Microwave surface resistance of colossal magnetoresistance manganites

We present here a systematic study of the microwave surface resistance R_s of both ceramic and single-crystal colossal magnetoresistance manganites over both a large (2-60 GHz) frequency and a wide (77-400 K) temperature range. In many cases, R_s shows an anomalous dip just below the Curie point which has no analogous feature in the dc resistivity ρ. This minimum shifts in temperature with both frequency and application of a magnetic field. We demonstrate that this 'anomaly' is a consequence of the dynamics of a conducting ferromagnet and holds promise for use in developing new microwave devices. In addition, high-resistivity material shows an inversion effect in R_s. That is, R_s varies as ρ^-1/2 rather than ρ^1/2, the conventional result for a bulk conductor. This phenomenon eludes a complete explanation.