Giant magnetoimpedance and high frequency electrical detection of magnetic transition in La0.75Sr0.25MnO3

We show that high frequency electrical transport is an efficient technique for detecting the magnetic transition hidden by the scattering of charges at grain boundaries in colossal magnetoresistive oxides, even in the absence of any external magnetic field. The dc resistivity which shows only a weak anomaly at the Curie temperature in La0.75Sr0.25MnO3 transforms into abrupt jumps in both resistive (Z') and reactive (Z'') parts of the ac impedance, Z(f, T, H) = Z'(f, T, H) + jZ''(f, T, H) at higher frequencies (f = 0.1-5 MHz). The anomaly in Z' and Z'' at T-C decreases as much as 19% and 15%, respectively, in a dc magnetic field of H = 65 mT compared with 1% dc magnetoresistance, suggesting a possible giant low-field magnetoimpedance effect which could be exploited for room temperature practical applications. We interpret our observations due to changes in the magnetic penetration depth induced by the spontaneous ordering of spins and by the applied field.