Low-field microwave absorption in Ga-doped polycrystalline La0.6Sr0.4MnO3in the frequency range from 0.1 to 4 GHz

We investigated the magnetic-field-dependent microwave absorption in polycrystalline La(0.6)Sr(0.4)Mn(1-x)Ga(x)O(3)samples (x = 0.0-0.3) over a wide frequency spectrum (f = 0.1-4 GHz) and up to a magnetic field ofH(dc)= 2.5 kOe at room temperature. The field dependence of microwave power absorption (Delta P) in each sample exhibits a peak at a critical value ofH(dc)and the peak moves towards higher fields as frequency increases. The shift is at a faster rate forx = 0.2 and 0.3 compared tox = 0.01 and 0.1. Based on the line shape analysis, we attribute the observed features to ferromagnetic resonance forx <= 0.1 and a combined effect of electron spin resonance and superparamagnetic resonance forx >= 0.2. Our results are also verified by magnetic resonance spectra recorded using a coplanar waveguide spectrometer. We extracted the gyromagnetic ratio, saturation magnetization, and anisotropy field in our samples. Frequency-tunable microwave absorption at resonance found in these materials will be of interest for high-frequency device applications.