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Broadband Magnetoresistance in Ferromagnetic and Paramagnetic Samples of La0.7Ca0.3-xSrxMnO3

We have studied the room-temperature magnetoimpedance of paramagnetic (x = 0.06) and ferromagnetic (x = 0.1) samples in La0.7Ca0.3-xSrxMnO3 series using a radio-frequency impedance analyzer and also microwave power absorption using a network analyzer. In both measurements, samples were enclosed tightly inside a copper stripcoil and impedance or reflection coefficient of this copper stripcoil was measured as a function of the applied magnetic field for different frequencies of current (f = 0.1 to 2.5 GHz). The direction of the applied magnetic field was perpendicular to the alternating magnetic field produced by the coil. In the ferromagnetic sample (x = 0.1), magnetoresistance shows a peak around zero field for lower frequencies but a peak appears at H away from the origin at higher frequencies. The position of the peak shifts towards higher fields with increasing frequency. A similar trend is also found for the paramagnetic sample (x = 0.06) but the peak occurs at a higher field compared to the ferromagnetic sample for the same frequency. Microwave power absorption also shows features similar to magnetoresistance. Line shape analysis of the data was performed by fitting the data to a Lorentzian function. It is concluded that the observed features are imprints of ferromagnetic resonance in x = 0.1 and paramagnetic resonance in x = 0.06 samples.