Giant ac electrical response of La0.7Sr0.3MnO 3 in sub-kilogauss magnetic fields

Abstract

We report ac electrical transport in the metallic ferromagnet La 0.7Sr0.3MnO3. Both ac resistance (R) and reactance (X) were measured as a function of temperature (T= 400-100 K), frequency of the ac current (f = 100 kHz - 20 MHZ) and external dc magnetic field (H = 0-100 mT) applied parallel to the current direction. It is shown that, while R(H = 0 T) decreases smoothly around the Curie temperature (T C) for I = 100 kHz, an abrupt increase followed by a peak close to TC occurs for f ≥ 500 kHz. The peak decreases in magnitude, broadens and shifts down in temperature with increasing values of H. The peak in R is completely suppressed under H= 100 mT resulting in a huge low-field ac magnetoresistance (ΔR/R= -53 % for f= 2MHz) whereas the dc magnetoresistance only -31 % even at H = 7 T. While the reactance X(H = 0 T) also shows an abrupt increase at TC for f < 10 MHz, it decreases abruptly at TC for f ≥ 12 MHz. The magnetoreactance is largest (ΔX/X= -47 %) at f = 100 kHz and it changes sign from negative to positive with increasing frequency. It is suggested that the observed huge ac magnetoresistance arises from decrease of magnetic permeability which enhances skin depth under a magnetic field. Our results indicate that the extraordinary sensitivity of the ac magnetoresistance to low dc magnetic fields can be exploited for device applications. © 2010 Materials Research Society.