Effects of dimensionality in the rare-earth manganates

We report measurements of magnetization, magnetoresistance and ESR on syste ms with the 3-D and layered structures, to study the effect of reduced dime nsionality. Compositions of the systems Pr0.7Sr0.3-xCaxMnO3 and La2-2ySr1+2 yMn2-zCrzO7 were synthesized, which have the 3D and quasi-2D structures, re spectively. The magnetic and transport properties are markedly affected by the reduction in dimensionality. Large values of magnetoresistance have bee n observed in the layered materials even at low temperature, in contrast wi th the behavior of the 3D compositions. ESR measurements on the Pr0.7Sr0.3- xCaxMnO3 system show a single resonance line in the temperature range of ou r study. However, compositions of the La2-2ySr1+2yMn2O7 system with y=0.3, 0.4 and 0.5 show a complex behavior. As the samples are cooled, a single re sonance line is first observed, which can be described by a Lorentzian. Bel ow similar to 2T(c), for compositions with y = 0.3 and 0.4, a complex lines hape evolves, which can be resolved into two Gaussian lines. This crossover in the lineshape indicates a transition from a homogeneous to an inhomogen eous spin system, which can be attributed to the nature of the ferromagneti c ordering between the bilayers. The composition with y = 0.5 shows a diffe rent behavior, which might be due to the antiferromagnetic ordering exhibit ed by this composition. A detailed analysis of the experiments is presented .