High Frequency Electrical Transport in La $_{0.67}$ Ba $_{0.33}$ MnO

We report four probe ac electrical transport in La_0.67Ba _0.33MnO₃. Ac resistance (R) and reactance (X) were measured simultaneously as a function of temperature (T) and magnetic field (H) over a wide frequency range (f = 1-22 MHz). While the insulator-metal transition around the Curie temperature (TC) is accompanied by a primary peak in the dc resistivity, a secondary peak develops in R in zero field just below the primary peak with increasing f and it supersedes the primary peak as f increases above 5 MHz. On the other hand, X in zero field for f ≤ 5 MHz shows an abrupt increase at T = 321 K which coincides with the ferromagnetic Curie temperature (TC) . Well below TC , a step-like anomaly develops at T = 190 K while cooling. For f ≥ 10 MHz, both the anomalies seen at TC and around 190 K are more pronounced in R as compared to X and are sensitive to applied H. We report huge values of ac magnetoresistance (-45%) and magnetoreactance (-40%) at TC for ΔH = 1 kG for f = 5 MHz. It is suggested that the low-T anomaly is due to a rhombohedral to orthorhombic structural transition in the compound. Our study also suggests that ac electrical transport in this compound not only results in a high value of ac magnetoresistance, but it is a valuable tool to detect structural transition which is not visible in the dc resistivity.