetic properties of the layered ma nganite (La1-gRmu)(2-2x)Sr1+2xMn2O7 (R = Pr, Nd. Sm. Eu. Gd: x = 0.3) with the fixed doping concentration. The average ionic radius of the La/Sr site <r(A)> is varied by the replacement of the La ion with different rare earth ion. For <r(A)> similar to 1.3963 Angstrom, we observed the metal-insulato r transition accompanied by the ferromagnetic (FM) ordering near 75 K and s ubsequently the spin-glass-like transition near 30 K. For <r(A)> less than or equal to 1.3895 Angstrom, the metallic FM state disappears and instead t he non-metallic spin-glass-like phase appears around 30 K. With decreasing <r(A)>, the FM double exchange interaction is weakened and resultantly the antiferromagnetic (AFM) superexchange interaction is developed. These magne tic and transport properties in the layered manganite system are found to b e primarily dependent on the size of <r(A)>.