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1256-N04-01

Giant ac electrical response of La0.7Sr0.3MnO3 in sub-kilogauss magnetic fields A. Rebello, V. B. Naik, S. K. Barik, M. C. Lam and R. Mahendiran Department of Physics, Faculty of Science, National University of Singapore, 2 Science Drive 3, Singapore -117542, Singapore ABSTRACT We report ac electrical transport in the metallic ferromagnet La0.7Sr0.3MnO3. 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 (TC) for f = 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 (∆R/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 (∆X/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. INTRODUCTION Perovskite manganese oxides of the general formula R1-xAxMnO3 (R = trivalent rare earth ion, A = divalent alkaline earth ion) have been a topic of intensive investigation for the past one decade due to the colossal magnetoresistance effect shown by them.1 While several exotic electronic and magnetic phases exhibited by these oxides shifted recent interest in these materials towards more fundamental studies on strongly correlated electron systems,2 technological exploitation of the colossal magnetroresistance effect at room temperature has been hampered by the need of magnetic field of µ0H > 1 T to induce more than 10 % magnetoresistance (MR). Extensive efforts have been under taken to enhance the magnitude of magnetoresistance at low fields, particularly in milli and micro Tesla range using different techniques such as artificial grain boundary in epitaxial films3, trilayer tunnel junctions4, step edge junctions5 or exploiting magnetoresistance arising from the spin polarized tunneling of electrons between magnetic grains in polycrystalline manganites.6,7 Although tunneling magnetoresistance in polycrystalline manganites can be 40-50 % in a field of 0.5 T at T = 10 K, it decreases with increasing temperature and becomes zero near the ferromagnetic Curie temperature.8 One of the aims of our present work is to enhance the low-field magnetoresistance using an alternative strategy. We w