Enhancing the Low-Field Magnetoresistance by AC Current Excitation in La0.7Sr0.3MnO3

Although colossal magnetoresistance (MR) with direct-or low-frequency currents (f < 1 KHz) in polycrystalline La0.7Sr0.3MnO3 sample has been extensively reported in the literature, magneto-transport in megahertz to gigahertz range is seldom studied. Here, we report the magnetic field dependence of ac resistance (R) and reactance (X) of polycrystalline La0.7Sr0.3MnO3 sample carrying high-frequency current (f = 1 MHz-3 GHz) in axial dc magnetic fields (-1 kOe < H < 1 kOe) at 300 K. While R(H) shows a single peak centered around H = 0 for 1 <= f <= 30 MHz, the peak transforms into a dip at H = 0 accompanied by double peaks at H = +/- H-P for higher frequencies. The position of the double peaks shifts to higher magnetic fields with increasing frequency. Alternating current MR at 1 kOe increases from -0.7% for f = 1 MHz, -20% for f = 50 MHz, and + 28% for 3 GHz at H-p. The transition from the single to double peak behavior is also seen in X(H) with additional features near-zero field for f >= 2 GHz. While the enhanced value of the negative MR is attributed to the increase in magnetic skin depth due to the suppression of transverse permeability, the positive MR is attributed to the contribution from induced strong transverse permeability and ferromagnetic resonance.