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 4MHz, showing the effects of phase separation or grain boundaries. At 30MHz, 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 4MHz, La0.67Ca0.33MnO3 sol–gel manganite under H=1kOe shows negative ac magnetoresistance ΔR/R0, magnetoreactance ΔX/X0 and magnetoimpedance ΔZ/Z0 below 275K, 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 4MHz. At f=30MHz, the absolute value of negative ac ΔR/R0 increases monotonically with a decrease of temperature for 175K