Low-field magnetoimpedance in La0.7Sr0.3MO3 (M=Mn,Co)

We compare the ac magnetotransport in two ferromagnetic metallic oxides, La0.7Sr0.3MO3 (with M=Co and Mn) which show similar dc electrical properties. The ac resistance (RR) and inductive reactance (X=ωLX=ωL) of the complex impedance (Z=R+jXZ=R+jX) are studied as a function of temperature at a few fixed frequencies (f=10kHz to 7 MHz) of the ac current excitation in zero and a low external dc magnetic field (μ0Hdc=60mT). It is shown that as the frequency (ff) increases, RR in zero field shows a transition from a smooth decrease to an abrupt increase and a peak close to the ferromagnetic Curie temperature (TC)(TC). A rapid increase in XX at TCTC was also found for frequencies above 10 kHz. We report a large low-field ac magnetoresistance of 33% (7%) for Mn (Co) in μ0Hdc=60mT at f=2MHz, comparable to the high field (μ0Hdc=7T) dc magnetoresistance. It is suggested that the low-field ac magnetoresistance in these compounds arises from the change in the magnetic penetration depth and suppression of magnetic fluctuations close to TCTC.