Giant magnetoimpedance and grain boundary effect in La0.75Ba0.25MnO3 sol–gel manganite

The La0.75Ba0.25MnO3 sol–gel manganite shows a typical dc low field magnetoresistance (LFMR) effect at low temperature. Besides the sharp resistance peak representing the metal–insulator transition around Curie temperature, another broad peak associated with the effect of grain boundaries could be observed around 260 K. At low frequencies, such double peaks in ac resistance also occur. With increasing ac frequency, skin effect gradually enhances, the peak associated with the grain boundaries becomes less pronounced. At 0.5 MHz, beside the peak of ac magnetoresistance ΔR/R0 at Curie temperature, the ac ΔR/R0 increases with decreasing temperature, since the ac resistance is strongly dependant on dc resistivity at low frequency. With increasing ac frequency, the peak of ac ΔR/R0 at Curie temperature becomes weak and almost disappears, as the ac ΔR/R0 strongly depends upon variation of magnetic-permeabilities at high frequencies. The magnetoreactance originates mainly from the change of permeability induced by the field. At 300 K, the value of magnetoimpedance ΔZ/Z0 for La0.75Ba0.25MnO3 sol–gel polycrystalline sample can reach −49% at f=4 MHz under a very small field of 500 Oe.