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 = ω L ) of the complex impedance ( Z = R + j X ) 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 ( μ 0 H d c = 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 μ 0 H d c = 60 mT at f = 2 MHz , comparable to the high field ( μ 0 H d c = 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 .