Comparison of dc and microwave resistivity in polycrystalline La0.7-xYxCa0.3MnO3 samples: Influence of Y at grain boundaries

A series of polycrystalline La0.7-xYxCa0.3MnO3 samples were studied, where x=0, 0.07, 0.1, 0.15, and 0.2. Measurement of the dc resistivity, microwave surface resistance, ac susceptibility, and dc magnetization were made. As expected, the magnetic state becomes more frustrated as Y is added. It is f ound that the dc resistivity is approximately two orders of magnitude large r than the 9 GHz resistivity for all samples, however, both increase with Y doping by the same multiplicative factor. From circumstantial evidence and estimation of the grain boundary response time, it is found that the micro wave measurement short circuits the grain boundaries. We assume that the dc resistivity is dominated by grain boundary properties and the microwave re sistivity reflects intragranular behavior. Analysis of the electrical trans port at dc and microwave frequencies is carried out and a phenomenological model proposed which agrees well with observation. The model implies that t he influence of Y is modified by the underlying static disorder in which it is placed. (C) 2000 American Institute of Physics. [S0021- 8979(00)05519-5 ].