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 La_0.67Ca_0.33MnO₃ manganite are investigated at various frequencies. The ac resistance R with absence of field shows a peak around the metal–insulator transition temperature T_MI 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, La_0.67Ca_0.33MnO₃ 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<T<275 K, whereas the ΔX/X₀ is positive below 245 K but negative above 245 K. The ΔZ/Z₀ under H=1 kOe is positive below 238 K but negative above 238 K. Such a positive ΔZ/Z₀ is due to an increase of permeability induced by field. With an increase of field, the impedance Z experiences a peak, which is connected with the effect of transverse magnetic anisotropy.