Broad band magnetotransport at room temperature in La0.7Sr0.3-xCaxMnO3: Electrically detected magnetic resonances

We report the room temperature magnetoimpedance in bulk samples of La0.7Sr0.3-xCaxMnO3 (0 <= x <= 1) subjected to ac current excitation of frequency f = 1 MHz to 3 GHz and dc magnetic fields (- 5 kOe < H < 5 kOe). The ground state changes from ferromagnetic (FM) for x <= 0.20 to paramagnetic (PM) for x >= 0.24 at 300 K. Ac magnetoresistance is not only higher in magnitude than dc magnetoresistance but shows interesting features: It is negative and shows a single peak at H-dc = 0 below certain frequencies in FM samples but transforms into positive double peaks at H-dc = +/- H-k and the double peak feature shift towards higher magnetic fields with increasing frequency of the ac current. Surprisingly, the ac magnetoresistance in PM samples is also negative and displays a single peak at 500 MHz, which transmutes into an abrupt increase at a critical field value for f > 700 MHz. A plot of H-dc corresponding to the double peak/critical field versus frequency suggest that the curves are linear for PM samples and non-linear for FM samples. We suggest that the observed anomalies in the ac magnetoresistance are due to electron paramagnetic resonance for x >= 0.24 and ferromagnetic resonance for x <= 0.20. Thus, we are able to electrically detect paramagnetic and ferromagnetic resonances in these conducing magnetic oxides through a simple magnetoimpedance measurement.