Microwave surface resistance of colossal magnetoresistance manganites

We present here a systematic study of the microwave surface resistance Rs 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, Rs 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 Rs. That is, Rs varies as ρ-1/2 rather than ρ1/2, the conventional result for a bulk conductor. This phenomenon eludes a complete explanation.