EFFECTS OF DIMENSIONALITY IN THE RARE-EARTH MANGANATES

Abstract

We report measurements of magnetization, magnetoresistance and ESR on systems with the 3-D and layered structures, to study the effect of reduced dimensionality. Compositions of the systems Pr 0.7 Sr 0.3- x Ca x MnO 3 and La 2-2 y Sr 1+2 y Mn 2- z Cr z O 7 were synthesized, which have the 3D and quasi-2D structures, respectively. The magnetic and transport properties are markedly affected by the reduction in dimensionality. Large values of magnetoresistance have been observed in the layered materials even at low temperature, in contrast with the behavior of the 3D compositions. ESR measurements on the Pr 0.7 Sr 0.3- x Ca x MnO 3 system show a single resonance line in the temperature range of our study. However, compositions of the La 2-2 y Sr 1+2 y Mn 2 O 7 system with y =0.3, 0.4 and 0.5 show a complex behavior. As the samples are cooled, a single resonance line is first observed, which can be described by a Lorentzian. Below ~2 T c, for compositions with y =0.3 and 0.4, a complex lineshape evolves, which can be resolved into two Gaussian lines. This crossover in the lineshape indicates a transition from a homogeneous to an inhomogeneous spin system, which can be attributed to the nature of the ferromagnetic ordering between the bilayers. The composition with y =0.5 shows a different behavior, which might be due to the antiferroma.gnetic ordering exhibited by this composition. A detailed analysis of the experiments is presented.