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 (R) and inductive reactance (X = omega L) of the complex impedance (Z = R + jX) are studied as a function of temperature at a few fixed frequencies (f = 10 kHz to 7 MHz) of the ac current excitation in zero and a low external dc magnetic field (mu H-0(dc) = 60 mT). It is shown that as the frequency (f) increases, R in zero field shows a transition from a smooth decrease to an abrupt increase and a peak close to the ferromagnetic Curie temperature (T-C). A rapid increase in X at T-C was also found for frequencies above 10 kHz. We report a large low-field ac magnetoresistance of 33% (7%) for Mn (Co) in mu H-0(dc) = 60 mT at f = 2 MHz, comparable to the high field (mu H-0(dc) = 7 T) 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 T-C. (C) 2009 Elsevier Ltd. All rights reserved.