The ac transport and giant magnetoimpedance for sol-gel La0.67Ca0.33MnO3

In the present work, the temperature dependences of ac transport and magnetoimpedance for sol-gel La0.67Ca0.33MnO3 manganite are investigated at various frequencies. The ac resistance R with absence of field shows a peak around the metal-insulator transition temperature TMI for the dc case. A low magnetic field can shift the metal-insulator transition temperature more easily at high frequency than at low frequency. The valley phenomenon is observed in the temperature dependence of reactance X at 2 and 4 MHz, showing the effects of phase separation or grain boundaries. At 30 MHz, the peak of X appears instead of the valley, since the contribution of insulator capacitance term drops greatly and the inductance of metal dominates the X term with an increase of ac frequency in the sample. At 2 and 4 MHz, La0.67Ca0.33MnO3 sol-gel manganite under H=1 kOe shows negative ac magnetoresistance ΔR/R0, magnetoreactance ΔX/X0 and magnetoimpedance ΔZ/Z0 below 275 K, but positive ac ΔR/R0, ΔX/X0 and ΔZ/Z0 near room temperature much higher than the Curie temperature. The peak temperature of R/R0 moves to low temperature with an increase of the ac frequency from 2 to 4 MHz. At f=30 MHz, the absolute value of negative ac ΔR/R0 increases monotonically with a decrease of temperature for 175 K