Magneto-transport and ferromagnetic resonance studies of polycrystalline La0.6Pb0.4MnO3 thin films

To understand the nature of grain boundaries in polycrystalline materials, magneto-transport and ferromagnetic resonance measurement have been performed in polycrystalline La_0.6Pb_0.4MnO₃ (LPMO) thin films prepared by pulsed laser deposition. Films are found to undergo a semiconductor to metal transition at 230 K and re-enter into the semiconducting state below 130 K. Microwave absorption measurements carried out as function of applied field show two components of resonant absorption signal. First component is in accordance with ferromagnetic transition of grains at Curie temperature and the second component shows antiferromagnetic transition of grain boundaries at 160 K. An additional non-resonant absorption signal centered at zero field has also been observed that supports transition from conducting to insulating grain boundaries at ∼160 K. Further, temperature dependence of resistance in semiconducting state at low temperatures is in accordance with coulomb blockade model indicating insulating nature of AFM grain boundaries.