Microwave Response of CMR Manganites

We report on the microwave absorption of CMR manganite R_0.67M_0.33MnO₃ (R = La, Nd; M = Ba, Sr) films at 10 and 36 GHz using a cavity perturbation technique. Films were grown by pulsed-laser deposition onto LaAlO₃ substrates and the thicknesses ranged from 1000 to 2500 ÅThe dc resistivity ρ varied from 1 to 200 mΩ-cm, depending upon the temperature and sample. For films thinner than the skin depth δ, the conventional microwave result is that the surface resistance R_s scales with ρ. In contrast, for these highly resistive films R_s scales with the dc conductivity σ, as confirmed by both the temperature and field dependence of R_s. This type of behavior of R_s upon σ is usually expected for insulating materials. It is interesting to note that σ for these films is greater than 10^3 Ω^-1-cm^-1, the Mott minimum conductivity.